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Blown Power Transformer?

Discussion in 'Shock Brother's DIY Amps' started by HBamps, Oct 2, 2020.

  1. HBamps

    HBamps Tele-Meister

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    Hey all,
    I built a 5F2A variant a few months ago and have been enjoying it very much without issues. I was playing at low volume today, like I do almost every day, and all the sudden the amp makes a crackle and all the sound and light goes out.

    Checked the fuse and it's blown.
    Pull all the tubes, still blows fuse in 3 seconds.
    Disconnect B+ rail from rectifier pin, still blows fuse in 3 seconds.

    I did not disconnect the heater circuit but I did remove the tubes and inspected the heater wiring. all looks ok there.

    Is there any reason to believe this is something other than a blown PT?

    BTW, it's a Weber WO22772EU
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    A quick search for the Weber WO22772EU brings up a comment in the *offsetguitars* site from our *muchxs* about the euro spec PT not holding up to the demands of the amp.

    From your description it sounds like the winding has shorted.
    What tubes were you using and how hot was the bias?
     
  3. gusfinley

    gusfinley Tele-Afflicted

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    Do you have a Weber copper cap installed (WY3GT) ? I know you said "Pulled all the tubes" but it would make a difference if you still have a copper cap installed.
     
  4. gusfinley

    gusfinley Tele-Afflicted

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    I would check all of your unused secondary windings on the EU version to see if any of them have shorted together also.
     
  5. HBamps

    HBamps Tele-Meister

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    just a single 12AX7, 6V6 and a 5Y3
     
  6. HBamps

    HBamps Tele-Meister

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    This is an excellent idea, thank you.
     
  7. Nickfl

    Nickfl Friend of Leo's

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    Build and use a lightbulb limiter.

    Even if you don't have a burned out PT now, if you keep blowing fuses, eventually you will.
     
  8. HBamps

    HBamps Tele-Meister

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    I'm assuming the PT is blown. Everything else looks ok. My worry is wondering what took it out? just a fluke?
     
  9. Nickfl

    Nickfl Friend of Leo's

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    I doubt it's just a fluke. Did you check to see if you have any of the unused primary wires have shorted as someone suggested? If it wasn't that it could have been a short in a tube or somewhere else.

    First of all, build a light bulb limiter if you're going to keep working on amps. It's a cheap, easy to build and necessary tool. Once you have one of those, disconnect your power transformer secondaries then plug into the light bulb limiter and see if it indicates a short with the secondaries connected to nothing if so then your power transformer is ****ed.

    For what it's worth in that forum they seem to be talking about a Princeton reverb, so the much lower current in this amp is not likely to have been an issue just because it was there. I don't know if the issue michxs was talking about was quality control or inadequate current rating, though either way it seems like that part number is discontinued now so maybe Weber had recurring problems with them.
     
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  10. King Fan

    King Fan Poster Extraordinaire

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    What @Nickfl said.

    Not only will a light bulb limiter save you fuses, it'll protect your amp while you test, and used systematically it can find the sector that's shorted. Your all-tubes-pulled test is certainly down the chain of steps, but if I'm right that still leaves a couple steps to localize further, including Nick's steps above.

    Though Weber transformers aren't considered bad per se -- they're seen as inexpensive and functional -- failure over time is one of the issues you read about, whether or not they're overstressed.
     
  11. HBamps

    HBamps Tele-Meister

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    The light bulb limiter makes sense. I'll look into making and using one. Im the meantime, I have to order a new PT. I picked this one because it gave me the ideal B+ I was looking for with it's 600V secondary.

    Thanks for all the help!
     
  12. sds1

    sds1 Tele-Afflicted

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    Every time I thought the transformer was bad, it wasn't.

    You are ahead of yourself I think, ordering a new PT, as you haven't ruled out more likely causes like the fellas say.

    I am rooting for you either way, best of luck and following!
     
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  13. D'tar

    D'tar Friend of Leo's

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    +1
     
  14. HBamps

    HBamps Tele-Meister

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    I "think" that if the fuses in blowing in seconds with no tubes and the B+ rail is disconnected (standby) what else could it possibly be?

    I checked the other set of secondaries and EU primary wires. Nothing looks shorted. I originally cut them staggered and individually taped to prevent shorting (but undid and checked anyway).
     
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  15. Wally

    Wally Telefied Ad Free Member

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    Lift all secondaries and make each end safe. Fire the amp up. does a fuse blow? If so the primary is shorted. If not, then put a meter on all of those secondaries...making sure to check to see if any of them have shorted to the frame of the PT...which is ground when in the chassis. If you find a sort, it might be of interest to note which one. It could indicate where you might want to check for problems in the circuit. And...if you get a new PT, you are going to want a current limiter device before applying voltage to that new PT.
     
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  16. FenderLover

    FenderLover Poster Extraordinaire

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    Disconnected B+ from the rectifier? Just pull the rectifier tube, it could be shorted. With the tube out, the first filter cap could be shorted.

    If your 6.3V measure good, just disconnect the PT leads going to the first filter cap. I'd suspect the rectifier or the cap before suspecting the PT, but it can happen.
     
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  17. sds1

    sds1 Tele-Afflicted

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    You are on the right train of thought here, you are looking at the amp and I am not, honestly my post is kind of a "gut feeling" drive-by more than anything...

    But in such a tight chassis especially, the possibility of a filament wiring short for example (pin to pin) or across the pilot lamp, stuff that is hard to see... fully disconnect the filament secondary to rule this one out.

    You could be totally right about the PT man. I just think you skipped a few troubleshooting steps. Without a full arsenal of test gear maybe you have to.
     
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  18. Nickfl

    Nickfl Friend of Leo's

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    Disconnect all your secondaries and try again just to be sure, you could have a short at a socket, between pins or something. As I said earlier you should do it with a light bulb limiter to avoid blowing fuses and protect your components, but if all of the power transformer secondaries are disconnected and capped off and it still blows a fuse the power transformer was shot anyway so all you lost from not using a limiter was another fuse.

    If you determine the transformer is bad and order another one you should still do some forensic work on the old transformer . Measure the resistance across the various windings and from the various windings to each other and try to figure out which one failed as that will give you a clue as to what took it out. It may be that the transformer just straight up failed, but if it was something else you want to find out before you put that new transformer into service.
     
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  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Yeah, I don't know either.
    Observation: I don't think the draw of the 5F1 has to be much lower than the princeton. The 5F1 can easily idle at ~50mA. (JJ tube) Where the princeton is usually set cold for vibrato function and may be idling at < 40mA and momentarily peaking at ~60mA.
     
  20. sds1

    sds1 Tele-Afflicted

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    I would hate HATE doing this without a lightbulb limiter.
     
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