Hi Tan,
One more thing for today.
On this 2230 I have, the voltage at the gate of Q935 is 0.211 volts more
positive than the Kathode of the SCR, so it must be very close to firing the
SCR!
The 0.054 volt drop across the 10 Ohm R948 is much more current than it
would take to fire the SCR if the voltage was just a bit higher.
I believe these sensitive gate SCRs only need about 200 uA of current to
fire.
tom jobe...

----- Original Message -----
From: "Tan Chor Ming" <jonray03@...>
To: <TekScopes@...>
Sent: Thursday, January 26, 2012 5:48 AM
Subject: RE: [TekScopes] Re: Power supply problem with Tek 2220

> Thanks for all who responded.
>
> Special thanks goes to Tom Jobe and JC for giving me the detail
> measurement.
>
>
>
> First of all, my scope is now working (or at least the PS is working).
>
>
>
> Measurement on R907 is 0.225 volts when cold and 0.24 volts when warm
>
>
>
> R949 is 0.75 when cold and 0.79V when warm.
>
>
>
> Everything seem ok though the current on R949 is little high.
>
>
>
> Note that the R949 is not exactly 0.51 ohm as in the 2230 schematics. See
> the explanation on how the fault is fixed
>
>
>
> Fault Finding log on power supply shutdown
>
> Refers to 2230 schematics but the PS is identical to the 2220
>
>
>
> Shutdown persist despite the following
>
> - Measurement on Pin 15 of U930 shows 0.295V, however, voltage on R907 is
> less than 0.25 --> Unlikely to trigger shutdown unless there is a spurious
> high curremt in R907
>
> - I have replaced the P9070 and placed it on a separate heatsink. This
> means the thermal shutdown board is not heated and thermal shutdown should
> not happen --> Eliminated the shutdown circuit
>
> - Connected a 1000p In parallet to C919 to bring down the oscillation
(which
> now becomes audible) --> Unlikely to be due to oscillation being too high
>
> - TP940 and TP950 is +42.5V so crowbar Q985 will not fire
>
> - Remove the connector of the P9070 and solder the leads directly
>
> - Change some filter caps which are hot. I do not suspect this to be the
> problem but just ran out of ideas.
>
> - Tried to lower the series resistance (R909, 33 ohm) connected to the
gate
> of Q9070 but heat is still the same. Note this resistor is not present in
> 2235.
>
> This resistor is a offers some protection for the Q908 and U930 in case
> P9070 fails but it reduces the turn off time of Q9070.
>
>
>
> When I disconnected the +5V to the digital storage board, the PS is able
to
> sustain for at least 15-30 minute before it shutdown and on again and stay
> for a few minutes before shutdown again.
>
> This provided a clue that the shutdown is related to the load. It is
> probably at the threshold without the storage board connected.
>
>
>
> At this point, I was using the 2235 circuit to debug the 2220 and I ran
out
> of ideas because all possible cause to cause shutdown were eliminated
>
>
>
> I decided to refer to the 2230 schematics and compare the difference. I
> notice a difference in the circuit which I was not aware earlier.
>
> The 2230 has 4 shutdown protection. The 2235 only has 2 (item 1 and 2)
>
> - 1. Overloaded preregulator (via R907)
>
> - 2. Over voltage of pre-regulator supply (via VR35 firing Q935
if
> greater than 51V)
>
> - 3. Thermal shutdown (only in 2230 using a separate board)
>
> - 4. Overloaded inverter (via R949 and connected to the gate of
> Q935 - only in 2230 )
>
>
>
> I have eliminated 1,2,3 and only realized the 4.
>
> I decided since it is at the shutdown is at overload threshold, I could
> raise the output current and if it is due to 4, shut down should no
happen.
>
>
>
> Immediately, I place a 1 ohm resistor across R949 (which is 0.51 ohms for
> 2230 but it is 1 ohm for 2235). This effectively raise the current in
R949
> by 33%
>
>
>
> Power on, and shutdown was not seen.
>
> Connect the +5V to the digital storage board, and it works now without
> shutdown
>
>
>
> Decided than I should increase the parallel resistor to 2 ohm which
> effectively raise the current by 20%
>
> Repeated the above test, and everything was working stably. In fact the
> scope has been tested for more than an hour without shutdown happening.
>
>
>
> Voltage measure after modification
>
> R907 - 0.23V when cold, 0.24V when warm --> Looks ok
>
> R949 - 0.74V when cold, 0.79 to 0.8V when warm --> looks ok but may be a
> tad higher than normal
>
>
>
> Inverter switch
>
> I tried to change the Q947 and Q946 in an attempt to lower the load
current.
> After popping out the transistors,
>
> I realized that the transistors I bought was TIP31C which is of low
current
> rating of 3A. I have misread the TIP41C and as TIP31C in the mail.
>
> There is a good chance that it will work but decided again taking the risk
> since the scope is already working.
>
> I decided the test the removed Q946 and Q947 for leakage. Testing the
> Collector - Emiiter, the resistance is infinite which shows that the
> transistor is not leaky.
>
> Pop back the original transistor and cover the scope.
>
>
>
> Temperture
>
> I am still concerned about the temperature. It is quite hot without the
> outer casing on.
>
> It is probably in the 70~80 C range. Strangely this is different from
Tom's
> observation.
>
> Note that the my tek 2220 is connected to the 230VAC.
>
>
>
>
>
>
>
>
>
> _____
>
> From: TekScopes@... [mailto:TekScopes@...] On
Behalf
> Of tom jobe
> Sent: Thursday, January 26, 2012 8:27 AM
> To: TekScopes@...
> 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.
> >
> >
> >
> >
>
>
>
>

