Jcm 900 Sl-x 2100 Br3 Rectifier Mod

Discussion in 'The Workbench' started by troyAmpallen, Mar 3, 2019.

  1. troyAmpallen

    troyAmpallen Member

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    I just aquired a JCM 900 SL-X 2100. It’s in great condition, and sounds good. Eveything seems to work except I have to crank the volume and gain quite a bit (like to 7) to get to band practice level running a JCM900 cab.

    I ordered a new tube set, and while waiting for the shipment, opened up the chassis. First this I noticed is the BR3 rectifier has been removed. How can the amp even work like this? Doesnt it supply plate voltage to the preamp tubes? Is this a known mod? PLEASE HELP
     

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  2. ampmadscientist

    ampmadscientist Well-Known Member

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    BR3 is DC power supply for DC filament of preamp tube.
    It could be bypassed, and AC could run the filaments.
    It would probably be noisy with AC filament, but it would work.

    2100slxfilaments.png
     
  3. Matt_Krush

    Matt_Krush Well-Known Member

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    BR3 (on the schematic I looked at) is for the Phase Inverter heater...
    If they removed the bridge rectifier, did they just wire the heater off an adjacent tube?
     
  4. troyAmpallen

    troyAmpallen Member

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    I pulled the PCB and it has been modded with an 8 ohm rectifier, attached to back of PCB. I have read this is a common mod. Just surprised me when it was gone. So, now the volume issue. Should be a lot louder I think. Hoping the new tubes will fix it. All the input jacks look clean.
     

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  5. Gunner64

    Gunner64 Well-Known Member

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    Br3 is the rectifier for the preamp tubes, the first 3 tubes. The phase inverter is still a.c.

    In My 2100 sl-x I just wired a larger br3 in the existing pcb, and hardwired the burnt traces.
    I have seen people wire the br3 rectifier off the board, maybe this is the case here.
     
  6. troyAmpallen

    troyAmpallen Member

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    Correction 200 a, 600 v is what in there (the new rectifier)
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    It looks pretty bad.

    Take out V3 phase inverter.
    Set voltmeter to DC.
    Turn amp power ON, set standby switch on "warm-up." Do not turn standby to "operate."
    Measure between pins 4/5 and 9 of V3 socket, read the DC voltage.
    Should be 6 volts DC or higher, indicating that BR3 is functional.
     
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  8. Gunner64

    Gunner64 Well-Known Member

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    That's Pretty much what I had to do to mine. I used an 8 amp (you meant 8amp..not ohm right?) Just like yours but wired it on top and bypassed the traces with buss wire..quite a bit neater than yours,:) but basically the same deal.
     
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  9. Gunner64

    Gunner64 Well-Known Member

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    The p.i. tube heater isn't rectified, or fed from br3..
    The mod you have is a remedy for a known defect in the sl-x..an under spec'd factory rectifier... and is pretty common. No offense intended, but yours is done in a fairly sloppy manner however. I would clean that up some if it were mine.
     
    Last edited: Mar 3, 2019
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  10. troyAmpallen

    troyAmpallen Member

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    Ok, I did as you advised and dc volts is 7.54 on both 4/5 to 9
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    When you measure between 4 and 9 what is the DC voltage?
    Red probe on 4, black probe on 9.
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    [​IMG]
     
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    I just wanted to make something very clear:
    It is not necessary to install an over-sized rectifier.

    The amp worked for decades with the stock sized rectifier - you don't need to modify it or over-size it.

    Why the rectifier goes bad: the filter capacitors get too old and draw too much current....OR you have a bad preamp tube.

    Over-sizing rectifiers: this is something that is done by inexperienced people who do not know how to troubleshoot the actual problems.
    Don't install over-sized parts.
    Don't trash your circuit board.

    This type of modification is reprehensible slop. DO NOT allow anybody to do this in your amplifier.
    The circuit board is DESTROYED by a hack....for no real reason...and this is totally unnecessary......for no real reason, what so ever.

    A. Replace with stock sized rectifier.
    B. Replace filter capacitors with good quality.
    C. Allow a PRO to do the soldering.

    [​IMG]
     
    Last edited: Mar 13, 2019
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    What is the result of the tests above?
     
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    There is absolutely nothing wrong with the factory rectifier.

    When somebody tells you to "over-size" the parts: RUN AWAY.
     
  16. Gunner64

    Gunner64 Well-Known Member

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    It's been adressed over, and over and over...it is under spec'd in the slx..mine had 3 tubes rectified by the factory part, which was a 1a part. In this case a larger value part IS the fix.
     
    Last edited: Mar 13, 2019
  17. ampmadscientist

    ampmadscientist Well-Known Member

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    How did the amp work for 30 years with the "wrong part" installed? :lol::lol::lol:

    You guys really think you are better designers than the engineers at the Marshall Factory? I doubt it.
    This is where I really have to question the logic behind doing this type of modification. :scratch::scratch::scratch:
     
    Last edited: Mar 13, 2019
  18. Gunner64

    Gunner64 Well-Known Member

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    Whatever dude. Every time the slx comes up you have the same thing to say..But you don't offer an alternative solution..This is the fix.
    I didn't say I was a better designer than Marshalls engineers. Marshall makes mistakes. Look at the 2KDSL.
    The amp worked for 30 years with the wrong part, overheating as it did until the pcb roasts so bad the traces go. Every one of these that I have seen (4) has had this condition.
     
  19. ampmadscientist

    ampmadscientist Well-Known Member

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    This is the alternative solution:
    1. Install a rectifier that fits the footprint on the board.
    2. Do not try to install a giant rectifier by hacking up the circuit board.
    3. (the real problem) replace the filter capacitors.
    4. Have a professional do all the repairs and soldering. (If you do not have the professional soldering equipment and skills)

    The board traces are cooked by heat: because the filter capacitors get too old and draw too much current.
     
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  20. Gunner64

    Gunner64 Well-Known Member

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    My circuit board wasn't hacked up to install a giant rectifier. It was damaged by the heat of the underspec'd br3. It is a known problem on the slx. Any other slx owners out there??? I have researched this up and down when I had problems with my slx, which was a recapped unit. If it were anything other than the rectifier itself things all on down the line would be burning up as well..not to mention blowing fuses. Later.:io:
     

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