Bringing a '79 MM 112 Sixty-Five (no 12AX7 tube) back up to speed. When I got it, it worked but was noisy (pops, hisses, occasionally sounded like it was picking up radio signals). Replaced the two 100uf/450v under the doghouse, and all five 20uf/25v on the main board. Was out of 150u/50v caps, so couldn't replace the two on the bias board and two on the main board. They are on order.
Powered up fine, but was so quiet I had to check that the tubes were lit and speaker plugged in. Only got sound when I hit a plugged in guitar fairly hard, and then the sound was distorted and intermittent, staccato like, somewhat akin to a strangled tremolo effect. Clearly not correct. Both Tremolo and Reverb were turned down/not engaged.
I double checked that my cap orientation was correct (I mark the negative end with a black dot before cutting out the old cap), and that no new cap leads or solder were accidentally touching nearby component leads. Also checked the 4558 opamp was in correctly. I previously had pulled the opamp and all eight transistors, cleaned their sockets and carefully reseated. All looks correct.
So...seems like the possibilities are:
1 - one of the newly installed caps is bad
2 - one or more of my solder joints is bad, or I have a bridged connection, or a loosened lead under board
3 - one or more of the unchanged old caps has gone bad
4 - the bias is totally off
5 - the 4558 opamp is bad
Any other ideas?
Yes
1. Are you sure the speaker is working and the connection is sound? (Including the speaker jack & plug)
2. Is the problem the same with both input channels?
3. If you inject signal into the reverb return RCA do you get sound?
4. What are your DC voltages? Are your IC's getting correct supply voltages?
5. Is there signal present at the master volume control?
Sincerely,
Lars Verholt
Problem Solved!
Since the amp worked before I did the partial recap, I decided to check my work again. First I determined that no leads or solder were making contact with anything they shouldn't, Then I tested each 20uf/25v cap installed with my meter on continuity. Sure enough, I discovered a shorted cap, grounding out the signal. Fortunately I had one more 20uf/25v cap on hand. Installed the new cap, and all is well again. Sometimes you get lucky...
Thanks.