Hi,
It is not clear, are we talking about assembly A5 U22, LM361N ?
Regards,
Jay_Diddy_B
Yes, that's exactly right.
Must be a common failure mode. I fixed an HP 4274A a while ago with the same symptoms - alternating series/parallel LEDs and apparent
lockup of the unit. Same bad part, the comparator on the A/D board!
Nice observation.
I also noted that there is a capacitor (see attached picture) at the input of the comparator, C70* in the schematic, that is not installed in my unit (older serial numbers).
Although adding capacitance at the input of the comparator degrades the performance by slowing it down, I think this capacitor was added to avoid a possible problem of high frequency oscilation of the comparator caused by any small noise in its input, such oscillations would overstress the IC and eventually kill it (that's my wild guess).
Nicely done. It never ceases to amaze me how much HP managed to cram into those instruments - very little wasted space. Congrats on getting it back up and running!
-Pat
Yep, back in those days there were no arduinos or fpgas and everything had to be done at a much lower level of detail using many components for every simple task. Luckily the digital part of the instrument was fine; otherwise, it could be a nightmare troubleshooting it without a signature analyzer.
Thank you for all the comments

Schematic detail of the assembly 5 (page 283 of service manual)
https://drive.google.com/file/d/0BwElEpRLIAVNMGNXRXpzYlV4Wjg/view?usp=sharingLink to the service manual
https://drive.google.com/file/d/0BwElEpRLIAVNQnBmNXlpMmlxeUE/view?usp=sharing