Hi Tan,
Thank you for the update on your progress.
You have provided us with many things to think about!
I wonder where you will go from here with your 2230?

The voltage you have at R949 is now about 0.8 volts. The diode CR948 is a
silicon diode so its forward voltage is going to be 0.7 volts or less, this
means that there is a little bit of voltage arriving at the Gate of the SCR
and R935. Q935 is a "sensitive gate" SCR and it does not take much voltage
and current to fire the SCR.
Those original SCRs often fail, and the Mod kit gave you a new SCR that was
probably an MCR72-4 (8A, 200 volt). I could never find an MCR72-4 but the
higher voltage part MCR72-6 (8A, 400 volt) was easy to find.

As for the temperature I reported yesterday, it was VERY wrong. I ran the
2230 for 30 minutes this morning and the coldest part of the chassis was 22
C, and the hottest part was 52 C. I must be more careful with my sloppy
comments and testing!
This 2230 has new TIP41C transistors in it at Q946/Q947, for whatever that
is
worth.

Francis brought up a very good point about how different transistors have
different characteristics. I don't know what most of us can do about that,
other than have blind faith in the parts we buy, and just see if they work
properly.
A year or two ago, Francis taught me how to make a test fixture and how to
test some transistors that had to do with someone's 2245A inverter thermal
problems. It was scary to see how the same part number transistor from
different manufacturers differed just in their switching characteristics!
We are lucky to have Francis taking an interest in these problems as he is
very knowledgeable.

The earlier 22xx's such as the 2230 used 151-0476-02 transistors for Q946
and Q947. On the Internet these seem to cross over to a TIP31C.
The later 22xx's used the 151-0852-00 which crosses over to a TIP41C.
The power supply modification kit that Tektronix provided for some of the
22xx's had 151-0852-00 transistors included.
I don't know where to buy the 151-0852-00 transistors, so I'm using TIP41C's
that you can buy anywhere, until we find something better.

About that extra connection the 2230 has from R949 to the base of the SCR
Q935, the same connection was added as part of the 22xx power supply Mod
kit.
The 22xx update kit used a 1N4152 silicon diode and a 10 ohm resistor in a
wire you
add to your 22xx. The Power Supply Mod kit only covered certain 22xx's such
as 2213, 2215, 2213A, 2215A, 2235 and 2236.
Tektronix described this as: "a diode-resistor network is added between the
emitters of Q946/Q947 and the gate of Q935 to ensure power supply shutdown
if an overvoltage occurs".
tom jobe...

PS When you get around to working on your 2235, you might consider doing all
of the changes in the 22xx Mod kit. The later production scopes came with
the mod kit parts installed, so you might already have the Mod kit installed
in your 2235, especially if it is a military 2235 called AN-USM-488. If you
have the Mod kit installed on your 2235, there will be the shrink tubing
covered wire with the diode and resistor, going from the front of R949 to
the back of R935 on the top side of the main board.

