E3631a repair and performance - Page 1
Forum section: electronics / repair

author: Spike
author_rank: Regular Contributor
timestamp: March 08, 2025, 12:15:25 am
content: Hey all,I recently picked up a parts or repair Agilent E3631a on eBay. The +-25v supplies appeared to be functioning properly but the 6v supply was non-functional. The repair is not actually the topic of this post as I managed to get the 6v supply working without too much trouble (thanks to many posts here as well as some YouTube videos).After the repair I decided to run though a calibration and at least some of the performance verification steps in the service manual. The things that I checked were mostly fine but checking the ripple and noise I saw that the 6v and -25v suppies were sitting at about 350uV rms and 300uV rms respectively. Within spec but just barely. The +25v supply, though, was sitting at about 1.5mV rms, about 4x the 350uV maximum specified, with some readily discernible 120Hz ripple on the output. Neither of the other channels had discernible ripple.I pulled and checked the larger electrolytics on the bias supply and the +-25V supplies, C44, C45, C34-C36 and C54-C56 on the top board (schematics attached). They all checked out tiny bit low in capacitance but within spec. and other parameters seemed fine as well. I replaced them anyway and afterward the ripple was reduced but I was still left with overall ripple and noise of about 850uV rms on that channel.If anyone can offer any tips or suggestions for where to start probing around to see why this is so, I'd be grateful. I'm a bit of a novice and am still trying to learn my way around. I realize that this is not that far out of spec and is probably good enough for me but I'd like to try to learn a bit more about how these supplies work and I may end up selling it anyway as I bought It kind of knowing I didn't have any more bench space. So If I can get it within spec I'd like to try.It should be pointed out that my measurement methods are surely shoddy but regardless I have the other channels to compare to as well as another E3631a that measures significantly lower using the same methods. Anyway, thanks in advance for any help or at least discussion.

author: IanJ
author_rank: Unknown
timestamp: March 08, 2025, 10:22:38 am
content: To measure ripple use an isolated method to start with, i.e. hook up a DMM (set to AC) direct to the PSU output.Scope the output of the various regulators, it all starts with the supply voltages.Scope the DAC signals and follow the circuit through from source right through and see where the ripple is originating.I repaired a 3631A a few years ago (see my YT video) and if I remember correctly the PSU had a number of bad resistors.I describe how the circuit works so worth following along/comparing.Ian.

author: Spike
author_rank: Regular Contributor
timestamp: March 08, 2025, 08:19:10 pm
content: Quote from: IanJ on March 08, 2025, 10:22:38 amThanks, IanI’m using my HP 3400a to measure, along with a scope to get a look at the output. Hopefully it’s good enough for my purposes.It was actually your video that helped me track down one of the faults with the 6v channel. The 6v CV REF signal was not making it past op amp U10 on the top board, as was the case for your supply if I recall correctly. After replacing that, I was able to trace that signal to the bottom board right up to the faulty analog opto-isolators. After replacing them all was well with the 6v channel. I hadn’t touched the +-25v channels up until measuring the ripple and noise.I guess I’ll start probing voltage regulators and see what happens. If anyone has dealt with this before and knows of a common issue that contributes to this, I’m all ears.

author: Kwakerman
author_rank: Regular Contributor
timestamp: March 10, 2025, 02:32:37 pm
content: A while back I had an issue with mine caused by high ESR on the capacitor soldered to the back of the 6v output terminals, I think this was allowing oscillations on the 6v line and caused a few issues with the control circuitry. From memory I think it was 1000uF 25v, might be worth checking to see what the 25V outputs capacitors are and connecting two over the terminals (use the front of the bananas) to see if that cleans things up.

author: Spike
author_rank: Regular Contributor
timestamp: March 10, 2025, 08:03:41 pm
content: Quote from: Kwakerman on March 10, 2025, 02:32:37 pmThanks, KwakermanI’ve just checked the output terminal capacitors on all channels and they all seem within spec on both capacitance and ESR. Adding capacitance has little to no effect on output ripple.I’m at a bit of a loss at this point. Though I still need to finish checking the bias supplies, I’ve checked the ones that seem involved in the +25v supply circuit and found no smoking guns. I’m failing to see from looking at the +-25v supply schematic where the ripple could be coming from. I mean, yes, there’s the obvious bridge rectifier and smoothing caps but in theory those are all good. I guess I’ll just keep probing.

author: Spike
author_rank: Regular Contributor
timestamp: March 12, 2025, 01:31:56 am
content: Quote from: Spike on March 08, 2025, 08:19:10 pmIn poking at this a bit more I stumbled across something that I find weird. Perhaps others will not. After checking various voltage regulators in the bias supplies, all of which looked fine. I decided to back up from the output terminals and measure at various points in the +-25v supply. The very first measurement was between j7, pin 5 on the top board and the Ground (A) side of R66 in the schematic attached above.I found none of the aforementioned ripple. I removed the front panel to get a look at the backside of the terminals and found no issues there. I re-measured from the output terminals with the front panel off and this time the ripple was virtually gone. The total ripple and noise now sat at about 300uV rms on the +25v channel and about 200uV on the other two, all within spec. When I reassembled the front panel, taking care not to get any wiring wedged in the transformer, the ripple did not return.was this coupled in from the transformer? I'm a bit baffled by this because I would have thought that the transformer would be accounted for in the design. I don't have this issue with my other E3631a. I wasn't the first one in there, is it possible that there was an issue created from a previous disassembly. Anyone experience or hear of something like this before with one of these supplies?

author: martinr33
author_rank: Frequent Contributor
timestamp: June 03, 2025, 07:36:57 pm
content: Never mind, it looks like I picked up the wrong manual!The units I am looking at do make measuring the ripple challenging. Part of the problem may be the loads - a 1R wire wound resistor may have too much inductance as. the AC noise measurement (3458A) more than triples to about 1.5mV. The programmable load can be better or worse. Lead discipline is important. Shielding, grounding, twisting.HP documents the ripple measurement method in the manual.The setup uses a coax cable with 50 Ohm parallel termination at the meter, and 50 Ohm series with a 0.1uF capacitor at the power supply.I am somewhat suspicious of this configuration, as it seems that it will aggressively reduce low frequency noise. And yes, 2x attenuation which will make passing a lot easier.I have not yet set up this configuration.Measuring with a 3458A, I find that the units tend to be close to noise spec, or over it, on the 6V range. I have to be very careful with wiring.Post showing the cable configuration for a switching PSU:https://www.eevblog.com/forum/blog/eevblog-594-how-to-measure-power-supply-ripple-noise/msg409879/#msg409879