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From: Tan Chor Ming
To: TekScopes@...
Sent: Friday, January 20, 2012 11:01 PM
Subject: RE: [TekScopes] Power supply problem with Tek 2220

?

Thanks Gary ,

Do you suspect that it’s a cap ESR issue?

Why would it cause such a behaviour?

I believe you can infer a lot from the behaviour than blind guesses that’s where I am trying to tap from the experienced engineers.

?

I suppose I have more than the basic knowledge because I am an electronics engineer with working knowledge in vacuum and solid state electronics.

I have built a scope from scratch, otherwise I would not attempt so work with EHT.

?

Rgds,

Chor Ming

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From: TekScopes@... [mailto: TekScopes@... ] On Behalf Of Terry Gray
Sent: Saturday, January 21, 2012 1:11 AM
To: TekScopes@...
Subject: Re: [TekScopes] Power supply problem with Tek 2220

?

?

At least check the incircuit power supply electrolytic capacitors for ESR. If you don't have an ESR tester, buy one or build one. There is lots of ESR information on the web and also in the archives of this yahoo group. ?You definitely need one if you're considering working on this type of electronic equipment and problems. Otherwise give the problem to someone else who has this essential test equipment and basic electronic knowledge to assist in diagnosing your problem. You are over your head without basic skills and test equipment.

?

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From: Tan Chor Ming
To: TekScopes@...
Sent: Friday, January 20, 2012 8:45 AM
Subject: Re: [TekScopes] Power supply problem with Tek 2220

?

?

Has anyone seen this behaviour repairing Tek2000 series scope power supply?

?

Behaviour

1. When power on from cold, it will fire up properly and stay on for a few minutes

2.? Once it warms up, the power supply shut down and try to on again and it continues in this on/off mode

3.? Pre-regulator out = 43V? (between TP940 and TP950)

4.? All voltages when the unit is stable for a few minutes are +8.6, -8.6, +5.2, +100 are all ok.

?

What are likely problems before I start ripping the components from the circuit?

?

?ttesenq@... suggested that it could be the pre-regulator MOSFET P9070.

?

I though that it could be a electrolytic cap but it doe not look likely because electrolytic failure would not have allowed the unit to fire up for a few minutes

It is more likely a active component that weaken with increase temperature, so ttesenq@... could be right.?

This would mean Q947 and Q946 are also likely components

?

Any other possible lead of similar experience.

?

?

?

----- Original Message -----

From: Tan Chor Ming

To: TekScopes@...

Sent: Friday, January 20, 2012 11:37 PM

Subject: RE: [TekScopes] Power supply problem with Tek 2220

?

Thank you Tom for taking the time to point out specifically the likely cause of the problem.

I know it takes effort to go through the details and I really appreciate it.

Just to let you know where I come from, so that there is less to assume.

I am an electronics engineer from Singapore above 50 so very much familiar with vacuum and solid state electronics.

I do not have the schematics for 2220 so like you I am assuming it is quite similar to 2235 which I have it downloaded.

I have both 2220 and 2235 and there appear to be quite similar though the PCB layout is slightly different.?

I have complete understanding of how the power supply works.

Pre-regulator - PWM driving Q908 to produce +43V across TP940 and TP950

Final Regulator – DC-DC push-pull inverter through Q947 and Q946.

I have another scope 2235, unfortunately, it too went down at the same time, so the only instrument I have is a multimeter.

Anyway, I do not have an isolation transformer so even with the 2235 working, I will need to get an isolation transformer before I could take any measurement.

Any idea what VA rating is needed for the isolation transformer?

Based on the power needs, it appears to be at least 200VA.

Initially, I was asking about R907, not because I was suspecting the resistor but I intend to check the load current by putting an ammeter in series with the resistor.

For that to be useful I need to know what is expected.? Like you. I am suspecting that the voltage at R907 has gone up after the set warms up thus shutting down U930. ?

I will probably go with the cap changes you have recommended since I have some time during the Lunar New Year break.?

Unfortunately, I do not have ESR meter, but I don’t think that a big issue just replacing the caps.

