Well, further investigation shows there's indeed a problem there. I'm not sure if the hours of work during open heart surgery has maybe worsened something that was there before, or I just didn't notice, or whatever it was.
But the symptoms are this:
- The 10V HI terminal is sensitive to mechanical action. Fiddling with it a bit can restore it to its about 4ppm high which is what I regard is its "stable" or "true" value.
- Sometimes it'd just drift away from there. Touching a bit the HI terminal restores its level.
- Touching it can also make it jump but a few tens of ppm.
- It can also drop to "zero" - in fact, always about 14mV. I have no good explanation for this (other than that the cap and others parts on these terminals - 10V HI and LO - have some sort of leakage that elevates HI from closer to zero. Then, why 14mV?...
I've cracked open the front panel - note this "PCB configuration" doesn't seem documented in either of the manuals I am aware for the 732A - and the best I can come up with is the #23 wire being connected to the HI binding post via a solder joint that looks pretty untidy, and then also a trace. Also, if the LO terminal has everything soldered together, including the nut, the HI doesn't, so it's possible, because it relies on the mechanical contact between the nut, washer and the board pad, it may not connect perfectly. That #23 has to do with supplying U1 internally, so maybe that node being mechanically sensitive to action on the front terminals can explain the full drop of the output.
I dread working on the front terminals - almost as much as inside the oven - but I could:
- Infiltrate a micro-drop of deoxit at the nut-to-pad interface
- Rework that solder joint, and clean well with IPA.
- Gently tighten the nut to enhance the galvanic contact with the board PCB.