Twin Reverb AB763 (intermittent/one-time?) grounding issue

Discussion in 'Glowing Bottle Tube Amp Forum' started by Zack_repairs, Nov 21, 2019.

  1. Zack_repairs

    Zack_repairs TDPRI Member

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    Hello. I’m working on a Fender Twin Reverb AB763 that might have a grounding issue… that’s intermittent? It happened once. Here’s the situation.

    I’ve done some general work replacing old electrolytic capacitors, testing it, finding a few other faulty areas. It came to me with a grounded power cable installed… and also a death capacitor and wired polarity switch. I removed both.

    I tested various voltages in it against the schematic, tested signals, confirmed the bias, played it at my workbench and thought it was solid. I took it for a final test to a rehearsal space with a guitar. A friend (who has played for many decades so I trust his experience) said he felt it tingling <strikethrough>without</strikethrough> touching the strings, so we shut it down.

    EDIT: He just clarified that he touched the strings to feel this. He said it was like a live wire at a very low voltage. (I thought he felt it like static in the air.) He said that it didn't feel like it discharged like a static shock, that it maintained its low voltage "tingle."

    At the same time, we were testing another amp. After stopping with the Twin Reverb, he used the same guitar, cable, and outlet/power strip with no issue on the second amp.

    Other points that might be factors but just could be noise:

    -We plugged both into the same power strip and powered them up for several minutes.
    -He wore the guitar, plugged the cable into it, then plugged the cable into the amp.
    -He tried the normal channel. Then after stopping, he wanted to retry with the vibrato channel. Same process of plugging in the cable with the same result.
    I know of no power issues in the space. I tested the power strip the next day and found no issues. (Expected AC between Hot and Neutral and Hot and Ground, no voltage between Neutral and Ground.)

    I retested at my workbench and then again in the space with the guitar, measuring from the case to the power strip ground but got no voltage. I get 4mVDC between the instrument cable tip and case ground but that seems irrelevant (yes?).

    Before the incident, I had no power problems. I don’t know how putting the amp back together and moving it could have caused one? After retesting at home (no issues), I brought it back to the space and couldn’t recreate the issue.

    In-between at home, I found:

    -Moderately cold solder points inside the speaker 1/4-inch cable I fixed. (Eh.)
    -Deteriorated heater wiring, especially on the first power tubes closest to the transformer. Small parts of the wiring had lost the colored outer shield, but I couldn’t find continuity with my meter. It seemed like there was an inner shield that was exposed but not conductive, but both leads were twisted together in a space without the outer shield. I replaced this section of wire.
    -Mesh shielding on the top of the wood case has a few strands poking loose and down. They run about a cm long and don’t look like they’d poke/reach anything. But I wonder if reassembling this could cause some kind of fault where one of those touched something wrong?

    EDIT: Old heater wiring pictured below.
    IMG_4962.JPG IMG_4965.JPG

    I’d very much like to repeat this problem and/or find a clear fault to fix. Does anyone have any tests to try to hunt it down or reconfirm the safety of this otherwise? (I’ve run many more tests than noted here, but welcome any ideas or repetition.)

    Any speculation about the cause? The mesh shielding strands seem like my best guess, and that still feels tenuous.

    Thanks much! Long time listener, first time caller.
     
    Last edited: Nov 22, 2019
  2. Wally

    Wally Telefied Ad Free Member

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    Pics?? And....I wonder how one feels ‘tingling’ without being in contact with both that amp through the guitar strings/ground and some other conductive variable like a mic or another guitar/amp?? Being old school and raised on two wire AC amps, I can hear the noise of reversed polarity, but I cannot feel it....until I touch another amp, guitar, or mic.
     
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  3. corliss1

    corliss1 Friend of Leo's

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    I think he means as soon as he touched the chassis? If there is an AC on the chassis somehow and you flip the power switch that would be enough. If you're not able to reproduce though...............I'm not sure. Could have been something else plugged in that night? Or maybe just not related to the amp/electrical at all.

