Received a DOA 1978 RP65 recently, all the PO knew was that it sat in a garage and would not power on.
Seems to be all original except for two (incorrect) chassis nuts and (sloppily) replaced flyback diode and snubber cap on one power tube socket. Also one opamp (in IC-6 socket) is not the normal LM1458, it's some kind of TI that doesn't cross reference to anything familiar. Came with (probably replacement) clean and clear Telefunken EL-34s.
Cleaned all sockets and jacks, checked for anything loose, broken or burnt. Installed a correct 3A Slo-Blo fuse, turned amp Power on (with Hi-Standby-Low switch in Standby), the pilot light came on and the tubes began to glow.
Could I have gotten lucky that the amp only needed a new fuse to work again?
No. As soon as I switched from Standby to Low power, the fuse blew and the pilot light went out.
Removed all tubes and repeated the power on sequence with the same result.
Replaced the main filter caps with fresh ones, same result.
The rectifier diodes (MR510) in the doghouse measured a little flakey, so I lifted one end of each. Now the amp would stay powered on in Low or Hi power mode. So bad rectifier diodes, right? I replaced them with modern equivalents (1N5408-E3/54 3.0 Amp 1000 Volt from Mouser) and powered on again. And again, blown fuse.
Seemed like I was finding a lot of grounding points on the board under the doghouse, so I studied the schematic and traced the circuit back. Found that the red cloth covered HT lead from the OT that connects to a R/W wire feeding the doghouse board was grounded from the OT! Disconnecting it from the board, power stayed on in Hi or Low mode.
Does this definitively indicate a bad OT? I am getting good resistance readings at the OT wires at the power tube plates, but should the red HT lead measure grounded? Is it possible to open the OT and repair? Or do I just need to track down a replacement 5-75 OT?
Thanks for any advice.