Jcm 900 2100 Slx - Power Board Burnt At 50/100w Switch

Discussion in 'The Workbench' started by magnetbox, Jan 14, 2019.

  1. magnetbox

    magnetbox New Member

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    Hey everyone, I'm looking for any advice / experience dealing with this problem, since it seems (on the surface) like a common issue. (yes I know how to discharge caps and have done some minor board repairs...)

    I picked up a 2100 SLX (100W) cheap a few weeks ago in non-working condition, took a gamble on it. Came with a full retube (5881 power section). Immediately changed out a blown 4A mains fuse and put the correct 500mA fuses on the tube failovers (these were also 4A!).

    It fired up and sounded good— put about a half hour of play time on it. Other than scratchy pots the only thing out of the ordinary was a burning smell that came around as the amp got up to temperature. No smoke thankfully. Pulled the chassis and found a burn mark and some leads missing(?) near the 50/100W switch:

    IMG_8567.jpg

    Did some research and this seems like a common problem, probably from someone toggling that switch with the power on, right?

    From the schematic it looks like RD1 is absent (behind the board probably) and WH2 is hanging on by a thread... these go to the output tube plates. While testing the amp I did try both 50 and 100w switch positions (YES I powered down beforehand!) and both seemed to work-- not sure if there was any noticeable difference in sound so the switch might be bypassed. I'm expecting to see either a shorted wire or a crude repair when I pull the power board. Ran out of time but that's next on the list.

    Anyhow, any insight / advice or sympathy is appreciated. Thanks!
     
    Last edited: Jan 14, 2019
  2. magnetbox

    magnetbox New Member

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    Ok! Finally had a moment to pull the board and look below. It's pretty rough. Think this is worth fixing, or is it toast? I'm fine with taking it to a professional, but an hour of bench time is about $80 and I don't want to throw my money away if it's a lost cause.

    The lead to the plates, RD 1, is flopping around loose. Yikes.
    burn_bottom_board_1.jpg
    More damage on the bottom of the board:
    burn_bottom_board_2.jpg
     
  3. Adieu

    Adieu Well-Known Member

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    Destroy the PCB leads to them and replicate damaged portions with point-to-point or on breadboard or something
     
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  4. Matt_Krush

    Matt_Krush Well-Known Member

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    Think I'd be making a project build out of it.
     
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  5. magnetbox

    magnetbox New Member

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    Yeah, if all else fails I'll keep the shell / chassis / trannies and build something, maybe a 2203 clone? Or I could probably part it out on eBay / whatever and break even on what I paid for it.
     
  6. anitoli

    anitoli Well-Known Member

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    First thing you need to do is find out the reason why this happened.
     
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  7. magnetbox

    magnetbox New Member

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    Agreed, I'm a novice so I'm sort of collecting clues from around the internet. Arcing caused by impedance mismatch? My first thought was that the switch was toggled by the previous owner while powered on, which everyone says is a no-no.

    Have also read that the switch itself is under-spec (or close) for the voltages.

    Looking a little deeper at the schematic, it looks like all the switch does is tie the screen grid to the plate voltage to make them run as triodes. Since one lead (RD1) is disconnected and the joint on the other (WH2) is not looking too healthy then I think that my switch is essentially bypassed right now, and my amp is stuck in pentode mode... which would be fine with me. Would it be unreasonable to just clip these two leads from the tube sockets, put some tape over the switch and call it good? :fingersx:

    Screen Shot 2019-01-19 at 1.09.06 PM.png
     
  8. anitoli

    anitoli Well-Known Member

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    Check continuity across the switch in all positions. That appears to be a flyback burn.
     
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  9. magnetbox

    magnetbox New Member

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    Switch seems to be functioning properly.

    Forgot to mention that the power tube fuses-- neither blown-- were 4A when I took ownership (I replaced with 500mA before I ever turned it on). The fuse holder is just to the left of the burned area (looking at the bottom) and the burn seems to reach one of the fuse pins.

    The 100k resistor right before the fuse LED is totally disconnected now 'cause the trace is missing behind one leg. Hard to tell if this was electrical or just a byproduct of the arc / fire.

    One of the screen resistors seems a bit wobbly too, going to touch up that joint and poke around more.
     
  10. anitoli

    anitoli Well-Known Member

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    Did you check for continuity across the fuse holder?
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    Yes have seen that happen before.
    Yes have seen that happen before.
    It will cost a very large amount of money to fix that.
    The rear board and the sockets are FUBAR.
    May as well re-build the power amp from scratch, point to point instead.

    Pay a tech to fix that? It's not worth it.
     
  12. magnetbox

    magnetbox New Member

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    Yeah, fuse holder seems to work as normal. One thing I noticed before powering it up again was that one 500ma fuse had blown the first time I ran the amp, though nothing cut out. Hmm. Given, that was before I knew I had any loose wires floating around inside the amp! So hopefully just the result of a short.

    I removed the last lead and taped off the two disconnected wires with electrical tape and scraped as much carbon off the board as I could without disturbing anything. Then I replaced the blown tube fuse, set the switch to 100W mode (where the leads would normally be disconnected mechanically) and fired it up. Played the amp for the better part of an hour with no obvious signs of trouble, no noticeable burning smell. So I've got a mostly-functional amp, as I see it, but certainly not "fixed" in the proper sense.
     
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    Holy sheep shit, you played it?

    Yeah I know a guy who has a 900 with a hole burned through the board and his still works too.
    He keeps playing it although I told him not to.
     
  14. magnetbox

    magnetbox New Member

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    Ha! Am I risking grave bodily harm? I do keep a fire extinguisher handy...
     
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  15. ampmadscientist

    ampmadscientist Well-Known Member

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    "I do keep a fire extinguisher handy..."

    Yes I have recommended that before, to others.
     
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  16. Gutch220

    Gutch220 Active Member

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    why not try to fix it?
    just wire everything P2P in that area, no?

    Why do you think caused this to happen? Maybe the switch was "in-between" settings?
     
  17. ampmadscientist

    ampmadscientist Well-Known Member

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    A speaker mismatch will cause plate or screen to arc to filaments or ground. And so the plate and screen are both wired to the switch.
    They probably arced to a ground on the board.

    I might say maybe that somebody spilled a drink into the amp, but I have seen this same thing happen before so I know it was not a drink that caused it.

    All the charred wires and burned sockets will have to be replaced, and all the black carbon will need to be cut out / removed.
    It is possible to fix it maybe, but a butt load of work for sure...

    Yes I am probably crazy enough to try to fix that.
    Rewire it all point 2 point and get rid of the board entirely.
     
    Last edited: Jan 22, 2019

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