JCM900 SL-X 2100 EL34 Problem #2 (Loss of Output)

Discussion in 'The Workbench' started by jeff.gt, Feb 25, 2020.

  1. jeff.gt

    jeff.gt Member

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    Starting a new thread to t/s 2nd problem with the JCM900 (same from the hot bias thread). Before I replaced the bias resistors, the amp output would cut out after about 20 minutes of use, playing at residential volume at about a two. Flip the standby switch off, wait a second and power back on, volume comes back. Amp has brand new electo harmonix EL34EH and new JJ 12ax7's. Had the same problem with the old groove tubes installed (found 1 dud power tube in the mix).

    With the incorrect resistors in the bias circuit I could not set bias any lower than 65ma, but now that it's correct I get 35ma right on the money right (500v plate at pin 3). This amp sat unused for quite some time, at least 10 years, probably a lot longer. If this amp was ran for a considerable amount of time with 65ma (or way higher) bias, could that have damaged anything besides the power tubes? Into a 1960B, The amp tone is rather thin...lots of treble, I'm going to blame that on volume at a two though (or maybe EL34EHs)!

    Thanks y'all,

    Jeff
     
  2. MickeyJ

    MickeyJ Well-Known Member

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    dry solder joint. could be anywhere.
    just melt em all every one you can see, maybe add a bit here and there.
     
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  3. jeff.gt

    jeff.gt Member

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    Does putting a patch cable in the effects loop have any effect on a Sl-x 2100? Possible oxidation in those jacks?
     
  4. MickeyJ

    MickeyJ Well-Known Member

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    If the amp ran hot tubes for a while it won't matter one bit maybe the tubes had a hard time and wore out some maybe they didn't. Get it running properly by re-heating every solder joint you can see and then play it hard and have a listen . Worn out output tubes sound a little pissy low bass low treble i think you'll know. The jcm900 is a good amp I like them.
    For now- after the amp stops working check to see if the tubes still have the orange glow. if some don't you know your heater connections are dry soldered it could be anything though one end of a signal cap
    It could be under the boards on a jcm900.
    Theyre not hard to pull apart anyway.
     
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  5. jeff.gt

    jeff.gt Member

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    Will get back into it tomorrow!
     
  6. Gunner64

    Gunner64 Well-Known Member Premium Member

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    Yep, look at the preamp tubes for filiment glow. Not only for shitty solder connections, but the 3 preamp tubes ( not the p.i ) have rectified dc filiments, and the bridge rectfier that does this , br3, overheats, fails and burns the traces on the pcb leading up to it in some of the sl-x's.

    If the filiments aren't glowing when the amp cuts out look at br3 on the pcb for evidence of overheating, and if so the related C8 should be changed as well.
     
    Last edited: Feb 25, 2020
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  7. MickeyJ

    MickeyJ Well-Known Member

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    the thing is with this amp it works then it doesn't.
     
  8. MickeyJ

    MickeyJ Well-Known Member

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    gunner64's info is good, if the amp was mine I'd change out that rectifier pronto for a higher rated one and save the traces on my board if not already damaged. I'd sniff out the regulator for the dc supply if it has one too and upgrade that too.
     
  9. Lance Chambers

    Lance Chambers Well-Known Member

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    Post up detailed pics....................:coffee:
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    Did you connect a patch cable between send and return?
    Yes the jacks oxidize and fail.
    This might be the only problem.
     
  11. Esa Martikainen

    Esa Martikainen New Member

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    When it fails next connect the guitar directly to FX loop Return and if amp gives sound connect guitar back to normal input and if it is still quiet there you know the power amp section is not causing it and problem is on preamp side.

    However I recall that there are two different level standards for FX loop and if yours has different than the newer Marshalss use you might need to use for example an overdrive pedal between guitar and FX Return to gain up the Signal.

    Esa
     
  12. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    Before you guys go too deep just re,ember that this amp stood idle for 10 years or more, electrolytics dry out over time, just consider that first.
     
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  13. jeff.gt

    jeff.gt Member

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  14. jeff.gt

    jeff.gt Member

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    Lance I'll take some more pics after work when I open it back up...link above is the last thread with pics of the PCB
     
  15. Gunner64

    Gunner64 Well-Known Member Premium Member

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    Just read the link to the other thread. Sitting for 10 years in that humid environment everything is suspect. Especially corrodable contact points..Every switch contact, every jack, every tube socket...and what Ken said in regards to the electrolytics. Thats a long time to sit in humid conditions.
     
  16. Lance Chambers

    Lance Chambers Well-Known Member

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    It appears (from viewing the pics) that "C13" has overheated. I believe it is responsible for oscillations. It needs replacing! :hmm:
     
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  17. jeff.gt

    jeff.gt Member

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    On it, what value cap at c13?
     
  18. MickeyJ

    MickeyJ Well-Known Member

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    I recently fired up an amp that had been sitting next to an open window for years maybe five years.
    it didn't work.
    First thing i did is wiggle the speaker lead plug a little the amp came to life.
    The ( cliff ) jacks were all slightly coated with whatever.
    Anyway i spent a bit of time making sure all the jacks were clean.

    Someone has already mentioned this jack business I believe but if you want to try stuff that doesn't mean pulling those boards off ( it's not hard it's fun ) then clean those jacks and clean them real good.

    if you want to fix this amp real nice change out that snub cap the oscillation one for a higher voltage rating.
    R31 looks slightly overheated it's sitting next to vl2 if it is overheated change that too. ( is that the nfb resistor? it goes to a purple wire maybe it's a bad solder joint right under that r31 resistor just re-melt all your joints it's piss easy)
    Don't disregard gunner64's advice about heater dc components change em out and change em to a higher voltage rated value. if one day you melt your filament traces you'll rue the day you didn't upgrade those components, it's only gonna cost ten bucks or something.
     
    Last edited: Feb 25, 2020
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  19. jeff.gt

    jeff.gt Member

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    Gunner, what value cap should I replace BR3 and C8 with?
     
  20. jeff.gt

    jeff.gt Member

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    snub cap...C13? What should I drop in there? building a parts list...
     

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