Author Topic: HP 54502A Failing Analogue Trigger (Read 1644 times)

BogdanTheGeek · « **on:** July 13, 2021, 09:59:04 pm »

Hi Everyone,
I just got an HP 54502A scope and before anyone jumps at me, yes I have ordered a new dallas NVSRAM chip. Strangely enough this one doesn't seem to have failed(or has it) even though the date code is '89.
I absolutely love how this scope looks and I hope it can be good enough for my home use, I have a nice Rigol DS1054Z at work which I love but it feels like a toy. The HP also has an absolutely massive screen compared to anything I've ever used.
Back to the issue at hand. I have looked everywhere on the internet(including this forum), but I cannot anyone else having an issue with the analogue trigger alone.
Ch1 fails the self test for it, Ch1 passes, but fails the hysteresis test. Also, when cold the DAC fails the self test, but it passes once warmed up.
Anyone know what the failure codes mean(000 000 000 0111 for analogue trig)?
I have managed to connect the external trigger and trigger off of that up to about 10Mhz, after that it was not very stable, but to be fair the signal i was feeding wasn't the best.
This thread seems to be the only thing about this issue and there isn't much to it:
https://www.eevblog.com/forum/repair/hp-agilent-54502a/msg1197125/#msg1197125

PS: I will attach the service manual here for people looking for it as it was a pain to find (had to use the wayback machine on keysights website).
Still can't find any schematics online.

Runco990 · « **Reply #1 on:** July 14, 2021, 09:11:17 pm »

First check that all power supply voltages are in spec, both for voltage AND ripple! These series have capacitor leakage problems in the power block.

Then be mindful of the cable you use to run the scope calibration. It is picky.... found out the trial and error way.

Damn fine scopes as far as the construction goes. Still hold it's own, except no digital phosphor.

BogdanTheGeek · « **Reply #2 on:** July 14, 2021, 09:46:04 pm »

I have played around with the calibration, but I will try again with a better bnc to bnc.
All supply voltages are in spec. I will check ripple too.
It looks like a great scope, I just want it to work. Its the third scope I bought this month, all 3 broken with my luck.
First was a Hitachi V1050-f where everything was pretty much broken(sweep generator, trigger, attenuators on both channels), gave up after replacing all the caps on it(only managed to fix the sweep generator and to an extent the attenuators).
Than I got a Hameg 604 That has some weird vertical shift where the traces are only 1/4 of the way up from center even when level nob is all the way to the top on either channel. Comes and goes which is the worst part.
And then the HP with the trigger.
Good part is that I am learning a lot about analogue circuits and scopes in general.

BogdanTheGeek · « **Reply #3 on:** July 16, 2021, 06:59:02 pm »

Measured the noise on the PSU, everything around 50mV of high frequency noise, 5.2V line had about 150mV p-p. I couldn't find a table with the tolerances for the rails in the service manual.
Still waiting on the DALLAS chip to arrive, maybe that will fix it.
Any other ideas are very welcomed.

james_s · « **Reply #4 on:** July 16, 2021, 07:10:34 pm »

I don't think the NVRAM is going to change anything, when that goes bad it will lose settings when the power is off but as long as the power is on it makes no difference.

Have you tried other cables as someone else suggested? I don't have any experience with HP scopes but Tek scope calibration routines are insanely fussy.

BogdanTheGeek · « **Reply #5 on:** July 17, 2021, 12:10:03 am »

waiting on some 50ohm coax to make a bnc to bnc cable. I only have my probes and I used a 1x probe from the Calibration port into CH1 before. I've seen people do it even with alligator clips, but also some that say that it needs a good cable to succeed.
Yeah, I will replace it just because its 30 years old, so probably will have to do it at some point. Got a socket for the next guy who has to change it.
I don't see how the calibration would affect the self test, though.

BogdanTheGeek · « **Reply #6 on:** July 18, 2021, 06:55:58 pm »

Haven't gotten the RG58 cable I was waiting for, but decided to give it a try with the crappiest 75 $\$\Omega\$$ TV cable I have ever seen, but the only coax I have. The results are in, triggering passed on both channels, hysteresis failed on CH2 as well as all the gain and delay calibration points which is to be expected. Hopefully I will get the "good" cable sometime next week and will be able to do a full calibration

. I have high hopes that it will pass
I will keep the old NVRAM in for now, just to see how long it lasts, but it will be good having a replacement on hand.

BogdanTheGeek · « **Reply #7 on:** July 18, 2021, 07:04:02 pm »

Analogue Self Test still fails and cant find trigger on a signal.

I guess trying to get a full calibration to succeed is the only thing left to do and then hope it will fix it.

bd139 · « **Reply #8 on:** July 18, 2021, 07:31:41 pm »

Ah so you bought that one off ebay that was lurking on my watch list

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/

Smith · « **Reply #9 on:** July 19, 2021, 08:03:09 am »

Just a few thoughts. I used simple coax for self test and calibration, this worked ok. It does need to warm up at least half an hour to complete both successfully.

You can take out the hybrids and clean under them. I have heard about troubles with dust under the hybrids.

Also check the impedance of the scope, I've seen multiple inputs blown, and sometimes they impedance goes way off of goes fully open. Sometimes they can be repaired, by placing a resistor on the ceramic.

As said, do check the power supply. I have never seen one with and original working power supply. As I remember correctly you can also adjust the outputs

BogdanTheGeek · « **Reply #10 on:** July 20, 2021, 07:14:17 pm »

Lucky me

BogdanTheGeek · « **Reply #11 on:** July 20, 2021, 07:17:48 pm »

I will try to clean the hybrids, let it warm up and retry the test. Still no 50 $\$\Omega\$$ coax in the mail.
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.

BogdanTheGeek · « **Reply #12 on:** July 20, 2021, 08:14:57 pm »

Cleaned the hybrids, let it warm up an hour or so, redid the calibration with the crappy tv cable converted to BNC. I don't know its its the same issue, but the symptoms are the same, no triggering.
The Hysteresis for CH2 failed along with the GAIN and the DELAY for either channel, but trigger passes.
If In the self test, the analogue trigger still fails. I guess that the delay parameter is part of the analogue trigger test, and maybe that is what is failing.
I have attached a photo of the calibration results.

james_s · « **Reply #13 on:** July 24, 2021, 05:26:23 am »

TV coax is 75 ohm, a scope is going to have 50 ohm inputs. I don't think it's even worth messing with the calibration until you have a cable that it at least the right characteristic impedance. Calibration routines can be really picky.

BogdanTheGeek · « **Reply #14 on:** July 24, 2021, 03:53:20 pm »

Finally got some RG58 50ohm coax, will make a cable this weekend and try again. To excited to receive satellite images now though.

BogdanTheGeek · « **Reply #15 on:** July 24, 2021, 10:17:49 pm »

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

SoundTech-LG · « **Reply #16 on:** July 25, 2021, 02:29:25 pm »

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 · « **Reply #17 on:** July 25, 2021, 02:43:23 pm »

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

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/

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 · « **Reply #18 on:** July 25, 2021, 02:49:12 pm »

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

MarkL · « **Reply #19 on:** July 25, 2021, 03:38:41 pm »

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.

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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Author Topic: HP 54502A Failing Analogue Trigger (Read 1644 times)

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