gearmaker
Hello, Thanks for all of the great information on this site. Very helpfull. I have an 4582-2 15KW ultra source portable that I bought used in unworking condition. The rotor was bad. Very little use on unit observed. I tried to interest several motor companies in my area, Durham NC, to repair rotor. No luck. I found the break in the winding and repaired with SS swages and revarnished and that repaired the generator. Used it for a few years as backup generator for my home with no problems. This year after 14 hours stopped making power. Tough to find as the broken brush still ohmed out correctly not running. Took brush holder out and it was hosed. Put new one in and no joy. Flash rotor at diode with 12 volts ran fine and held. Wire 4 -2.86 VDC, wire 14- 12.7 VDC, R1- 25 ohm, rotor- 12.7 ohm.  Cleaned slip rings with wood block  with 400 grit and running motor. Careful here! 2.86 VDC to #4 at brush could not figure the voltage drop.  Diode was the problem! Diode itself was fine, diode case was intermittingly grounding to case. Isolated from case and everything is fine. Almost went to constant flash kit. Testing individual components following flow chart drove me nuts as everything should have been fine and Ohms law said my drop should have been correct but it was not. But the grounded diode was not enough to blow 10amp fuse. Hope this helps someone. Thanks for the great forum
Quote
Peddler
The diode is strictly to prevent VR from pushing high DC voltage back into the 12 VDC system.  It flashes the field continuously theoretically but as the VR takes over it is simply a blocking device.  You should have about 5-7 volts on the brush side of the diode when cranking, this is the field flash, then as the VR takes over the brush side voltage will go above 12VDC depending on the load on the generator.  Not sure what you are saying.  If you say you are able to flash the field momentarily with 12 VDC then remove that and it holds normal voltage then I would say your problem is in the flash circuit, diode ,resister or source (fuse).  If the voltage falls off as soon as the flash voltage is removed you have a bad voltage regulator.
Quote
gearmaker
Yes, my problem was in the flash circuit. What I did not notice was the body of the diode was intermittenly grounding to the case. The little silicone washer the isolates the diode from  case ground had worn just enough to increase the  the voltage drop I was experiencing . So from wire 14 through R1 was only yielding 2.86 VDC. Componenets measured correct by themselves but not when wire 14 thru R1 thru D1 to wire 4 connected. Now that I have isolated the Diode case from the unit case I have proper field flash voltage. Generator runs great! Took me a couple of days to find this error. I just wanted to post if another was having field flash issues to save time in diagnoses. Thanks!
Quote