    I had a modern amp a couple months ago where the user was reporting shocks. Well, they tried to do some work on it and missed one of the ground points when they put it back together, so the entire preamp board was floating up to 180VDC. No guitar required to feel that zap when you touched the input jacks :D
     
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  4. schmee

    schmee Poster Extraordinaire

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    The AC wires must be wired correctly, not just random. Otherwise interacting with anything else may cause a mild shock.
    [​IMG]
     
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  5. Zack_repairs

    Zack_repairs TDPRI Member

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    Yeah, I misunderstood what happened and updated the original post. He _did_ touch the strings. It felt like a very low-voltage live wire to him that maintained that sense; it didn't just discharge like static electricity would feel.

    EDIT: I posted pics of the original heater wiring above. Let me know if you had something else in mind, thanks.
     
    Last edited: Nov 22, 2019
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  6. Zack_repairs

    Zack_repairs TDPRI Member

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    I'll triple-check on this, thanks. I wired it so that the polarity switch has no effect but left it installed in the case. I'll have to look inside again in a few hours, but I think it's the same as your diagram but with the white path soldered together in its wire/winding versus landing on the ground switch.
     
  7. Zack_repairs

    Zack_repairs TDPRI Member

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    [Deleted--can't find delete option beyond this.]
     
  8. Zack_repairs

    Zack_repairs TDPRI Member

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    Yeah, I'm interested in any theories about what else it could have been if not the amp, thanks. We played it last night for three hours without any issues, but I'm going to haul it back to my bench again today.
     
  9. BobbyZ

    BobbyZ Doctor of Teleocity

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    Put an ohms meter from the chassis to the ground prong on the AC plug. That should read zero ohms or really close.
    If that checks out ok and the outlet is wired right I'd start thinking your friend is maybe over sensitive.

    Like Wally said you only get shocked off the strings if you complete a circuit. As long as you don't touch a Mic or stand on concrete bare foot, you can play with 120ac on your strings and not know it.
    I don't recommend doing that ! But I have done it.
     
  10. Zack_repairs

    Zack_repairs TDPRI Member

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    Yeah, thanks. I tested this and forgot to mention. I looked now and have 1.2 Ohms between the ground plug and chassis.
     
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  11. King Fan

    King Fan Friend of Leo's

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    I like your smart, thorough approach. Backing way off and looking at it, I try to picture all the options:

    1. Your friend is oversensitive -- very possible, but also the 'least safe' decision if there *is* a problem.
    2. Sounds like you did a thorough check of the power strip at the scene. Did you use a multimeter or outlet tester? The latter is simpler but more complete and foolproof, IME. Did you test all the wall outlets and strips? Did he have anything else (mic, effects, etc.) plugged in on the same or other outlets? What's the flooring there (concrete? wood? etc.)
    3. Your obvious concern is you revised the power wiring somewhat. I agree with omitting the ground switch and death cap from all wiring. I can't be sure from the pic, though: You left the acc. outlet wired, and ran black to tip of fuse, and then on to switch? And white goes straight to the other PT primary?
    4. Intermittent here would be scary. How/where is the green household ground attached to the chassis? Are the power cable, plug, the green wire, and its anchor 100% bomber?
    5. Did you test for continuity not only from the household ground prong to the chassis, but from the ground prong to the guitar cable sleeve? And to the speaker jack sleeve (or its speaker wire)?
     
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  12. Zack_repairs

    Zack_repairs TDPRI Member

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    Hey, thanks for the ideas. Yeah, I agree to take this seriously, and while it has worked fine since, I'd really like some strong theories or something conclusive about what happened. I ran some of your test ideas already but reran what I could now.

    I tested the strip with a multimeter. I didn't write down the numbers, but I got roughly 117VAC between hot and neutral and hot and ground. And zero on neutral to ground. I didn't test the wall outlet, and my test was also the day after this happened. The floor is a concrete warehouse space with some carpeting and a rug in our room.

    We had nothing else plugged in other than the second amp. I felt a little off about putting them on the same strip but went with it. Both were powered up. This is the second amp that had no issues and was tested with the same guitar and cable.

