|
Hi I'm reposting something I put in the forum a while ago, as it may be hard to find with some edits for this situation and model >>
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, the field windings, field excitation circuits and regulators seem to be working. I have a 1969, On this plane one regulator feeds both alternators. On your aircraft, I looked up what I believe is the schematic for the 1974. It looks like this aircraft has two regulators and both of them have their own alternators with diode blocking to set which one is driving the electrical system. Since your problem appears to effect the front alternator, I would suspect a problem of higher than normal resistance in the leads going from the buss supply to the regulator for the front alternator, meaning check all the connections from the buss to the B input on the regulator. High resistance anywhere in this lead will starve the regulator for current, leading to erratic output voltage from the alternator. The regulator is exceptionally sensitive to this issue. It doesnt take much to cause a problem. 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. They are much better with electrical issues than your typical A&P.
|