Crate CR-285B Amp blowing fuses, need help TS

Discussion in 'Technical Repair' started by derryckw, Aug 15, 2017.

  1. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    0.1uf ???
    For smoothing caps ?? o_O

    Smoothing caps are C6 and C7.
    By the way, "HUM" can also be coming from the preamp or the current mirror circuit. ( a failing or leaky SS transistor ).
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    C6,7 are the smoothing capacitors 6800uF,...
     
  3. derryckw

    Thread Starter New Member

    Aug 15, 2017
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    That makes sense!!! So both C6 and C7 measure pretty close to 6800uf but like I said C6 is not holding a charge, at least it appears that way, am I looking at that correctly? If C7 has 39v on it shouldn't C6 also?
     
  4. AlbertHall

    Distinguished Member

    Jun 4, 2014
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    So it still sounds like (some of) the diodes in the rectifier may be duff.
     
  5. JoeJester

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    C7 and C6 ... their common connection is connected to ground. From ground, you should have the +45V on C6 and -45V on C7 .

    If you measured from C6 (+) to C7 (-) you should have read closer to 90V.
     
  6. derryckw

    Thread Starter New Member

    Aug 15, 2017
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    That's exactly what I expected, 30 some volts was measured with power removed as I was just checking voltage before I laid an iron on the board, but if both caps were getting voltage then they should both discharge at the same rate so I'm thinking AlbertHall is correct and something is wrong with the rectifier, either way diodes are on order, we shall see.
     
  7. AlbertHall

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    Unless one of those capacitors is defunct. You can check them, sort of, with a multimeter. On the ohms ranges you should see a low reading gradually climbing as they charge and you can compare the two and they should be similar.
     
  8. derryckw

    Thread Starter New Member

    Aug 15, 2017
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    I have a capacitance meter and they both read with in a few uf of expected value, no signs of bloating etc I doubt they are bad. Ohm meter test also good.
     
  9. AlbertHall

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    OK, back to the diodes.
     
  10. derryckw

    Thread Starter New Member

    Aug 15, 2017
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    On order, would have felt better if one of them was at least shorted or open
     
  11. derryckw

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    Aug 15, 2017
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    Ok I'm back, replaced the rectifier and all the caps, pretty much rebuilt the power supply, fired it up +46 and -46v, disconnected the power and still have voltage on both Caps - and + respectively. So far so good, hook up the amp and get the same hum for about 30 sec then it's drops out and Magic smoke arises from the circuit, woohoo I love smoke. Best I can tell it was C20 (1uf/50v) and R23 (47ohm) that toasted, both on the +45v side of the circuit, the connection of the +45v may not have been good since when I took it apart the wire fell out of the connector, would the absence of the 45v caused this? I've replaced both but haven't fired it up, anything else I should check before making more smoke?
     
  12. JoeJester

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    Can we assume you checked C20 and R23?
     
  13. derryckw

    Thread Starter New Member

    Aug 15, 2017
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    Charred and Open, Cap exploded and resistor fried both have been replaced
     
  14. JoeJester

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    Cool. R23 is the one I question because it drops the +45 to something lower. The capacitor on the other hand, a short would have cause the faults in the diode.
     
  15. derryckw

    Thread Starter New Member

    Aug 15, 2017
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    So you think I'm safe to apply voltage? There is nothing else in the circuit before that so I would assume I'm good.
     
  16. derryckw

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    Aug 15, 2017
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    Well I checked everything in that +45v line and everything looked ok, plugged it up and it's blowing the fuse again, back to square one. Disconnected the power supply and it checks good so back to the amp board, guess I'll pull those transistors and make sure I didn't fry them
     
  17. derryckw

    Thread Starter New Member

    Aug 15, 2017
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    Q11 is blown, but I now know what the problem was, the Q10 and 12 that I installed where both NPN!! Same part number as the PNPs I was replacing, time to order more parts.
     
  18. derryckw

    Thread Starter New Member

    Aug 15, 2017
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    Good Power Transistors installed, good power supply but now I have no audio at all, powers up and stays on but no audio, obviously more than the 2 components fried on me.
     
  19. JoeJester

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    where have you checked to see the audio?

    preamp output - line output jacks seems like a good place to start.

    what test equipment do you have?

    DMM?
    Oscilloscope?
    Signal generator?
    Spectrum Analyzer?

    Since it was "working" somewhat before you started soldering,
     
  20. derryckw

    Thread Starter New Member

    Aug 15, 2017
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    I have a DMM, a simple signal gen, and I'm going to try and borrow an Oscope for the weekend. I was at least getting noise from the output where it's dead silence now.
    So far I've got a 1KHz tone on the input and I can't find it coming from the preamp "signal" line going to the Amp board. From the Gen I'm getting about .6v on the Signal line I'm getting 0, from the input board to the preamp board .6v, so it appears the problem is on the preamp board with the Master Vol.
     
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