New interesting item on the bench

Discussion in 'The Workbench' started by chuckharmonjr, Jun 10, 2019.

  1. chuckharmonjr

    chuckharmonjr Well-Known Member

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    A friend of mine asked me to look at his Hot Rod DeVille. Kept redplating on him. He broke a tube, which popped the fuse. When he put new tubes in, now it redplates. Started looking at it yesterday. Run away bias. Had to do a double take to make sure it wasnt a DSL..lol. Sure enough, at pin 5 the bias voltage slowly climbs up to zero from -54 or so. Anybody got any ideas?
     
  2. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Check the caps and resistors in the bias circuit. Do you have a schematic ?
     
  3. chuckharmonjr

    chuckharmonjr Well-Known Member

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    I do indeed, and am very suspect of one cap in particular. Exactly where I was going with it.
     
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  4. neikeel

    neikeel Well-Known Member

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    I had one of these.
    Probably bias caps. They have flash back diodes on the sockets iirc. New tubes and rebias was pretty straightforwards.
    Not bad (very loud) clean tones and good pedal platform. Not great crunch/gain tones.
     
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  5. ampmadscientist

    ampmadscientist Well-Known Member

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    Take the knobs off, undo the nuts, cut the wire ties and disconnect the OT (label wires). Take the screws out of the board...
    Draw the preamp board straight down till the pots clear, then swing the pots side out of the chassis.

    At that point you will see the underside of the circuit board, which may have some crispy / heat damaged copper traces.

    At that point you can put a piece of cardboard between the board and the chassis.
    Allowing you to turn the power on and trouble shoot / replace parts.

    I usually just replace the caps / zeners / power resistors and bypass the cooked circuit tracks with thick solid wire.
    Clean the controls and jacks, resolder the board sometimes.

    Then there's one more thing that's the PI plate resistors, make sure those are good or replace them w/ better ones.
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    Fender deville.png
     
  7. myersbw

    myersbw Well-Known Member

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    AMS said "At that point you will see the underside of the circuit board, which may have some crispy / heat damaged copper traces."

    ....and, those crappy ribbon cables? They're not above suspect either. :( Bad, bad construction. I am not a fan of that one.
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    Both Hot Rod Deluxe and Hot Rod Deville are prone to the cooked circuit traces. (in the low voltage power supplies)
    (it's not the ribbon cables, it's the copper circuit tracks)
    These amps are built on the same circuit board.
    But at the same time these are the most popular guitar amps in the world, sound great. Very widely used.
     
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  9. myersbw

    myersbw Well-Known Member

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    Job security, right??? :O ...don't forget the Blues series...
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    The "Blues JR." has those really crappy pots and jacks. Job security.
     
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  11. Goldguitarguy

    Goldguitarguy Active Member

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    Pretty funny you're fixing a hot rod Deville now because I just got one in three weeks ago with a cooked Power Transformer from the owner trying to work on it. Also found R79, R78, R57 were toast.

    The replacement transformer I've gotten is part # 41752 for the newer Devilles and the one on the schematic is #41753. I was assured that the parts were the same before I ordered it. Amp fired up no problem and sounds great. My issue is when I checked the plates voltages I'm sitting at 514vdc which seems high for a 6L6GC. Thinking maybe the transformer is not the same or this is the cause of the problem in the first place.

    Can't find specs on the old one anywhere nor does anybody sell them new but the voyages are the same according to the two different schematics of the Deville and Deville iii.
     
  12. mickeydg5

    mickeydg5 Well-Known Member

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    41753 is the export transformer and 41752 is the standard transformer, noted on the schematic.
    If purchased a Hammond it may have a bit higher HT.
    Also transformers usually have +/- tolerances and fluctuate accordingly with the utility power.
     
  13. Goldguitarguy

    Goldguitarguy Active Member

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    Yeah I’m an idiot...lol..I understand there is tolerance differences between PT’s and read amp voltages. I’m just concerned that the voltage may be too high for the 6L6’s. It’s 30vdc higher than what the Schematic calls for and 14vdc over the plate max for the 6l6’s. Not sure if that matters because I’ve never came across the situation before.
     
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    The plate volts will be high until you load the power supply.
    Once the power tubes are in, bias correct, the plate volts will drop quite a bit.
    When the bias is dialed in, the plate volts will be about 460.
     
    Last edited: Jun 13, 2019
  15. LPMarshall hack

    LPMarshall hack Well-Known Member

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    Good to “see” you chuck!
     
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  16. Goldguitarguy

    Goldguitarguy Active Member

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    I biased at 60mA and it's still 499vdc, just seems like a lot. I'm not a tech by any standard though
     
  17. fifteenohms

    fifteenohms Well-Known Member

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    Also known as “magic Pelosi Bubbles.”

    You know. The #ResistanceIsTooDamnHigh effect, where it can’t take the heat it created for itself then tries to get away from the Board.
     
  18. mickeydg5

    mickeydg5 Well-Known Member

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    Which PT did you use? If it is the Hammond replacement at 117VAC and your wall is approximately 120VAC then you are going to have 500VDC on the plates instead of 485VDC.
     
  19. Goldguitarguy

    Goldguitarguy Active Member

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    It was a Fender original replacement from Amplifiied
     
  20. mickeydg5

    mickeydg5 Well-Known Member

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    Yes it is designed at 117VAC primary.
    What is your current wall voltage and power tube plate voltage? Calculate the ratio difference.
     

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