Repair of HP54502A - Page 1
Forum section: electronics / repair

author: PerArdua
author_rank: Regular Contributor
timestamp: November 11, 2015, 12:11:28 am
content: Hi everyone,I bought a HP54502A digital oscilloscope just over a year ago, and have grown to like it. However, recently it started to switch off and then back on again after being used for a couple of minutes. It passed the selftests okay, and as the restarts occurred more often after time progressed, I was thinking it could be some thermal issue in the power supply. I opened it up, and it appears very clean for its age. Upon opening the power supply, two large capacitors (just after a bridge rectifier?) seemed to have a slight bulge, so I replaced them. However, this has not fixed the issue.Does anyone have any idea as to what the problem might be? If you would like pictures/video let me know and I will try to get some. I am fortunate enough to be at uni, so I have a plethora of multimeters/scopes/whatever to take measurements if needed.Many thanks for all of your time, I really appreciate it.Regards,PAEDIT: I should like to add that a quick look over the other boards shows no obvious signs of damage/etc.

author: Stray Electron
author_rank: Super Contributor
timestamp: November 11, 2015, 04:16:41 pm
content: In my experience, the Model 54502 is scarce. But I suspect that it's very similar to the HP 54501 and 54510 and there are many threads on this forum and others about troubleshooting those so I would look for that and see if the information for those helps. I know that there is a current thread on troubleshooting a 54510 PSU on this forum right now.Keep us posted on what similarities you find and if you find the problem.

author: PerArdua
author_rank: Regular Contributor
timestamp: November 11, 2015, 04:50:03 pm
content: Dear Stray Electron.Many thanks for your reply. Indeed, it does seem that the power supplies are at least very similar (if not, the same). I'll have a search for those models you mentioned, and after lectures tomorrow, I should have the whole afternoon to do some probing if need be.Your time is most appreciated.PA

author: Smith
author_rank: Frequent Contributor
timestamp: November 12, 2015, 05:48:06 pm
content: It's a pain to fix the PSU. There's a reverse engineered schematic online, as a third party made these PSU's. I replaced all caps, as electrolyte was running all over the PCB but none of the caps measured bad, or had sings of defects. I also cleaned the whole PCB and replaced some discrete components that where affected by the electrolyte (wich has been there for at least 7 years) After that all was fine again.I heared about a lot who failed to repair the PSU.

author: wn1fju
author_rank: Frequent Contributor
timestamp: November 12, 2015, 11:50:07 pm
content: I've worked on two of these supplies. One I never got to work correctly, but the other one I eventually did. On the one that I did fix, there was leaking electrolyte all over and I pulled and replaced 29 caps. Out of the 29 I pulled, 20 were leaking and about six had pretty severe bulging along the bottom seal. But, as the previous poster also experienced, none of the caps tested bad! After I installed everything, I could only get about 4.2V on the 5V rail - the other voltages seemed mostly OK. So I started turning all the various adjustment pots in desperation, including the ones that set the switching frequency and the reference voltage used for the over-voltage protection circuitry (the small auxiliary board that sticks up perpendicular). Still no joy. I finally removed the optocoupler to break the switcher's feedback loop and there was absolutely no effect. So there wasn't any regulation going back to the switching transistor - it was running open loop and something was dragging the 5V line down on the secondary side. There is an SCR(?) in the protection circuitry and I clipped two of the three leads. The thing sprang to life and my 5V was where it was supposed to be. In retrospect, while I was chiseling off the epoxy for the -12V pot while I first tried to adjust things, I destroyed the pot. This gave me an overvoltage which gets or-ed with all the other voltages in the protection circuitry and managed to drag the 5V line down. Once I had the protection circuitry disabled, I fixed the pot, adjusted it to -12V and then put the protection circuitry back. Everything works fine (and has for a few months). In other words, it seems that you can get this supply horribly misadjusted so that the protection circuitry kicks in and prevents you from correcting the problem. Seems kind of counter-intuitive, since one would think you could simply readjust it back to working. But it had me fooled for a while!

