Author Topic: Help Repairing Tek T922 Analog Scope - Mysterious Triggering Problem  (Read 471 times)

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Offline Smauel

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I need a little bit of troubleshooting and repair help.

I recently acquired an old Tektronix T922 analog scope. It seemed to be in fine working condition (there was some external damage in shipping, which I was able to get a refund for, and then repair mostly).

However, when I made a second order active low pass filter to turn a square wave from a 556 timer circuit into a sine wave, the triggering started acting up, on that signal source especially, and even more strangely, the more the potentiometer for the filter is turned (adding more series resistance into the input of the filter, filtering more high frequency out), the more it acts strangely.

Firstly, the region in which I can turn the trigger level and still get a stable trigger seems narrower than it should be. Not only that, but when I turn the trigger level, it is clearly changing something about the triggering, but I don't think its the trigger level, as the voltage level at the very start of the display when shifted all the way to the right is not constant, but always changing rapidly. Additionally, pressing the slope switch does not seem to change anything.

Here are two short clips of the weirdness:
https://photos.app.goo.gl/u8sVKzDo2Eq8g7mn7
https://photos.app.goo.gl/F8mwiigAFQ8rjpoC8

Changing the trigger settings have the result that it either triggers "normally" (a stable trigger, but the level where it triggers at is very binary, at either the very top or the very bottom, no in-between when adjusting the trigger level), or it triggers twice roughly 180 degrees out of phase with itself making two signals that slide around, or it triggers automatically at preset time intervals, making a chaotic display.

I can't make much sense of what I'm seeing. One observation that is halfway a guess is that it seems to only trigger at or near the extrema of the signal, if at all, but I don't know what that means.

Please tell me if you have ever witnessed anything like this, so I can see about figuring out what is causing it to try and fix it.

Thanks in advance for any help you can offer!
 

Offline jdragoset

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Re: Help Repairing Tek T922 Analog Scope - Mysterious Triggering Problem
« Reply #1 on: April 15, 2021, 12:58:01 am »
Exercise (hundred times or so) Triggering source and mode switches.
Switch the source to "ext" and jump the vertical signal to the ext trigger input.
Test for any change in triggering response.
Trigger appears to be in "Lf reject" as is on many newer scopes, as in no DC continuity in the source switch contacts.
This same type of problem will show up on vertical input as a + or - spike instead of each input square wave transition.
Very old wiping switch contacts will oxidize but often (not always) clean up with simple exercise without adding lube such as Deoxit fader lube. 
 

Offline wn1fju

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Re: Help Repairing Tek T922 Analog Scope - Mysterious Triggering Problem
« Reply #2 on: April 15, 2021, 11:32:12 am »
For what it's worth, I had triggering problems on my similar T912 scope (auto/norm wasn't right and multiple traces).  I worked my way through the triggering schematic to the point where the trigger signal makes its way onto the horizontal board.  There are a couple of 7400 logic gates that process that signal - one of them was bad.  As soon as I replaced the offending chip, everything worked fine.

If I remember, it is possible to lift the vertical boards out of the way and probe the horizontal board while the scope is running.
 

Offline Smauel

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Re: Help Repairing Tek T922 Analog Scope - Mysterious Triggering Problem
« Reply #3 on: April 16, 2021, 05:13:03 am »
jdragoset, thanks for the suggestions.

Exercising the switches did not appear to help unfortunately.

Switching the source to external and plugging the vertical input into the external input also did not make any difference. However, switching to X-Y mode does work as expected. That was a helpful suggestion because that tells me that the switch is probably fine since the X-Y mode works, but there is definitely something wrong with the triggering circuit since the external trigger doesn't work properly either.

I assume by "Lf reject" you mean the AC-GND-DC coupling switch. It is in AC mode, but changing to DC mode does not make a difference to the triggering issue, as the signal I'm using is relatively high frequency, so it passes through anyway.


wn1fju, thanks for your input as well.

Glad to hear of someone with a similar issue. I'll take a look at the schematic and see if I can find some datasheets. Then maybe I can get some voltage measurements and such to test those 7400 chips and I guess the rest of the horizontal circuit while I'm at it.

That sounds like really promising advice. I'll update when I get a chance.

Thanks again!
 

Offline Smauel

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Re: Help Repairing Tek T922 Analog Scope - Mysterious Triggering Problem
« Reply #4 on: April 22, 2021, 10:57:29 pm »
I took apart the scope a bit, and I've had a long look at the schematic and the voltage levels I'm seeing in the horizontal circuit. I am seeing some strange voltages, some of which are definitely intermittent, which was a bit of a surprise, and makes it more difficult to solve of course.

Unfortunately, the schematic is not clear about most of the voltages I should expect to see, but it states two voltages, both of which are intermittently incorrect. Also, when one is correct, the other is incorrect, but I think this is incidental. The first is -4V at a voltage divider, which is sometimes -7.3V instead, and the other is 2.5V at the emitter of a transistor Q2124, which is sometimes 5.3V instead.
Voltages on power supplies are all good.
From what I can tell, the 5.3V on the 2.5V part are simply a consequence of the -7.3V on the -4V voltage divider.

That voltage divider is between ground and -8V to create -4V at the base (pin 2) of a transistor U2126, part of a five transistor 3086 DIP array. However, the voltage divider sometimes reads -7.3V instead of -4V, meaning additional current is being drawn from R2127, the one with one [attachimg=1]lead to ground, causing the voltage to drop.

