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Fixing a 2246A with a dead channel 3

 

I've posted a quick story of how I fixed a dead channel 3 in a 2246A (Mod A) bought at an auction:



I wonder about the unsoldered resistor -- it looked as though it has never been soldered in the first place, but that's simply not possible. Is that what a cold solder joint looks like after it breaks?

Also, I wonder about why my service manual did not correspond to the main board I had. It was fine in the channel preamp area, but really diverged in the back of the main board, where the control ICs were.

All in all, a success story, and I'm a happy user now.

--J.
Re: TEK parts, a reliable supplier

 

I have bought Tek parts from QService as well and have been very impressed with their excellent service and parts. Your story is a good one and shows service above and beyond.

George

--- In TekScopes@..., "viagioz" <viagioz@...> wrote:

Dear community...

This is not publicity, nor an advertisement, but just let me share with you the excellent experience I had with this supplier:


support@...

I`ve ordered one Hybrid that failed once assembled, because I forgot to connect back R1913 and R1920 in a 2465B. The hybrid was and is OK!
The bellow email is the proof that there are still serious guys playing around:

"Hello Again

According to our records and your delivery note, we sent you a 155-0242-01 ic with Production date code = 516684

All ICs are actively tested before sending out, and this was tested as well, in any case, and unlike other online vendors, we guarantee all parts for correct operation, so you can send the IC back to

QSERVICE ELECTRONICS
SKEVOU ZERVOU 10
RHODES 85100 GREECE
TEL 22410-33837

Please use registered airmail postal service shipping as we sent it to you, and advise what was the amount paid, we will cover that

Once received, we will check the IC in a test oscilloscope, and will send another out, we have many of these,

However, although the typical failure symptom for this ic are the the dots appearing on the screen, display sequencer ic, ot the HV assy itself may as well cause the same problem.

In any case we have no problem replacing the suspected IC

Thanks and have a good evening

Dinos"
TEK parts, a reliable supplier

 

Dear community...

This is not publicity, nor an advertisement, but just let me share with you the excellent experience I had with this supplier:


support@...

I`ve ordered one Hybrid that failed once assembled, because I forgot to connect back R1913 and R1920 in a 2465B. The hybrid was and is OK!
The bellow email is the proof that there are still serious guys playing around:

"Hello Again

According to our records and your delivery note, we sent you a 155-0242-01 ic with Production date code = 516684

All ICs are actively tested before sending out, and this was tested as well, in any case, and unlike other online vendors, we guarantee all parts for correct operation, so you can send the IC back to

QSERVICE ELECTRONICS
SKEVOU ZERVOU 10
RHODES 85100 GREECE
TEL 22410-33837

Please use registered airmail postal service shipping as we sent it to you, and advise what was the amount paid, we will cover that

Once received, we will check the IC in a test oscilloscope, and will send another out, we have many of these,

However, although the typical failure symptom for this ic are the the dots appearing on the screen, display sequencer ic, ot the HV assy itself may as well cause the same problem.

In any case we have no problem replacing the suspected IC

Thanks and have a good evening

Dinos"
Re: Power supply problem with Tek 2220

Francis
 

Hi Tom and all,

I would like to add my one cent opinion here...

I have made many trials on this kind of inverters, and I am
convinced that TIP41C is definitely NOT the right choice for
this application, due to their huge storage time.
After trying everything I could find in my drawers, I discovered
a bag of NOS TIP31, not even A,B or C, mostly made by Motorola.

Believe it or not, their behaviour regarding switching is VERY
similar to the original Tek components, including some softness
in the transition which gives a good collector waveform, with less
spikes than other types.

Ok, they are only 40v Vce max.
Only? I made a very simple 250v DC supply and found that all had
a 150v Vce capability, with floating base, and at least 250v with
base tied to emitter. I tried two of them on a 2245 spare PS, and
got an excellent result, working flawlessly during hours.

Hope that this will help somebody.............

--- In TekScopes@..., "tom jobe" <tomjobe@...> wrote:

Hi Tan,
I see that JC has reported that his properly working 2230 has 0.23 volts
across R907.
I remembered that I had a 2230 so I found it and a note on it said that I
had worked on it in 2009 and that I had put some new parts in the power
supply.
I took the outer covers off of the 2230 and ran it on the bench. R907 had
0.245 volts while cold, and 0.246 volts after 30 minutes. The side of the
chassis where the heatsink for Q9070, Q946, Q947 is attached, barely got
warm to the touch in 30 minutes (maybe 2 or 3 degrees C more?).
I also checked R949, it was 0.683 volts cold, and 0.687 volts after 30
minutes.

The voltage on R949 came up on Irwin Zosa's 2235 when he discovered that one
of the inverter transistors (Q946-Q947) had a leaky emitter to collector
junction (in one direction only). Irwin replaced the transistor pair with
some new TIP41C transistors and saw some improvement in his heat shut down
problem.
Re: 155-0242-01: Hybrid decapsulated

 

Hi Tom!
I believe that those silicon chips are custom ASICS. Specially because the substrate is tied to a -5V supply and the silicon have the TEK logo in a corner¡­

From what I investigated, Tek had its own design:
Tektronix Hybrid Components Organization was originally
created in 1970.
In 1994, this organization was spun out as a joint venture
between Maxim and Tektronix, as Maxtek.
It was reacquired by Tektronix in 2000 and renamed Tektronix
Component Solutions

"Maxim officially completed the acquisition of Tektronix's monolithic integrated circuits (IC) operation
located in Beaverton, Oregon on May 28, 1994. The deal included a joint venture with Tek to operate the hybrid
IC business of Tek's wholly owned subsidiary, Tektronix Components Corp." in Maxim Engineering Journal.

To develop a SMT replacement for this hybrid, I would need to know a lot more about these chips. 20 years past, patent protection is not an issue anymore, I think¡­ If those guys from component-solutions could share some details, it would be a start.

For reverse engineering it seems to be quite hard¡­ the driver IC is quite complex, mixes analog and digital signals¡­ I would need to pass 100's of hours with eyes sticked to a >100x magnification microscope!

My idea for now is to understand the failure mode to eventually add a protection to VZ OUT and try to keep the scope alive a few more years...

Regards,
Jose.

--- In TekScopes@..., "Thomas Miller" <tmiller11147@...> wrote:

Very nice work, Jose. Any idea what the chips are?

Keep up the great work.


Tom


----- Original Message -----
From: viagioz
To: TekScopes@...
Sent: Wednesday, January 25, 2012 9:37 AM
Subject: [TekScopes] Re: 155-0242-01: Hybrid decapsulated





I published two photos from the those 3 silicon chips inside. One driver, two similar power amps. Will try to get bigger magnification, localize the failure...

Cheers! CT1DGN

The LINK:


--- In TekScopes@..., Jerry Massengale <jmassen418a@> wrote:
>
>
> Jose,
>
> Great pics. Thank you.
>
> jerry
>
>
>
>
>
>
> Jerry Massengale
> jmassen418a@
>
>
>
>
> -----Original Message-----
> From: viagioz <viagioz@>
> To: TekScopes <TekScopes@...>
> Sent: Fri, Dec 16, 2011 9:08 am
> Subject: [TekScopes] 155-0242-01: Hybrid X-Ray Photos
>
>
>
>
> HI frinds,
>
> I share with you the inside from TEK24xx Z Axis Hybrid:
>
>
>
> At this level investigation everything seems to be OK inside (the 3 die attach, bounding...)
>
> It is related with the subject "2465B no trace".
>
> I try now to borrow an hybrid to be sure it is the source of the problem.
>
> Then I will remove the ceramic attached underneath and I will share with you microscope pictures.
>
> Nice weekend, maybe Merry Christmas! I will not get the new hybrid before next year, I guess!
>
> 73'
>
> Jose
> CT1DGN
>
Question on other's experience with T935 A intensified by B mode

 

I have acquired a T935 scope for essentially free. After cleaning it up, it seams to work very nicely. I am actually very impressed with the design, especially the mechanical. It is miles ahead of the 465/475, which came out only a few years earlier.

Mine has an issue with the intensity modulation in A intensified by B mode. With the A trace at normal or low viewing intensity, the intensified B sweep is so bright that it blooms the display. The intensified section appears nearly 5 mm thick.

That can't be normal, but I was wondering if other members who have this scope see something similar. These is no adjustment for the z axis intensity during the B sweep portion like older scopes had. It is determined by the current through a network of about 5 resistors when in the A intensified by B mode, and also depends on the output impedance of a standard 7400 NAND gate. Before I poke around a bit to see what is going on, I was wondering if others see something similar.

The circuitry is not easily accessible BTW, as the horizontal board sits underneath the vertical input amplifier board.

- Steve
Re: 7704A Readout Board

 

Make sure the readout isn't turned off.
Some readout boards have a slide switch on them which shuts it off.
And, some mainframes have a RO off or RO brightness control on the front panel
?
HankC, Boston
Re: Photos of a non-illuminated graticule; WAS 576 versus 577

 

Hi Ed --
There's no separate 2nd anode wire to the funnel. I don't recall one on
any bistable I ever worked on (could be wrong). The color looks a
lot more like P1 than P31.
-ls-

"Ed Breya" <edbreya@...> wrote:

What's the anode voltage on the 577 CRT? If it's more than about 12 kV
(I doubt it), it could be that there's some or more lead (Pb) in the
CRT faceplate glass for X-ray protection, causing more UV attenuation.
Or, maybe the bistable storage phosphors are different enough to not
work. If so, either way, it seems like you can't do much about it. So
maybe direct frontal illumination with visible LEDs would be worth a
try.

