Electronics > Repair

HP 54502A Failing Analogue Trigger

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BogdanTheGeek:
The new cable didn't make a difference. I hope I don't have to go down chasing capacitors.

SoundTech-LG:
What you need to do is narrow down the problem. I think you said it is limited to CH 1. If that is the case, disassemble, and pull the mainboard. Swap CH 1 attenuator with one from a working CH (easy to do as they are held only by screws). Reassemble, power on..  See if the failure follows the attenuator from CH 1. This not only identifies a possible bad attenuator, it also tells you if it is the CH1 attentuator, or somewhere else in CH 1 circuitry. Let us know the result.

SoundTech-LG:

--- Quote from: bd139 on July 18, 2021, 07:31:41 pm ---Ah so you bought that one off ebay that was lurking on my watch list  :-DD

Artek have full schematics available for $15 for this. Usually referred to as CLIP guides. Search for 54502A here: http://artekmanuals.com/manuals/hp-manuals/

--- End quote ---

I was surprised to see Artek had the CLIP for the 54502A. I was under the impression there were no schematics available for the 545xx series at all.

bd139:
Yeah they have a few. Never the one for the scope I inevitably end up with though  :-DD

MarkL:

--- Quote from: BogdanTheGeek on July 20, 2021, 07:17:48 pm ---...
I have checked the power supply, as mentioned above, looked fine. I have gotten the new NVSRAM, but I want to try the simple things first.

--- End quote ---
Part of the power supply is also any voltages derived locally from the main power supply.  In looking at photos of a 54502A board posted here:

  https://www.eevblog.com/forum/reviews/hp-54502a-nearly-vintage-digitizing-scope/

I'm counting at least seven TO-220 devices which are likely linear regulators.  Unless you want to do some circuit reverse-engineering, you should purchase the schematics (see bd139's post) to check the output voltage and ripple of these local regulators.

If you have failures occurring in one channel and not the other, one simple thing to do is to swap any swappable components (namely the attenuators and acquisition hybrids) to see if the problem moves.  Label them before you swap them so you can put them back where they came from.

Are you unable make the scope trigger at all in normal operation?  If so, try slowly sweeping the trigger level to extreme settings.  Maybe it will trigger once, or at a completely incorrect level, which could be a clue where to look next.

Also, if possible please post a screen shot of the self-test results.

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