I was playing and all of the sudden....

Discussion in 'The Workbench' started by matttornado, Nov 8, 2020.

  1. matttornado

    matttornado Well-Known Member

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    I did check all voltages at all tubes and they all were dead on to spec. more or less.

    yes i definitely need to reflow some solder joints. I’ll have to lift up the circuit board and check my work underneath. my amp is the 1st year with PCB so solder joints are underneath. I have, however, done some cut and solder from the top already.
     
    Last edited: Nov 11, 2020
  2. matttornado

    matttornado Well-Known Member

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    i reflowed solder joints that i could easily get to.

    I did the click test while checking voltages starting at V7. got pops when probing and all voltages and bias was good for all 4 power tubes. Next I continued on with the PI. got louder pops on pins 1/6 and 2/7. All voltages measured up and were in spec.
    Got all the way to V1 with the loudest pops on pins 1/6 and 2/7!

    After that I chop sticked every component hoping to hear a sudden increase power put nothing! I know something must have failed because i heard the sound suddenly die like something broke as i was playing. it was like someone turned the volume down to barely on.

    time to hit the books! it’s probably something so dumb and easy to find.
     
  3. matttornado

    matttornado Well-Known Member

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    i still have to lift the board up by removing all the pots and input jacks.
     
  4. KraftyBob

    KraftyBob Well-Known Member

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    I have a similar situation as you. I bought a non-working Tweaker 15 to fix and re-sell and it has a similar problem. The volume is really low, overly distorted and this one cuts out too. The OT was bad so I replaced that so at least I have sound now, but it’s not a good sound lol.

    I’ve done every measurement possible, poked and prodded every component, replaced all the tubes, reflowed all the tube sockets, etc., and still can’t find the problem.

    My next step will be to do a signal trace with a waveform generator and oscilloscope. However, I put this amp on hold as I need to setup my workbench in the basement as I haven’t done that since our move. I’ll probably get this done in the next few weeks and will update here if/what I find.

    The other thing I’m going to do as long as I have the amp apart is replace all the electrolytic caps. One is starting to leak, and for about $40 I can replace them all. I know this cap is not the problem, however, as this one is in the preamp section and that works fine. My problem is in the power amp section.

    Good luck with your repair
     
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  5. matttornado

    matttornado Well-Known Member

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    thanks for sharing. please report back. i will as well.
     
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  6. Gunslinger1

    Gunslinger1 New Member

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    Check the rectifier. It is probably solid state and is only about a $40 fix. I have a 2008 JCM 2000 DSL 401 ( the original series) made in England. I had the same issue about eight years ago. Put new tubes in, new speaker, new speaker harness. Would be fine for awhile but would go out again. Spoke with a tech at a different shop and said those amps are notious for the rectifiers going out. Called my tech and he had just come to the same conclusion. Don't use the amp much but haven't had that problem again.
     
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  7. Crikey

    Crikey Well-Known Member

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    Clean input jack? Try different cables & guitars?
     
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  8. matttornado

    matttornado Well-Known Member

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    yep!
     
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  9. Travis398

    Travis398 Well-Known Member

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    old corroded input jacks can create a similar problem, cleaning may not restore the contact surfaces.

    If all else fails change the jacks.
     
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  10. matttornado

    matttornado Well-Known Member

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    Found the problem. Loose screen grid R on V7 along with solder joints that have become soft and re-cooled. there’s signs of that on all of the output tube’s screen resistors. also, the yellow wire connecting the grid resistors at pin 6 were cooked at the tube socket pins, again this is on all power tubes.
    then i found the 56ks compromised on my screen filter caps.
     

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  11. matttornado

    matttornado Well-Known Member

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    here’s another picture F379DDBC-93B5-490B-8E68-CEC411D7A4D6.jpeg
     

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  12. pedecamp

    pedecamp Well-Known Member

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    My question is why? What caused all this problem?
     
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  13. mickeydg5

    mickeydg5 Well-Known Member

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    So you are Jack Whiting off , are you?
     
  14. neikeel

    neikeel Well-Known Member

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    What makes you think the balance resistors on your screen filters are compromised? I see QC die remnant and a solder burn on the wire - that is all.

    It would be unusual for a rectifier issue to cause this issue - you say all your voltages are dead on.

    A dodgy joint was the best outcome and dead/dying OT the worst.

    Does it work properly now?
     
  15. RickyLee

    RickyLee Well-Known Member VIP Member

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    I have had the screen grid resistors de-solder themselves on the tube sockets before. Good find and easy fix. Just put new ones on there while you are in there.
     
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  16. Seanxk

    Seanxk Well-Known Member

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    These Welwyn W21 1K versions are common around this period ( 69-75 ), interestingly they are actually only 3W too.
     
  17. matttornado

    matttornado Well-Known Member

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    Yes exactly! Well, the one 56K looks dodgy. It wasn't like that the last time I was looking around....
    Faulty Screen filter cap maybe?
     
  18. matttornado

    matttornado Well-Known Member

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    They were replaced about two years ago and are 7 watt 1k rated, not 3 watt.
     
  19. matttornado

    matttornado Well-Known Member

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    It's not QC die. those marks are new and look as though heat got to them. I guess the photo isn't the best. plus both measure open but I din't desolder one side yet to re-measure.
     
  20. matttornado

    matttornado Well-Known Member

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    I'm going to fix it this week sometime and take it from there.
     

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