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Occillating power tranny primary?

Discussion in 'Amp Tech Center' started by Doc Simons, Aug 5, 2018.

  1. Doc Simons

    Doc Simons TDPRI Member

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    1971 Pro Reverb working perfectly a few day ago. Chassis then removed to try a mod for friend. The amp has new caps but previously no mods other than blackfacing. He brings it over, we fire it up, sounds fine. Turn it off for a few minutes, blows a fuse. Never has blown a fuse before. Then it began to blow fuses alternating with peiods of sounding great. Eventually, it stopped turning on completely. Here's the kicker. I put the amp in series with a bulb. When all the secondaries are open, totally disconnected, it still occillates. Anyone actually see this happen?
     
  2. Inglese

    Inglese Tele-Meister

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    You mean with no load at secondary you still have quite high primary current in? Enought to blow the fuse?
     
  3. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Yeah, I'm not sure "oscillation" is the right word here.

    If the only thing you have connected is the switch, the fuse, and the primary side of the PT, and you're blowing fuses, you have a bad PT.
     
  4. dsutton24

    dsutton24 Doctor of Teleocity Ad Free Member

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    What was the 'mod' you tried?
     
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  5. Doc Simons

    Doc Simons TDPRI Member

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    Didn't before the blowing fuses.
     
  6. Doc Simons

    Doc Simons TDPRI Member

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    I figured. The bulb in series went from bright to dim about 1 per sec. Very wierd. I thought it would be a short or an open coil, but where does this on off on off come from?
     
  7. dan40

    dan40 Friend of Leo's

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    Are you saying that the bulb flashes even with all of the secondary leads unsoldered and disconnected from the circuit?
     
  8. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    I still want to know what "oscillation" means in regards to this issue :D
     
  9. Old Tele man

    Old Tele man Friend of Leo's

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    Tubes can make inductors and capacitors oscillate, but inductors (transformers) can't oscillate by themselves.
     
  10. ranjam

    ranjam Tele-Holic

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    That's not entirely true, if at all. I see a lot of old Hammond transformers up here in Canada. A lot of the really old units can have a core that actually vibrates against the core. The only way I could fix that was to wedge in a small piece of wood between the windings and the core, and a dab of Goop kept it in place. The transformer suddenly gets 'quiet'. It can happen. I hear this as a low frequency 'buzz', if this is what you mean. If not, please disregard what I am telling you. ;)
     
  11. RLee77

    RLee77 Friend of Leo's

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    What changes did you make for the mod? The most logical explanation is that the changes you made were not entirely undone, or a connection is now intermittent and cycles on/off as it cools/heats. I’ve seen cases where a solder blob fell into the wiring, and made intermittent connections.
    The other possibility is that the PT has an intermittent internal shorted winding, that makes connection as the PT heats up and expands.

    I’d start by looking really closely at the areas you were working around.
     
  12. wanderin kind

    wanderin kind Tele-Meister

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    did you disconnect the heater winds also?

    disconnect all sec winds and see what happens,

    there is a thing called ferro resonance, but his is usually associated with three phase systems.

    note that when you disconnect all loads, flux goes up so high as to saturate the core, as the transformer is trying to cancel out the primary voltage by creating a back EMF via Lenz's Law and all that,

    this is why you will hear a power transformer get noisy when there is no load connected to it. this can be lam rattle or coil/core rattle.
     
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