Hi drew*,
BTW, sorry for the slow reply as I was visiting the Skyline chili capital this past weekend.
nineno wrote:I removed J4-8, and meaured the voltages. Everything seemed good, except J4-1 and J4-2. They're listed as 67V and 65V. I get 120V (which is what I expected, since that's the power inlet, right?)
I dunno what I had planned there - I must've measured the AC referenced to the chassis? But you are correct.
nineno wrote:I began trying to set the bias. TP1 dropped down to 0.550V pretty quickly. TP2 however, I couldn't get to go low enough. I assumed I had a resister stuffed wrong, so I turned my attention to the other amp. I figured if I could get that one dialed in then I could check the non-compliant one verses a working one.
However, I had the EXACT same results with the second amp. This time, though, I left the amp plugged in longer and the squealing stopped after about 5 minutes, but then I couldn't set the bias at all. Both TP1 and TP2 were registering zero. As I probed around the board, checing voltages the squealing came back suddenly. I think It was when I touched the probe to pin 6 of the 6SN7...?
Sounds like your basic oscillation...
nineno wrote:I had the Rx48 and Cx21 in series on the 8Ohm outputs of the x-former, nothing at all on the input (not even wires).
This probably the problem - short the input for now. Also, it may not be a bad idea to disconnect the NFB for now, too. You won't have to re-bias when hooking it back up.
nineno wrote:Also, did Rx47 turn into R49and Cx20 turn into C8 in a manual revision? It appears that is the case, but I'd like that verified.
Correct.
Go ahead and:
- remove the NFB
- short the input
- jumper the chassis temporarily to the PCB
- double check that your output tranny is ground referenced to the PCB at J4-3
- try again (shut down if oscillation re-occurs) and report back
Good luck!
Shannon