I am aware of the direct 43Vdc injection, but thank you for telling me the detail that Q908 needs to be disconnected.

Unfortunately, I do not have a power supply with this voltage output.? I only have a power supply of 18V max 2A.? So I will have to hold off this approach.

One more thing which I did not mention earlier is that I detached the +5.2V connection to the digital board (for the storage function of 2220) and behaviour remains unchanged.

Rgds,

Chor Ming

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From: TekScopes@... [mailto:TekScopes@...] On Behalf Of tom jobe
Sent: Saturday, January 21, 2012 7:30 AM
To: TekScopes
Subject: Re: [TekScopes] Power supply problem with Tek 2220

?

I saw your 2220 question the first time you posted it a few days ago, and I
did not have a nice short answer for you. Since you have not gotten the
answer
you were looking for, let me comment a bit on this.
First off, I don't own a 2220 or have the service manual for it, and none of
the download sites I checked seem to have it either, including
ArtekMedia.com having it for purchase. So I will assume it is similar to the
better 22xx's such as the 2235, at least around the power supply. I will
also assume that you do have a service manual to work with. U930 monitors
the voltage drop across R907, and in your case it probably thinks the
current is too high after a few minutes, so it shuts down.
I have worked on lots of 22xx scopes and I have not seen the value of R907
drift and cause this problem, but others have reported that as a
possibility. Odds are, that this is not your problem.
An over current situation can be caused by anything after R907 within the
inverter, the transformer itself or on the secondary side of the main
transformer.
How you go about finding the problem depends on what equipment you have
available, and what your testing preferences might be.
If you don't have much to work with, just start changing components. In my
experience the components in the right rear corner of the mainboard cause
the most grief so I would start there with C925, C942 and C943. If you have
some decent soldering equipment you can change all three capacitors quickly
without removing the mainboard.
If you have access to an ESR meter you could check all of the aluminum
electrolytic capacitors in circuit. Most of the electrolytics give no
trouble, especially the six(?) 840 uF caps in front of the heat sink for
Q9070 (but you would want to check all of the electrolytic caps if you had
an ESR meter handy).
Another approach would be to put in the 43 volts DC from an external power
source until something got hot or went up in smoke. If your power supply
allowed you to adjust the current limit you could sneak up on it and just
get the problem part(s) nice and warm. You will need to at least disconnect
Q9070 when you apply the external 43 VDC at TP940 and TP 950 from the bottom
or top of the mainboard. I lightly tack on two wires to the bottom side to
make the 43 VDC connections. You can use the scope in its normal way when it
is powered with the external 43 VDC.
The external 43 VDC idea came from a fine Tekscopes member named Hakan, many
years ago, and he has an excellent document you should read at:

A couple of years ago I made up a document listing some commonly available
components you might use to replace some of the original Tektronix part
numbers that often fail. I can send you that directly or you can find it in
the Tekscopes Message archive if you like.
Another thing you might do is take resistance readings from chassis ground
to each of the voltage test points before you get very far into this repair
(with the scope not connected to the mains of course!).
There are lots of other ideas to add to this, but this should get you
started.
This will be a simple fix, so just take your time and understand what you
are doing.
tom jobe...

----- Original Message -----
From: "Tan Chor Ming" <jonray03@...>
To: <TekScopes@...>
Sent: Friday, January 20, 2012 6:45 AM
Subject: Re: [TekScopes] Power supply problem with Tek 2220

> Has anyone seen this behaviour repairing Tek2000 series scope power
supply?
>
>
>
> Behaviour
>
> 1. When power on from cold, it will fire up properly and stay on for a few
> minutes
>
> 2. Once it warms up, the power supply shut down and try to on again and
it
> continues in this on/off mode
>
> 3. Pre-regulator out = 43V (between TP940 and TP950)
>
> 4. All voltages when the unit is stable for a few minutes are +8.6, -8.6,
> +5.2, +100 are all ok.
>
>
>
> What are likely problems before I start ripping the components from the
> circuit?
>
>
>
> ttesenq@... suggested that it could be the pre-regulator MOSFET
> P9070.
>
>
>
> I though that it could be a electrolytic cap but it doe not look likely
> because electrolytic failure would not have allowed the unit to fire up
for
> a few minutes
>
> It is more likely a active component that weaken with increase
temperature,
> so ttesenq@... could be right.
>
> This would mean Q947 and Q946 are also likely components
>
>
>
> Any other possible lead of similar experience.
>
>

?