Ed

--- In TekScopes@..., "Dennis Tillman" <dennis@...> wrote:

An update on the 577/D1 storage curve tracer: Nothing I have tried
causes
the phosphor of the 577 to glow. I covered the spectrum from 385nM
(UV/purple), blue, aqua blue, green, yellow, orange, red, and IR.
The only
interesting response I saw at all was the UV/purple LED, which
caused most
of the other CRT phosphors to glow green, causes a purple reflection
in the
577/D1 phosphor. It doesn't appear to be a phosphor glow but more of
a
reflection. It must be activating the phosphor of this CRT
differently than
all the others because none of the others had a purple reflection.
But the
response of the 577/D1 phosphor is so short lived that it looks more
like a
reflection than a glow. It goes away the instant the LED is off.



On the other hand I have had great success taking photos of the
Wiltron 640
RF Analyzer's CRT by first exciting the phosphor with the UV/Purple
LED. The
glow fades out slowly for 8-10 seconds. This gives me plenty of time
to take
the shot with the graticule illuminated by the phosphor.



Dennis



From: Dennis Tillman, Tuesday, January 24, 2012 9:38 AM



<snip>

Unfortunately my 577 (the CRT looks more green) and SC502 (the CRT
has a
pale green/white appearance) which are both storage CRTs barely glow
at all
so this isn't going to help me to take pix of transistor
characteristic
curves.



------------------------------------

Yahoo! Groups Links
Re: PG508 - Using 10-turn pots

 

That's great - seems like it should work OK. I should have clarified that any wirewound pot will have lots of inductance - even a one-turn type, which would have a toroidal element. The multi-turn ones actually have helical elements, with as many loops as there are turns of shaft rotation - and even more inductance.

Ed

--- In TekScopes@..., "Bruce Lane" <kyrrin@...> wrote:

Hi, Ed,

Thanks. I just checked the schematic, and it looks like the pot in question does nothing more than set DC bias level on the emitter side of an NPN transistor, configured in common-base.

Time to investigate multi-turn pots...

* REPLY SEPARATOR *

On 26-Jan-12 at 04:05 Ed Breya wrote:

If the pot has to vary a fast risetime or wide bandwidth signal, a
ten-turn style will likely not work. Those are built with many turns of
manganin wire wrapped around a linear mandrel, then rolled into a toroidal
resistance element - it will have a lot of inductance.

If the pot just needs to set a DC bias or control a low frequency signal,
no problem.

Old pots of any style can sometimes be worked around enough to clean up
the wiper contact degraded from years of idleness or bad storage
environment - but it can take a lot - maybe maybe hundreds of full
rotations. It may be worth trying that first.

Ed

--- In TekScopes@..., "Bruce Lane" <kyrrin@> wrote:

Fellow Tekkies,

I got my PG508 in and, with a bit of cleaning of the controls, found it
to be nearly pristine and very functional indeed.

There is one issue: I'm finding the variable control on pulse duration
does not give me precise enough control, and (like most older pots) tends
to jump a bit.

My gut reaction to curing this is to try interfacing a 10 or 20-turn pot
to it, preferably through a blank plug-in so I don't have to deface the
front panel.

Thoughts on this idea, and how to best accomplish it, would be welcome.

Thanks much.


Bruce Lane, Owner & Head Hardware Heavy,
Blue Feather Technologies ()
Assoc. member, AZA & AAZK for many moons.
"Salvadore Dali's computer has surreal ports..."



------------------------------------

Yahoo! Groups Links




__________ Information from ESET NOD32 Antivirus, version of virus
signature database 6827 (20120125) __________

The message was checked by ESET NOD32 Antivirus.


-=-=-=-=-=-=-=-=-=-=-=-
Bruce Lane, Owner & Head Hardware Heavy,
Blue Feather Technologies --
kyrrin (at) bluefeathertech do/t c=o=m
"Quid Malmborg in Plano..."
Re: PG508 - Using 10-turn pots

 

Hi, Ed,

Thanks. I just checked the schematic, and it looks like the pot in question does nothing more than set DC bias level on the emitter side of an NPN transistor, configured in common-base.

Time to investigate multi-turn pots...

* REPLY SEPARATOR *

On 26-Jan-12 at 04:05 Ed Breya wrote:

If the pot has to vary a fast risetime or wide bandwidth signal, a
ten-turn style will likely not work. Those are built with many turns of
manganin wire wrapped around a linear mandrel, then rolled into a toroidal
resistance element - it will have a lot of inductance.

If the pot just needs to set a DC bias or control a low frequency signal,
no problem.

Old pots of any style can sometimes be worked around enough to clean up
the wiper contact degraded from years of idleness or bad storage
environment - but it can take a lot - maybe maybe hundreds of full
rotations. It may be worth trying that first.

Ed

--- In TekScopes@..., "Bruce Lane" <kyrrin@...> wrote:

Fellow Tekkies,

I got my PG508 in and, with a bit of cleaning of the controls, found it
to be nearly pristine and very functional indeed.

There is one issue: I'm finding the variable control on pulse duration
does not give me precise enough control, and (like most older pots) tends
to jump a bit.

My gut reaction to curing this is to try interfacing a 10 or 20-turn pot
to it, preferably through a blank plug-in so I don't have to deface the
front panel.

Thoughts on this idea, and how to best accomplish it, would be welcome.

Thanks much.


Bruce Lane, Owner & Head Hardware Heavy,
Blue Feather Technologies ()
Assoc. member, AZA & AAZK for many moons.
"Salvadore Dali's computer has surreal ports..."



------------------------------------

Yahoo! Groups Links




__________ Information from ESET NOD32 Antivirus, version of virus
signature database 6827 (20120125) __________

The message was checked by ESET NOD32 Antivirus.


-=-=-=-=-=-=-=-=-=-=-=-
Bruce Lane, Owner & Head Hardware Heavy,
Blue Feather Technologies --
kyrrin (at) bluefeathertech do/t c=o=m
"Quid Malmborg in Plano..."
Re: PG508 - Using 10-turn pots

 

If the pot has to vary a fast risetime or wide bandwidth signal, a ten-turn style will likely not work. Those are built with many turns of manganin wire wrapped around a linear mandrel, then rolled into a toroidal resistance element - it will have a lot of inductance.

If the pot just needs to set a DC bias or control a low frequency signal, no problem.

Old pots of any style can sometimes be worked around enough to clean up the wiper contact degraded from years of idleness or bad storage environment - but it can take a lot - maybe maybe hundreds of full rotations. It may be worth trying that first.

Ed

--- In TekScopes@..., "Bruce Lane" <kyrrin@...> wrote:

Fellow Tekkies,

I got my PG508 in and, with a bit of cleaning of the controls, found it
to be nearly pristine and very functional indeed.

There is one issue: I'm finding the variable control on pulse duration
does not give me precise enough control, and (like most older pots) tends
to jump a bit.

My gut reaction to curing this is to try interfacing a 10 or 20-turn pot
to it, preferably through a blank plug-in so I don't have to deface the
front panel.

Thoughts on this idea, and how to best accomplish it, would be welcome.

Thanks much.


Bruce Lane, Owner & Head Hardware Heavy,
Blue Feather Technologies ()
Assoc. member, AZA & AAZK for many moons.
"Salvadore Dali's computer has surreal ports..."
Re: Photos of a non-illuminated graticule; WAS 576 versus 577

 

What's the anode voltage on the 577 CRT? If it's more than about 12 kV (I doubt it), it could be that there's some or more lead (Pb) in the CRT faceplate glass for X-ray protection, causing more UV attenuation. Or, maybe the bistable storage phosphors are different enough to not work. If so, either way, it seems like you can't do much about it. So maybe direct frontal illumination with visible LEDs would be worth a try.

Ed

--- In TekScopes@..., "Dennis Tillman" <dennis@...> wrote:

An update on the 577/D1 storage curve tracer: Nothing I have tried causes
the phosphor of the 577 to glow. I covered the spectrum from 385nM
(UV/purple), blue, aqua blue, green, yellow, orange, red, and IR. The only
interesting response I saw at all was the UV/purple LED, which caused most
of the other CRT phosphors to glow green, causes a purple reflection in the
577/D1 phosphor. It doesn't appear to be a phosphor glow but more of a
reflection. It must be activating the phosphor of this CRT differently than
all the others because none of the others had a purple reflection. But the
response of the 577/D1 phosphor is so short lived that it looks more like a
reflection than a glow. It goes away the instant the LED is off.



On the other hand I have had great success taking photos of the Wiltron 640
RF Analyzer's CRT by first exciting the phosphor with the UV/Purple LED. The
glow fades out slowly for 8-10 seconds. This gives me plenty of time to take
the shot with the graticule illuminated by the phosphor.



Dennis



From: Dennis Tillman, Tuesday, January 24, 2012 9:38 AM



<snip>

Unfortunately my 577 (the CRT looks more green) and SC502 (the CRT has a
pale green/white appearance) which are both storage CRTs barely glow at all
so this isn't going to help me to take pix of transistor characteristic
curves.
Re: Tek W Plugin 7000 & up serial.

technite2001
 

I decided to rescan the later W manual I found. The BAMA scan is from 2003 of an older manual I had and the later manual has updated Circuit Descriptions and Parts Lists. It should be completely scanned by weeks end.
regards,
Jerry
W2JI

--- In TekScopes@..., "Aaron" <ataylor@...> wrote:

Jerry,

A scan would be much appreciated. Thanks for taking the time to do this!

