No power Vox Brian May SS amp

Discussion in 'Amp Tech Center' started by Mhat, Sep 19, 2016.

  1. Mhat

    Mhat Tele-Meister

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    I have a buddy's vox Brian May signature amp. It won't power up. I told him I'd look at it. The fuse is fine, as is the AC line. On the PT secondaries I'm getting roughly 6V AC per lead when disconnected from the board. As soon as I connect it to the board, I have no voltage reading. The secondaries go right to the bridge rectifier, would it be fairly safe to assume that the rectifier is the problem?

    Here's the schematic.
    [​IMG]
     
  2. mRtINY

    mRtINY Tele-Afflicted

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    Maybe... It's a cheap part to replace.
     
  3. Mhat

    Mhat Tele-Meister

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    Yeah, I just picked up a larger rated rectifier for 2 bucks. I'll give it a shot.
     
  4. Mhat

    Mhat Tele-Meister

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    Hm, swapped the bridge rectifier and still no DC voltage . I rechecked the PT secondaries and I only get 3.5VAC on each one. I was going off my memory and was wrong about the 6VAC I originally posted. I'm not sure what the PT voltage should be as it's not listed in the schematic but the first filters caps are rated at 25volts. Can a PT go bad a still produce voltage but a reduced voltage?
     
  5. mRtINY

    mRtINY Tele-Afflicted

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    There's likely a short somwhere...

    Can you disconnect the DC connections so the power supply has no load?
     
  6. RLee77

    RLee77 Friend of Leo's

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    Difficult to guess that sort of thing, without having hands-on to test some parts, but if I were to take a guess it would be IC3, the LM1875 power amp chip. In my experience with them (not that specific one, but similar ones), they are prone to overheating, especially if the thermal bond to the heat sink becomes loose.
    If you don't see any obvious discolored or cracked components, and metering around doesn't reveal anything suspect, you could remove that chip and see if the supply comes back up.
     
    Last edited: Sep 22, 2016
  7. andresound

    andresound TDPRI Member

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    I am late to this thread, but I ahve been asked by a freind to replace his VBM1 PT. I assumed the spec would be approx 15-0-15v secondaries?
     
  8. Mhat

    Mhat Tele-Meister

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