Troubleshooting a 78 Super Reverb

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by marty pollard, Feb 14, 2008.


  1. marty pollard

    marty pollard Member

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    Just got it unpacked, plugged the power tubes, turned it on and...
    -
    -
    -
    not a sound. :(

    Tubes all glow, speakers are wired up, tried all 4 inputs; nothing.

    So that eliminates all the obvious, common sense stuff that I can try on the outside of the amp.

    What's the first thing to look for?
     
  2. Vincec811

    Vincec811 Member

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    I assume you have turned the STANDBY to ON. Do you get a small pop when you flip the Standby to ON? Is there any hum out of the speakers at all? Could try the phase inverter (12AT7) tube --it could be bad. Swap the one in the Reverb section with the one immediately to the left of the power tubes (as you face the back of the amp). If that doesn't work you will have to go inside the amp (or have your tech do it).
     
  3. marty pollard

    marty pollard Member

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    I assume if the PI is glowing, it's 'working'.
    I do know about standby, thanks. :)
    No hum sound at all.

    So, when I go inside (Mike, John Phillips, et al), where do I start poking around?
    And yes, I know the risks and will be careful.
     
  4. Axekisser

    Axekisser Member

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    It might be obvious but check the speaker connections (jacks,etc.) to make sure all is intact. Sometimes things get loosened up if it was shipped.
     
  5. Rosewood

    Rosewood Member

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    You do have the speakers plugged into the speaker jack on the left, not the external speaker jack, correct?
     
  6. marty pollard

    marty pollard Member

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    Please, a little credit here.
    I know you're trying to help but I'm not stupid.
    I've done all the external troubleshooting I can and will need to crack the chassis to continue. Just wanting to affirm the correct troubleshooting sequence.

    I'll first check the speaker output jack terminals and then...?
     
  7. Mike T

    Mike T Member

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    I just had a similar problem with a SF Twin. Could not for the life of me figure it out. Come to find out it was a bad filter capacitor.
     
  8. Blue Strat

    Blue Strat Member

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    Is the rectifier tube plugged in? This would be to the left of the 2 power tubes when facing the back of the amp.

    Tubes can glow and not work at all. Tube swaps would be the first thing I would suggest.
     
  9. marty pollard

    marty pollard Member

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    Hmmm-
    Just pulled the chassis and looked in the dog house and I see 2 smaller caps closest to v1 and 3 big blue Atoms next with space for 2 more but they're not there. Are they supposed to be? I told the seller (a store) to not fix the buzzing; that I would do it for a discounted price. Did the seller just tell the tech to button it back up 'as is' and ship it?

    It looks like a couple of caps were cut out at the terminals.

    Are all the schematics the same 'basically'?
     
  10. mark norwine

    mark norwine Member

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    The power caps are gone?

    That's like buying a car that's missing its cam shaft........
     
  11. marty pollard

    marty pollard Member

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    Well, I don't know; one of the schematics at ampwares shows two smaller value caps and three big value caps so maybe that's all there is in this model too.
     
  12. mark norwine

    mark norwine Member

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    The doghouse of a super reverb should have 5 electrolytic caps
     
  13. hasserl

    hasserl Member

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    Well you're on the right track. The next thing to do is get a full set of voltage readings through out the amp. If there is something wrong in the filter cap section you'll find out pretty quickly.
     
  14. Blue Strat

    Blue Strat Member

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    2+3 = 5 which is the correct number. When you say there's "space" for 2 more, do you mean that there are 2 pairs of unoccupied eyelets?

    Your description is a confusing.
     
  15. phsyconoodler

    phsyconoodler Member

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    A picture would end all speculation.
     
  16. marty pollard

    marty pollard Member

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    Yes, two spaces with eyelets are unoccupied but again, there are 2+3 for a total of 5 and the schematic implies the same so I guess that's ok.
    I have a friend who's an electrical engineer coming over tonite to help.
     
  17. marty pollard

    marty pollard Member

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    Ok, my friend found out that the ss rectifier had a problem. When we flipped the little square turret board over we found on of the wires had corroded in two. Fixed and had just a little scratchy output at full volume..

    He narrowed it down to either the OT or the fact that the 470 ohm resistors were missing from both power tube sockets. Is that a mod of some kind? It's obvious something USED to be there because you can see the old solder. Do I need to replace those to verify that they are not the problem?
     
  18. Rosewood

    Rosewood Member

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    Should be a 470 ohm 1 watt across pins 4 to 6 of each tube. I use 470 ohm 5 watt wire wounds.
     
  19. Blue Strat

    Blue Strat Member

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    If you've got full volume, whoever modded the amp did something to bypass the 470 ohm resistors. The amp won't work without them.

    Best recommendation is to figure out what has been done, remove it, and restore the 470 ohm screen resistors to stock configuration.
     
  20. phsyconoodler

    phsyconoodler Member

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    There are some interesting things developing here.
    One is the mention of diodes in a 78 super reverb.They originally had a tube rectifier.Then you mentioned the 'turret' board for the diodes.No super reverb I've seen ever had turrets.
    I'm thinking this amp has been modified inside and there are some things that are very amiss.I have not seen diodes that 'corrode' either.A picture of the inside of that amp would be worth a thousand words.
     

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