Exploding Capacitor

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by K-man, Feb 3, 2006.


  1. K-man

    K-man Member

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    I recently tried to fix a '68 Super Reverb for a friend (my first foray into amp repair). Turns out it was a bad driver tube, but I found all sorts of other problems so I gave the amp a full cap job and replaced the plate resistors per an article I found from Hoffman amps.

    When I finished and turned the amp on (still on standbye), it made a very loud static sound. After a minute the sound went away, so I flicked the stanbye switch and poof, one of the bypass caps I replaced exploded.

    I had put in a bias probe to check the bias, and noticed that one of the power tubes wasn't glowing. Turns one of the tabs in the socket was bent so one of the tube pins wasn't seated properly.

    Could that cause a cap to explode?
     
  2. RussB

    RussB low rent hobbyist Silver Supporting Member

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    Shame on you...messing with a friend's valuable vintage Fender, and damaging it! :BITCH


    I rekon you installed the cap backwards!
     
  3. K-man

    K-man Member

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    I told him up front I couldn't make any promises...

    No, the cap was installed the correct way.
     
  4. Blue Strat

    Blue Strat Member

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    Which cap was it? What was it connected to?
     
  5. TubeAmpNut

    TubeAmpNut Member

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    If you flipped the standby switch and a cap blew, you either had the polarity reversed on a filter cap or seriously underrated a filter cap. If the tubes werent glowing, you likely didn't have the heaters working. Having poor contact on pin 2 or 7 (the power tube heater connection) would not cause a filter cap to blow. No heater, no conduction.

    I reccommend you bring the amp to a tech.

    BK
     
  6. K-man

    K-man Member

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    It was a cathode bypass cap, I think to the reverb driver. After it blew I called it quits and suggested my friend take the amp to a tech. I was just curious what could have caused it. I didn't change any of the wiring, just took the old caps out and put new ones in. It is possible a wire could have come loose from underneath the eyelet board during the process of changing the caps...
     
  7. K-man

    K-man Member

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    Let's say the 80uf filter caps were wired in reverse (although I could have sworn I put them exactly like the old ones), could that have caused any other damage?
     
  8. K-man

    K-man Member

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    Or what about the diode in the bias circuit? The Fender schematic shows the cathode facing the transformer, but it also showed a + sign at the cathode side of the symbal? I think I wired it with the cathode towards the transformer.
     
  9. Dave C

    Dave C Gold Supporting Member

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    Hi K-man,
    If you'd care to take a ride with the amp I'd be glad to have a look at it for you . I've serviced a ton of old Fenders thru the years . I stock parts from resisters to trannies. I'm a 30 minute ride down I-95 from the RI border in SE Ct. I just hate to see anybody get into trouble on their first amp repair. Give me a call and maybe we can set something up.
    Dave C (860)447-0577
     
  10. John Phillips

    John Phillips Member

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    K-Man, take Dave up on his offer :).

    If the amp made a loud static sound when the standby was off, that's a component which is directly connected to the PT failing. It sounds like you may have done something very major wrong in the B+ circuit, upstream of the standby switch - like wired the first stage filter caps backwards. Or one of them... this is quite easy to do since they are opposite ways round on the board - they're wired in series, so there is a common connection between the positive end of one and the negative end of the other.

    Could this then cause the reverb driver bypass cap to explode? Possibly - this stage is fed from high up the B+ chain and only via the reverb transformer, with no resistor to limit the current, so reverse polarity or AC on the B+ supply could blow it... that won't happen unless the rectifier tube had shorted as well, but if one or both main filters were fitted backwards this is a definite possibility, and may have been the arcing noise you heard as the rectifier tried to deliver a huge current, until it finally gave up and shorted, which would stop the noise. I would have thought the fuse should have blown then, but possibly not... you did have the correct value in?

    You could also have wired the bias supply diode or cap backwards. You did realise that the bias supply is supposed to be negative? I can't see why that should blow the bypass cap though, so I'd put a (small) bet on the first scenario.

    I could be totally wrong, but that's the only way I can see that the blown cap could have been caused by something you did, as opposed to just being faulty.
     
  11. TubeAmpNut

    TubeAmpNut Member

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    I agree with JP,

    After reading your post a second time, I bet you miswired the first filter cap bank. These are in series and appear 'backward' when seen from above.

