A wee update.
Plane went down for service, engineers let me loose with a voltmeter. Concluded that the bus was running at 13.7V so boosted it back up to over 14V. I didn't think this was going to solve anything because if a voltage regulator is altering its set voltage over night once it's going to do it again.
I was right it was on again the next morning. I might add it was fine on the positioning flight back that day.
Engineers were as pissed off as we were so 2 days later one of them turned up with so he thought everything.
Armed with a circuit diagram we started checking every wire for continuity and earthing. Wires OK.
Engineer looses plot and starts swapping out everything.
New alternator
New overvolt regulator
New Low Voltage light
New capacitor on the back of alternator.
New regulator.
On tracing through the circuits we found that a 50ohm resistor linking the alt warning light to earth was dead. The engineer hadn't brought one of them.
Thankyou to NATS tels engineers who did have one in there jar of resistors.
The end of the story is that I have done 6 hours in it today and the light hasn't come on once.
We still don't have a clue what actually solved the problem.
In the end we had
3* alternators
2* voltage regulators
3* low voltage lights
1* over voltage regulators
1* cap
1* 50 ohm resistor
I hate electrical problems
MJ
PS If anyone wants a fax with the wiring diagram of the PA38 send me a PM