Reverse engineer and repair a generator AVR

[Alex_Baker]

Howdy people

I have a "vintage" Honda EM500A portable generator from the early 80s in need of repair. When I first got it it had carburetor issues, but now that I have that sorted out I discovered that it was outputting low voltage, around 90v or so.

I was able to acquire a service manual and check that all the windings in the unit matched the resistance specs, so my focus went to the little AVR unit. Unfortunately there is no schematic available, or replacement parts. There are virtually no replacement parts available for this generator as a whole. The AVR was also completely sealed in resin. Once I decided to take the leap and disassemble the AVR, I let it sit in paint stripper for over a week to remove the resin. This process nearly destroyed the electronics, but there is just enough left that I think I can repair it with this forums help.

I have attached the schematic that I came up with, as well as the generator's wiring diagram. Does this schematic make sense? I also need help figuring out what the component labeled TH is, it disintegrated in the solvent, but it seemed to be filled with black grit, which made me think that it was a varistor.

[fzabkar]

That looks like a typical automotive regulator, apart from the AC windings. I don't know how important the thermistor is, though. ISTM that the positive tempco of the zener could be chosen to offset the -2mV/C tempco of the diodes and the transistor B-E junction.

[MarkF]

Here is a video for a EM500 with a lot of good info.
Unfortunately, I think you're looking at a new AVR.

   https://www.youtube.com/watch?app=desktop&v=feRasa6NCi0


Watch to the very end for a view of the de-potted AVR.
He says there is NO info or circuit diagram for your AVR available.

[Alex_Baker]

I spent some time and rebuilt the AVR using the values I had in the schematic and a 10k potentiometer in place of the component labeled TH. I used new capacitors and resistors but kept the transistors and rectifiers.

To my surprise it actually worked! I was able to get the voltage up and could tune it with that potentiometer. No load to full load the voltage sags a bit more than I would like, about 10 volts, but I wont worry about that too much for now.

[coromonadalix]

yep 

you have to depot it and try to figure out many things,  but if the chips or transistors are nit buffered out,   things are possible ?

AVR teardown

[fzabkar]

What did you use for TR1? A value of 13K for R0 would require a very high hfe in order to get an appreciable field current (1A ?). Is TR1 a Darlington pair?