Thermistor in a MC240 blew, violently

Discussion in 'McIntosh Audio' started by EvanF, Mar 9, 2018.

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  1. EvanF

    EvanF AK Member

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    I imagine this would be because of a short down the line on the primary side, but could capacitor charge cause this? Installed all new power supply caps and checked against the schematic. Will post photos if wanted. Obviously the AC primary would measure a low DC resistance from the windings, but is 1.4Ω too low?
     

     

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  2. Pio1980

    Pio1980 AK Member Subscriber

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    A reversed doubler diode or doubler elyticap could cause this. Do you have the correct rated primary and secondary fuses in place?
     
  3. EvanF

    EvanF AK Member

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    Yes. I installed a new rectifier diode in place of the selenium one with a 470Ω resistor in series. Both old Si diodes are in place. The new one is in the correct orientation. https://www.mouser.com/ProductDetai...qZeNK75brD/uWlUs3o99ImXIcY5KVkFjf27xzLdLP0w== will work?
     
  4. EvanF

    EvanF AK Member

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    I was an idiot not putting the amp on my variac.... The caps had about 100V or so, then the thermistor opened. Didn't charge all the way up.
     
  5. Pio1980

    Pio1980 AK Member Subscriber

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    You may have a bad or backward elyticap in the power supply. A fuse should have gone first.
     
  6. EvanF

    EvanF AK Member

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  7. EvanF

    EvanF AK Member

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    Those are two 270µF 250V. Blue wire goes to cathode of a diode which goes to fuse to red lead from power transformer.
     
  8. Pio1980

    Pio1980 AK Member Subscriber

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    The elyticap on the left appears to be backward, turn it around. It may be ruined, cross your fingers
     
  9. Pio1980

    Pio1980 AK Member Subscriber

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    Also, double check wiring against schematic, it doesn't look right.
     
  10. EvanF

    EvanF AK Member

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    It looked right. The green wire on the ground connection on the left cap is part of the winding with the 12µF cap. Both 270µF are tied at negative, going to red winding. Positive of left should be at ground and positive of right should be at cathode of SR2 and black wire side of the main choke.
     

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  11. EvanF

    EvanF AK Member

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    Both caps go from low resistance to 400kΩ on my meter (diodes and .01µF film cap and filter cap insulation all parallel).
     

     

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  12. EvanF

    EvanF AK Member

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    I may have used an undervalued diode for SR3, maybe avalanche breakdown pulled too much current? The power supply fuse 2/10 amp did blow.
     
  13. Pio1980

    Pio1980 AK Member Subscriber

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    The two doubler elyticaps should be in series without the diode between, striped ends groundward. One red wire from the power transformer goes to the junction between the elyticaps. The two rectifier diodes are in series across the two series elyticaps, with the other red transformer wire to their junction. The top of the bridge formed by this arrangement is the b+ source to the choke.
     
  14. Pio1980

    Pio1980 AK Member Subscriber

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    The elyticaps aren't properly wired, both striped ends should go towards ground (IF the stripe indicates negative polarity) with them directly in series, no diode between.
     
  15. EvanF

    EvanF AK Member

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    So the one on the left is reversed? Looks like the schematic is wrong. Dang it.
     
  16. Pio1980

    Pio1980 AK Member Subscriber

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    Schematic is incorrect, you are right.
     
    Last edited: Mar 10, 2018

     

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  17. EvanF

    EvanF AK Member

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    Flat end of the schematic symbol is positive. Flat end goes to ground. Take a look at the pic I posted.
     
  18. Pio1980

    Pio1980 AK Member Subscriber

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    Negative goes toward ground, positive up, I've done one of these amps.
     
  19. Pio1980

    Pio1980 AK Member Subscriber

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    Banded end of diode is positive source, the two diodes should be in series, this across the series elyticaps. The two red leads from the power transformer each to the joints between the diodes and the elyticaps. This forms the voltage doubler bridge network.
     
  20. Pio1980

    Pio1980 AK Member Subscriber

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    Schematic is wrong, C23 is shown backwards. Turn it around. What I took for a diode between the elyticaps may be a solder joint. Confirm please.
     

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