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1978 Fender Twin Reverb No Reverb/Vibrato

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  • 1978 Fender Twin Reverb No Reverb/Vibrato

    Have resistance on reverb tank leads, continuity on footswitch, no reverb or vibrato.

  • #2
    Have you checked it isn’t just bad valves? ie in V3 and V4 for the reverb, V5 for the trem.
    I suppose a 1978 will be the 135W version?
    How do the idle voltages compare to the schematic nominals?
    https://el34world.com/charts/Schemat..._135_schem.pdf
    My band:- http://www.youtube.com/user/RedwingBand

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    • #3
      What pdf64 said^^^. And, on the reverb, do you hear the springs klaaaannnnggg with reverb volume turned up some, or not? Yes, then the problem may be in the drive circuit, V3 and drive transformer and associated components. No, then we're looking at the return circuit, one of the V4 triodes and associated components. If it's on the drive side, sometimes the cathode resistor on V3 gets roasted because of a shorted tube, that always stops the drive circuit in its tracks.

      For the vibrato, when you have the control pedal plug shorted - with pedal, cheater plug, or gator clip lead - do you see the neon lamp flashing? It's on the side of the "bug" facing rear of amp. Might need to observe with dim/no ambient lighting. On the other end, the photocells in amps of this series (1963 thru @ 1980 or so) are failing with age quite often.

      Write back & let us know.
      This isn't the future I signed up for.

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      • #4
        Reverb leads should measure < 5R for input coil and about 150R for the return coil. Does that come close to your measurements?
        Does thre reverb humm when turned up and pedal leads disconnected? Then your retrun stage is OK.

        The send stage you can check on your scope and even Vac meter. Should give > 1Vrms

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        • #5
          Also you need to measure theVac signal coming out of the reverb transformer.
          It could well be bad.

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          • #6
            Braided footswitch wire was broken, so waiting on switchcraft RCA plug to fix. Was able to get vibrato to work, but not reverb yet due to footswitch.

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            • #7
              The footswitch is not required for the reverb. So if reverb is not working, look for the problem elsewhere.
              "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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