JCM 2000 TSL Lights Up Current Limiter

Discussion in 'The Workbench' started by 12AX7, Jan 13, 2020 at 4:45 PM.

  1. 12AX7

    12AX7 New Member

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    I have a JCM 2000 TSL I just installed the Dr Tube Stable Bias Mod, new bias trim pots and a Classic Tone choke into. While testing I'm plugging it into a current limiter that I built out of a 300 watt light bulb and receptacle like the one Uncle Doug uses on his youtube channel. When I turn the power switch to the amp on the power switch light lights up and the bulb on the current limiter glows (very dimly just the filament in the bulb glows slightly) then after a couple mins on standby I take it off standby and it burns a lot brighter and then the power light on the amp switch goes out after a few seconds. Things I did to recheck my work... I took the mother board back out of the head and found that pin 5 (that I had isolated from the motherboard) on 3 of the sockets had pulled almost out of the sockets and was almost protruding out the top side. So I pushed them back into place and twisted them like the others so they wouldn't pull through the slots again when removing the tubes. I checked out all my solder connections on the bias board I built with a continuity meter to be sure none were bridged and they all passed. I double checked all my wire connections to the bias board mod and they were in the right places and none bridged. I had already soldered in a deans connector (all I had on hand) on the wires of the choke so I could unplug it to do future servicing/modding ect.. ,so I unplugged it from the choke and alligator clipped the R71 resistor to the other end of the deans connector so it was back into the circuit to test and it still lights up. I unplugged all the tubes and tried it and the bulb doesn't light up, so then I put the 2 outside tubes in and it lit up removed them and put in the 2 inside tubes and it lights up. The tubes I'm using are the old ones the amp had in it before installing the bias mod (they haven't had much use on them at all since this is a spare amp) I do have a new set of Groove Tubes but don't know if I should try them yet or not don't want to damage them. What should I try next?
     

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    Last edited: Jan 13, 2020 at 4:51 PM
  2. neikeel

    neikeel Well-Known Member

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    Did you install the bias mod prophylactically or because it was playing up with bias drift. If you have had bias runway and red-plating you may find that they are the issue.
    A way of checking the board and amp is to leave the tubes out and fire the amp up on standby and check that you have negative grid volts of around -45vdc and the plate voltages (pin 3) of 480vdc and screen volts of around 475v (all approximate).
    That will reassure that you have the correct voltages to risk your new tubes.
     
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  3. 12AX7

    12AX7 New Member

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    Thanks neikeel! The head did have a run away bias issue. It red plated the set of tubes that were in it before this set. This set Ive never seen red plate but my son had the amp for a little while they could have red plated with him. I got the head back and ran it while checking the bias and they ran away pretty bad never red plated for me though. Help me understand this test procedure correctly , Check for -44vdc on pin 5? Check for 480 vdc on pin 3. Which pin for the screen volts? I'm not sure of the pinout of the tube I'll be going by the numbers on the sockets. Thanks again.
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Need to go through and make measurements.
    Don't put the tubes in or try to play the amp until everything has been verified.

    I. TSL DSL 50 100 board insulation test

    Un-plug power from the amplifier. Disconnect Mains power entirely from the amplifier.

    Set meter for ohms. Set meter for highest ohm scale:

    Your scale may vary.

    Remove all output tubes.

    Connect red meter probe to pin 4 of the output tube socket.

    Connect black meter probe to pin 5 of the output tube socket.

    This is what you should read:

    Ohm Reading will climb up, up up. Then meter will read infinity (or "OL.")

    "OL" means: Open Load. No Resistance.

    This reading is a normal reading.

    Test all output tube sockets and make sure that you get the same normal reading as above.

    Fail Reading:

    The meter reading will not climb up, up up. (as above)

    The reading will stay low, and steady. It will not climb.

    Example: reading between pin 4 and 5 of the output tube socket is 658 ohms, steady and is not climbing.

    Example: reading between pin 4 and 5 of the output socket is 250K ohms and steady.

    This indicates the board may be bad.

    All output tube sockets must be tested as above. Be sure to test all of the of the output tube sockets.

    Write down your readings for each socket as you test.

    The board must be repaired or replaced (options) before any other tests are made.

    If the test reading above is Fail: stop here.

    Do not install tubes or play the amplifier. This could damage new tubes or cause more damage to the amplifier.

    A. Replacement Board is available.

    B. Dr. Tube stable bias modification kit : which will repair the existing board.

    Next Step: bias stability test. Do not skip this test.

    TSL DSL 50 100 Bias stability test

    Part I

    Take out all output tubes for this test.

    Set meter for DC volts.

    Connect DC volt meter black probe to metal chassis ground, use clip lead if possible etc.

    Connect red probe to pin 5 of output tube socket.

    Turn on power, but leave standby switch in warm-up position.

    Do not turn the standby to operate position.