Aaron



--- In TekScopes@..., "technite2001" <precisioncarb@> wrote:

Greetings to the group.
I found a copy a later W manual with the FET mod. If there is a need I will scan and post it online. Its listed in the change section and is 13 pages (schematic and pictures).
Let me know.
Regards,
Jerry
W2JI
Re: Tek 2230 - Storage does not work.

 

On Wed, 25 Jan 2012 13:10:19 -1000, "David Sidrane" <david_s5@...>
wrote:

I did not realize the noise was that bad on the 234-0408-20.
It was annoying enough that I was willing to risk repairing the
oscilloscope and even undoing the modifications to support a
155-0022-00.

Do you have a 155-0022-00? Where can I find the mods you are speaking of?
I have the two 155-0022-00 channel switches which I got from junked
storage boards and did not use but before you decide if you want one,
check and see if you are up to doing the modifications.

I suspect you can get away with just changing the output inductors and
resistors. The problem is going to be the frequency and temperature
compensation in the emitter networks.

You may be better off grabbing a junked 2230 for either the new
channel switch, the old channel switch and a complete set of discrete
parts, or the whole storage board.

If not, I would consider your 234-0408-20, as I would rather have one
storage chan with noise then none.
It took me weeks to track down that bad part and replace it so I had
it framed.

From: TekScopes@... [mailto:TekScopes@...] On Behalf
Of David
Sent: Wednesday, January 25, 2012 12:56 PM
To: TekScopes@...
Subject: Re: [TekScopes] Re: Tek 2230 - Storage does not work.


Buy my what? The 234-0408-20 with popcorn noise? It makes channel 2
all but unusable in my opinion. I think you would be better off
getting one of the common 155-0022-00 chips and making the minor
modifications needed.

On Wed, 25 Jan 2012 22:43:54 -0000, "david_s5y" <david_s5@...
<mailto:david_s5%40usa.net> >
wrote:

David,

Can I buy yours?

David

--- In TekScopes@... <mailto:TekScopes%40yahoogroups.com> ,
David <davidwhess@...> wrote:

First I got a couple of junked 2230 storage boards which was how I
discovered there are two versions of the storage channel switch in the
2230 and that the 155-0022-00 will oscillate in a 234-0408-20 design.
I was looking into modifying my 2230 to use the more common
155-0022-00 when a friend who I was consulting with took pity and sent
me a part pull that he thought might be bad but actually worked fine.

The 155-0022-00 was used in a number of earlier oscilloscopes like the
7000 series but I do not know about the 234-0408-20. The only extra
234-0408-20 I have or know of at the moment is the bad one with
popcorn noise that I replaced.

You can find the 155-0022-00 specifications in the Tek Made Integrated
Circuits Catalog.

On Wed, 25 Jan 2012 19:15:06 -0000, "david_s5y" <david_s5@...>
wrote:

I just looked at the pictures. Where did you get the 234-0408-20 from?

David

--- In TekScopes@... <mailto:TekScopes%40yahoogroups.com> ,
David <davidwhess@> wrote:

Beware that there are two versions of the 2230 storage channel switch
and while they use the same circuit board, there are some discrete
parts changes which need to be made if you replace one with the other.
When I swapped a 234-0408-20, the one with popcorn noise that I posted
about some time ago, with a 155-0022-00 I got uncontrolled
oscillation. I managed to procure a replacement 234-0408-20 instead
of making the changes.

If you measure at pin 21 of the A/D converter, the voltage should be
between about 0.25 and -2.25 volts as you adjust the vertical position
control. If it adjusts within that range and you have no displayed
trace, then the problem is not before this point. I suppose you could
remove R2293 and apply about 1.0 volts directly to the A/D converter
which should generate a vertically centered trace to verify everything
past this point.

On Wed, 25 Jan 2012 17:12:53 -0000, "david_s5y" <david_s5@>
wrote:

I am trying to trouble shoot the missing storage traces. Does anyone
have the DC voltage levels for Pin 6 and 11 of a working U2101 (MO36E). I am
seeing +5.6 Volts which seems intuitively wrong give the 511 Ohm to -5
Volts.

I measured -2.1 volts in store mode and -1.25 volts in non-store mode
for the 155-0022-00.

I measured -1.62 volts in store mode and -1.11 volts in non-store mode
for the 234-0408-20.

In store mode the voltage changes a bit depending on the channel
selection and alt/chop selection.

+5.6 volts does sound high. Schematic 11 shows the power connections
for the chip on pins 3 and 5 which are not shows on schematic 16. They
should be about 7.4 volts and 1.8 volts although I think it depends a
little bit on which version you have.

For that matter with a 1Khz 1V sin wave applied to chan 1 set to .5 V
per. What should I see on 2 and 15 and 12 and 13?

I measured about 0.02 volts peak-to-peak to ground at pins 2 or 15
which may seem low but is expected since the impedance is about 50
ohms.

Pins 12 and 13 are the transconductance amplifier outputs into a
cascode so the voltage change there is too small to bother measuring.

I measured about 0.4 volts peak-to-peak at the collector of Q2101.
Re: Power supply problem with Tek 2220

tom jobe
 

Hi Tom,
None of us have the proper 2220 service manual, nor do we know where to find
it or buy it. By several accounts, the 2220 is just a crippled 2230 (much
like the 2224 was just a crippled 2232).
I'm working off of a 2230 manual I bought from Artekmedia, but there are
free 2230 manuals available everywhere it seems. I buy the Artekmedia
manuals just because I enjoy using the higher quality scans when I either
work on my own equipment, or follow the discussions on Tekscopes.
I believe that Tan added some capacitance to C919 and brought the switching
frequency down into the audible range with no improvement in his situation.
I don't think he knew what frequency it was before, or what it is now.
Irwin Soza brought his switching frequency down from 72 MHz to the 60 MHz
the manual talked about (I think?).
Thanks again for your ongoing help to all of us!
tom jobe...

----- Original Message -----
From: "Thomas Miller" <tmiller11147@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 6:00 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220


Thanks Tom.

I don't even have the proper manual for the 2220 and was just looking
at the 2232 schematics. Also, the earlier messages are gone and I was just
following
the last two days by email.

We still need to see if he has measured the switching frequencies that you
recommended
before as that would sure cause this issue.

I suspect you get the same enjoyment as I do in helping others with this
equipment. Plus, it keeps us out of the bars :).



Regards,
Tom M





----- Original Message -----
From: tom jobe
To: TekScopes@...
Sent: Wednesday, January 25, 2012 8:37 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220



Hi Tom,
I believe that Tan has said that he replaced Q9070 (although he called it
something else), and that he had put Q9070 on its own heat sink. He
thought
this ruled out the "thermal shut down board" as being the problem.
We will have to wait and see what Tan does next, but I thank you for
taking
an interest in this 22xx problem! Normally us beginners are left to try
and
help one another on these 22xx's, so it is a luxury to get help from a
knowledgeable person such as yourself!
tom jobe...

----- Original Message -----
From: "Thomas Miller" <tmiller11147@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 5:33 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

Well, assuming that R907 is correct, the current is not excessive. I would
guess the inverter is working correctly. Does the overtemp sense measure
the
heat from Q9070? If so, that would be my first target. The Q908 is used to
shut down the Mosfet gate capacitance. If that is not working right, the
gate will drop allowing the Mosfet to go through a linear region and will
produce a lot of heat.

R908 limits the gate current and if it has drifted high, the switch
(Q9070)
will not turn on or off fast enough.

But you are right. The first thing to check are the two resistors, R907
and
R908.

Regards,
Tom M

----- Original Message -----
From: tom jobe
To: TekScopes@...
Sent: Wednesday, January 25, 2012 8:03 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

Hi Tom,
Thank you for your highly valued opinion on this matter!
The voltage drop Tan is seeing across his R907 is very normal (even though
this involves the dangerous assumption I'm making that his R907 is of the
correct value!).
Maybe Tan will look into the health of his Q946 and Q947 transistors
before
he does anything else?
tom jobe...

----- Original Message -----
From: "Thomas Miller" <tmiller11147@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 4:47 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

Hi Tom,

I don't think I would disable the thermal shutdown circuit if the
transistors are really getting as hot as Tan has stated. It would be like
taking a pain killer to mask a broken arm. But I do think you are on to
something with the leaky transistors. Or even the main switching
transistor
could be bad.

Regards,
Tom

----- Original Message -----
From: tom jobe
To: TekScopes@...
Sent: Wednesday, January 25, 2012 7:27 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

Hi Tan,
I see that JC has reported that his properly working 2230 has 0.23 volts
across R907.
I remembered that I had a 2230 so I found it and a note on it said that I
had worked on it in 2009 and that I had put some new parts in the power
supply.
I took the outer covers off of the 2230 and ran it on the bench. R907 had
0.245 volts while cold, and 0.246 volts after 30 minutes. The side of the
chassis where the heatsink for Q9070, Q946, Q947 is attached, barely got
warm to the touch in 30 minutes (maybe 2 or 3 degrees C more?).
I also checked R949, it was 0.683 volts cold, and 0.687 volts after 30
minutes.

The voltage on R949 came up on Irwin Zosa's 2235 when he discovered that
one
of the inverter transistors (Q946-Q947) had a leaky emitter to collector
junction (in one direction only). Irwin replaced the transistor pair with
some new TIP41C transistors and saw some improvement in his heat shut down
problem.

