| Author | Message | ||
| D_Rom |
I'm checking out a MM 112 RP sixtyfive for a friend, probably of late seventies vintage. The Chassis # is 2165-RP. The amp just worked one day and didn't the next. I first suspected some mechanical failure, but found no evidence of such. A preliminary check of the guts seems to point to a power-supply failure. I haven't gotten around to taking any components off board yet, but it looks like a few diodes might have gotten hot (enough to discolor the board) and burnt out. Also looks like they've been replaced before (their are about 5 germaniums and 3 silicons). The board is either a GP-2 or GP-3. It wasn't marked but it's got the 620-ohm resistors in the power section. Are there any "notorious" problems with the transformers or power supply sections of these amps? Anyone ever see anything similar? Thanks -d | ||
| D_rom |
actually- I guess the black ones could just be modern epoxies, too | ||
| Steve Kennedy |
While it could be almost anything, if the electrolytic filter caps in the power supply are the originals, one may have shorted. Is the fuse blown? Does the pilot light stay on but no sound? Do the tubes glow unnaturally? | ||
| D_Rom |
Fuse is not blown. I have not plugged it in since it died, but everything _looked_ ok at the time - light was on, fuse intact, tubes starting to heat up. It just didn't pass any signal and there was a LOT of hum when the volume was turned up. The PS transformer seemed to be doing its job, but I don't know the specs to compare ratios. I came up with the following output voltages at 12V input: 1. 7.28V (tapped to 3.65V per side) 2. 31.3V (tapped to 10.4V and 20.9V) 3. .9V (to the heaters) |
Tue, 03/25/2014 - 09:57
#1