?

Hi


Allow me to offer a different opinion on this isolation matter. I worked on a 7D13 with an isolated(floating) section this weeikend. I used my isolation gear so that I could tie my scope ground(common) to the floating common. In such cases isolation is desirable. In the 7603, the power supply transformer provides all the isolation you need. Using another level of isolation allows the 7603's chassis to float and that is not good. It is an unneeded safety hazard.


Jerry Massengale
jmassen418a@...

?

?

...I identified Q180 (2N3906) as having failed . C180 was also damaged (open). R180 and R216 had signs of cracked epoxy but their values were good. I replaced all of these components and the unit is running fine.

Trying to troubleshoot my 475 that quit working.
Have the service manual but being new to scope repair
i need some help.

The symptoms: a very very faint/blury horizontal line 1 div wide.
that traces only once when switching trigers or
playing with time/div nob goin from XY mode to slow
sweep rates.

In XY the blury 1 div wide line can be controlled horizontaly but nothing
verticaly. And it never wants to triger anything in any mode.

I have traced the vertical signal from the input to
the tube and it appears to be getting vertical deflection signal
but i am not clear on what amplitude and dc bias it should
be.

Sweep generator appears to run only once.

Right now i am guessing that something is wrong around the sweep
generator . But also do not understand why i am not getting
a dot in XY. like there is some problem with CRT circuit.


Any input of what should i look at would be appreciated.

thanx

Dan

Has anyone seen this behaviour repairing Tek2000 series scope power

Behaviour

1. When power on from cold, it will fire up properly and stay on for a few
minutes

2. Once it warms up, the power supply shut down and try to on again and

continues in this on/off mode

3. Pre-regulator out = 43V (between TP940 and TP950)

4. All voltages when the unit is stable for a few minutes are +8.6, -8.6,
+5.2, +100 are all ok.



What are likely problems before I start ripping the components from the
circuit?



ttesenq@... suggested that it could be the pre-regulator MOSFET
P9070.



I though that it could be a electrolytic cap but it doe not look likely
because electrolytic failure would not have allowed the unit to fire up

a few minutes

It is more likely a active component that weaken with increase

so ttesenq@... could be right.

This would mean Q947 and Q946 are also likely components



Any other possible lead of similar experience.

The effect was not visible for risetimes longer than about 500 ps.

Hi Achilles,

I'm wondering whether you've made further progress on your SG-503? Would like to hear the outcome when you have a chance.
Thanks,

Patrick Wong AK6C

BTW a 74LS14 with a resistor and mica capacitor provide a general-purpose 100 ns pretrigger delay. It seems stable to better than 20 ps rms if you happen upon a "good" inverter.

Hi Mike,
Strange things in the dot pattern happen indeed in Random mode, but I don't remember a case of time-reversal.
I think much depends on the repetition rate of your signal generator. I think the 264 and S-52 gave me no problems. I don't have faster repetition fast pulses.
Long ago I have seen negative rise and fall time with 3S76/3T77A. And that was sequential mode while showing the square wave output of a 284.
Albert

--- In TekScopes@..., "Mike" <zuckerme@> wrote:

David, Albert-

I have vaguely similar issues with both my 7T11 and 7T11A in random mode. I see large gaps in the horizontal dot density, and even misplaced (time-reversed!!) dots. For example by moving 7S11 delay, 7T11 time offset, and trigger position I can display a 100 ps step with positive, zero or even negative risetime (all with clean repeatable triggering).

---
Mike

David, Albert-

I have vaguely similar issues with both my 7T11 and 7T11A in random mode. I see large gaps in the horizontal dot density, and even misplaced (time-reversed!!) dots. For example by moving 7S11 delay, 7T11 time offset, and trigger position I can display a 100 ps step with positive, zero or even negative risetime (all with clean repeatable triggering).

---
Mike