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Power transformer diagnosis

Discussion in 'Amp Tech Center' started by parknride, Nov 29, 2020.

  1. parknride

    parknride TDPRI Member

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    Hello all,

    I recently purchased a Bugera V55 infinium in a lot of damaged appliances at a local returns outlet. Obviously it is in non working condition. I powered it up and got the Bugera blinking light but no noise. Some searching seemed to suggest a tube issue. I changed a busted tube, and tested the others (all are now in good condition. Amp would turn on, no noise and the blinking light. Tubes were not getting warm in any of the above scenarios.

    I began testing the power supply from the outlet in and here is what I discovered.

    AC IN from wall (117.8 vac)

    Power transformer:
    Heater voltages: 1. Across the total winding (no center tap): 3.26vac
    2. One lead to ground : 16.57vac
    3. second lead to ground: 17.82vac

    High voltage: 1. Across the total winding (no center tap): 183.7vac
    2. one lead to ground : 71.6vac
    3. second lead to ground : 1.48vac

    Bias voltage: 1. Across total winding (center tapped) : 20.8vac
    2. one lead to center tap: 10.5vac
    3. second lead to center tap: 10.5vac

    The only one that conforms to how I understand how a power transformer functions is the bias circuit. I'm trying to determine if the transformer itself is blown.

    I did also check continuity between the windings:
    1. Primary to secondary : No continuity
    2. High voltage winding (total winding): Continuity
    3. Heater winding : Continuity
    4. Bias winding: Continuity

    Thanks for your time.
     
  2. Joshtheguitarguy

    Joshtheguitarguy TDPRI Member

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    If I were you I would replace the whole transformer, just to get that variable out of the equation.
     
  3. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Yeah, bias looks good, rest not so much.

    Was the PT completely disconnected from the board when performing those tests? If your measurements are correct, I would also not expect the tubes to glow............so, did they glow?
     
  4. Dacious

    Dacious Poster Extraordinaire

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    It sounds like that is a 230-240 unit, or its been wired that way. No way will the amp work.

    Check the fuse holder - sometimes, flipping one way or the other changes the primary from 120-220.
     
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  5. dogmeat

    dogmeat Friend of Leo's

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    sounds like its shorted internally to me

    edit: let me back up.... are you measuring with the tranny still plugged into the board?
    I found a diagram. the hi volt side is 2 yellows (F1 in one leg). goes to a full wave 4 diode bridge and becomes "A" which is 460 volts. should be able to read that with the standby switch open. between the leads voltage will probably be 700 or so (check your meter to see if it can read that high if you try this)

    if it's hard wired in and you want measure the main voltage out you could pull F1 and measure from the fuse holder to the other lead. that should get it off the board

    heater voltage should be 6.5

    https://schems.com/bmampscom/bugera/V55-Schematic.pdf
     
    Last edited: Nov 29, 2020
  6. dogmeat

    dogmeat Friend of Leo's

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    heres a cool, easy to make tool for checking transformers

    https://www.premierguitar.com/articles/the-super-secret-transformer-tester-1

    I built one some years ago. what they don't tell you is it doesn't work on the primary side because you would have to put power on the secondary. if so, it becomes a step down transformer that results in a lower voltage than the battery or what ever you use for a power source (I have a variable power supply). a neon needs about 80 volts or so
     
    Last edited: Nov 29, 2020
  7. parknride

    parknride TDPRI Member

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    The transformer was disconnected from the board when I took the measurements. The tubes did not glow.
     
  8. parknride

    parknride TDPRI Member

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    I think This is the best approach but I don't have another transformer with a bias tap.
     
  9. parknride

    parknride TDPRI Member

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    I pulled the transformer from the circuit before the measurements and just hooked up my voltmeter to the loose leads.
     
  10. parknride

    parknride TDPRI Member

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    If it's a 230-240 wouldn't that also effect the bias circuit? According to the schematic it's designed to put out 20 vac and that part at least appears to be spot on.
     
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  11. dogmeat

    dogmeat Friend of Leo's

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    yes, and the bias tap appears to be OK. the other 2 secondary windings are not. sometimes the easy way out is a new stock transformer. if it's not available, or way too stupid money there are ways to adapt other existing trannys, who knows, there may even be a drop in. have to do some research
     
    Last edited: Nov 30, 2020
  12. Jon Snell

    Jon Snell Tele-Holic

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    The V55 was for the EU market and is probably a 230/240volt primary.
    Screenshot 2020-11-30 at 11.31.17.png
     
  13. Dacious

    Dacious Poster Extraordinaire

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    If the 21 volts is on the schemo it's the result of the rectified bias circuit after its been dropped in the bias resistor.
     
  14. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    The bias circuit cannot provide +25V and -25V rectified if the voltage to the centre tap is 10VAC. I'm reading that as a 20-0-20 winding, which gives another winding set providing half voltages. Pictures of any labelling of the transformer would help. Even if the primaries aren't identified, a part number can nail whether or not you have a European spec transformer.
     
  15. parknride

    parknride TDPRI Member

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    I will check later on today, but its looking more and more like you guys are correct about the type of transformer. Thanks
     
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  16. Joshtheguitarguy

    Joshtheguitarguy TDPRI Member

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    Oh. I don't know then. I say you shouldn't spend too much to fix this amp unless you're intending to flip it and make a quick buck.
     
  17. parknride

    parknride TDPRI Member

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    You guys were right it was european spec. And you caught it quick too. I already replaced the transformer with one I had lying around that fit the bill. I'm going to use another center tapped transformer from a solid state amp for the separate bias voltage. The only other issue is that the new transformers high voltage is center tapped. Can I just use one of the leads and the center tap of the new transformer to sub in for the non center tapped high voltage leads of the first transformer? Thanks again for all your help.
     
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  18. parknride

    parknride TDPRI Member

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    I am just going to flip it. My wife would kill me if I tried to use a 55 watt tube head in the house. I got the whole lot (50 appliances) for about $150. I though attempting to fix the amp would be give me some more experience, because I've just started building and repairing my own. And of course I learned something right away (read the schematic, and check the transformer for proper primary voltage). Thank you for your help.
     
  19. Joshtheguitarguy

    Joshtheguitarguy TDPRI Member

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    Sure thing. I hope It goes well selling it.
     
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