Basketcase late '76/early '77 210 Sixty-Five with 12AX7 phase inverter arrived recently (they follow me home, I swear!). Was as described: cosmetically pretty good but very noisy and barely passing any signal. Initial observations:
- Wrong power tubes (6L6 instead of 6CA7/EL34)
- One power tube broken
- Speaker Impedance set to 4 ohms instead of 8
- Footswitch plugged in backwards (black plug end in red marked jack, etc.)
- Star washers installed on wrong side of control panel (outside instead of inside)
- Doghouse missing two screws
The amp powered up, tubes lit, but Channel 1 only buzzed, no signal passing, the Reverb whined, and the Tremolo didn't work. Bright and Deep switches didn't work either.
Opened up the chassis, more fun inside:
- One of the Tremolo transistors was partially unplugged from its socket
- One of the five 150uf/50v electrolytic caps was replaced with an ancient cardboard tube cap, wrong value and voltage and installed backwards
- All other original caps in bad shape
Did my usual cleaning of all jacks,sockets, pots and switches, plus a full electrolytic recap (four in the doghouse, five on the chassis boards). Reseated the Tremolo transistor and reinstalled all control pots correctly. Resoldered some questionable connections at the power tubes. Swapped around the LM307H ICs to see if any were bad (I suspect at least one is).
Channel 1 now works well. Deep switch works, Bright switch still does not. Tremolo is stong with good Intensity and Speed range. Haven't addressed the Reverb yet, I'm betting the tank is shot.
Here's the weird part: Channel 2 now passes good signal, but only when the Tremolo Intensity control is turned up. That is, the Channel 2 Volume and Master Volume controls both work but only when the Tremolo Intensity is turned up - Intensity is acting like a Master Volume control. I pulled both Tremolo transistors to stop the Tremolo from pulsing while chopsticking around - nothing jumped out as the cause.
Guessing it could be a bad dual-gang Volume pot, or any of the components/joints in the signal path. Is there a specific mix or summing resistor I should check first? Could it be the suspected bad LM 307H (I have spares I will swap in to test)?
Thanks for any suggestions.