First of all, apologies for hi-jacking this discussion. I am hoping Scott grurs on this forum can help with a puzzling problem I am having.
I was given an LK-72 back in the '80s and it was working but needed the 0.1uf caps feeding the 7591s replaced. It sit idle for many years because of other work/life priorities. However eventually I did manage to replace all the electrolytics and film caps plus matched resistors where required. The last time I used it after the changes I noted one channel was playing at lower (considerable?) volume as well as having the occasional loud pops when volume knob was turned. In trying to debug it, I seem to have made matters worse. This lists what I have done to date:
1. I cleaned all pots/switches and tube sockets with DeoxIt D5.
2. Checked voltages on all tubes and noted voltages on V6 pins were very different than that on V5 (Pin 6 70.1 vs 59.9, Pin 8 73.1 vs 63, Pin 9 69.9 vs 59.9). So I rewired and replaced all caps and resistors for V6.
3. I adjusted the bias voltages for the output tubes to around -18v as mentioned in past Scott LK-xx threads.
4. I should have stopped there and check if the problem were solved. but I went on to add DC bias as well as cathode resistors (10 ohm, 1/4 w).
Once done with all that I checked the voltage on the newly added DC Bias pots and was puzzled that it was reading around -70 volts (instead of -44 volts before). The DC voltage at the replaced silicon rectifier now reads something like -70.5 volts. All B+ voltages are within spec. (Even if no current was flowing to the pre-amp tube filaments, should the voltage be so high?)
I suspected one of the 12ax7s was bad but after checking each individually (by connecting 12v to the filament pins) I think that was ruled out (?) (they each read around 140 ma).
I am now wondering if the rectifier or power transformer could be bad but don't think that makes sense either.
Would really appreciate any and all suggestions on how to troubleshoot this problem.
Thanks greatly in advance!
Fred I