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JTM45 vs Idiot

Discussion in 'The Workbench' started by _Steve, Jun 9, 2021.

  1. _Steve

    _Steve Well-Known Member

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    Was really trying to avoid posting for help but this has me totally stumped!

    I've been firing up my (metro spec) JTM45 clone for the 1st time. After investigating a very low/weak output I tracked it down to the phase inverter (V3). The PI is only outputting ~1V pk-pk and only on the RIGHT-hand triode (pins 1-3). The left-hand triode isn't outputting anything.

    I've tried isolating the PI by pulling all the other tubes (except the rectifier) and it still behaves the same with a test signal injected directly onto the left-hand grid. Ive also tried swapping out for a different 12AX7.

    Ive checked and double checked the components and they seem to be correct. I've also measured the resistance of the grids & cathodes to ground and get exactly the values I expect based on the schematic.

    Here is the schematic for reference:
    66JTM45Schematic.gif

    Here are my voltages. Note how the plate of the left-hand triode (pins 6-8) is at ~ full B+, and the right-hand side is at a more expected voltage. I have double checked that the pin is wired to the correct node. Also the grid voltages are different between each triode and I would have expected them to be the same:
    20210609_171806.jpg

    Here is a pic of the board. Note the NFB has been disconnected eliminate it as a cause:
    20210609_170059.jpg

    Any clues in there? Something else I could check? I really hope im missing something really basic and stupid.
     
  2. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Your plate resistor looks fried, by how dark the wire leads are, maybe?

    (the 82K)
     
    Last edited: Jun 9, 2021
  3. Chris-in-LA

    Chris-in-LA Well-Known Member

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    That also looks like a possible cold solder joint at the end of that dark lead.
     
  4. myersbw

    myersbw Well-Known Member

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    On the pair of 220K that feed the bias voltage...that "yellow" band looks fairly orange. Pull all your tubes and measure each resistor after the caps are drained. (Including the 82K...all of them in that circuit.) The dark leads of the 82K might just be severe oxidation? I usually take an Exacto knife and scrape leads lightly on every component. Seconds of extra time, but SO worth getting solid solder joints. And, that silver mica is 47pf and not 470pf, correct? ) Never hurts to check...

    Also, I'm not a fan of wire soldered from the bottom of a turret board because you can't see it. Position a dental mirror to check the underside for any wire drop-outs that might've happened when the top components were added.
     
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  5. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Check the value of each resistor. Be sure they're in spec.
     
  6. neikeel

    neikeel Well-Known Member

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    With those voltages and your pics I suspect dry joint on that nos plate resistor.
    Suggest polish ends and reflow.
    The supply rail looks ok as pin 6 voltage is good.
     
  7. neikeel

    neikeel Well-Known Member

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    Oh and btw ods in a JTM45 - interesting choice
     
  8. _Steve

    _Steve Well-Known Member

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    Thanks all, ill try these suggestions this morning. I'm skeptical it's a cold solder, but we'll see! :)
     
  9. _Steve

    _Steve Well-Known Member

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    I replaced both the plate resistors and snubber cap, flowed it with plenty of solder but unfortunately its the same symptoms.

    Any other ideas? Im thinking it has to be an incorrect connection under the board but ive checked that as best I can from the top by measuring resistances and continuity where I think there should be...

    20210610_103938[20651].jpg

    (fyi I left the snubber off and jumpered one in to double check)
     
  10. PelliX

    PelliX Well-Known Member

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    How about the resistance between the pins of the PI socket to ground, anything surprisingly low there? Any excessive current draw or heat when fired up?
     
  11. william vogel

    william vogel Well-Known Member

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    Pin 6 voltage is way high. It should be close to pin 1 voltage. Measure at the junction of the 82k/100k plate load resistors. It’s probably 340 whatever volts. Sounds like the 100k resistor is not 100k ohms.
     
  12. _Steve

    _Steve Well-Known Member

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    Yeah thats what I thought but I double checked them and even just swapped them out with brand new resistors. I think the underlying issue is the triode is not conducting therefore there is no current flowing through that resistor and hence no voltage drop. WHY, however is another question.

    Im going to try some of the other suggestions now and look again at the bias diff which is quite large..
     
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  13. Pete Farrington

    Pete Farrington Active Member

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    Are the female contacts in the valve socket ok?
    It’s weird how the pin7 voltage is higher than that at pin2.
     
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  14. _Steve

    _Steve Well-Known Member

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    That was a great suggestion and I was hopeful but alas the pins are fine :(
     
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  15. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Check the PI heater connections.
     
  16. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Check the tube socket. Check that every pin is wired correctly, too.
     
  17. neikeel

    neikeel Well-Known Member

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    Good suggestions

    the only under board wire relevant here is the blue wire to turret. I’m sure the voltage on the other plate is high because the other plate is not pulling any juice.
    What happens if you clip a croc lead between the 82k close to the body and the pin ie bypass the blue wire and turret?
     
  18. StuC

    StuC Member

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    Pin 1 seems low. Are you sure the HT isn't wired to the pin 6 end of the 100k? What voltage do you have on the turret with the 82k and 100k?
     
  19. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    It looks like the two green grid wires are reversed. Pins 2 & 7. But that may be the angle of the photo. might be worth checking.
     
  20. _Steve

    _Steve Well-Known Member

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    Finally found the issue!!

    I noticed that the tube wasn't heating up as much as the other tubes. The left hand triode finally started conducting when I reflowed the solder on its heater pin. This is despite me testing and re-testing each socket pin with a probe on BOTH top and bottom.

    Thanks for all the help and suggestions. Lesson learnt about cold solders!
     
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