Some other test results from the past include:

Tektronix 2232,
R907 = 0.278 volts(!), R949 = 0.776 volts (R949 is 0.51 ohms on this
scope)

Tektronix 2236,
R907 = 0.189 volts, R949 = 0.497 volts (R949 is 0.51 ohms on this scope)
This is a late model 2236 and R912 is 575 ohms compared to 549 ohms for
the
earlier 2236's.
The higher ohms on R912 increases the shutdown current threshold across
R907.

All of the above and your symptoms, makes me wonder about the health of
your
Q946 and Q947 transistors?
Also, could you disconnect W950-1, W950-2 and W950-3 to completely
disconnect the thermal shutdown board A18?
Your voltage readings across R907 seem to be quite normal.
tom jobe...

----- Original Message -----
From: "Tan Chor Ming" <jonray03@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 1:46 AM
Subject: RE: [TekScopes] Re: Power supply problem with Tek 2220

> Hi Tom,
>
> Thank you for the detail of the problem which is similar to my tek 2220.
>
>
>
> I am still unable to nail down the problem. I believe the scope is
working
> normally, because when it fires up it works normally.
>
> My feeling is that it poorly designed thermally for 230V operation
>
>
>
> Do you have voltage measurement for tek 2235 on R907?
>
> I am getting 0.198V (with the +5V to the storage board disconnected,
> effectively turning off the storage function).
>
> This means that without the storage function connection, the current on
+43V
> is slightly below 1A.
>
> I am not sure if this is a normal condition but it should be similar to
the
> tek 2235.
>
> Under this condition, the scope could operate for quite a while without
> problem
>
>
>
> Once the +5V is connected to the storage board, voltage at R907 goes up
to
> 2.3V~2.5V or about 1.15 to 1.25A on +43 and the power supply will shut
down.
>
>
>
> Following done with little effect or improvement
>
> - Change 3 filter caps (only those that are feels hot) - C960, C962,
C964
> (for -5V for 2220 only)
>
> - Change NMOS P9070. When changing P9070, I used a separate heatsink for
> P9070 and still such down happens which means it is unlikely to be
caused
by
> the thermal shutdown board.
>
> - Shutdown is more likely cause by spurious overload in R907
>
> - Change C907
>
> - Change CR907 (two diode in parallel)
>
> - Added C919 with 1000pf in parallel. Lowered switching freq but still
> having a heated P9070 which shuts down. Not sure what freq but it is
> definitely audible.
>
>
>
> Rgds,
>
> Chor Ming
>
>
>
> _____
>
> From: TekScopes@... [mailto:TekScopes@...] On
Behalf
> Of tom jobe
> Sent: Monday, January 23, 2012 11:17 PM
> To: TekScopes@...
> Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220
>
>
>
>
>
> Hi Tan,
> This morning I remembered about an interesting 2235 repair that was done
by
> a very crafty person by the name of Irwin Zosa during 2007 and 2008.
> Irwin lives in your part of the world in the Philippines where it is
almost
> as hot as in Singapore. His basic problem was the same as yours, the
2235
> would shut itself down with a thermal problem.
> There was not much interest in his 2235 problem on Tekscopes at that
time,
> so the email exchanges we had went off group as he worked his way
through
> the problem. Once he had the problem solved, he posted a nice summary of
his
> findings to the Tekscopes group.
> Irwin's repair summary is in message number 36518 and it was posted on
Jan
> 6, 2009.
> He found a number of problems as he went along, but his final discovery
> might amaze you.
> It sure amazed me because I would have never found it!
> tom jobe...
> PS I will paste in Irwin's 2235 repair summary below to save you the
trouble
> of getting it from the Tekscopes Message archive.
>
> Hello to all: Way back in the first quarter of 2007, I had this
> problem of my 2235 shutting down after a few minutes from switching
> it ON. After a lot of interaction from forum members I finally
> nailed down the problem. Here is a brief summary of this process:
> First, I checked all the voltages according to the service manual and
> they were OK. I also checked the ESR of the capacitors and they read
> good but the unit being about twenty years old or so, I decided to
> replace the secondary caps (C960, C961, C962, C963, C968, C970) with
> ultra-low ESR, 105-degree C Nichicons. I could not find 840uF units
> so I used 1000uF. The problem still did not go away although the
> length of time-to-shutdown increased. I also replaced the FET
> switcher Q9070 (IRF710)with a higher rated device (IRF840). I then
> replaced Q946 and Q947 with TIP41 units. At every change that I
> made, slight improvements were observed. I also replaced R912 (357
> Ohms)with a 390 Ohm part to increase the shutdown threshold of this
> 2235. Then I replaced the secondary rectifiers CR954, CR955 (MR814),
> CR956, CR957 (MR812), CR960, CR961, CR962, CR963 (MR812). The
> replacements that I used were the MUR160 ultra-fast rectifiers. All
> of these were not done "shotgun" style and in the exact order as
> described but rather, followed logical steps of checking one section
> at a time. I even thought of thermal runaway because T944, T948, and
> Q9070 would heat up so much (really hot!). I also noticed early on
> that the frequency of the pre-regulator section (U930) was about
> 72KHz and in the service manual it is listed as about 60KHz. It
> suddenly dawned on me to try to reduce the operating frequency of
> this section so I replaced R919 and now it is running at 60KHz. The
> excessive heat has gone and no more shutdown. I tried to run the
> scope for about half a day and still no shutdown. This 2235 has all
> of the power supply improvements already installed from the factory
> (but why was the pre-regulator section running at 70KHz ?). I would
> like to thank the forum members who gave their insights. Special
> thanks goes to Tom Jobe, who stayed with me all the way in this
> repair project. He took the time to take measurement readings for
> me, gave his analyses, and even offered parts that I may need for
> free. At some point I was considering another brand of oscilloscope
> but I held on to this (even keeping it in storage for almost a year
> because of this problem) because from my research, these TEK scopes,
> during their time, were among the best. I think they still are.
>
>
>
>
Re: Power supply problem with Tek 2220

 

¿ªÔÆÌåÓý

Thanks Tom.
?
I don't even have the proper manual for the 2220 and was just looking
at the 2232 schematics. Also, the earlier messages are gone and I was just following
the last two days by email.
?
We still need to see if he has measured the switching frequencies that you recommended
before as that would sure cause this issue.
?
I suspect you get the same enjoyment as I do in helping others with this equipment. Plus, it keeps us out of the bars :).
?
?
?
Regards,
Tom M
?
?
?
?
?

----- Original Message -----
From: tom jobe
Sent: Wednesday, January 25, 2012 8:37 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

?

Hi Tom,
I believe that Tan has said that he replaced Q9070 (although he called it
something else), and that he had put Q9070 on its own heat sink. He thought
this ruled out the "thermal shut down board" as being the problem.
We will have to wait and see what Tan does next, but I thank you for taking
an interest in this 22xx problem! Normally us beginners are left to try and
help one another on these 22xx's, so it is a luxury to get help from a
knowledgeable person such as yourself!
tom jobe...

----- Original Message -----
From: "Thomas Miller" <tmiller11147@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 5:33 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

Well, assuming that R907 is correct, the current is not excessive. I would
guess the inverter is working correctly. Does the overtemp sense measure the
heat from Q9070? If so, that would be my first target. The Q908 is used to
shut down the Mosfet gate capacitance. If that is not working right, the
gate will drop allowing the Mosfet to go through a linear region and will
produce a lot of heat.

R908 limits the gate current and if it has drifted high, the switch (Q9070)
will not turn on or off fast enough.

But you are right. The first thing to check are the two resistors, R907 and
R908.

Regards,
Tom M

----- Original Message -----
From: tom jobe
To: TekScopes@...
Sent: Wednesday, January 25, 2012 8:03 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

Hi Tom,
Thank you for your highly valued opinion on this matter!
The voltage drop Tan is seeing across his R907 is very normal (even though
this involves the dangerous assumption I'm making that his R907 is of the
correct value!).
Maybe Tan will look into the health of his Q946 and Q947 transistors
before
he does anything else?
tom jobe...

----- Original Message -----
From: "Thomas Miller" <tmiller11147@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 4:47 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

Hi Tom,

I don't think I would disable the thermal shutdown circuit if the
transistors are really getting as hot as Tan has stated. It would be like
taking a pain killer to mask a broken arm. But I do think you are on to
something with the leaky transistors. Or even the main switching
transistor
could be bad.

Regards,
Tom

----- Original Message -----
From: tom jobe
To: TekScopes@...
Sent: Wednesday, January 25, 2012 7:27 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

Hi Tan,
I see that JC has reported that his properly working 2230 has 0.23 volts
across R907.
I remembered that I had a 2230 so I found it and a note on it said that I
had worked on it in 2009 and that I had put some new parts in the power
supply.
I took the outer covers off of the 2230 and ran it on the bench. R907 had
0.245 volts while cold, and 0.246 volts after 30 minutes. The side of the
chassis where the heatsink for Q9070, Q946, Q947 is attached, barely got
warm to the touch in 30 minutes (maybe 2 or 3 degrees C more?).
I also checked R949, it was 0.683 volts cold, and 0.687 volts after 30
minutes.

The voltage on R949 came up on Irwin Zosa's 2235 when he discovered that
one
of the inverter transistors (Q946-Q947) had a leaky emitter to collector
junction (in one direction only). Irwin replaced the transistor pair with
some new TIP41C transistors and saw some improvement in his heat shut down
problem.

Some other test results from the past include:

Tektronix 2232,
R907 = 0.278 volts(!), R949 = 0.776 volts (R949 is 0.51 ohms on this
scope)

Tektronix 2236,
R907 = 0.189 volts, R949 = 0.497 volts (R949 is 0.51 ohms on this scope)
This is a late model 2236 and R912 is 575 ohms compared to 549 ohms for
the
earlier 2236's.
The higher ohms on R912 increases the shutdown current threshold across
R907.