author: Smith
author_rank: Frequent Contributor
timestamp: November 13, 2015, 06:53:28 am
content: The HP board is nice, easy to adjust and easy to work on. As mentioned earlier, the PSU is NOT made by HP. Its not build to be repaired, it can only be adjusted (with the pots). The service manual says to replace the PSU when it's broken.After all, replacing it with another (just as old) unit will probably give you the same problems in the years to come.As I remember, there where no unobtainium parts in the PSU, only the transformer perhaps.

author: PerArdua
author_rank: Regular Contributor
timestamp: November 13, 2015, 08:23:04 pm
content: Many thanks for your replies.The board itself is actually beautifully clean. No signs of leaked electrolyte.Today I added wires on the test points to allow me to probe the outputs when it was running (did not fancy putting my hand next to the crt). Whilst a couple of voltages were not perfect, they were okay enough. So I waited for it to switch off. Alas, it did not happen. 20 mins went by and not a single problem. I wondered if this was due to the cover being removed, but even when the cover was replaced, it appears to now work fine.One thing I would like to ask those who have experience with this supply, is how warm does the supply case tend to get? Mine wasn't exactly 'hot', but it was rather warm indeed.As a side note, I have been considering finding a replacement fan - I suspect that modern fans will be much quieter and able to provide a greater throughput of air - which may help with the cooling... Furthermore, I will probably go around and replace all the capacitors anyway, when I get home. Alas, the irons students have at uni are a measly 14W and the big copper pours wick away heat like nothing else. The technicians have big Weller irons, but watching how one guy used the iron to scrape away at a couple of solder joints (scratching the substrate) I do not feel comfortable letting them desolder the entire board. Ah well.I shall endeavour to upload some photographs later this evening.Many thanks again for everyone's time.PA

author: Stray Electron
author_rank: Super Contributor
timestamp: November 13, 2015, 10:02:59 pm
content: Quote from: wn1fju on November 12, 2015, 11:50:07 pmDid you happen to keep notes about the pinouts and which pots controlled which voltages and so forth? If so, can you post them? They could be a great help to others.Any idea why you could never get the second PSU working?How bad or good is the quality of the capacitors that HP's subcontractor Used in these things? I hear of a LOT of them failing. Also did you have to recalibrate the scope after fixing the PSU?

author: wn1fju
author_rank: Frequent Contributor
timestamp: November 13, 2015, 10:30:13 pm
content: For Stray Electron:There is a pdf floating around from a French web site that has a pretty good reverse engineering of the schematic. The link to it has been given in several related forum entries (I only have the document somewhere, not the original link). That has all the adjustment pot locations.In retrospect, the first P/S that I ultimately did not fix might have had the same problem with the protection circuit kicking in. But I never went back to it because I scavenged quite a few parts from it in an attempt to fix the second P/S!I can't comment on the quality of the electrolytics that the manufacturer originally used other than to say that many of them were leaking in the first P/S and almost all of them were leaking in the second P/S. My guess is that these supplies need a bit more cooling than is supplied by the host piece's fan.

author: PerArdua
author_rank: Regular Contributor
timestamp: November 14, 2015, 06:35:51 pm
content: The electrolytic caps are all Nippon as far as I can see. Most have a rating of 105C - might be worth replacing with 125C options in the future?The supply schematic can be found here:http://www.radiocollection.be/images/restaurations_img/HP54503/supplyHP54503.pdf] http://www.radiocollection.be/images/restaurations_img/HP54503/supplyHP54503.pdfAfter my repair, I did not need to recal the scope.Also worth bearing in mind is that these supplies are getting old. The transistor in mine has a '90 date code. That makes it 25 years old! Coupled with some (perhaps) slightly warmer temps, and its not surprising things are failing.Regards,PA