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I think it helps when trouble shooting to think of an alternator as a current controlled voltage source. The output voltage depends on the field excitation current, with a higher current value leading to a higher output voltage. The regulator looks at the output voltage of the alternator and then raises or lowers the field excitation current for the field winding in the alternator to maintain the correct output voltage from the main winding. Since the output voltage is not where it should be, but not zero, and varies with speed, the field windings, field excitation circuits and regulators seem to be working. I have a 1969, On this plane one regulator feeds both alternators. The regulator switch chooses which one. There is a common wire that goes from the buss, through the field circuit breaker, to the regulator selector switch, to the regulator, back to the regulator selector switch, then to the alternator selector switch where it splits to feed the front and back alternators. Since your problem appears to effect both alternators equally, I would suspect a problem of higher than normal resistance in the common leads, meaning check the connection from the buss, through the field circuit breaker, to the regulator selector switch. Also the lead from the regulator selector switch to the alternator switch. High resistance in this common lead will starve the regulator for current, leading to low output voltage from the alternator. The regulator selector switch has two switches, one of which feeds the buss voltage to the regulator, and one of which feeds the regulator output to the alternators. I had a problem with voltage instability that was plaguing me for some time. When I put in a new avionics stack, I had the avionics shop look into it. They did a really smart thing and starting jumpering from the buss bar forward, bypassing components until they saw a stable output result, then replaced all the crimps between the buss and the good result point. I had already had my A&P replace all the switches in an attempt to fix it without a good result. Voltage is now rock solid. If you decide to try this yourself, DO NOT bypass the regulators. This could fry your airplane as the output of the alternators in an uncontrolled state can exceed 60V. Based on my experience if you have an avionics outfit that is good I would take the plane there. The are much better with electrical issues than your typical A&P.
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