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Tektronix 2430A occasionally fails some tests

Goran Parezanovic
 

My experience with similar problem as described was that one of the sense voltages coming back to power supply was around 5 % lower that what it was supposed to be ( I think it was 8 V ). The sense voltage goes through 1 ohm resistor in the power supply and the voltage drop through 1 ohm resistor causes regulator to reduce voltage ( in case that there is a short somewhere through 8 V supply ). So my 8 V was draining some small amount ( 5 % ) of current more than max. designed for and from the sense voltage drop the 8 V supply would drop to few percent down and therefore the calibration would fail. I tested components (as far as I could ) but couldn't find what is taking extra current. At the end I decided to reduce the resistor of 1 ohm in feedback branch - I replaced it with 0.9 ohm same watts and everything worked fine since ( 2 years ). Occasionally I open it and check that the voltages are still the same ( since I was a bit nervous playing with the circuit designed to limit the current ) but so far all is good.

user920
 

The scope has the lithium (keeper 2) battery. I checked it's voltage and it's 3.52 V, so I guess it's still fine. I was planning a recap of the PSU, as soon as I get some vishay replacement I'll do it. Do you think it's necessary to replace caps also on the other boards?
Thanks for your help

Bob Koller
 

It is possible, I see many scopes with bad PSU, that will appear to work, sort of. I am quite sure that yours needs caps. The PSU may have been repaired in the past, many were. Likely poorly, with incomplete cap replacement, or cheap underrated parts.

The caps are easily replaced.
Does your instrument have the Dallas parts, or the lithium cell? If the lithium part, it is is probably fine. The Dallas parts if original are on their last leg.

user920
 

The scope was never touched internally (the nvram battery is still the original one and no recap was made). I guess I found a "solution" to the problem: if I let the scope warm up for like 10-15 minutes, I can do all the self cals and diagnostics I want and no error is thrown. This way, the scope works fine indefinitely. Is this possible in your experience?

Bob Koller
 

Have you recapped the power supply? ALL of this model need to have ALL the small electrolytic caps replaced as a matter of course. Until the power supply is solid, you cannot be sure of any other problem.
Also, as you may know, you cannot operate this scope out of the case for more than a very few minutes without providing external airflow over the main board hybrids. Also, do not block the internal fan, as it cools the power supply.

When the PSU is known to be good with new caps, you have performed a Self Cal, and intermittent failures persist, one can consider a CCD or PA failure. It does occur occasionally in my experience of having worked on 100+ of the 24xx digital scopes.
You can swap the hybrids between the channels and observe if the problem follows. You can also try cleaning the hybrid connector contact fingers, sometimes this will correct a problem.

Miguel Work
 

CCD KAPUT :(

-----Mensaje original-----
De: TekScopes@groups.io [mailto:TekScopes@groups.io] En nombre de user920
Enviado el: jueves, 11 de julio de 2019 10:54
Para: TekScopes@groups.io
Asunto: [TekScopes] Tektronix 2430A occasionally fails some tests

Hello, I recently bought this oscilloscope as partially working, the owner said it worked fine but once it failed some tests (PA and TRIGS). At home, no errors appear and the scope seems to be working. While I was trying some signals using an android app for function generation, the signal disappeared from the screen. I proceeded to make the self diag and there were two fails (CCD and PA, all relative to gains). Here are the pictures: https://imgur.com/a/DNgRcPG After I restarted the scope, however, there were no errors and I continued to test the scope without trouble. I am not sure what the problem can be, if there is any. Do you have any ideas of what I can check? Thank you

user920
 

So I was testing it, and found out that, if I run the self test, the scope doesn't glitch as long as I keep it on. If I turn it on, after some minutes it starts to have those gain problems, but as soon as I run the tests (after the warm up) all is good and I can use the oscilloscope as normal. Is it possible that the battery is failing and causing this problem? Are self-cal values stored in the nvram? I really hope it's not a ccd fault...

user920
 

Hello, I recently bought this oscilloscope as partially working, the owner said it worked fine but once it failed some tests (PA and TRIGS). At home, no errors appear and the scope seems to be working. While I was trying some signals using an android app for function generation, the signal disappeared from the screen. I proceeded to make the self diag and there were two fails (CCD and PA, all relative to gains). Here are the pictures: https://imgur.com/a/DNgRcPG
After I restarted the scope, however, there were no errors and I continued to test the scope without trouble. I am not sure what the problem can be, if there is any. Do you have any ideas of what I can check? Thank you