HP/Agilent 54825A Oscilloscope Power Supply Issues - Page 1
Forum section: electronics / testgear

author: G0HZU
author_rank: Super Contributor
timestamp: July 02, 2022, 10:35:32 pm
content: HiIn recent months my old HP/Agilent 54825A Infinium scope has been playing up and it has slowly been getting worse and worse. To cut a long story short I think the PSU is generating an alarm flag and this cripples the scope such that it shuts down some of its operation and the trace freezes and often I see a red error message that reads something like "fine interpolator gain cal failed".This error can happen right from the first power up of the day or it can happen after 30 seconds or several hours. Often a reboot restores operation but the scope has been getting worse and worse in that this is becoming a stubborn fault. I've noticed a few symptoms:The AUX OUT signal dies at the same time the scope trace freezes and I think this is a normal reaction for the scope when it receives the alarm flag from the PSU. After a few seconds or minutes the scope then displays the (false?) "fine interpolator gain cal failed" error message.After much wiggling of things and messing with settings and temperature I kept coming back to the power supply and I now think I've found what is causing the scope to go into partial shutdown.By monitoring a test point on the PSU I can tell from the moment the scope is powered on if it is going to be able to run the scope loader program. This program dumps some files into a ramdisk drive and then launches the scope application under Windows 98.However, if the PSU alarm flag is already set then I know the scope launch is doomed to failure. It typically freezes during the loader operation if this flag is set and doesn't even get as far as loading the scope app into the ramdisk.However, if the alarm flag is OK then the scope will launch just fine. Unfortunately, if I monitor the alarm flag then I can tell within a few seconds when the scope trace is going to freeze followed by the "fine interpolator gain cal failed" error message appearing on screen. This happens right after the PSU alarm flag is set by the PSU itself. If I monitor my 5V 12V -12V and -5V rails from the PSU they all look fine except the 12V rail has a small amount of spiky ripple on it. These rails seem to start up correctly and stay at the same voltage regardless of the PSU alarm flag.Has anyone seen this behaviour before? I'm left wondering if one of the supply rails has drifted a bit out of tolerance and the PSU has condemned itself via the alarm flag. If I isolate the alarm flag from the rest of the scope then the scope always seems to work fine but I know this is asking for trouble further down the line.Has anyone worked on these PSUs before? Any info about how to adjust them or test them?Thanks in advance for any replies...

author: tautech
author_rank: Super Contributor
timestamp: July 03, 2022, 12:01:53 am
content: Quote from: G0HZU on July 02, 2022, 10:35:32 pmMe too.How many runtime hours and boots ?PC based mainboard with SMPS PSU ?Have you tried subbing in another PC PSU to see if it goes away ?Sure smells like dicky PSU or mainboard caps to me.

author: G0HZU
author_rank: Super Contributor
timestamp: July 03, 2022, 11:39:59 am
content: Hi TautechThis scope is old... probably 25 years old and so the run time will be huge. It's had the official factory upgrade pack so the motherboard/CPU/memory/HDD has been upgraded and it has the LS120 floptical drive. But the PSU looks to be original. The PSU is really complicated and not easy to work on.There's some images of the PSU in this old thread below. This looks to be the same model as my PSU. It would be nice to have a manual for the PSU or a circuit diagram or some idea how the PSU interfaces with the rest of the scope. I've traced out some of the interface circuit but I'm not sure what this alarm/status flag actually means. It might not even be an alarm/status flag but it causes the scope to hang when it gets set.https://www.eevblog.com/forum/repair/agilent-54825a-infiniium-oscilloscope-dead-power-supply/Presumably the PSU can test itself in some ways. This might include temperature and current. The PSU consumes about 5W in standby and typically 170W when the scope is running. This is measured using a wattmeter on the AC inlet. There might be other reasons the scope needs to communicate with the PSU. There is a separate 'line' feed to the scope board but I think this is just for a line triggering option. Apart from the usual large white 5V/12V/-12V/-5V connectors from the PSU there's also a smaller 11 wire interface from the PSU to the scope mainboard.I suspect the alarm may have something to do with the spiky ripple on the 12V output. But then again the 12V supply probably doesn't have to be that clean so maybe this is normal...