----- Original Message -----
From: "Tan Chor Ming" <jonray03@...>
To: <TekScopes@...>
Sent: Thursday, January 26, 2012 5:48 AM
Subject: RE: [TekScopes] Re: Power supply problem with Tek 2220

> Thanks for all who responded.
>
> Special thanks goes to Tom Jobe and JC for giving me the detail
> measurement.
>
>
>
> First of all, my scope is now working (or at least the PS is working).
>
>
>
> Measurement on R907 is 0.225 volts when cold and 0.24 volts when warm
>
>
>
> R949 is 0.75 when cold and 0.79V when warm.
>
>
>
> Everything seem ok though the current on R949 is little high.
>
>
>
> Note that the R949 is not exactly 0.51 ohm as in the 2230 schematics. See
> the explanation on how the fault is fixed
>
>
>
> Fault Finding log on power supply shutdown
>
> Refers to 2230 schematics but the PS is identical to the 2220
>
>
>
> Shutdown persist despite the following
>
> - Measurement on Pin 15 of U930 shows 0.295V, however, voltage on R907 is
> less than 0.25 --> Unlikely to trigger shutdown unless there is a spurious
> high curremt in R907
>
> - I have replaced the P9070 and placed it on a separate heatsink. This
> means the thermal shutdown board is not heated and thermal shutdown should
> not happen --> Eliminated the shutdown circuit
>
> - Connected a 1000p In parallet to C919 to bring down the oscillation
(which
> now becomes audible) --> Unlikely to be due to oscillation being too high
>
> - TP940 and TP950 is +42.5V so crowbar Q985 will not fire
>
> - Remove the connector of the P9070 and solder the leads directly
>
> - Change some filter caps which are hot. I do not suspect this to be the
> problem but just ran out of ideas.
>
> - Tried to lower the series resistance (R909, 33 ohm) connected to the
gate
> of Q9070 but heat is still the same. Note this resistor is not present in
> 2235.
>
> This resistor is a offers some protection for the Q908 and U930 in case
> P9070 fails but it reduces the turn off time of Q9070.
>
>
>
> When I disconnected the +5V to the digital storage board, the PS is able
to
> sustain for at least 15-30 minute before it shutdown and on again and stay
> for a few minutes before shutdown again.
>
> This provided a clue that the shutdown is related to the load. It is
> probably at the threshold without the storage board connected.
>
>
>
> At this point, I was using the 2235 circuit to debug the 2220 and I ran
out
> of ideas because all possible cause to cause shutdown were eliminated
>
>
>
> I decided to refer to the 2230 schematics and compare the difference. I
> notice a difference in the circuit which I was not aware earlier.
>
> The 2230 has 4 shutdown protection. The 2235 only has 2 (item 1 and 2)
>
> - 1. Overloaded preregulator (via R907)
>
> - 2. Over voltage of pre-regulator supply (via VR35 firing Q935
if
> greater than 51V)
>
> - 3. Thermal shutdown (only in 2230 using a separate board)
>
> - 4. Overloaded inverter (via R949 and connected to the gate of
> Q935 - only in 2230 )
>
>
>
> I have eliminated 1,2,3 and only realized the 4.
>
> I decided since it is at the shutdown is at overload threshold, I could
> raise the output current and if it is due to 4, shut down should no
happen.
>
>
>
> Immediately, I place a 1 ohm resistor across R949 (which is 0.51 ohms for
> 2230 but it is 1 ohm for 2235). This effectively raise the current in
R949
> by 33%
>
>
>
> Power on, and shutdown was not seen.
>
> Connect the +5V to the digital storage board, and it works now without
> shutdown
>
>
>
> Decided than I should increase the parallel resistor to 2 ohm which
> effectively raise the current by 20%
>
> Repeated the above test, and everything was working stably. In fact the
> scope has been tested for more than an hour without shutdown happening.
>
>
>
> Voltage measure after modification
>
> R907 - 0.23V when cold, 0.24V when warm --> Looks ok
>
> R949 - 0.74V when cold, 0.79 to 0.8V when warm --> looks ok but may be a
> tad higher than normal
>
>
>
> Inverter switch
>
> I tried to change the Q947 and Q946 in an attempt to lower the load
current.
> After popping out the transistors,
>
> I realized that the transistors I bought was TIP31C which is of low
current
> rating of 3A. I have misread the TIP41C and as TIP31C in the mail.
>
> There is a good chance that it will work but decided again taking the risk
> since the scope is already working.
>
> I decided the test the removed Q946 and Q947 for leakage. Testing the
> Collector - Emiiter, the resistance is infinite which shows that the
> transistor is not leaky.
>
> Pop back the original transistor and cover the scope.
>
>
>
> Temperture
>
> I am still concerned about the temperature. It is quite hot without the
> outer casing on.
>
> It is probably in the 70~80 C range. Strangely this is different from
Tom's
> observation.
>
> Note that the my tek 2220 is connected to the 230VAC.
>
>
>
>
>
>
>
>
>
> _____
>
> From: TekScopes@... [mailto:TekScopes@...] On
Behalf
> Of tom jobe
> Sent: Thursday, January 26, 2012 8:27 AM
> To: TekScopes@...
> 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.
> >
> >
> >
> >
>
>
>
>

On Thu, 26 Jan 2012 13:54:41 -1000, "David Sidrane" <david_s5@...>
wrote:

>3 to gnd 1.38 - Where is this tied?
>
>5 to gnd 6.8

Good. These are the IC power pins. They are connected through a
couple of diodes to ground and the 8.6 volt supply as shown on
schematic 11.

