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Have resistance on reverb tank leads, continuity on footswitch, no reverb or vibrato.
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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 -
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.Comment
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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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Also you need to measure theVac signal coming out of the reverb transformer.
It could well be bad.Comment
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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.Comment
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The footswitch is not required for the reverb. So if reverb is not working, look for the problem elsewhere.Originally posted by Enzo
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Reverb tank tested 200 and 0 resistance so installed a known good tank. Reverb now working, vibrato will work once I'm done with footswitch RCA plug.Originally posted by g1 View PostComment
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I'd suggest you measure the known good tank and compare, also try the old tank again. Close to zero and 200 are normal measurements for good Fender tank. Maybe it was just a dirty connector or something.Originally posted by ca7922303 View PostOriginally posted by Enzo
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Reverb and vibrato both working now after replacing RCA plug on footswitch with known good tank. Original tank still tests about 200 and 0 but will not work when installed in twin reverb?Originally posted by g1 View PostComment
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Known good tank measures 250R and 70R(4eb3c1b), original tank measures 220R and 0R(4AB3C1B).Originally posted by g1 View PostComment
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Well, ther it is, the 4AB3C1B is the right one for Fender tube drive, the 4EB3C1B is right for solid state drive reverb.Education is what you're left with after you have forgotten what you have learned.Comment
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The 4eb3c1b is working in the '78 twin reverb, the 4ab3c1b is not. Is a '78 solid state drive reverb?Originally posted by Enzo View PostComment
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nopeOriginally posted by ca7922303 View PostIf I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.Comment
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The 4AB tank should be the correct one for the Twin.
It's possible the one you have is bad, or there is not sufficient drive from the drive transformer. The 4EB is easier to drive and that may be why it works, but it won't work as well as the proper 4AB tank.
If you have another vintage fender amp, you could test the 4AB tank in there.
Otherwise, the cable that connects to the tank 'input' should be able to drive an 8 ohm speaker. If you have a test speaker, you could hook it up and listen.
Also, post the DC voltages at pins 1 and 3 of V3, the 12AT7 reverb drive tube. (schematic in post #2)Originally posted by Enzo
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