author: G0HZU
author_rank: Super Contributor
timestamp: July 03, 2022, 01:02:13 pm
content: Things are looking hopeful... I'm going round it with an impedance/ESR meter and I've found one totally dead and corroded/leaky electrolytic cap. It's near the 12V section so this might be the cause of the fault. It's hard to get to some of the caps but I'm going to try and access and test them all this afternoon.I'm using an Analog Discovery 2 and "TheStuffMade" (Jaxbird) ESR software/jig to trace out the caps over frequency. Obviously, I've made sure the PSU caps are fully discharged before testing.I keep meaning to buy a DER-5000 but the setup above works really well if set to fast sweep mode over (say) 100Hz to 100kHz.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 03, 2022, 01:29:11 pm
content: That is an interesting Impedance Analyzer for Analog Discovery.Thanks for sharing!

author: G0HZU
author_rank: Super Contributor
timestamp: July 03, 2022, 02:15:30 pm
content: Yes, that impedance software works really well with the AD2 although I only use the passive test jig with it.I found three dead and leaky/corroded electrolytic caps in the PSU using the ESR setup and one of them had an ESR up at hundreds of ohms. The others were at about 40 ohms ESR. It isn't possible to see the leaky corrosion until the cap is removed from the PCB. Then it's obvious the cap is dead.I've tacked in some temporary replacements (sub 1.5R ESR) but I'm going to order some decent branded 105degC caps to put there. So far it has booted with no alarms and no trace freezing.Of course, the moment I think it really is repaired it will probably generate an alarm... but so far, so good...

author: G0HZU
author_rank: Super Contributor
timestamp: July 03, 2022, 06:23:45 pm
content: It's been fine for several hours now and this includes about a dozen successful reboots. In recent days the longest it would work for (if it managed to boot up at all) was about 30 minutes but usually it failed within seconds or maybe 5 minutes of starting the scope app.The low level spiky ripple on the 12V supply is still there but is because I wasn't referencing the ground connection to the correct ground pins on the PSU header. By connecting to the ground pins associated with the 12V smoothing cap there was hardly any ripple and I think it would have been the same as this even before the repair. The main supply rails from the PSU were always fine even when the PSU alarm flag was set. This was what was so confusing.When the scope froze during the loader phase or during normal operation it could never be coaxed back into life unless a proper power cycle was carried out. Attempting to CTL-ALT-DEL and cancel/restart the loader would never work once the scope had frozen. Restarting Windows (but without a power cycle) would also always result in the scope loader failing after the scope had frozen for the first time. It would only work again after a proper power cycle but even that became a rare event in recent days. Apart from the scope app everything else was always fine under Windows 98 even after a scope freeze although I did check the RAM and do various HDD checks and I cleaned all the cable connectors and card connectors more than once during the early diagnostic checks I carried out.So that's what kept bringing me back to the PSU despite it always showing correct and stable outputs on all the voltage rails. I also tried using a couple of scopes to trigger on any slight positive (and negative) transient on all of the main PSU rails so see if there was a brief brownout causing the freeze but this only showed the PSU outputs were always stable and fine. This was what made me search for the presence of a PSU status/alarm flag and it was a huge relief to find this as suddenly everything made sense and I could predict how the scope was going to behave as the status/alarm flag is sticky once set and only clears with a power cycle.There are a lot of electrolytic caps in this PSU and all the main ones were fine. It was three of the much smaller ones that had failed and these are presumably part of the control and regulation circuits. Only one of the three I replaced had failed properly, the other two were showing raised ESR and would probably have failed properly within a few more days or weeks. If anyone has one of these old 54825A scopes or the 54810A and 54815A 54835A and 54845A then it's worth checking the ESR of the caps on the PSU. It doesn't take that long to do.

author: G0HZU
author_rank: Super Contributor
timestamp: July 03, 2022, 06:54:36 pm
content: See below for a screenshot I took a few days ago. This is the scary message the scope throws up shortly after the trace freezes. The trace then disappears and the message below appears on screen.Fine interpolator gain cal failed: Service is requiredNot what you want to see at any time! Note that trying to run the lengthy scope calibration routine doesn't help. I'm not sure why the scope gives this message when the PSU alarm flag is set but I was able to cause the message within seconds by setting the flag myself.Often the scope would just hang when trying to load the scope app via the scope loader program but sometimes it would show the other message below. I hope this info is useful to anyone else seeing similar symptoms with one of these old scopes.

