Jcm 2000 Tsl 122 Bias Help

Discussion in 'The Workbench' started by Sapphire Sounds, Apr 13, 2019.

  1. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    unplug the internal speakers from the head. There's a plug, right? Test that...

    upload_2019-5-7_14-41-28.png
     
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  2. ampmadscientist

    ampmadscientist Well-Known Member

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    You didn't follow the directions.
    You don't have good data.
    Go back and start over again and this time get all the readings.

    If it tested -36 volts the circuit board could be bad.

    Take out the tubes. Turn power on, turn standby to "warm-up."
    What is the voltage on pin 5 of all output sockets?
    Adjust bias pots for -46 volts DC on pin 5. If it won't adjust to -46 VDC the board could be bad.
    If it won't adjust to -46 DC volts stop there.
    Don't turn the standby to operate position.
     
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  3. ampmadscientist

    ampmadscientist Well-Known Member

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    We will take ALL the time we need to get this amp right.
    Do not put the tubes in yet.
    Do not play the amp yet.
    Concentrate efforts on getting the bias right FIRST, before you put any tubes in.

    Do not try to skip the testing. Make sure it's 100% FIRST.
     
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  4. Sapphire Sounds

    Sapphire Sounds Member

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    I used 200 OHM on my tester

    The reading was around 140 ohms
     
    Last edited: May 9, 2019
  5. Sapphire Sounds

    Sapphire Sounds Member

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    I used 2M OHM on my tester

    The reading typically jumped to 580 and started climbing for all four of them
     
  6. Sapphire Sounds

    Sapphire Sounds Member

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    I set the meter to 500V DCV

    The readings were -48 for two on one side and -49 on the other two this time in standby warm up when I checked asap.

    I adjusted the bias pots down and the reading is -46 for all four. It climbed to -47 on all four so I adjusted the bias pots to get all of them down to -46 again. I left it alone for fifteen minutes and just checked and got -50 on all four.

    20190509_162654.jpg


    I set the meter for 500 DCV and turned the pots while watching the voltage readings on the meter. Each side was starting above -50 and had a smooth number sweep going up or down.

    The bias pots adjust the opposite side that they are on. I was not able to bring them all down to -46 volts but got them all to -48 with both fully turned down, this is after it had been climbing for 25-30 minutes now.

    My tester has just run out of battery I believe..
     
    Last edited: May 9, 2019
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    A. Did you install the Dr. Tube bias Mod board?
    B. Did you replace the bias pots?
    If you didn't replace the bias pots, go ahead and do that.
    The bias should not be drifting, the pots are shot probably.

    The bad circuit board causes the pots to fry out.
    The B+ voltage is bleeding into the bias circuit, and it cooks the bias pots.

    There is another test if you didn't install the Dr. tube bias board.

    But don't put the tubes in, or try to play the amp until you have adjustable / steady bias voltage on pin 5 of all the output tube sockets.
    The bias voltage (on pin 5) should adjust up and down, then hold steady (no tubes installed).
    If it keeps drifting around (it's drifting now):
    With the standby in warm-up, it should hold steady and not drift after you replace the bias pots.

    But don't turn the standby to operate yet....prove that it's all 100% and steady. Prove that it holds -46 volts DC on pin 5.

    At this point: do not put the tubes in.
    Do not turn standby to operate position.
    Do not attempt to play the amp.
     
    Last edited: May 9, 2019
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  8. Sapphire Sounds

    Sapphire Sounds Member

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    A. I did not install the Dr. tube mod
    B. I did not replace the bias pots

    Okay I am looking at what I'm going to order for the amp so I can get it all at once
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Turn power off, unplug mains power from the amp.
    The power is UN-plugged?

    Take tubes out.
    Set meter for ohms.
    Set meter for 1 Meg scale, or highest scale if it does not have 1 Meg.
    Black probe connect to pin 4 of output tube socket.
    Red probe connect to pin 5 of output tube socket.

    Reading of ohms should climb up up up. (this is normal)
    If reading stays low, and does not climb, (example 640 ohms steady) there is a bad spot on the circuit board.

    Test all output sockets pin 4 and 5 with the ohm meter.
    Write down the result for each socket.
    Report the readings to the forum.

    If you DO have the bad spot on the board:
    you need to replace the board, or install the bias mod kit.
     
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  10. Sapphire Sounds

    Sapphire Sounds Member

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    Okay meter was set at 2 Meg OHM

    Tested all sockets pin 4 and 5

    All power tube sockets climbed up up up and started out around 580
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    OK replace the bias pots and see if it holds steady.
    It doesn't look like the board has any shorts...
    Do you live in a humid climate? Like New Orleans? Houston? Honolulu?
     