    Note DC voltage of pin 5 for each output tube socket, test all output tube sockets.

    Write down the DC voltage reading for each socket and keep track of the readings.

    Part II

    Adjust bias trim controls and observe DC voltage on pin 5 of output tube socket.

    You should be able to adjust the two trim pots for -42 volts DC on all output tube sockets pin 5.

    The voltage should hold steady adjust up and down smoothly for both bias adjust pots on all output sockets.

    If the DC pin 5 bias voltage is missing, or if the voltage will not adjust up and down smoothly,

    replace the bias pots and or repair bias power supply and retest for correct operation.
     
  5. Lance Chambers

    Lance Chambers Well-Known Member

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    Throw it in the dumpster! :applause:
     
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  6. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    No, no, first you test it. Then you throw it in the dumpster.
    We have procedures to follow.
     
  7. 12AX7

    12AX7 New Member

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    Tested all 4 output tube sockets with a fluke 87III on Mohm setting and get around 0.500 Mohm on each. it will continue to shed a number every 10 seconds or so after reaching this point. on the kohm settings they all show OL right as soon as I plug the tester into the sockets no count down or count up at all..
     
  8. 12AX7

    12AX7 New Member

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    Tested pin 5 voltage on all output sockets results are (looking at the tubes from the backside of chassis starting with closest to pre amp tubes) -42.0vdc,-042.0vdc,-028.46vdc,-028.46vdc. Trim pot to the set of tubes mesuring at -42vdc has good adjustable range trim pot to the last two tubes has almost no effect range is -28.45 - -28.49 both of these trim pots are new I just installed them and this one appears to be bad.
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    OK so you can't operate the amp until you get -42 volts DC adjustable to all 4 output sockets pin 5.
    Don't put the tubes in.
    Maybe there's a broken circuit track where the bias pot is soldered to the board?

    Keep testing with the standby set to "warm-up," until you get the bias to adjust.

    Then there is more tests after you get that working.
     
    Last edited: Jan 14, 2020 at 6:45 AM
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  10. Adieu

    Adieu Well-Known Member

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    Uhm.... i'm not sure WHAT a 300w (?) current limiter should or should not indicate, but fyi a "100w" (output) JCM 2000 TSL is rated as a 375w appliance for power consumption
     
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  11. neikeel

    neikeel Well-Known Member

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    As above really.
    You have an issue with one of the new bias pots, it’s installation whatever.
    Need to pin down where so check the tracks, the multi pin connector and the pot itself.
    in the original post you said you replaced the bias pots (was this a new board plugin or did you unsolder and drop in new ones ?). The pots are not very robust and the possibility of heat damage or even dry joint should be considered before retesting.
     
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  12. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    The bias controls the current of the amplifier.
    When the bias voltage is wrong on pin 5, the tube will draw mega-current.
    And that is part of what is lighting up the current limiter ( it's too far over current).

    Because of mega current condition, the one ohm output tube cathode resistor will blow...
    It has an avalanche affect, one thing causes the next thing to fail.

    So that's why when you test this stuff remove the output tubes, don't turn the HT on, etc...
     
    Last edited: Jan 14, 2020 at 9:00 AM
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  13. Chris-in-LA

    Chris-in-LA Well-Known Member

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    A lightbulb limiter will glow dimly on standby and more brightly when standby is put in the ‘run’ position. That’s not a problem. It’s when the bulb glows really bright, that’s when you have a problem. It might be normal that the light went off on the switch, due to lower voltage in the system. The amp will not run right plugged into the limiter.
     
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  14. 12AX7

    12AX7 New Member

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    I soldered the new pots in. I'm going to take the chassis back appart latter today and see if I can fix this issue. Thank you all for the help!
     
  15. 12AX7

    12AX7 New Member

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    Ok changed trim pot and I got -42vdc on all 4 output tubes. Whats the next test?
     
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  16. scott-colorado2016

    scott-colorado2016 Active Member

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    Off topic, but not...
    I've seen this term "dry solder joint" for the first time ever over the past week in several posts. I've been soldering for 35-40 years and never heard it before. Are we talking about a cold solder joint or one that has not been soldered? There have been numerous vintage amp repair/refurbishments done by the likes of Uncle Doug and the Guitologist where there was a resistor that was wrapped but never got soldered, so I know in the hand-made days that happened. Is that what "dry joint" refers to?
    Honest question, thanks.
     
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  17. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    probably depends on what country you're from (& translation), but, I believe, cold & dry mean the same thing...
     
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  18. 12AX7

    12AX7 New Member

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    Good question. I was wondering myself I assumed it meant cold solder.
     
  19. scott-colorado2016

    scott-colorado2016 Active Member

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    Makes sense. I think I saw Ken Underwood mention it in a post the first time I saw it. He's in the U.K., I'm from the U.S.A..
     
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  20. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    well, maybe they'll chime in & confirm (or deny)... :)...
     
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