This project's still active. I've not had a ton of time to spend on it, but during the winter holidays there may be some additional time so I am getting this back to the front of the docket. I am hoping for some input on what's going on here and where to head next.
A quick and probably partial recap and status report.
1. The unit initially was completely dead, which I tracked to A19 not being supplied with 120VAC. Which meant all outguard modules were essentially off.
2. Once the above was corrected, the unit started largely working. Except that, when I attempt to output more than about 60V, the unit initially successfully outputs the requested voltage (say, 70V), but just for a few moments, then it drops to about 57V, which triggers the fault mode "OUTPUT LIMIT FAULT. OUTPUT UNDER VOLTAGE."
3. My next step at this point was to swap modules (A4, A5, A13/12, and I may have tried others) from my fully working (successfully repaired) 5440A with inconclusive results.
Then I went ahead and tracked the symptoms above (the lower than set output voltage) backwards through the output board and back to A13 FILTER B board. Technically, I think the issue resides on what Fluke calls the "HV Loop," and they have some pretty contorted and not illuminating (at all, to me) steps in section 4-23.
I've done this by going by section 4-19, where I've applied a DCV proportional to the set output vs. full scale between A4 (OUPUT board, NOT A5, as per Service Manual, so please note the error in the manual) A4TP5 and A4TP13. For instance, a set 70V out would want this applied DC to be about 112mV. I did this with my HP8904A, which has the great convenience of being able to key-input voltage settings directly. This step is in order to disable the monitoring system, so an out-of-bounds condition (more than 5% difference between the output and the set voltage) would not reset the unit and further troubleshooting would be possible to make at length.
Under these conditions, the events at #2 above occur and stay active past the few seconds it'd take the unit to actual its reset. To me, this confirms the monitoring is operating correctly and the issue originates with the generation of the HV.
4. I looked at +TSV and -TSV by probing A13TP1 to A13TP2, and I am seeing the voltage go up to about 5V (as Vout stabilizes to 70V), then dropping to about 2.15V (as the voltage out is falling from the 70V set to the 57V is settles for), and then going back up a bit to something like 2.5V or so. I do see some ripple riding on top of that (per my Fluke 189), though it varies pretty wildly (it starts low, maybe mV, but as the fault occurs, it jumps to maybe 250-300mV, then goes down in the tens of mV and stays there). I am not 100% sure what I should be seeing or looking for at A13TP1 and A13TP3, so I don't have a satisfying analysis of my own of what this means.
5. I am not seeing anything significant happening over R20 (the +TSV .33 ohm) when the fault occurs. Barely in the mV range if even that. So there seems to be no protection actuation on the HV that could be causing this.