All of the above and your symptoms, makes me wonder about the health of
your
Q946 and Q947 transistors?
Also, could you disconnect W950-1, W950-2 and W950-3 to completely
disconnect the thermal shutdown board A18?
Your voltage readings across R907 seem to be quite normal.
tom jobe...

----- Original Message -----
From: "Tan Chor Ming" <jonray03@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 1:46 AM
Subject: RE: [TekScopes] Re: Power supply problem with Tek 2220

> Hi Tom,
>
> Thank you for the detail of the problem which is similar to my tek 2220.
>
>
>
> I am still unable to nail down the problem. I believe the scope is
working
> normally, because when it fires up it works normally.
>
> My feeling is that it poorly designed thermally for 230V operation
>
>
>
> Do you have voltage measurement for tek 2235 on R907?
>
> I am getting 0.198V (with the +5V to the storage board disconnected,
> effectively turning off the storage function).
>
> This means that without the storage function connection, the current on
+43V
> is slightly below 1A.
>
> I am not sure if this is a normal condition but it should be similar to
the
> tek 2235.
>
> Under this condition, the scope could operate for quite a while without
> problem
>
>
>
> Once the +5V is connected to the storage board, voltage at R907 goes up
to
> 2.3V~2.5V or about 1.15 to 1.25A on +43 and the power supply will shut
down.
>
>
>
> Following done with little effect or improvement
>
> - Change 3 filter caps (only those that are feels hot) - C960, C962,
C964
> (for -5V for 2220 only)
>
> - Change NMOS P9070. When changing P9070, I used a separate heatsink for
> P9070 and still such down happens which means it is unlikely to be
caused
by
> the thermal shutdown board.
>
> - Shutdown is more likely cause by spurious overload in R907
>
> - Change C907
>
> - Change CR907 (two diode in parallel)
>
> - Added C919 with 1000pf in parallel. Lowered switching freq but still
> having a heated P9070 which shuts down. Not sure what freq but it is
> definitely audible.
>
>
>
> Rgds,
>
> Chor Ming
>
>
>
> _____
>
> From: TekScopes@... [mailto:TekScopes@...] On
Behalf
> Of tom jobe
> Sent: Monday, January 23, 2012 11:17 PM
> To: TekScopes@...
> Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220
>
>
>
>
>
> Hi Tan,
> This morning I remembered about an interesting 2235 repair that was done
by
> a very crafty person by the name of Irwin Zosa during 2007 and 2008.
> Irwin lives in your part of the world in the Philippines where it is
almost
> as hot as in Singapore. His basic problem was the same as yours, the
2235
> would shut itself down with a thermal problem.
> There was not much interest in his 2235 problem on Tekscopes at that
time,
> so the email exchanges we had went off group as he worked his way
through
> the problem. Once he had the problem solved, he posted a nice summary of
his
> findings to the Tekscopes group.
> Irwin's repair summary is in message number 36518 and it was posted on
Jan
> 6, 2009.
> He found a number of problems as he went along, but his final discovery
> might amaze you.
> It sure amazed me because I would have never found it!
> tom jobe...
> PS I will paste in Irwin's 2235 repair summary below to save you the
trouble
> of getting it from the Tekscopes Message archive.
>
> Hello to all: Way back in the first quarter of 2007, I had this
> problem of my 2235 shutting down after a few minutes from switching
> it ON. After a lot of interaction from forum members I finally
> nailed down the problem. Here is a brief summary of this process:
> First, I checked all the voltages according to the service manual and
> they were OK. I also checked the ESR of the capacitors and they read
> good but the unit being about twenty years old or so, I decided to
> replace the secondary caps (C960, C961, C962, C963, C968, C970) with
> ultra-low ESR, 105-degree C Nichicons. I could not find 840uF units
> so I used 1000uF. The problem still did not go away although the
> length of time-to-shutdown increased. I also replaced the FET
> switcher Q9070 (IRF710)with a higher rated device (IRF840). I then
> replaced Q946 and Q947 with TIP41 units. At every change that I
> made, slight improvements were observed. I also replaced R912 (357
> Ohms)with a 390 Ohm part to increase the shutdown threshold of this
> 2235. Then I replaced the secondary rectifiers CR954, CR955 (MR814),
> CR956, CR957 (MR812), CR960, CR961, CR962, CR963 (MR812). The
> replacements that I used were the MUR160 ultra-fast rectifiers. All
> of these were not done "shotgun" style and in the exact order as
> described but rather, followed logical steps of checking one section
> at a time. I even thought of thermal runaway because T944, T948, and
> Q9070 would heat up so much (really hot!). I also noticed early on
> that the frequency of the pre-regulator section (U930) was about
> 72KHz and in the service manual it is listed as about 60KHz. It
> suddenly dawned on me to try to reduce the operating frequency of
> this section so I replaced R919 and now it is running at 60KHz. The
> excessive heat has gone and no more shutdown. I tried to run the
> scope for about half a day and still no shutdown. This 2235 has all
> of the power supply improvements already installed from the factory
> (but why was the pre-regulator section running at 70KHz ?). I would
> like to thank the forum members who gave their insights. Special
> thanks goes to Tom Jobe, who stayed with me all the way in this
> repair project. He took the time to take measurement readings for
> me, gave his analyses, and even offered parts that I may need for
> free. At some point I was considering another brand of oscilloscope
> but I held on to this (even keeping it in storage for almost a year
> because of this problem) because from my research, these TEK scopes,
> during their time, were among the best. I think they still are.
>
>
>
>
Re: Power supply problem with Tek 2220

tom jobe
 

Hi Tom,
I believe that Tan has said that he replaced Q9070 (although he called it
something else), and that he had put Q9070 on its own heat sink. He thought
this ruled out the "thermal shut down board" as being the problem.
We will have to wait and see what Tan does next, but I thank you for taking
an interest in this 22xx problem! Normally us beginners are left to try and
help one another on these 22xx's, so it is a luxury to get help from a
knowledgeable person such as yourself!
tom jobe...

----- Original Message -----
From: "Thomas Miller" <tmiller11147@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 5:33 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220


Well, assuming that R907 is correct, the current is not excessive. I would
guess the inverter is working correctly. Does the overtemp sense measure the
heat from Q9070? If so, that would be my first target. The Q908 is used to
shut down the Mosfet gate capacitance. If that is not working right, the
gate will drop allowing the Mosfet to go through a linear region and will
produce a lot of heat.

R908 limits the gate current and if it has drifted high, the switch (Q9070)
will not turn on or off fast enough.

But you are right. The first thing to check are the two resistors, R907 and
R908.


Regards,
Tom M

----- Original Message -----
From: tom jobe
To: TekScopes@...
Sent: Wednesday, January 25, 2012 8:03 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220



Hi Tom,
Thank you for your highly valued opinion on this matter!
The voltage drop Tan is seeing across his R907 is very normal (even though
this involves the dangerous assumption I'm making that his R907 is of the
correct value!).
Maybe Tan will look into the health of his Q946 and Q947 transistors
before
he does anything else?
tom jobe...

----- Original Message -----
From: "Thomas Miller" <tmiller11147@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 4:47 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

Hi Tom,

I don't think I would disable the thermal shutdown circuit if the
transistors are really getting as hot as Tan has stated. It would be like
taking a pain killer to mask a broken arm. But I do think you are on to
something with the leaky transistors. Or even the main switching
transistor
could be bad.

Regards,
Tom

----- Original Message -----
From: tom jobe
To: TekScopes@...
Sent: Wednesday, January 25, 2012 7:27 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

Hi Tan,
I see that JC has reported that his properly working 2230 has 0.23 volts
across R907.
I remembered that I had a 2230 so I found it and a note on it said that I
had worked on it in 2009 and that I had put some new parts in the power
supply.
I took the outer covers off of the 2230 and ran it on the bench. R907 had
0.245 volts while cold, and 0.246 volts after 30 minutes. The side of the
chassis where the heatsink for Q9070, Q946, Q947 is attached, barely got
warm to the touch in 30 minutes (maybe 2 or 3 degrees C more?).
I also checked R949, it was 0.683 volts cold, and 0.687 volts after 30
minutes.

The voltage on R949 came up on Irwin Zosa's 2235 when he discovered that
one
of the inverter transistors (Q946-Q947) had a leaky emitter to collector
junction (in one direction only). Irwin replaced the transistor pair with
some new TIP41C transistors and saw some improvement in his heat shut down
problem.

Some other test results from the past include:

Tektronix 2232,
R907 = 0.278 volts(!), R949 = 0.776 volts (R949 is 0.51 ohms on this
scope)

Tektronix 2236,
R907 = 0.189 volts, R949 = 0.497 volts (R949 is 0.51 ohms on this scope)
This is a late model 2236 and R912 is 575 ohms compared to 549 ohms for
the
earlier 2236's.
The higher ohms on R912 increases the shutdown current threshold across
R907.

All of the above and your symptoms, makes me wonder about the health of
your
Q946 and Q947 transistors?
Also, could you disconnect W950-1, W950-2 and W950-3 to completely
disconnect the thermal shutdown board A18?
Your voltage readings across R907 seem to be quite normal.
tom jobe...

