JCM2000 DSL100 crackle at startup and other issue

Discussion in 'The Workbench' started by James Morris, Dec 11, 2019.

  1. James Morris

    James Morris New Member

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    Howdy fellas... newbie here.. I have a DSL100 on the bench, it's a 1998 but I think the tube board has been replaced sometime in the past. Its an Issue7 board with the resistors in place of two LDRs. ]The amp has a nasty crackle when turned on out of standby - it stops after a short time. I am thinking the first filter caps could be the problem. Any other advice would be welcome.

    The other issue is with the Classic Gain channel - the clean setting has very low volume compared to the crunch setting. I would be surprised if this is normal. Pretty tough to follow the schematic. but I guess it is either a relay, LDR or transistor that could be malfunctioning.

    I have recently replaced the power tubes with a fresh matched quad. The bias is working as it should although I think I will oreder up some 56k grid stoppers and replace the carbon film bias 220K resistors with 1% metal film r's. I had it idling for over an hour and it didn't drift but still going to replace them.

    The power tube sockets are in good condition and snug. The amp is pretty clean otherwise with only a pot and a couple of resistors haveing been replaced.. The resistors that were replaced are R55 and R56 on the 60-00 tube board..

    thanks
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    "I am thinking the first filter caps could be the problem."

    Yes it is true that sometimes rarely the first filter cap will cause a crackle.
    Try cleaning the sockets first. You can use lubricated non conductive cleaners. (such as LPS-1 Deoxit G-100 etc...)
    Second swap the tubes out with good ones one at a time. You don't need a whole set of tubes to do this. Retest each time you swap one out (when the amp is cold).
    Monitor the one side of a de- coupling cap for DC leakage after the amp is cold and the power first turns on.

    2. Freeze Spray: after the amp is warmed up you can freeze components one at a time. This may produce a crackle reaction if the components are not thermally stable. You may find that freezing resistors, caps, connections, etc could reveal a crackle.
     
    Last edited: Dec 12, 2019
  3. MickeyJ

    MickeyJ Active Member

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    Standby switch maybe?
    Seeing as you don't even need a standby switch on your amplifier at all for any reason, try leaving the standby switch on all the time and use just the power switch and see if it still crackles.
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Take out the matched quad of power tubes and put the old ones back in. Now re-test.
    Just because tubes are new does not mean that you should assume.
    Report the results.

    Don't replace any parts till you do more troubleshooting to narrow down the cause.
    Don't randomly replace any parts.
    Don't try to fix the problem based on guessing.
     
  5. James Morris

    James Morris New Member

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    The power tubes were replaced due to one that failed on my Hickock tube tester. The other three, while they still checked within specs for an EL34, they are not very strong. I do not replace power tubes indiscriminately. I mentioned it so that we can rule out the power tubes, if it crackles with two sets of tubes then..... All the 12Ax7s tested strong. All tube sockets have been cleaned with deoxit and checked for pin tension. With all the preamp tubes and PI removed and with stdby switch not in standby the amp powers on and then in around 15-20 seconds there is a loud scratchy crackle through the speaker. The scratchy crackle sounds like an arc or bad connection somewhere in the power supply. I checked the amp in total darkness and there is no visible arc anywhere and nothing unusual looking in the power tubes either. Some previous person has reflowed the solder joints on the power tube sockets, and left flux residue all over the place. Looking more closely at the pins it looks like a couple of them are still not properly soldered. I mentioned the two 330uf filter caps because the cap on top of one of them looks different than the other.. the one is dull like it has been heated and the other is shiney black which according to all the pictures I have seen of these boards is they way they normally look.

    with no preamp tubes installed and the problem still there - indicates problem in the power end of the amp.. more later thanks

    How about the low volume on the Classic Gain clean setting.... anyone.
     
  6. James Morris

    James Morris New Member

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    I'll post up the schematic links later today.

    I should also mention that when I first checked the amp out, the bias was quite high... going to 110mv+ per side, that's like 55mv or more per tube. So I adjusted it down right away to 87-88mv per side. Who knows how long the ELs were operated at high bias levels.. My thought was that someone may have popped a new quad in there without checking and adjusting bias.. or it had drifted up to that level over time..
     
