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Demon 250 amplifier blows fuses

Discussion in 'Amplifiers' started by klj2497, Jun 7, 2019.

  1. nomadradio

    nomadradio Analog Retentive

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    Two numbers to consider.



    Voltage rating.

    Current rating.

    The 32-Volt fuse was better than nothing, but tends to blow more slowly than a fuse meant for 125 or 250 Volts. Either of those is appropriate, but the 32-Volt fuse is not a good idea, as a general rule. Either 125 or 250 is fine, but 32 Volts is a not-so-good second choice.

    The current rating for five tubes would be closer to 10 Amps, max.

    And that's why I suggested that the fuses may or may not have protected the transformer when they tripped. They were a bit large. The rating was small enough to protect your house from burning down, but a bit large to properly protect the transformer from harm.

    Might have gotten away with it. Only one way to find out.

    73
     

  2. klj2497

    klj2497 Member

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    Hi, i found 2 wires on the center of the hi low wattage switch the green one goes to the bad diodes. and the pink one goes to the bad resistor and capacitor combo. Which one do i disconnect.(y)
     
  3. nomadradio

    nomadradio Analog Retentive

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    Only the one going to the diodes needs to come loose.

    73
     
  4. klj2497

    klj2497 Member

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    Thank you, now i see why i need to check the transformer before i replace all the diodes, caps and resistors.
     
  5. klj2497

    klj2497 Member

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    correction on the wire colors. Pink to diodes, green to cap/resistor
     
  6. nomadradio

    nomadradio Analog Retentive

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    Pulling the pink wire loose will break the circuit and remove any load from the transformer's high-voltage winding.

    If it still pops a fuse this way, the transformer is most likely bad.

    We use a variable-voltage AC transformer, or "Variac" for this. A selector switch puts a 1-Amp breaker in line with the power cord. This minimizes collateral damage. No matter how much current the linear will draw when it's keyed, the standby current would never be more than 1 Amp.

    The small breaker limits the surge stress on other parts when troubleshooting a short.

    Simply putting a 1-Amp fuse in the thing and flipping the switch might or might not tell you the same thing. A legitimate turn-on surge might trip it. Turning up the AC voltage slowly prevents that sudden surge and allows you to troubleshoot an overload safely.

    Well, more safely.

    73
     
    Shadetree Mechanic likes this.
  7. klj2497

    klj2497 Member

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    Success, I pulled the pink wire from the diodes. turned it on and, the transformer is okay so far. I Replaced all the diodes and the two big caps /resistors. Then i turned it on. success. Then i replaced the tubes one by one . and Success. tomorrow i will test it.
     
  8. klj2497

    klj2497 Member

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    Its working pretty good for having been doa with 4watts into the demon it produces about 220watts dead key and swing about 280watts . and it swings about 285watts on side band.
     
    Dmans likes this.

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