Help with a Mitsubishi MGE4800 AVR

Thread Starter


Joined Jul 15, 2009
Hi Guys,
I bought a Mitsubishi MGE 4800 generator, the engine runs smoothly, starts on the first pull.

However, the generator side does not work as I only get about 5V AC at the 120V receptacles.

I have done the following diagnoses and I think my AVR is bad.

The wirling diagram can be found here, on page 30: Products+MGE4000z-4800z-5800z-6700z.pdf

Basically, it is a 120V+240V generator , it has 2 main coils, the AVR is a strange one, it has 6 wires:
1. 2 yellow wires are connected to EC (Exciter Winding)
2. Red + White are connected to one of the MC (Main Winding)
3. Light Green+Brown are connected to FC (Field Winding).

With AVR's 6 wires disconnected, I have measured:
1. 9.5 ohm on the generator Field Winding (FC in the schematic diagram, between light Green and Brown)
2. ~0.9 ohm on the engine Exciter Winding (EC in the diagram, between 2 yellow wires)
3. with engine running and AVR disconnected, Exciter Winding (2 yellow wires) generates about 20 VAC
4. with engine running and AVR disconnected, and me applying about 12VDC from a car battery on Field Winding, the generator generates about 80VAC at the 120V receptacles.

With engine running and 2 yellow AVR wires connected, but other 4 AVR wire-plug diconnected, I measured:
1. between the 2 yellow exciter winding, the VAC is about 18VAC (down from 20VAC with no load)
2. between the Light Green + Brown pins from the 4-wire plug, the VDC is about 18~19 VDC. So this **seems** good for AVR.

However, as soon as I connected the 4-wire plugs:
1. between the light Green + Brown, the VDC is dropped to only 1.0 VDC.
2. Voltage at the receptacle is ~5V VAC.

So, it seems AVR can regulate the VAC from Exciter Winding and generate about 20VDC at the FC output (Light Green+Brown) without load, but it cannot maintain the voltage under the FC load.

So I think the AC winding (MC) and FC etc are working, and the problem is the AVR is bad. The AVR is coated with epoxy, but I cannot find any burnt component, and there is no burnt smell.

This AVR sells about US $150 on ebay which is 3x what I paid for the generator, so I would rather fix the AVR if possible.

I have spent several days tracing the schematic diagram, and I think it is "correct", please see attached pictures. It is hard to trace, because the PCB is coated with epoxy, and many of the components don't have markings on them, so I have guessed 2 of them are Zener diodes because they have voltage drops on both sides, and I guessed 2 big "capacitor" like components are MOV, one of them is totally coated with epoxy, the other has 390D10 marking on it. The other component that I am not 100% sure is the C4. I cannot measure its value.

There are 2 SCRs, I have taken them off and measured them with a multimeter, and it seems they are good.

I also took off the A1029 (I think it is a 2SA1029 a PNP transistor) and I tested it with a multimeter and it seems it is good.

I tried to take off one zener diode, and it broke. :( so I don't know its spec or part number.

Before I go ahead and purchase replacement components, could you please take a look at the schematic, and let me know which component(s) go bad can give me the symptoms I described above? Is my schematic correct? :)

Thanks a lot!