HP E3631A Repair - 6v CC reference section problem  - Page 1
Forum section: electronics / repair

author: NE666
author_rank: Frequent Contributor
timestamp: October 10, 2025, 10:24:44 am
content: My HP E3631A PSU functioned well for many years and then during an extended period of operation (about 3 hours) on my bench, brought up the 'UNREG' annunciator on the 6v channel.When it's powered from cold, it will operate normally for around a minute and then the issue reappears. When the annunciator is lit, it's not possible to set an output voltage on that channel, and its output fluctuates wildly. The +/-15v channel is unaffected, which combined with information from other posts here on the common failures of this supply, pointed me at the four 'control' circuit blocks of the 6v channel on the bottom PCB.My thermal camera shows nothing of interest in any of these blocks, except for R48 (1/8w, 1206 SMT, 261 ohm) of the +6v CC reference block, which was lit up like a Christmas tree at over 90 Celsius. Clearly, that's not right.I've attached the schematic of that section, taken from the HP service guide.As R48 measured at its expected 261 ohms, I initially suspected Q6, which I replaced. The effect was to make R48 run even hotter, over 100 Celcius, causing the PCB pads to lift slightly. I've gone on to replace the linear optocoupler (U18), the op-amp providing feedback to it (U22) and R48. The problem remains.All resistors measure as expected, there's no evidence of shorts on that side of the coupler, and the supply voltages are correct (ADC REF is +5v).I'm now at the point where my casual hobbyist knowledge doesn't offer up any further ideas as to what is going in this circuit element, such that R48 is passing nearly 50ma of current (I've measured it). I'd therefore be grateful for any insight someone else can share.

author: Kean
author_rank: Unknown
timestamp: October 12, 2025, 12:33:49 pm
content: This circuit is a linear opto-isolator - probably using an HCNR200 or similar part.You can find out a bit more about how/why this works by watching this video by IMSAI Guy.The design allows up to +/-240V across the isolation barrier between the 0-6V supply and the +/-25V supply. Note that the two sides of the isolation barrier have different "ground" references. One is GND (common with the +/-25V supply and some of the control logic) and one is "A" (the 0-6V positive output, not the negative as you might expect, which is also used for a digital logic ground "D"). Note that the front panel uses the -16.4V rail as a logic ground, and the GPIB/RS-232 circuits are also isolated and use Earth as ground reference.If you look closely at the four isolation circuits used for the 0-6V supply section, two use the "A" ground reference of the output (right side), and two use it on the input (left side) - because two are control inputs and two are for monitoring outputs. The isolated sides using common GND are also using +15/-15V power rails, and the side using "A" are using +15V_O/-15V_O.It also looks like there is at least one error in this part of the schematic, as I believe pin 1 of U18 should be going to -15V_O, like the circuit around U17 does.Not sure where you fault lies, but maybe the above helps to clear up the circuit operation as it can be a bit mind bending to follow the different isolated sections.Hopefully I didn't get anything wrong in the above...

author: NE666
author_rank: Frequent Contributor
timestamp: October 12, 2025, 01:22:43 pm
content: Many thanks, appreciate your input. The circuit is indeed based around the HCNR200.I did study the series of videos from IMASI guy that you referenced, I agree that they are a good intro to the linear optocoupler concept for anyone wanting to study it. To my limited, hobbyist knowledge, the circuit he shows is quite a bit simplified from the one that HP has employed.To help me understand how HP's implementation actually works I created a LTspice simulation for it, which I'll attach here in case it might be of any use to someone else in the future, making the same kind of repairs. I think pin 1 of U18 is shown correctly connected, both because the simulation appears to work and I also measure +15v at that point on the DUT (referenced to the 'not A' ground, via U18 pin 4)? If there are errors in the schematic, then I have faithfully reproduced them in my simulation, so caveat emptor.In the simulation, I can of course find ways of introducing shorts and failures that lead to excessive current in R48. My frustration remains that I don't observe any of them on the actual DUT.

author: Kean
author_rank: Unknown
timestamp: October 12, 2025, 01:55:21 pm
content: OK, it appears I even managed to confuse myself and got the +/-15V and +/-15V_O mixed while switching back and forth between the schematic sheets.I tried to load your LTspice file, but it is missing the library components for "CNR200_A" and "LM358_AB". When sharing it is sometimes helps to embed these models.But I do see pin 1 of the optocoupler in that file going to -15V_O as I suggested.Next step might be to document the voltages (or waveforms if not steady state) on each pin of U17, U18, U22, and U23.Edit: OK I see you have added the components attachment now.

author: NE666
author_rank: Frequent Contributor
timestamp: October 12, 2025, 02:22:40 pm
content: Sorry, my bad on a couple of points. I've not shared anything but an elementary LTspice sim before and it slipped my mind that custom components and symbols wouldn't come along for the ride.Equally, on the polarity of supply to pin 1 U18. You're absolutely correct of course, my simulation does have it connected to -15V_O (rather than +ve, as shown in the schematic). The odd thing here, which confused my previous reply, is that this wasn't a conscious act on my part. I didn't recognise the error myself but by pure coincidence must have introduced a transcription error, which turned out to be a 'happy little accident'.When I get chance later this week, I'll probe the voltages again at the points you suggest.Thanks.

author: Kean
author_rank: Unknown
timestamp: October 12, 2025, 03:07:09 pm
content: No worries. I got the sim to run with those extra files you added.Unfortunately I don't have an E3631A here, so I can't make any comparison measurements but otherwise I am happy to try help out.

author: Swainster
author_rank: Frequent Contributor
timestamp: October 14, 2025, 07:37:17 am
content: Not specific to your symptoms, but on an Agilent branded E3631A I found that a number of thin film SMD resistors had failed. As I recall, they were green coloured, so easy to spot. I also seem to think that this supply comes in through-hole and SMT versions, so this fault may be something that only happens to later SMT models.

author: NE666
author_rank: Frequent Contributor
timestamp: October 14, 2025, 11:42:21 am
content: I'm not familiar with the history of this model but personally, I've never seen a documented repair of an E3631A that had all through hole components.My particular unit is an earlier example, being badged HP and not Agilent. These have some minor differences in PCB layout, as compared to the later Agilent branded ones (and thus the generally available Agilent Service Guide). There are no green coloured resistors, although the same values are used, so whether that implies a different supplier and a lesser rate of failure, who knows.In my case, when measured the resistors of the circuit section in doubt all agree with their values as given in the schematics.