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Shorted Pin 3 to Fuse holder, now problems

Discussion in 'Amp Tech Center' started by avspecialist, Oct 28, 2020.

  1. avspecialist

    avspecialist Tele-Meister

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    I recently rebuilt a Bassman 10 into a Super Reverb style AB763. I replaced the Bassman 10 output transformer with a Heyboer Super Reverb output transformer from Mojotone. Everything was working great. So one day I decided to try different output tubes and do some measurements with my DMM. I was on pin 3 or the right 6L6GC when my probe came in contact with the back end of the fuse holder, a quick spark and I immediately shut down the amplifier with the power switches. Luckily I was using my lightbulb current limiter. So now, I have some distorted sound on the normal channel, with funky noise on that channel and no sound on the Bass channel. I replaced all the tubes one by one which are 2 12AX7 input tubes, 1 12AT7 phase invertor and the 2 6L6GC. Still the same issue.

    I just ordered a ESR meter to check the caps under the dog house.

    Any Ideas where else to check??, output transformer?? When I get home later , I'll post some pictures from the scope. Thanks in advance for any recommendations, Bob
     
  2. dan40

    dan40 Tele-Afflicted

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    A full set of voltages on all of the sockets would help to narrow down the trouble spot. You can buy small mini clips that attach to your meter leads and lessen the chance of an accidental short. If you believe that one of the caps got damaged, you can simply tack solder a new cap across each cap one at a time and see of the problem goes away.
     
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  3. avspecialist

    avspecialist Tele-Meister

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    Thanks Dan, Thank you for the advice, I’ll take a look at it tonight, thanks
     
  4. tele_savales

    tele_savales Tele-Holic Platinum Supporter

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    I’ve been trying to remember to shrink tube my fuse holders and the leads to my power switches. Especially since the last time my fat forearm got zapped 313F39D2-ABAD-47B9-BF0D-54362A10830F.png
     
  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    It may be possible the coupling caps including the tone stack caps received VAC and were damaged. That could explain the bass channel not working.

    The speaker could have been damaged.
     
  6. avspecialist

    avspecialist Tele-Meister

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    I just got my De5000 meter today, I can now check the ESR of the caps to see what I may have shorted. Then Ill work on checking the voltages. I downloaded the 52 page English manual.


    DE-5000 ESR meter.JPG
     
  7. avspecialist

    avspecialist Tele-Meister

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    Lowerleftcoast, fortunately, I had the amp on my bench connected to my load center, good suggestion though. I'm thinking its either the caps or resistors. So far not the tubes, they all have been swapped out and same issue. It might even be the output transformer too. I have another super reverb one in stock, Ill probably swap that out last.
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The OT is possible but the bass channel not working... that symptom can't be the OT.

    The resistors should be able to handle a quick short, sharp, shock. (thanks P Floyd)

    Could the caps withstand the AC??? maybe not.

    Odd possibilities... Jack switches if the AC could make it to them. Pots.

    Get your voltages and do the pop test.
     
  9. avspecialist

    avspecialist Tele-Meister

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    OK, So I played around with the new De5000 ESR meter late last night. I'm still learning about it. Its a little overkill for what I need, I wonder if I should had gotten the blue one like Uncle Doug. Anyway it was late and I did some caparisons of new capacitors and the ones in the Bassman 10, most of the readings are similar. Tonight when I get a minute, I post the measurements. so Far I don't think I found anything out of the ordinary. I have to get some filter caps to compare the ne Sprague's in there. I think the 3 for the preamp ones red a little high, but I don't have any thing to compare to unless I open up one of my other amps.

    This could be a good learning lesson for a lot of here on the forum.
     
    dan40 likes this.
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