Let me give a f/u, however unsatisfactory as it is.
Finally got around to trying to troubleshoot with a VOM.
You folks have warned me about the difficulties. Trying to do anything under the panel is downright dangerous to your neck and potentially dangerous to your breathing ability with the passenger seat still in place. But even with that seat out , I question how much direct visibility will exist for lead testing. Taking pictures with a thin IPhone and translating that to a reasonable view without disassembly is not easy.
Well what could I do to get some info? I just wanted to start creating a list of knowns to help in the diagnosis, even if they confirm the obvious.
I took out the KT76A to give a view of the transistors. I tried to take out the KX170B above that but turning the cam about 4 turns ended in a hard stop without movement of that radio, (I did not want to force it) unlike the KT76A which allowed multiple turns that backed that radio out.
Given the tiny area open to me, I started by using the continuity function. I could barely get the lead to touch the case of the transistor and one screw holding it down. Besides the fact that the transistor mounting seemed totally solid, there was no continuity with the avionics stack frame and that transistor or screw. There was continuity with the screw and transistor case.
Next I used the Voltage tester. There was no voltage detected from frame to transistor case without turning on any switches but the following info was determined: initially, Zero volts detected after master alone turned on and Zero volts with the right POT switch sequentially turned on and knob rotated.
But on second try, 0 after Master alone ,but adding the POT exhibited 12.14 on full turning, then 12.49 at near closing (I noticed the panel lights were now on.), then Zero when Pot switch closed.
For comparison, the POT on the Left for the Nav lights/TailBeacon was 11.69 after master and avionics master turned on and POT turned all the way, then 11.90 when just before the turn off point.
So what was learned. The initial try showed the fault not allowing the panel lights to activate and nothing at the transistor collector, but that second try allowed voltage to be picked up at the collector, and was variable, and turned on the panel lights.
Once that happened, further testing, even if I had access from the backside, would not have been productive this session. The ambient temperature was about 75 degrees, so maybe the fault will me more prevalent and last the historical 10 minutes in the cold.
So besides waiting to remove the seat, or possibly gaining better access by removing the right essentially radioless adjacent part of panel, I could try for slightly greater access by figuring out how KX 170B comes out, and maybe the GNS430W.
I also may just tolerate this as it’s little more than an nuisance some of the time. Maybe, the smarter thing to do is wait for next annual and ask my IA to just swap that right transistor for a new one ( which has been suggested by you ) and hope for the best. That is cheap and simple to do, even for me if allowed after getting those radios out of the way.
Another thought is to purchase some interesting test leads with right angles and maybe half loops, if they exist. Also, the use of an inexpensive fiber optic video device, like those used to examine cylinders and valves, could then be used to turn in conjunction with a testing lead to more easily complete the direct testing from my stack opening.