----- Original Message -----
From: "Tan Chor Ming" <jonray03@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 1:46 AM
Subject: RE: [TekScopes] Re: Power supply problem with Tek 2220

> Hi Tom,
>
> Thank you for the detail of the problem which is similar to my tek 2220.
>
>
>
> I am still unable to nail down the problem. I believe the scope is
working
> normally, because when it fires up it works normally.
>
> My feeling is that it poorly designed thermally for 230V operation
>
>
>
> Do you have voltage measurement for tek 2235 on R907?
>
> I am getting 0.198V (with the +5V to the storage board disconnected,
> effectively turning off the storage function).
>
> This means that without the storage function connection, the current on
+43V
> is slightly below 1A.
>
> I am not sure if this is a normal condition but it should be similar to
the
> tek 2235.
>
> Under this condition, the scope could operate for quite a while without
> problem
>
>
>
> Once the +5V is connected to the storage board, voltage at R907 goes up
to
> 2.3V~2.5V or about 1.15 to 1.25A on +43 and the power supply will shut
down.
>
>
>
> Following done with little effect or improvement
>
> - Change 3 filter caps (only those that are feels hot) - C960, C962,
C964
> (for -5V for 2220 only)
>
> - Change NMOS P9070. When changing P9070, I used a separate heatsink for
> P9070 and still such down happens which means it is unlikely to be
caused
by
> the thermal shutdown board.
>
> - Shutdown is more likely cause by spurious overload in R907
>
> - Change C907
>
> - Change CR907 (two diode in parallel)
>
> - Added C919 with 1000pf in parallel. Lowered switching freq but still
> having a heated P9070 which shuts down. Not sure what freq but it is
> definitely audible.
>
>
>
> Rgds,
>
> Chor Ming
>
>
>
> _____
>
> From: TekScopes@... [mailto:TekScopes@...] On
Behalf
> Of tom jobe
> Sent: Monday, January 23, 2012 11:17 PM
> To: TekScopes@...
> Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220
>
>
>
>
>
> Hi Tan,
> This morning I remembered about an interesting 2235 repair that was done
by
> a very crafty person by the name of Irwin Zosa during 2007 and 2008.
> Irwin lives in your part of the world in the Philippines where it is
almost
> as hot as in Singapore. His basic problem was the same as yours, the
2235
> would shut itself down with a thermal problem.
> There was not much interest in his 2235 problem on Tekscopes at that
time,
> so the email exchanges we had went off group as he worked his way
through
> the problem. Once he had the problem solved, he posted a nice summary of
his
> findings to the Tekscopes group.
> Irwin's repair summary is in message number 36518 and it was posted on
Jan
> 6, 2009.
> He found a number of problems as he went along, but his final discovery
> might amaze you.
> It sure amazed me because I would have never found it!
> tom jobe...
> PS I will paste in Irwin's 2235 repair summary below to save you the
trouble
> of getting it from the Tekscopes Message archive.
>
> Hello to all: Way back in the first quarter of 2007, I had this
> problem of my 2235 shutting down after a few minutes from switching
> it ON. After a lot of interaction from forum members I finally
> nailed down the problem. Here is a brief summary of this process:
> First, I checked all the voltages according to the service manual and
> they were OK. I also checked the ESR of the capacitors and they read
> good but the unit being about twenty years old or so, I decided to
> replace the secondary caps (C960, C961, C962, C963, C968, C970) with
> ultra-low ESR, 105-degree C Nichicons. I could not find 840uF units
> so I used 1000uF. The problem still did not go away although the
> length of time-to-shutdown increased. I also replaced the FET
> switcher Q9070 (IRF710)with a higher rated device (IRF840). I then
> replaced Q946 and Q947 with TIP41 units. At every change that I
> made, slight improvements were observed. I also replaced R912 (357
> Ohms)with a 390 Ohm part to increase the shutdown threshold of this
> 2235. Then I replaced the secondary rectifiers CR954, CR955 (MR814),
> CR956, CR957 (MR812), CR960, CR961, CR962, CR963 (MR812). The
> replacements that I used were the MUR160 ultra-fast rectifiers. All
> of these were not done "shotgun" style and in the exact order as
> described but rather, followed logical steps of checking one section
> at a time. I even thought of thermal runaway because T944, T948, and
> Q9070 would heat up so much (really hot!). I also noticed early on
> that the frequency of the pre-regulator section (U930) was about
> 72KHz and in the service manual it is listed as about 60KHz. It
> suddenly dawned on me to try to reduce the operating frequency of
> this section so I replaced R919 and now it is running at 60KHz. The
> excessive heat has gone and no more shutdown. I tried to run the
> scope for about half a day and still no shutdown. This 2235 has all
> of the power supply improvements already installed from the factory
> (but why was the pre-regulator section running at 70KHz ?). I would
> like to thank the forum members who gave their insights. Special
> thanks goes to Tom Jobe, who stayed with me all the way in this
> repair project. He took the time to take measurement readings for
> me, gave his analyses, and even offered parts that I may need for
> free. At some point I was considering another brand of oscilloscope
> but I held on to this (even keeping it in storage for almost a year
> because of this problem) because from my research, these TEK scopes,
> during their time, were among the best. I think they still are.
>
>
>
>
Re: Power supply problem with Tek 2220

 

¿ªÔÆÌåÓý

Well, assuming that R907 is correct, the current is not excessive. I would guess the inverter is working correctly. Does the overtemp sense measure the heat from Q9070? If so, that would be my first target. The Q908 is used to shut down the Mosfet gate capacitance. If that is not working right, the gate will drop allowing the Mosfet to go through a linear region and will produce a lot of heat.
?
R908 limits the gate current and if it has drifted high, the switch (Q9070) will not turn on or off fast enough.
?
But you are right. The first thing to check are the two resistors, R907 and R908.
?
?
Regards,
Tom M
?

----- Original Message -----
From: tom jobe
Sent: Wednesday, January 25, 2012 8:03 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

?

Hi Tom,
Thank you for your highly valued opinion on this matter!
The voltage drop Tan is seeing across his R907 is very normal (even though
this involves the dangerous assumption I'm making that his R907 is of the
correct value!).
Maybe Tan will look into the health of his Q946 and Q947 transistors before
he does anything else?
tom jobe...

----- Original Message -----
From: "Thomas Miller" <tmiller11147@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 4:47 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

Hi Tom,

I don't think I would disable the thermal shutdown circuit if the
transistors are really getting as hot as Tan has stated. It would be like
taking a pain killer to mask a broken arm. But I do think you are on to
something with the leaky transistors. Or even the main switching transistor
could be bad.

Regards,
Tom

----- Original Message -----
From: tom jobe
To: TekScopes@...
Sent: Wednesday, January 25, 2012 7:27 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

Hi Tan,
I see that JC has reported that his properly working 2230 has 0.23 volts
across R907.
I remembered that I had a 2230 so I found it and a note on it said that I
had worked on it in 2009 and that I had put some new parts in the power
supply.
I took the outer covers off of the 2230 and ran it on the bench. R907 had
0.245 volts while cold, and 0.246 volts after 30 minutes. The side of the
chassis where the heatsink for Q9070, Q946, Q947 is attached, barely got
warm to the touch in 30 minutes (maybe 2 or 3 degrees C more?).
I also checked R949, it was 0.683 volts cold, and 0.687 volts after 30
minutes.

The voltage on R949 came up on Irwin Zosa's 2235 when he discovered that
one
of the inverter transistors (Q946-Q947) had a leaky emitter to collector
junction (in one direction only). Irwin replaced the transistor pair with
some new TIP41C transistors and saw some improvement in his heat shut down
problem.

Some other test results from the past include:

Tektronix 2232,
R907 = 0.278 volts(!), R949 = 0.776 volts (R949 is 0.51 ohms on this
scope)

Tektronix 2236,
R907 = 0.189 volts, R949 = 0.497 volts (R949 is 0.51 ohms on this scope)
This is a late model 2236 and R912 is 575 ohms compared to 549 ohms for
the
earlier 2236's.
The higher ohms on R912 increases the shutdown current threshold across
R907.

All of the above and your symptoms, makes me wonder about the health of
your
Q946 and Q947 transistors?
Also, could you disconnect W950-1, W950-2 and W950-3 to completely
disconnect the thermal shutdown board A18?
Your voltage readings across R907 seem to be quite normal.
tom jobe...