    Last edited: Dec 13, 2019
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    OK I got it.
    disconnect power unplug mains from amplifier.
    Remove tubes.
    Set meter for ohms, highest ohms scale.
    Connect ohm meter probes to pins 4 and 5 of output tube socket.
    Ohm reading should climb up up up and then read infinity.
    The reading should now be "OL." Open Load. No resistance between pins 4 and 5.

    If the ohm reading does not climb up up up to infinity:
    Then the circuit track insulation of the board is defective.
    This is a wide spread known defect in DSL circuit boards.

    Do this ohmmeter test for all the output tube sockets.
    Write down the result for all sockets are report the readings here in the forum.

    Example:
    If ohm reading between pin 4 and pin 5 is "500Kohms steady," and does not climb up up up to infinity, the board is defect.

    The board may be repaired using the dr Tube update kit OR
    New circuit boards are available.

    If you are convinced take out the preamp tubes.
    Disconnect capacitors one at a time then retest when the amp is cold.
    Only 1 leg of the cap needs to be disconnected.

    Process of elimination.
    If there is a bad cap causing the crackle it will stop when disconnected.
     
    Last edited: Dec 13, 2019
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    This is a sign of a defective circuit board, classic symptoms.
    Do the tests in the post above to verify that the board is defective.
     
  9. James Morris

    James Morris New Member

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    Ohms testing between G1 and G2 on the power tube sockets the reading goes up to appx 1.7M then slowing ramps down to near 290K and never really settles. And all the output tube sockets are the same .


    If you are convinced take out the preamp tubes.
    Disconnect capacitors one at a time then retest when the amp is cold.
    Only 1 leg of the cap needs to be disconnected.


    not sure what you're saying here above.. If I am NOT convinced? I can check all the caps with an LCR meter .. Have to pull the tube board.

    what Dr Tube kit? The only one I see is to move all the bias components off the board.. ist that the one?
     
    Last edited: Dec 13, 2019
  10. James Morris

    James Morris New Member

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    OK once you disconnect three capacitors...two pi coupling caps and C33 lytic,, then you can measure between EL34 socket pins 4 & 5 successfully. I then sat and watched the resistance rise as it charged up the PS filter caps and finally read infinite. So, there is no insulation breakdown of the FR3 substrate around those pins.. the removed caps tested good.
    I do however still have the distorted crackle sound on startup. The bias voltage is holding rock steady.. I am leaving the amp on and monitoring it for a while longer but don't anticipate any change.


    any other ideas would be appreciated especially the low volume I mentioned on the Classic gain channel
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    So far your board tests bad.
    There is no need to test filter caps at this moment.

    1. The board can be repaired with the Dr. Tube kit.
    https://drtube.com/en/modifications/jcm2000-stable-bias-mod
    Or the board can be replaced. There is replacement boards available.
    The insulation breaks down between the pin 5 control grid and the pin 4 screen grid. Eventually high voltage is bleeding into the bias voltage.
    The board will arc and burn between those 2 pins on the output socket (presumably the crackle you hear is this voltage arcing between the pins).
    The output tubes become UN-stable or just go red full time.
    This normally may take 30-60 minutes after the high voltage is turned on But, there could be many variations of the same problem.

    2. The bias adjust pots should usually be replaced. There is a replacement bias board with the pots already mounted.
    The bias pots go bad in 8 out of 10 amps we worked on.
    The symptoms are basically that the bias will not respond to the adjustment, or the tube is already red plating.
     
  12. James Morris

    James Morris New Member

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    Bias v is steady on both side.. as I stated prior .. Crackling distortion turned out to be the effects return jack.. replaced and fixed..

    as for the Classic Gain channel.. i got no response here.. so I'll figure it out myself..

    thanks
     
  13. James Morris

    James Morris New Member

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    Update the bias started acting up again. Took advice and ordered a new main board Issue 20 and installed it. Amp works fine now..

    Thank you for the help
     
  14. Lance Chambers

    Lance Chambers Well-Known Member

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  15. ampmadscientist

    ampmadscientist Well-Known Member

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    What was the bias doing exactly?
    Any information would help diagnose other amplifiers....
     

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