Smoke and stink...uh oh

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Leonc, Apr 5, 2008.

  1. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Well, after having sat unused for maybe 3 months, I decided to give my '65 Gibson Mercury II a whirl today. Last time I used it, it was performing well and sounding great.

    I plugged it in, turned it on, let it warm up...played for about 6 or 7 second, and then...pfft pop! No sound! I turned it off. I noticed very stinky smoke coming from rear, seemed to be near power section. Crap!

    Pulled chassis, looked. All caps (which were replaced with new ones about 5 months ago when I got it) appeared in tact and unburnt. When I got it, the filter caps had burst and there was big burn mark on the chassis all over the section where the big old filter caps had been.

    Checked fuse, it was blown. Put in another, amp powered on...but only for a second, and second fuse blew. This is a SS rectified amp. The diodes were also replaced back when the cap job was done. The amp had been working fine since then but has been unused for a few months, as I said.

    Any idea what might be going on?
     
  2. Blue Strat

    Blue Strat Member

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    Could be a shorted power tube taking out a screen resistor or PS divider resistor. The tube may have shorted due to incorrect bias.

    Those are my best guesses. It will require some detective work for sure.
     
  3. alltone

    alltone Member

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    Howdy :Here is a link for a connection using a standard 60 watt light bulb hooked in series with your mains AC cord.This will save you fuses and more possible frying.After replacing your filter caps(too bad you lost them all), use the BULB. This will give you a chance to check voltages down the line and troubleshoot your problem.It might be a good idea to power up without the output tubes first. Also, check for signs of carbon arching on output tube bases and sockets. If carbon traces present clean all of the residue off with sandpaper or blade . Good advice in the previous thread...that kind of whack can take out stuff in the high voltage section!!:horse
    http://antiqueradio.org/dimbulb.htm
     
  4. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Thanks fellas. I decided to take it to my local tech. Probably the smartest thing I could do at this point...
     
  5. ted01

    ted01 Member

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    You might try using a variac. I got one down at Fry's Electronics for about $70. This way you can slowly bring up the line voltage while watching for things like red plates and such. Try pulling all the tubes and then turning on the amp. If nothing happens bad, you probably have a shorted tube. If you have a multimeter, you can check the voltages at each stage of the power supply. Don't forget, those voltages are absolutely lethal and will kill you if you mess up. I use alligator clips and clip them to the point I want to measure and then get my hands completely out of the chassis before I begin to bring the line voltage up on the variac. Put one hand in your back pocket whenever workiing inside the chassis. Make sure you allow plenty of time for the filter caps to discharge before you do anything. I actually measure the voltage across the caps and wait until it drops to near zero. Remember that filter caps can "recharge" themselves, even after they appear to be fully discharged. I often use multiple DMM's to monitor voltages at several different points. A DMM can be had for as little as $2.99 from Harbor Freight. They aren't particularly accurate, but for measuring voltages in the power supply for safety reasons, they do pretty good.

    Ted
     
  6. SatelliteAmps

    SatelliteAmps Member

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    Absolutely the right thing to do.
     

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