----- Original Message -----
From: "Tan Chor Ming" <jonray03@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 1:46 AM
Subject: RE: [TekScopes] Re: Power supply problem with Tek 2220

> Hi Tom,
>
> Thank you for the detail of the problem which is similar to my tek 2220.
>
>
>
> I am still unable to nail down the problem. I believe the scope is
working
> normally, because when it fires up it works normally.
>
> My feeling is that it poorly designed thermally for 230V operation
>
>
>
> Do you have voltage measurement for tek 2235 on R907?
>
> I am getting 0.198V (with the +5V to the storage board disconnected,
> effectively turning off the storage function).
>
> This means that without the storage function connection, the current on
+43V
> is slightly below 1A.
>
> I am not sure if this is a normal condition but it should be similar to
the
> tek 2235.
>
> Under this condition, the scope could operate for quite a while without
> problem
>
>
>
> Once the +5V is connected to the storage board, voltage at R907 goes up
to
> 2.3V~2.5V or about 1.15 to 1.25A on +43 and the power supply will shut
down.
>
>
>
> Following done with little effect or improvement
>
> - Change 3 filter caps (only those that are feels hot) - C960, C962,
C964
> (for -5V for 2220 only)
>
> - Change NMOS P9070. When changing P9070, I used a separate heatsink for
> P9070 and still such down happens which means it is unlikely to be
caused
by
> the thermal shutdown board.
>
> - Shutdown is more likely cause by spurious overload in R907
>
> - Change C907
>
> - Change CR907 (two diode in parallel)
>
> - Added C919 with 1000pf in parallel. Lowered switching freq but still
> having a heated P9070 which shuts down. Not sure what freq but it is
> definitely audible.
>
>
>
> Rgds,
>
> Chor Ming
>
>
>
> _____
>
> From: TekScopes@... [mailto:TekScopes@...] On
Behalf
> Of tom jobe
> Sent: Monday, January 23, 2012 11:17 PM
> To: TekScopes@...
> Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220
>
>
>
>
>
> Hi Tan,
> This morning I remembered about an interesting 2235 repair that was done
by
> a very crafty person by the name of Irwin Zosa during 2007 and 2008.
> Irwin lives in your part of the world in the Philippines where it is
almost
> as hot as in Singapore. His basic problem was the same as yours, the
2235
> would shut itself down with a thermal problem.
> There was not much interest in his 2235 problem on Tekscopes at that
time,
> so the email exchanges we had went off group as he worked his way
through
> the problem. Once he had the problem solved, he posted a nice summary of
his
> findings to the Tekscopes group.
> Irwin's repair summary is in message number 36518 and it was posted on
Jan
> 6, 2009.
> He found a number of problems as he went along, but his final discovery
> might amaze you.
> It sure amazed me because I would have never found it!
> tom jobe...
> PS I will paste in Irwin's 2235 repair summary below to save you the
trouble
> of getting it from the Tekscopes Message archive.
>
> Hello to all: Way back in the first quarter of 2007, I had this
> problem of my 2235 shutting down after a few minutes from switching
> it ON. After a lot of interaction from forum members I finally
> nailed down the problem. Here is a brief summary of this process:
> First, I checked all the voltages according to the service manual and
> they were OK. I also checked the ESR of the capacitors and they read
> good but the unit being about twenty years old or so, I decided to
> replace the secondary caps (C960, C961, C962, C963, C968, C970) with
> ultra-low ESR, 105-degree C Nichicons. I could not find 840uF units
> so I used 1000uF. The problem still did not go away although the
> length of time-to-shutdown increased. I also replaced the FET
> switcher Q9070 (IRF710)with a higher rated device (IRF840). I then
> replaced Q946 and Q947 with TIP41 units. At every change that I
> made, slight improvements were observed. I also replaced R912 (357
> Ohms)with a 390 Ohm part to increase the shutdown threshold of this
> 2235. Then I replaced the secondary rectifiers CR954, CR955 (MR814),
> CR956, CR957 (MR812), CR960, CR961, CR962, CR963 (MR812). The
> replacements that I used were the MUR160 ultra-fast rectifiers. All
> of these were not done "shotgun" style and in the exact order as
> described but rather, followed logical steps of checking one section
> at a time. I even thought of thermal runaway because T944, T948, and
> Q9070 would heat up so much (really hot!). I also noticed early on
> that the frequency of the pre-regulator section (U930) was about
> 72KHz and in the service manual it is listed as about 60KHz. It
> suddenly dawned on me to try to reduce the operating frequency of
> this section so I replaced R919 and now it is running at 60KHz. The
> excessive heat has gone and no more shutdown. I tried to run the
> scope for about half a day and still no shutdown. This 2235 has all
> of the power supply improvements already installed from the factory
> (but why was the pre-regulator section running at 70KHz ?). I would
> like to thank the forum members who gave their insights. Special
> thanks goes to Tom Jobe, who stayed with me all the way in this
> repair project. He took the time to take measurement readings for
> me, gave his analyses, and even offered parts that I may need for
> free. At some point I was considering another brand of oscilloscope
> but I held on to this (even keeping it in storage for almost a year
> because of this problem) because from my research, these TEK scopes,
> during their time, were among the best. I think they still are.
>
>
>
>
Re: Power supply problem with Tek 2220

 

¿ªÔÆÌåÓý

Oh, also you should check R909, CR908, Q908, and R908 as any of these could cause the duty cycle switch (Q9070)?to overheat.
It may be worth a look (with proper isolation) at the gate waveform on Q9070.
?
?

----- Original Message -----
Sent: Wednesday, January 25, 2012 7:47 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

?

Hi Tom,
?
I don't think I would disable the thermal shutdown circuit if the transistors are really getting as hot as Tan has stated. It would be like taking a pain killer to mask a broken arm. But I do think you are on to something with the leaky transistors. Or even the main switching transistor could be bad.
?
Regards,
Tom
?
?
----- Original Message -----
From: tom jobe
Sent: Wednesday, January 25, 2012 7:27 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220

?

Hi Tan,
I see that JC has reported that his properly working 2230 has 0.23 volts
across R907.
I remembered that I had a 2230 so I found it and a note on it said that I
had worked on it in 2009 and that I had put some new parts in the power
supply.
I took the outer covers off of the 2230 and ran it on the bench. R907 had
0.245 volts while cold, and 0.246 volts after 30 minutes. The side of the
chassis where the heatsink for Q9070, Q946, Q947 is attached, barely got
warm to the touch in 30 minutes (maybe 2 or 3 degrees C more?).
I also checked R949, it was 0.683 volts cold, and 0.687 volts after 30
minutes.

The voltage on R949 came up on Irwin Zosa's 2235 when he discovered that one
of the inverter transistors (Q946-Q947) had a leaky emitter to collector
junction (in one direction only). Irwin replaced the transistor pair with
some new TIP41C transistors and saw some improvement in his heat shut down
problem.

Some other test results from the past include:

Tektronix 2232,
R907 = 0.278 volts(!), R949 = 0.776 volts (R949 is 0.51 ohms on this scope)

Tektronix 2236,
R907 = 0.189 volts, R949 = 0.497 volts (R949 is 0.51 ohms on this scope)
This is a late model 2236 and R912 is 575 ohms compared to 549 ohms for the
earlier 2236's.
The higher ohms on R912 increases the shutdown current threshold across
R907.

All of the above and your symptoms, makes me wonder about the health of your
Q946 and Q947 transistors?
Also, could you disconnect W950-1, W950-2 and W950-3 to completely
disconnect the thermal shutdown board A18?
Your voltage readings across R907 seem to be quite normal.
tom jobe...

----- Original Message -----
From: "Tan Chor Ming" <jonray03@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 1:46 AM
Subject: RE: [TekScopes] Re: Power supply problem with Tek 2220

> Hi Tom,
>
> Thank you for the detail of the problem which is similar to my tek 2220.
>
>
>
> I am still unable to nail down the problem. I believe the scope is
working
> normally, because when it fires up it works normally.
>
> My feeling is that it poorly designed thermally for 230V operation
>
>
>
> Do you have voltage measurement for tek 2235 on R907?
>
> I am getting 0.198V (with the +5V to the storage board disconnected,
> effectively turning off the storage function).
>
> This means that without the storage function connection, the current on
+43V
> is slightly below 1A.
>
> I am not sure if this is a normal condition but it should be similar to
the
> tek 2235.
>
> Under this condition, the scope could operate for quite a while without
> problem
>
>
>
> Once the +5V is connected to the storage board, voltage at R907 goes up to
> 2.3V~2.5V or about 1.15 to 1.25A on +43 and the power supply will shut
down.
>
>
>
> Following done with little effect or improvement
>
> - Change 3 filter caps (only those that are feels hot) - C960, C962, C964
> (for -5V for 2220 only)
>
> - Change NMOS P9070. When changing P9070, I used a separate heatsink for
> P9070 and still such down happens which means it is unlikely to be caused
by
> the thermal shutdown board.
>
> - Shutdown is more likely cause by spurious overload in R907
>
> - Change C907
>
> - Change CR907 (two diode in parallel)
>
> - Added C919 with 1000pf in parallel. Lowered switching freq but still
> having a heated P9070 which shuts down. Not sure what freq but it is
> definitely audible.
>
>
>
> Rgds,
>
> Chor Ming
>
>
>
> _____
>
> From: TekScopes@... [mailto:TekScopes@...] On
Behalf
> Of tom jobe
> Sent: Monday, January 23, 2012 11:17 PM
> To: TekScopes@...
> Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220
>
>
>
>
>
> Hi Tan,
> This morning I remembered about an interesting 2235 repair that was done
by
> a very crafty person by the name of Irwin Zosa during 2007 and 2008.
> Irwin lives in your part of the world in the Philippines where it is
almost
> as hot as in Singapore. His basic problem was the same as yours, the 2235
> would shut itself down with a thermal problem.
> There was not much interest in his 2235 problem on Tekscopes at that time,
> so the email exchanges we had went off group as he worked his way through
> the problem. Once he had the problem solved, he posted a nice summary of
his
> findings to the Tekscopes group.
> Irwin's repair summary is in message number 36518 and it was posted on Jan
> 6, 2009.
> He found a number of problems as he went along, but his final discovery
> might amaze you.
> It sure amazed me because I would have never found it!
> tom jobe...
> PS I will paste in Irwin's 2235 repair summary below to save you the
trouble
> of getting it from the Tekscopes Message archive.
>
> Hello to all: Way back in the first quarter of 2007, I had this
> problem of my 2235 shutting down after a few minutes from switching
> it ON. After a lot of interaction from forum members I finally
> nailed down the problem. Here is a brief summary of this process:
> First, I checked all the voltages according to the service manual and
> they were OK. I also checked the ESR of the capacitors and they read
> good but the unit being about twenty years old or so, I decided to
> replace the secondary caps (C960, C961, C962, C963, C968, C970) with
> ultra-low ESR, 105-degree C Nichicons. I could not find 840uF units
> so I used 1000uF. The problem still did not go away although the
> length of time-to-shutdown increased. I also replaced the FET
> switcher Q9070 (IRF710)with a higher rated device (IRF840). I then
> replaced Q946 and Q947 with TIP41 units. At every change that I
> made, slight improvements were observed. I also replaced R912 (357
> Ohms)with a 390 Ohm part to increase the shutdown threshold of this
> 2235. Then I replaced the secondary rectifiers CR954, CR955 (MR814),
> CR956, CR957 (MR812), CR960, CR961, CR962, CR963 (MR812). The
> replacements that I used were the MUR160 ultra-fast rectifiers. All
> of these were not done "shotgun" style and in the exact order as
> described but rather, followed logical steps of checking one section
> at a time. I even thought of thermal runaway because T944, T948, and
> Q9070 would heat up so much (really hot!). I also noticed early on
> that the frequency of the pre-regulator section (U930) was about
> 72KHz and in the service manual it is listed as about 60KHz. It
> suddenly dawned on me to try to reduce the operating frequency of
> this section so I replaced R919 and now it is running at 60KHz. The
> excessive heat has gone and no more shutdown. I tried to run the
> scope for about half a day and still no shutdown. This 2235 has all
> of the power supply improvements already installed from the factory
> (but why was the pre-regulator section running at 70KHz ?). I would
> like to thank the forum members who gave their insights. Special
> thanks goes to Tom Jobe, who stayed with me all the way in this
> repair project. He took the time to take measurement readings for
> me, gave his analyses, and even offered parts that I may need for
> free. At some point I was considering another brand of oscilloscope
> but I held on to this (even keeping it in storage for almost a year
> because of this problem) because from my research, these TEK scopes,
> during their time, were among the best. I think they still are.
>
>
>
>
Re: Power supply problem with Tek 2220