    Hope this isn't burying the lede, but my wiring on the AC had put the switch on the white/neutral side, pictured below. Could this have caused this issue, or is it just unideal? I'm about to fix it to go with the correct diagram.
    IMG_0142.JPG

    Since having this on the bench today, I desoldered the ground leads on the main board and rectifier board and lifted it up/apart a little bit to blow compressed air inside. Could a bit of something bridged another something inside? I didn't see any conclusive, big wire clippings fly out, but I did see a single, small strand of an old wire.

    After that, I resoldered the main ground point to the chassis. It was already soldered down before I began all this, but it looked a little cold. I used the 100-watt iron I just bought for this occasion.

    I retested the ground plug after this and am getting better results now (?). It's tough to get the probe on these cleanly sometimes, but all my readings are less than 1 Ohm this time, so maybe I did help the grounding. (I think I saw one or a couple before. EDIT: 1.2 on my reply above.) That's on all the points you called-out, from the ground plug to: the case, the instrument cable sleeve, the black speaker connection point.

    I also saw more deterioration on the heater wires from the lamp to the first tube and replaced that run.

    Thanks again for your thoughts! Please let me know if you have more questions or ideas.
     
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  13. corliss1

    corliss1 Friend of Leo's

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    That's a reasonable number for that length of wire to the chassis - sounds good to me. I know just my meter probes alone are about 0.3ohm when I touch them together.
     
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  14. King Fan

    King Fan Friend of Leo's

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    Good stuff. You're right that switching the neutral is not ideal -- but as I understand it it wouldn't *by itself* put any voltage on the guitar.
     
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  15. Wally

    Wally Telefied Ad Free Member

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    One would want the black hot wire fused and switched to one end of the primary, while the white neutral goes direct to the other primary end as in the diagram earlier in the thread. +1 on the revision, Zack. And kudos for reviewing your work with an open mind to the possibility that a human error occurred.
     
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  16. Zack_repairs

    Zack_repairs TDPRI Member

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    Thanks, Yeah. I switched it fully to the diagram in the thread. (EDIT: Well "fully" without the lugs on the polarity switch.)

    ...but the way it had been shouldn't have caused any issue, right? Or highly unlikely but maybe? I'm still grasping for theories.
     
    Last edited: Nov 23, 2019
  17. corliss1

    corliss1 Friend of Leo's

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    Nope. It sounds like there really isn't an issue with the amp at this point.

    If something else was ungrounded, or possibly had the ground pin pilled from the plug end, he could have touched both things at once causing the issue, but it doesn't sound like that was the case.
     
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  18. BobbyZ

    BobbyZ Doctor of Teleocity

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    Once the switch is turned on it doesn't matter if it's on the hot or common wire. So that isn't it.

    You want it on the hot to lessen the shock hazards when the amp is switch is off. Basically you want to cut power as close as possible to the outlet.
    There's two advantages to grounded plug done right in outlets done to code. Obviously the ground is one. The other is the hot and common always get plugged in the same way.

    With the old two prongers if the plug was inserted "wrong" they'd have voltage on the chassis switched on or off. Trouble was those old plugs fit fine either way. Then a lot of them have the ground switch, that's like flipping the plug, to really make it interesting.
    You literally have to check for "stray" voltage every time you plugged one if those in. That's why you'll hear guys talk about touching the mic stand with the back of their hands. Get a shock? Flip the plug or flip the ground switch.
    Oh the good old days. :rolleyes:
     
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  19. Wally

    Wally Telefied Ad Free Member

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    Bobby, old schoolers like me learn to hear that reversed polarity in an amp with a two wire AC cord. It was one of the first lessons I had from my teacher when I was 12....56+ years ago.
     
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  20. BobbyZ

    BobbyZ Doctor of Teleocity

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    I never had much in the way of formal lessons. Music teacher in the 6th grade (76-77) showed us how to tune and play a few chords. (said the F chord was real hard) That was it for me.
    Had to learn about how not to get shocked, by getting shocked! Then you figured out what you did and didn't do it again.
    The flip the plug hum thing a friend showed me when I was 17 or 18. Though that was pretty cool!
    And yeah by that time grounded cords were standard. But we played old amps. Or cut the ground off because dad's shop was wired in 1950 and the bars most likely had two prong outlets then too.
    To kids that shot each other with BB guns for fun and played with lawn darts, a few shocks here and there didn't seem so bad. :)
     
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