author: G0HZU
author_rank: Super Contributor
timestamp: July 03, 2022, 08:12:32 pm
content: QuoteLooks like you made a good call... the scope is still working since I swapped out those three caps. Each time I power cycle it I still hold my breath in case it freezes again. However, it does look like the scope is going to be reliable again!

author: tautech
author_rank: Super Contributor
timestamp: July 03, 2022, 08:22:53 pm
content: Quote from: G0HZU on July 03, 2022, 11:39:59 amYep old SMPS were a PITA and overly complex compared to SMPS in instruments today.Quote from: G0HZU on July 03, 2022, 06:23:45 pmQuote from: G0HZU on July 03, 2022, 02:15:30 pmVisual confirmation is only half the story as sometimes caps look just fine but are completely shagged.Now that it's been confirmed several are duds don't hold back but replace the bulk of them as irrespective of how they test they'll be well past their best before life.QuoteSMPS with daughter boards are awkward things and you don't really want to rework these things too much so go for a king hit and get most of them old caps out.IME the rectified mains bulk cap is the only one that's normally OK but not always so,Anyways, good progress !

author: G0HZU
author_rank: Super Contributor
timestamp: July 03, 2022, 08:50:01 pm
content: Thanks! The 3 caps that failed are all the same value at 10uF 16V and they are really tiny as shown in the image below. The other caps are physically bigger/longer but I agree that I should really replace them as well. It's difficult to access and read some of them so I wondered if there is a known list of cap values for this PSU that could be ordered as a kit.The three failures were all in the same part of the PSU as circled in the second image below. I'm a bit nervous to fully take the PSU apart because some of the heatsink and grounding arrangements look to be quite critical in terms of alignment and my mechanical/assembly skills aren't that great.

author: tautech
author_rank: Super Contributor
timestamp: July 03, 2022, 08:55:30 pm
content: Yep them tiny little mothers generally fail first.If you're not confident with your rework skills at least replace all the easy ones and by then you might even feel confident to tackle the rest.TBH I feel very uneasy giving advice to the RF Guru you are and imagined you'd done your fair share of this sort or work but maybe you haven't.

author: G0HZU
author_rank: Super Contributor
timestamp: July 03, 2022, 09:12:17 pm
content: In this case I'm nervous to take the PSU fully apart because I'm not sure if I'll be able to put it back together with the same thermal resistance behind all the various semiconductors that attach to the side walls of the PSU. I'm not sure if there's a special thermal interface there and I'm not sure if there needs to be a certain pressure from the clippy heatsink things that press against the semiconductors. If I get this wrong I could make the PSU overheat and this may cause significant damage.I have repaired loads of stuff over the years and experience has taught me when to be cautious if something looks like it might be critical in terms of the assembly. It probably is fairly easy to take the PSU apart and maybe the heatsink clips and the thermal resistance are easy to manage. If anyone reading this can advise then maybe I'll take the plunge and take it fully apart.

author: G0HZU
author_rank: Super Contributor
timestamp: July 03, 2022, 11:26:44 pm
content: I should have taken an image of my PSU to show the additional 11 wire interface to the PSU. This looks to be using an 11 way Molex connector. However, the image below is of a similar supply and the circled 11 way connector pokes out of the end of the PSU and this connects to the main acquisition board in the scope where the connections can be probed with a DVM or scope.This is where I was able to find the alarm/status flag from the PSU. Does anyone have any more info about this 11 wire interface? The scope seems to be able to use it to possibly isolate some of the more delicate parts of the scope if the PSU reports an alarm. I kind of reverse engineered some of the other connections. One of them allows the PSU to be started if it is grounded via a resistor. There's also some ground connections and I recall there is a 15V line there (even in standby?) and 5.1V and -5.3V and a few others. When I fit the properly rated 105degC 10uF 16V caps to the PSU I'll try and take a few images of this interface.I'm really pleased and relieved that the scope is working again. I've been meaning to investigate this issue for several weeks now but kept putting it off. In the last couple of days it had reallygot worse. I had a mini crisis over it today because it had virtually stopped working at all and I need it for some research work in the next few days. Since the repair it's still powered on and still working fine and I've tried a few more power cycles. It works every time now.