tom jobe
 

Hi Tom,
Thank you for your highly valued opinion on this matter!
The voltage drop Tan is seeing across his R907 is very normal (even though
this involves the dangerous assumption I'm making that his R907 is of the
correct value!).
Maybe Tan will look into the health of his Q946 and Q947 transistors before
he does anything else?
tom jobe...

----- Original Message -----
From: "Thomas Miller" <tmiller11147@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 4:47 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220


Hi Tom,

I don't think I would disable the thermal shutdown circuit if the
transistors are really getting as hot as Tan has stated. It would be like
taking a pain killer to mask a broken arm. But I do think you are on to
something with the leaky transistors. Or even the main switching transistor
could be bad.

Regards,
Tom


----- Original Message -----
From: tom jobe
To: TekScopes@...
Sent: Wednesday, January 25, 2012 7:27 PM
Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220



Hi Tan,
I see that JC has reported that his properly working 2230 has 0.23 volts
across R907.
I remembered that I had a 2230 so I found it and a note on it said that I
had worked on it in 2009 and that I had put some new parts in the power
supply.
I took the outer covers off of the 2230 and ran it on the bench. R907 had
0.245 volts while cold, and 0.246 volts after 30 minutes. The side of the
chassis where the heatsink for Q9070, Q946, Q947 is attached, barely got
warm to the touch in 30 minutes (maybe 2 or 3 degrees C more?).
I also checked R949, it was 0.683 volts cold, and 0.687 volts after 30
minutes.

The voltage on R949 came up on Irwin Zosa's 2235 when he discovered that
one
of the inverter transistors (Q946-Q947) had a leaky emitter to collector
junction (in one direction only). Irwin replaced the transistor pair with
some new TIP41C transistors and saw some improvement in his heat shut down
problem.

Some other test results from the past include:

Tektronix 2232,
R907 = 0.278 volts(!), R949 = 0.776 volts (R949 is 0.51 ohms on this
scope)

Tektronix 2236,
R907 = 0.189 volts, R949 = 0.497 volts (R949 is 0.51 ohms on this scope)
This is a late model 2236 and R912 is 575 ohms compared to 549 ohms for
the
earlier 2236's.
The higher ohms on R912 increases the shutdown current threshold across
R907.

All of the above and your symptoms, makes me wonder about the health of
your
Q946 and Q947 transistors?
Also, could you disconnect W950-1, W950-2 and W950-3 to completely
disconnect the thermal shutdown board A18?
Your voltage readings across R907 seem to be quite normal.
tom jobe...

----- Original Message -----
From: "Tan Chor Ming" <jonray03@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 1:46 AM
Subject: RE: [TekScopes] Re: Power supply problem with Tek 2220

> Hi Tom,
>
> Thank you for the detail of the problem which is similar to my tek 2220.
>
>
>
> I am still unable to nail down the problem. I believe the scope is
working
> normally, because when it fires up it works normally.
>
> My feeling is that it poorly designed thermally for 230V operation
>
>
>
> Do you have voltage measurement for tek 2235 on R907?
>
> I am getting 0.198V (with the +5V to the storage board disconnected,
> effectively turning off the storage function).
>
> This means that without the storage function connection, the current on
+43V
> is slightly below 1A.
>
> I am not sure if this is a normal condition but it should be similar to
the
> tek 2235.
>
> Under this condition, the scope could operate for quite a while without
> problem
>
>
>
> Once the +5V is connected to the storage board, voltage at R907 goes up
to
> 2.3V~2.5V or about 1.15 to 1.25A on +43 and the power supply will shut
down.
>
>
>
> Following done with little effect or improvement
>
> - Change 3 filter caps (only those that are feels hot) - C960, C962,
C964
> (for -5V for 2220 only)
>
> - Change NMOS P9070. When changing P9070, I used a separate heatsink for
> P9070 and still such down happens which means it is unlikely to be
caused
by
> the thermal shutdown board.
>
> - Shutdown is more likely cause by spurious overload in R907
>
> - Change C907
>
> - Change CR907 (two diode in parallel)
>
> - Added C919 with 1000pf in parallel. Lowered switching freq but still
> having a heated P9070 which shuts down. Not sure what freq but it is
> definitely audible.
>
>
>
> Rgds,
>
> Chor Ming
>
>
>
> _____
>
> From: TekScopes@... [mailto:TekScopes@...] On
Behalf
> Of tom jobe
> Sent: Monday, January 23, 2012 11:17 PM
> To: TekScopes@...
> Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220
>
>
>
>
>
> Hi Tan,
> This morning I remembered about an interesting 2235 repair that was done
by
> a very crafty person by the name of Irwin Zosa during 2007 and 2008.
> Irwin lives in your part of the world in the Philippines where it is
almost
> as hot as in Singapore. His basic problem was the same as yours, the
2235
> would shut itself down with a thermal problem.
> There was not much interest in his 2235 problem on Tekscopes at that
time,
> so the email exchanges we had went off group as he worked his way
through
> the problem. Once he had the problem solved, he posted a nice summary of
his
> findings to the Tekscopes group.
> Irwin's repair summary is in message number 36518 and it was posted on
Jan
> 6, 2009.
> He found a number of problems as he went along, but his final discovery
> might amaze you.
> It sure amazed me because I would have never found it!
> tom jobe...
> PS I will paste in Irwin's 2235 repair summary below to save you the
trouble
> of getting it from the Tekscopes Message archive.
>
> Hello to all: Way back in the first quarter of 2007, I had this
> problem of my 2235 shutting down after a few minutes from switching
> it ON. After a lot of interaction from forum members I finally
> nailed down the problem. Here is a brief summary of this process:
> First, I checked all the voltages according to the service manual and
> they were OK. I also checked the ESR of the capacitors and they read
> good but the unit being about twenty years old or so, I decided to
> replace the secondary caps (C960, C961, C962, C963, C968, C970) with
> ultra-low ESR, 105-degree C Nichicons. I could not find 840uF units
> so I used 1000uF. The problem still did not go away although the
> length of time-to-shutdown increased. I also replaced the FET
> switcher Q9070 (IRF710)with a higher rated device (IRF840). I then
> replaced Q946 and Q947 with TIP41 units. At every change that I
> made, slight improvements were observed. I also replaced R912 (357
> Ohms)with a 390 Ohm part to increase the shutdown threshold of this
> 2235. Then I replaced the secondary rectifiers CR954, CR955 (MR814),
> CR956, CR957 (MR812), CR960, CR961, CR962, CR963 (MR812). The
> replacements that I used were the MUR160 ultra-fast rectifiers. All
> of these were not done "shotgun" style and in the exact order as
> described but rather, followed logical steps of checking one section
> at a time. I even thought of thermal runaway because T944, T948, and
> Q9070 would heat up so much (really hot!). I also noticed early on
> that the frequency of the pre-regulator section (U930) was about
> 72KHz and in the service manual it is listed as about 60KHz. It
> suddenly dawned on me to try to reduce the operating frequency of
> this section so I replaced R919 and now it is running at 60KHz. The
> excessive heat has gone and no more shutdown. I tried to run the
> scope for about half a day and still no shutdown. This 2235 has all
> of the power supply improvements already installed from the factory
> (but why was the pre-regulator section running at 70KHz ?). I would
> like to thank the forum members who gave their insights. Special
> thanks goes to Tom Jobe, who stayed with me all the way in this
> repair project. He took the time to take measurement readings for
> me, gave his analyses, and even offered parts that I may need for
> free. At some point I was considering another brand of oscilloscope
> but I held on to this (even keeping it in storage for almost a year
> because of this problem) because from my research, these TEK scopes,
> during their time, were among the best. I think they still are.
>
>
>
>
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