A while back one of mine low power Onan power supplies failed, but I was able to figured it out and bring it back into operation. From what I read on the net, it seems that the Onan’s don’t have the best reputation, so I will share my experience to help you guys bring them back to work.
I noticed an odd ticking sound coming from the power supply, before I could turned it off, a big bang and some smoke came from it and of course it was no longer working. Time to bring it on the work bench and drill out the rivets. It seems that there is no service manual or schematics available for these supplies, but to get a clue on how the thing works I used the theory of operation section in the Quantar instruction manual. The Onan supplies seems to follow the same design, mains power is rectified after that there is a PFC / SMPS boost circuitry with bring the voltage up to 400V DC regardless of the input voltage. The 400V handled over to the DC -DC converter with gives us the output voltages. It was obviously that the two 270uF caps exploded, so I began looking at the boost circuit and did some measurements and noticed that the 400V are way off, was well over 500V, no wonder that the 450V rated caps let some smoke out. The boost circuit is build around an ML4812, same as in the Motorola supplies, hence I used a schematic from one of these to get a better understanding. I found four dodgy resistors, one open the others increased their resistance. After changing the caps and the resistors, the 400V where spot on, same with the output voltages.
After you drill out the two rivets on the side, remove the front plate and the screw next to the power switch, you can now carefully remove the back plate, disconnect the fan and slide the PCB out of the case. Disassemble the PCB from the heat-sink. If you have the PCB in front of you, on the right side you should see the four resistors.
Don’t trust them only because they look Ok, mine looked fine but all of them were defect.
On the back side of the board notice the brown color, lot of heat was there. On my unit the mains cable going to the switch in the front was laying directly on these resistors, no airflow for cooling, the constant overheating is presumably the cause of the problem.
I haven’t mounted the new resistors flat on the board, so they have a better cooling, also routed the mains cable different. After the repair I found that the resistors not running hot any longer.
From what I see, this could happen on the high power version as well, it seems that on the high voltage board the only difference is the lack of 3 caps and two MOSFETs on the 400V boost circuit.
The defective parts where:
4x 200 KOhm resistors
2x 270uF electrolytic capacitors (5x in the high power)
I could’t find any info on the net about repairing this exploded “fireball” Onan power supplies, if you have discovered other faults, it would be appreciated if you also contribute your experience and leave a comment.