If you can dig up the Tek-Made book, the data sheet for the
155-0022-00 is on page 5-7 although I do not think you need it yet.

>4 Ch 1 or Both 1.68

Good.

> Ch 2 4.0

Bad. But the biasing problem below might be the cause.

>14 Both Add 6.16
>
>14 Both Alt/Chop 1.44
>
>14 ch 1 Alt/Chop/add 1.44
>
>14 ch 2 Alt/Chop/add 1.44

All good. The voltages read about 1.4 volts high because the IC
ground reference at pin 3 is raised by two series connected diodes
shown on schematic 11.

>DC bias
>
>15 1.8
>
>16 1.04
>
>1 1.16
>
>2 1.94

Good.

>7 4.31
>
>8 3.51
>
>9 3.17
>
>10 3.93

Bad. These are way off. The external negative bias to -5 volts could
be missing and causing all of the problems.

Check R2101, R2102, R2109, R2105, and R2106. Look for an open
resistor or trace.

>Q2101/2 Base 5.04

Good.

What about pins 12 and 13?

>From: TekScopes@... [mailto:TekScopes@...] On Behalf
>Of David
>Sent: Thursday, January 26, 2012 10:59 AM
>To: TekScopes@...
>Subject: Re: [TekScopes] Re: Tek 2230 - Storage does not work.
>
>Have you checked the control input levels at pin 4 and pin 14 and the
>power supply pin voltages at pin 3 and pin 5?
>
>What about the DC bias levels at all of the input and output pins?
>They are marked on the schematic.
>
>The cascode bias at the base of Q2101 and Q2102 should be checked as
>well.
>
>On Wed, 25 Jan 2012 22:42:52 -0000, "david_s5y" <david_s5@...
><mailto:david_s5%40usa.net> >
>wrote:
>
>>Hi,
>>
>>This is getting weirder, but I am sure it means something to someone....
>>
>>Store is active.
>>
>>I set up to put a 3.5 V 1kHz signal into CH1, and turned the CH1
>sensitivity down to 5mV. *Ch2 is set to ground*
>>
>>Then Chn 1 displayed a clamped to a square wave version of the input.
>>movable by Chn 2' position to only show the half the signal (positive)
>peeks on the CRT.
>>
>>Now setting Vert Mode to both gives me 2 versions of the clamped. Chan 2 is
>2X the amplitude of chan 1.
>>
>>Chn 1's Position does nothing. But cha 2's position moves both wave forms.
>>
>>Setting it to Chn 2 gives me one waveform that is 2X.
>>
>>As I decrease the Chan 1 Sensitivity the waveform begins to round to a sine
>wave.
>>
>>Does this and the +5 V on pin 6 and 11 indicate a bad U2101?
>>
>>Next Q does someone have a TEK 234-0408B-20, they would like to sell?
>
>

Have you checked the control input levels at pin 4 and pin 14 and the
power supply pin voltages at pin 3 and pin 5?

What about the DC bias levels at all of the input and output pins?
They are marked on the schematic.

The cascode bias at the base of Q2101 and Q2102 should be checked as
well.

On Wed, 25 Jan 2012 22:42:52 -0000, "david_s5y" <david_s5@...>
wrote:

>Hi,
>
>This is getting weirder, but I am sure it means something to someone....
>
>Store is active.
>
>I set up to put a 3.5 V 1kHz signal into CH1, and turned the CH1 sensitivity down to 5mV. *Ch2 is set to ground*
>
>Then Chn 1 displayed a clamped to a square wave version of the input.
>movable by Chn 2' position to only show the half the signal (positive) peeks on the CRT.
>
>Now setting Vert Mode to both gives me 2 versions of the clamped. Chan 2 is 2X the amplitude of chan 1.
>
>Chn 1's Position does nothing. But cha 2's position moves both wave forms.
>
>Setting it to Chn 2 gives me one waveform that is 2X.
>
>As I decrease the Chan 1 Sensitivity the waveform begins to round to a sine wave.
>
>Does this and the +5 V on pin 6 and 11 indicate a bad U2101?
>
>Next Q does someone have a TEK 234-0408B-20, they would like to sell?