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210 65 Tremolo Intensity Pot acting as Volume Control

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.


bill_moore's picture

I had that problem on my DR build!

I found that I had misread a mixing resistor value, (2.2K instead of 220K).


210 65 Update

Thanks for the information, what was the resistor that was wrong, what function did it serve/where was it in the circuit?

I replaced all the LM307H transistors with spares I have on hand to see if that helped with the Trem as Volume issue, no difference.

Measured both the Channel 2 ganged volume pot and the Intensity pot, both within spec.

Measured all the resistors surrounding the Tremolo circuit, all in spec. Guess I'll do all of them to be sure.

Measured Main board diodes, all good. Need to check the rest.

Discovered an issue with runaway bias when switched to High power, All of the .047/600v caps on the Driver board and about half of the resistors were way out of spec, so I replaced them. I also suspected I might have a bad EL34 so I replaced the pair. Problem solved.

The Bright switch issue was due to a broken leg on the .015uf/200v Bright cap, tested with another cap and it works.

Probably time to use my audio probe to see where the signal goes in but doesn't come out...


Getting Closer

I isolated the Tremolo circuit (lifted the input signal at R35, 4.7k) to see what would happen, and "plain" Channel 2 does work, but at a greatly reduced volume. So the problem doesn't seem to be in the Tremolo circuit, but somewhere before it. Gotta be a bad resistor or cap in there somewhere...


More Clues

Replaced the bad Bright cap and the Bright switch now works on both Channels now as it should.

Resoldered an iffy green wire connection between the Channel 2 Volume and Master Volume pots.

Found and replaced the suspected bad LM307 transistor with a new one. For good measure, swapped all the LM307s from my working 65 to this one.

Discovered sound output is greatly reduced using the second Speaker Out jack, The jacks were wired differently than my working 65 Reverb head, so I rewired to match.

Found some continuity issues between caps and resistors around IC 3 and 4, pulled the board and reflowed some joints. Continuity issues resolved.

None of these things fixed the problem of the Intensity pot acting like a Master Volume.

Noticed the volume of the tremoloed signal gets louder when Intensity is turned up, but it's always just a single beat tremolo (even with trimpot adjusting). It never doubles up as MM Tremolo normally does above half the Intensity. The transistors are OK, worked perfectly in my good 65 head. The Tremolo transistors from my working amp into this amp didn't improve anything.

Plan to use my audio probe tonight to see where the signal is getting lost.


Problem Solved!

The audio probe once again saved the day.

Apparently C18, a 1mfd cap, decided to retire, no audio coming out of it getting to the rest of the circuit. Replaced, and everything now works as it should. Even the Tremolo now doubles at about halfway up.



Good on ya Ian. Keep up the good work.

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