    I still can't see why the reverb driver's cathode cap would blow. If you lost your first filter cap(s), the ripple would be huge. If one of your output tubes wasnt glowing, either the single tube is bad or your filament connection is bad. Either way, one working tube will generate sound. You should have heard a huge BUZZ in this case.

    Don't be afraid to diagnose what you did wrong here. You serve to learn a lot from it. The first thing you need to do inspect your work. Post photos of the doghouse (where the caps are) so we can see if there are any glaring errors. You need a good multimeter to do anything else (you do have one, right?). You also need a variac, a scope and a signal generator, but probably not to fix this...

    Once you are certain the caps are wired correctly (while your add it, replace the first pair that are wired in series), remove all your tubes. You are gonna want to check all your voltages without any load on the amp...

    We can pick it up once you get to this point. Let's hope the PT didn't burn out.

    BK

    -BTW, in the future don't wait a minute for the static sound to go away!
     
  12. Blue Strat

    Blue Strat Member

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    Over voltage is the most likely cause. The thing to do in cases like this is remove the blown up cap, and measure the voltage across the points it was connected to. If the voltage exceeds the max voltage of the cap, there's something wrong with the circuit.
     
  13. K-man

    K-man Member

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    Hey guys, thanks for all your help.

    I bet I did wire the first filter caps backwards. :Spank I know I wired them in series, but I may have had the positive end facing ground instead of the negative end.

    I'll contact my friend and see if he wants me to take another look at it, then Dave I may take you up on your offer.
     
  14. TheAmpNerd

    TheAmpNerd Member

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    I have. I've replaced quite a bit of them too.
    Typically the ones that I've seen blown are the
    bypass caps in the trem circuit (v5 - 25uf/25V).
    Fender later changed these, some I've seen are
    25uf/50V, 10uf/50V, and 5uf/50V.

    Incidently, I've received a number of Fenders that
    had the mains filters wired incorrectly, some from
    the factory (belive it or not) some from other techs,
    or folks just working on their amps. These have
    been a combination of:
    • Wired in series polarity reversed
    • Wired in parallel polarity reversed
    • Wired in series (should be parallel)
    • Wired in parallel (should be series)
    Amazingly the amps worked but sounded like chit.
    None of them blew caps either mains nor bypass.

    However, not that I would know :jo I did happen to
    find of my little bypass caps boiling over during burn in and test.
    Poor little 25V guy had about 70V on him for over 30 hours.
    You can bet I was surprised that he lasted that long!
     
  15. K-man

    K-man Member

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    Big shout out to Dave C. I took the amp to him today and we got it up and running. I did have the filter caps wired correctly. However I did not know that you have to power up new caps gradually. When I originally installed them I turned the amp on full power. :jo

    Thanks Dave, you are the man!
     
  16. Dave C

    Dave C Gold Supporting Member

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    You are welcome Kurt, it was a pleasure meeting you and helping out a fellow Gear Page member. Glad it all worked out as easily as it did.
    Dave C
     
  17. hasserl

    hasserl Member

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    Hmmm, I've never powered new caps up gradually & I've never had one explode on startup. Maybe the cap was old and dried up.
     
  18. Shea

    Shea Member

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    Same here. I'm sure I haven't replaced nearly as many filter caps as Dave has, but I have installed roughly 100 new filter caps in amps. I've never brought any of them up slowly, yet I've never had any of them fail on startup except for one time when I accidentally installed the rectifier diodes backwards.

    There was one time I had a cathode bypass resistor explode. It was in a cathode-biased 2xEL34 output section. I had foolishly omitted the grid bias resistors from the EL34s, so the voltage at the cathodes kept rising until it exceeded the rating of the capacitor and blew it up. The funny thing is, it sounded great until the cap blew.

    Shea
     
  19. VintageJon

    VintageJon Member

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    This is very interesting about the exploding cap.

    Were they current date code?
    What brand were they? (Have seen IC explode for no reason...)

    -Jon
     
  20. DEMENTED

    DEMENTED Supporting Member

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    Dave is da man, fixes my amps, no sweat. Glad all is now well with that amp K-man.

     

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