The base of the transistor is also decoupled to ground and to +8V. The collector of U2126 is connected directly to +8V as well, and the emitter is connected to the emitter of a second transistor on the 3086. Those emitters are connected by a 560 ohm resistor to -8V. Thus, the only path from the voltage divider other than the resistors is to +8V, ground, or through the base of the first transistor.

So the suspects have to be the resistors, the transistor, or some short. I poked and scraped at anything that might cause a short, but found nothing. I took the resistors out of circuit and measured them and they were fine, but they were old and crusty, so I replaced them while I was at it. So if the resistors are fine, the current has to be through the transistor. As far as I know, this means the transistor is drawing LOADS of current through its base, about 5 or 6mA by my calculations.

Obviously, that was an immediate red flag, but I took some voltage and diode drop measurements of the transistor in circuit, and they seemed okay. So, I desoldered it from the board and measured again and everything seemed nominal, as far as I could tell.

I don't have a component tester so I couldn't test the hFE or anything, but it still seems very bizarre to me that measurements of the transistor would be reasonable even though it is somehow passing 6mA through its base. I thought 6mA would destroy the base of a transistor. What would it take to destroy a BJT like that?

I assume that 6mA through the base will make it function improperly even if it reads okay out of circuit, so I will definitely replace the chip, but I would still like to know whether it could feasibly be okay for that to happen.

Am I missing something? Is that kind of current reasonable? Could the incorrect -7.3V at the -4V divider be because of something else affecting the emitter of the transistor?

In the meantime, I'll order that part and see if that fixes it, but I'd love to hear anyone's thoughts on this!
« Last Edit: April 22, 2021, 11:55:36 pm by Smauel »
 

Offline wn1fju

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Re: Help Repairing Tek T922 Analog Scope - Mysterious Triggering Problem
« Reply #5 on: April 23, 2021, 01:09:21 pm »
Don't know definitively whether the 3086 is the problem, but I've had a few of those 5-transistor arrays go bad over the years with somewhat strange faults.  I had one where there must have been some internal short to the substrate and that created havoc for everything connected to all 5 transistors.  I've had one where only one of the transistors tested bad when I pulled the chip out, yet several didn't work correctly when the chip was installed. 

I've also replaced these arrays with 5 discrete transistors (2N3904) soldered to an IC socket and then inserted into the board.  Seemed to work fine!  But I guess it depends on whether they are being used for simple switching or for higher frequency applications.

Good luck.
 

Offline Smauel

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Re: Help Repairing Tek T922 Analog Scope - Mysterious Triggering Problem
« Reply #6 on: April 23, 2021, 10:43:55 pm »
Good to know they can tend to test okay even if they aren't working correctly. That is really helpful.

The transistor that I am suspect of is in the trigger amplifier prior to the comparator that creates the trigger signal.

It seems the 3086 is uncommon now, but I found one I can buy from Jameco, which apparently they manufacture, or else I think I can find a comparable replacement. Anyway, I've ordered the Jameco 3086.

Hopefully it works out. I'll update again when I try it.

Thanks for the help again!

 


Offline Smauel

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I haven't had much time to work on the issue with classes wrapping up for the semester, and finals coming up in the next couple weeks.

However, I did install the new 3086, and it did not solve the problem. After fiddling some more, I have reason to believe the voltages in the region I suspected were correct all along, even if I don't fully understand them. Basically, switching to a different mode and/or making certain the body of the horizontal section is grounded properly with the screws change those particular voltages. Still could be this section, but I have nothing in particular to suspect, so I want to check other areas before I try to replace every component I can see.

Looking through the circuit more at what could be causing the problem, my suspicion has shifted to two other sections, the 7400 wn1jfu mentioned, and the source of the trigger signal in the vertical section.

The 7400 seems to be working normally, though I'm not certain. The output of one of the NAND gates varies in an analog way depending on the input voltage. I assume this is normal, as the datasheet says the output voltage is not guaranteed when it is between a 0 and 1 value for the digital voltage, however, it seems as though one of the other NAND gates varies only digitally, given a similar input. I'm not sure if this is normal operation.
The schematic of this circuit is attached. Pin 6 of the 7400 NAND gate chip is the one that varies digitally, pin 3 varies analogically.

The other suspect circuit is the trigger input from the vertical board, which might also make sense if the trigger signal amplifier is acting nonlinearly. However, since the display looks correct, the problem has to be after that, and I checked everything there and found no issue. Apparently a 4.3V Zener is a common failure, but it seemed to be okay. All the transistors were biased properly too. None of the voltages seemed wrong.

I've also heard bad solder joints and connectors can be a culprit, but the issue isn't intermittent, its very consistent, just with weirdly specific symptoms mostly to do with the input signal itself. Also, the joints look okay, and the connector pins are clean of corrosion. That said, I may go in and clean and resolder the connector pins between the vertical and horizontal board anyway at some point.

Not sure where to go from here, and I probably won't have much time to work on it for a little while, but as always, if you have any idea about whether what I'm seeing is normal, please tell me.

Thanks!
 

Offline wn1fju

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I'm always a bit suspicious when TTL logic gates aren't at TTL levels.  But assuming that 7400 chip is OK, and the trigger signal at pin 6 is OK, the trigger gets processed in the horizontal/sweep section by two more gates, ICs 2212 and 2234.  On my scope, that's where the fault was (I don't remember which chip of the two was bad). 

Not saying that my problems equal your problems, but 7400 chips are pretty cheap and plentiful and you could always simply swap in some new ones.

 


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