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Thread: 70's Fender Twin troubleshooting

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    70's Fender Twin troubleshooting

    Apologize in advance if my terminology is vague due to lack of specific knowledge. I recently bought a 1970 Twin from a friend for a good price and went into it knowing it'd need to be re-tubed and possibly need some other work because he said it hasn't been serviced since '75

    issues in order they presented themselves

    1. With harder pick strokes there's a popping like the signal is peaking

    2. The tremolo messes up some times and creates a "whooshing" sound. Still functions at times but there's definitely some sort of distortion happening with it

    3. The most glaring issue so far is after having the amp powered up for about an hour during a practice session I heard a strange bubbling noise and when I went to inspect it there was smoke coming out of the back of the amp and a brown or dirty red viscous fluid dripping out of the part I have circled in the picture provided. Smelled like solder but could have been anything, the brownish red substance dried to a wax like consistency - haven't powered the amp up since



    The twin I have looks slightly different than the one pictured here

    Any ideas what could cause these issues? Any guesstimates on repair costs?

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    Supporting Member Randall's Avatar
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    We can start by saying that it is the reverb drive transformer that is circled. It would be helpful to take a pic of the caps under the "doghouse" can.

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    It's weird, because it WAS working fine.....

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    Supporting Member TomCarlos's Avatar
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    Looks like the transformer for the reverb circuit. If it is smoking and dripping goo, not good. The question is, was that the sole problem or was something else the problem and it took out the transformer. So don't expect that it is just one part that is bad, it could be something else.

    In looking at the picture, it looks like there is a BIG speaker on the right. What about the speaker on the left. Is it the same? This is probably not related to your issue, but check to see if the speakers are the same and test them (if you can) to see what dc resistance you measure. You never know, your friend may have swapped speakers and not given thought as to the correct impedance required for this amp.

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    Quote Originally Posted by Randall View Post
    We can start by saying that it is the reverb drive transformer that is circled. It would be helpful to take a pic of the caps under the "doghouse" can.
    Got it, sorry again for not knowing the correct terms. Will get some pictures tomorrow

    Quote Originally Posted by TomCarlos View Post
    Looks like the transformer for the reverb circuit. If it is smoking and dripping goo, not good. The question is, was that the sole problem or was something else the problem and it took out the transformer. So don't expect that it is just one part that is bad, it could be something else.

    In looking at the picture, it looks like there is a BIG speaker on the right. What about the speaker on the left. Is it the same? This is probably not related to your issue, but check to see if the speakers are the same and test them (if you can) to see what dc resistance you measure. You never know, your friend may have swapped speakers and not given thought as to the correct impedance required for this amp.
    I don't think it's the problem but rather a symptom. Wasn't sure if it was an obvious indication of a specific problem that was common with Twins, will most likely be taking a trip up to the local amp tech after new years.

    Sorry if I didn't phrase it very clearly but the amp pictured is just a random photo I snagged online because I wasn't sure what the part that's circled was called. The speakers in the Twin I have are orangeback "Utah electronics" speakers.

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    Supporting Member TomCarlos's Avatar
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    Gotcha…. and I missed that part of our post.

    If you can, remove the chassis by disconnecting the cables. Its a heavy chassis - so be careful not to damage the tubes. Take pictures so you know what goes where. Once you have the chassis out, be careful NOT to touch any of the circuitry. You could still have VERY high voltages across the caps. Do a visual check for other burnt parts, including the caps under the rectangular cover (as mentioned by Randall). If nothing else, a visual might tell us if you have collateral damage!

    Tom

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    The reverb transformer is definitely having a serious problem, as dripping thecwax potting is definitely NOT normal or common.

    I just want to say outright: transformers are essentially THE most reliable parts in an amp. I am almost certain that there is something else wrong with the reverb circuit. If it hasn't been serviced in 44 years, I'd say it's time to read up on basic tube amp care and feeding.

    If you haven't seen here yet, look at the Tube Amp Debug Page on geofex.com by our very own R.G. Keen. It goes through preparation, safety practices, and a time-proven process to get an ailing amp up & running. Sure there are some "weird" issues not listed, but I'd wager a 6-pack that there's nothing wrong with your Twin that isn't covered there.

    Good luck, you're in the right place.

    Justin

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    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    I've seen a V3 12AT7 reverb driver conduct a lot of current, to the point of red plating, if it loses bias. I can imagine that could make its transformer really hot.
    In my case it was due to a bad solder joint on the 1M grid leak resistor.

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    Hack with a Workbench Tone Meister's Avatar
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    Quote Originally Posted by pdf64 View Post
    I've seen a V3 12AT7 reverb driver conduct a lot of current, to the point of red plating, if it loses bias. I can imagine that could make its transformer really hot.
    In my case it was due to a bad solder joint on the 1M grid leak resistor.
    And another common cause is a V3 cathode resistor whose value is too low. That is a common problem with some silverface amps. It would help if we knew what year the amp is and the correct circuit revision so the correct schematic can be posted.

    Can you post some clear pictures of the circuit in that area?

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    Supporting Member Randall's Avatar
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    My spidey sense suggests this OP isn't looking to get hands on with this, but just looking for some input before he brought it to a shop. And sometimes for safety's sake that's just as well.

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    It's weird, because it WAS working fine.....

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    Senior Member TimmyP1955's Avatar
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    Had a couple of new TR65RI that did something similar. Pulling the reverb driver tube fixed it. A different tube brought the issue back. Disconnecting the tank had no effect. We sent the amps back.

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