Well I could slap myself for not staying on task. I suspected a strong possibility for a non-working HV converter would be
dried out electrolytic caps. I pulled them from the DC converter an quickly measured the capacitance and found they were
both "normal". I didn't have any of these values on hand so I moved on. Distracted by the possibility of a bad coil in the transformer, I measured and found the last coil to be 90 ohm. Which is turns out is probably correct! So, I decided to just put in
whatever close capacitance values I had to see if it would come alive. It did. The surprise was that the HV converter is an inverting DC converter, 9v to -140v.
I verified the converter is working by adding a string of high R resistors as a divider. Measuring the last in the chain would not be significantly changed by a parallel 10Meg DVM nor current limit the converter. I also found the switching frequency was very close to the spec at 22Hz. I'll order new caps for the DC-DC converter as well as the head amp. If these were bad its a good idea to replace the head amp electrolytics as well.
I've already put some time into a new DC-DC converter retro-fit design so I'll go ahead and bread board one up. The IC I have can work with the 9V or 48v input and can be designed to output either +140v or with a couple of changes, -140v.
The moral of this story is don't trust standard capacitance readers for "good" or "bad" caps. Even if the cap reads good capacitance value, it may have high ESR and send you on a goose chase. I knew better so shame on me for falling for this after all these years!
I should know if the mic is fully working for sure in a few days.
P.S> Thanks to Stewart for sharing his experiences with his DC converters. Thanks to him I got back on track with a repair.
Regards,
Jeff