strange noise issue

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by billstets, Jul 2, 2008.

  1. billstets

    billstets Member

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    Well it's no longer an issue, but I wonder if anyone can explain why this happened. I built a JTM45 clone with a Metro kit, and I asked this question on the Metro board, but no one was able to answer it.

    I had excessive hum. Way excessive. What I finally traced it to was that the chassis was not connecting to the shielding in the bottom of the head cabinet. But here's the weird thing: It sounded fine with the chassis laying in the head cabinet, but when I bolted it down, the hum would get really loud as soon the as the bolts started to push the chassis down onto the bottom of the head cab. My solution was to put some additional pieces of shielding in the cab that lay on the cabinet shielding, and intersect and run perpendicular to the ends of the chassis. The ends of these shielding pieces touch (are actually under) the ends of the chassis when it's bolted down.

    But why would it not hum before it was bolted down?
     
  2. SatelliteAmps

    SatelliteAmps Member

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    That is odd. If you pull the chassis and run it with no shielding does it hum? My first inclination is a loose ground connection somewhere.
     
  3. mark norwine

    mark norwine Member

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    I've seen this before.....my guess:

    You have a bad solder joint somewhere. When you bolt the chassis down tight, you're stressing or twisting the chassis in some fashion, causing the questionable solder joint to fail.

    Seen it before.....
     
  4. billstets

    billstets Member

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    I would say out of the cab the hum is acceptable, though I'd like it a little quieter. I'm told this is normal. The cab helps with the hum a little.
     
  5. billstets

    billstets Member

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    I forgot to mention (see post above) that I think the hum is not unusually loud when the chassis is out of the cab altogether. Louder than in the cab, but normal hum. Also, why does adding the shielding to the cab as I described resolve the bad solder joint?
     
  6. JimmyR

    JimmyR Member

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    Because the extra shielding is acting to space the problem away from the cab. I agree with Mark. Have a look to see what components are close to the shielding when in the cab. This is one of those problems which would be difficult to solve over the 'net, easier if we could see the amp in front of us!
     

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