Help diagnose this problem please?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JamesHealey, Jan 24, 2008.


  1. JamesHealey

    JamesHealey Member

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    Hey I've got a JCM800 1987, it was slightly noisy so I decided to replace the caps, replaced the bias caps and the power filter caps, also replaced the pre-amp tubes and power amp tubes and biased it up.

    Well it's a lot less noisy in terms of background noise but when i turn up the volume it starts to pickup this really high pitched squeel like some component is screaching in the amp.

    this doesn't happen when I turn the amp up without anything plugged into the front, so if i have my guitar unplugged and I turn the amp up loud, it's totally fine little noisy but quite acceptably so.

    this seems to me like it's something in the pre amp? when i hit the choke with a pencil when the amp is on it makes a crunchy noise sometimes and when i touch any component inside the amplifier whilst it's on it echos, pickups this bassy frequency and starts to amplify it.

    really quite confused.
     
  2. Swarty

    Swarty Member

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    The crunch noise is probably a bad connection... you need to check every new solder joint. Did you pull the board to change the bias caps? For the squeal I'd swap a different tube into V1.
     
  3. JamesHealey

    JamesHealey Member

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    yea i moved the board out for the bias caps.. why?, i've tried different valves in V1.. im not sure if the pre-amp section is stock because it had been modded for a master volume, so wonder if someone had given the pre-amp section more gain? it almost sounds like the entire pre-amp section is microphonic? as for the crunchy sound it's somewhere around the preamp valves coz it doesnt happen when i tap it near the otherside of the chassis.
     
  4. tlpruitt

    tlpruitt Member

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    Sounds like a microphonic preamp tube. Does it make a funny noise when you tap on the preamp tubes themselves? Try a different 12ax7 in the V1 socket. If that doesn't fix it try a different 12ax7 in each of the other preamp sockets, testing after each to see if the problem goes away.

    Functionally, the old preamp tubes are probably fine and didn't need to be replaced unless you are going for a different sound. You can use the old preamp tubes for the swapping described above.
     
  5. JamesHealey

    JamesHealey Member

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    it's not pre-amp tubes I promise you, theres a lot of RF in background of the signal aswell, when i turn the amp's volume up it doesnt feedback like it would a microphonic tube the squeel changes pitch like a radio tuning in would do.
     
  6. 78tsubaki

    78tsubaki Member

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    I am with the others. You might have a lead dress problem if you just did a cap job. Overall it sounds like a possible cold solder joint (been there and done that) and a microphonic preamp tube.
     
  7. JamesHealey

    JamesHealey Member

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    I've solved the RF problem by moving the wires about around the jack input, kinda tucked em closer to the jacks and thats stopped all together..

    the sweeping tuning noise is still there and i've narrowed it down to the 0.22uf cap on the treble pot, I think it's heat damaged as it's slightly melted in the corner.. probably when i removed the master volume mod, or when it was installed.
     
  8. JamesHealey

    JamesHealey Member

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    what capacitor should i replace this with?
     
  9. JamesHealey

    JamesHealey Member

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    I've replaced this cap with no change to the problem..

    Ok so this is the deal I had a problem with background noise so i replaced the caps and tubes, i've done so without a dry solder joint as im 100% sure.
    The tubes are not microphonic as I have a selection of pre-amp tubes and i've tried them all in various combinations.

    I get a high pitch squeel on the volume I input, if i turn down the middle and treble this problem goes away.. and it also only happens when the amp is the right way around.. if the amp is upside down their is no problem.

    strange? well any ideas?
     
  10. Swarty

    Swarty Member

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    If this is a later version with the input jacks and pots mounted to the pc board, did you by chance remove the supressor cap on the first gain stage? If it is not a later version I'd suggest sheilding the grid leads on the first 2 stages.
     
  11. JamesHealey

    JamesHealey Member

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    i resoldered the jacks and swapped the two on the PCB over, it's now fine with me playing into channel 1 high.. it now only does this problem when im plugged into both Channel 1&2 together.. and it's no longer making a difference if i've got the amp upside down or the right way up.

    I think it could be a dodgy input jack socket.. so im going to replace the other on the pcb board.

    what would the supressor cap on the first gain stage look like? i dont really know an aweful lot about amps.. can u see it on a schematic?
     

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