I’m happy to close this one out with a solution and a lot of learning. Oh, and an AMU or two thrown out chasing the wrong culprit.
The problem started with a low bus voltage, around 13.2, but sometimes 13.5, sometimes 12.9. Relatively variable during flight. Looking back through engine monitor data, it has been slowly declining on average for 2 years. Before that, it was solid at 14.0.
Mechanic suggested replacing alternator at last annual so I did. Brushes were worn. I trust him very much on engine/cylinders/flight controls... in the future, I’ll do much more homework with electrical issues.
No change with alternator. Lived with it for a while but it was declining. Tried to use Zeftronics troubleshooting guide but needed just a little more specific help as I had no experience troubleshooting electrical. Now I do.
Talked to different mechanic. Talked me into new voltage regulator because “that’s cheaper than paying us for a few hours of troubleshooting.” No change in voltage. Although now I have a spare Zeftronics VR if anyone needs one.
Decided to try my own hand. Bought $20 voltmeter at Lowes. Spent couple hours quality time with airplane apart. Cowl off, battery/avionics compartment open, instrument panel cover off.
Identified two issues... Master Switch on my airplane is a single switch, dual pole. Not only does it Power the master relay, but it connects the alternator field wire. It isn’t the later model split switch. My switch is getting old. Voltage drop across it is the maximum allowable on the Zeftronics troubleshooting guide, .5 V. There is absolutely no access to change that switch without tearing out the panel. It’s on the list for next time the panel is apart.
The other issue was the bus voltage was good (matched battery relay) but power input to the VR was .4-.6 volts lower. Found out the “alt field” circuit breaker was actually the power for the VR. Tested drop across the CB... .5 volts. New circuit breaker cost me $20 on spruce. It was accessible from under the panel so I changed it. Just flew it... 14volts and rock steady!
Be aware, the electrical diagrams in the maintenance manual are model and year specific. I was surprised to see that both my field wire and my VR power wire went back behind the panel and then came back to the engine compartment. Also, that “battery master” switch on older models also cuts off the alternator field, so could be an area for resistance. They are cheap switches if you can get to them.
Anyone want a working alternator with worn brushes or a gently used Zeftronics VR??