  12. Sapphire Sounds

    Sapphire Sounds Member

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    Cape cod MA.. Sometimes humid but we get all kinds of weather here

    I came up with a list from reading all about TSL 122 mods...

    Id like to do what I can for the amp, I have not taken it apart to see the boards yet but when I do I will take clear pictures to identify which components it has inside and go through with whatever is recommended and agreed upon

    --------------------
    TSL 122
    --------------------

    Replace the bias pots?

    Speaker ground fix

    Test the FX loop switches (SW3)?

    Test FX loop jacks?

    The resistors for bias circuit and grid blockers need replacement and other bias circuit components

    220k control grid resistors to 5K6 ohm

    (Grid blocker resistors R7,R10,R66,R70 and others)

    R7 R10 R66 R70 to 5K6 ohm instead of 220k (6K8?)

    R66 and R70 replace with 5K6 metal film resistors

    R7 and R10 replace with 5K6 metal film resistors

    Lift one leg of: output of cap C6, R7, R66, R67 to surface wire instead then glue gun secure (do it for the other side?)

    R68 33K, R77 10K, R67 220K, R69 220K upgrade to 1 watt metal film resistors

    Replace resistors R69,R77,R67 with 1w metal film

    R69 change from (22K to 39K)?

    R76 to carbon film resistor if carbon composite?

    Remove all power tube sockets, drill out pin 5 of the control grid so it clears the PCB/pin of the tube socket when reinstalled then point to point the bias section and coupling section with new and correct value resistors (point to point with 1 watt resistors?)

    C6 and C7 replace with 22nf 630v coupling capacitors

    C36 and C37 replace the 100n 63v capacitors with 100n 250v capacitors

    C42 bias supply cap replacement

    C43 and C44 replace the 22uf non polar 50v capacitors (connected to W11, W13 and CON14)

    C46 (Its the 22uf 500K cap bridging the pins at R8) change for a 1000w 22uf or 1500v or 2Kv ceramic disk capacitor or replace C46 with 9 inches of open terminated RG58 (25pf resistance better in voltage and heat than a small capacitor)

    R2 R3 R5 change 82K and 100K splitter resistors for 2 watt type and mount in the air

    Heater current enamel scrape to add wire for the "food for tubes mod"

    Desolder R78 and R8 and solder to the sockets at valve V5?

    R62 1K B+ dropper to 10k (maybe not)? (W5 blue?)

    R6 check it out for damage aswell as R3-R4

    Remove R4 680K resistor completely?

    C38 (Samwha?) and C39 possibly domed/damaged replace?

    --------------------

    Electronic components

    --------------------

    22K linear Marshall bias pot x2

    5K6 ohm metal film resistors x4

    33K 1 watt metal film resistor

    10K 1 watt metal film resistor

    220K 1 watt metal film resistor x2

    39K 1 watt metal film resistor

    R76 carbon film resistor replacement?

    100n 250v capacitor x2

    22nf 630v coupling capacitors x2

    22uf 50v non polar capacitors x2

    22uf 1000w ceramic disk capacitor
    /
    22uf 1500v ceramic disk capacitor
    /
    22uf 2000v ceramic disk capacitor
    /
    9" open terminated RG58?

    82K 2 watt splitter resistor

    100K 2 watt splitter resistor

    R62 10K B+ dropper (W5 blue?)

    C38/C39/C42 replacements if needed

    1 watt resistors for point to point??

    -------------------

    I have no idea where to get alot of those components and which ones I should get if any/all to improve the amp. Thank you so much for the patience MarshallForum, I purchased a modern multimeter but I am awaiting opinions before placing any order on electronic components..

    Update: I found revision 20 TSL 122 boards in stock finally... When the new multimeter comes in I plan to further test my current board then take it out to get a good look at what it has on it and share my discoveries..

    Screenshot_2019-05-09-22-19-29.png
    Screenshot_2019-05-09-23-55-29.png Screenshot_2019-05-09-23-56-07.png Screenshot_2019-05-09-23-56-26.png

    ----------- service breakdown

    TSL 122 Dr.tube mod board shipped $58.27

    TSL 122 bias pots shipped $12

    5.6K ohm 1 watt metal film resistors shipped $8.30

    (Saves money at half cost, uses time, could snowball, tedious to avoid damaging materials)

    ----------

    TSL 122 revision 20 board shipped $158

    (Saves time but cost is twice or more, high quality board should last decades, has also needed the dr tube modification installed in some cases)
     
    Last edited: May 10, 2019
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    I am not sure that all those parts truly need replacing.
    Also the board is very easily damaged by soldering.
    I think I would get the bias steady and see if any of those things is really wrong. (most of them probably won't be wrong to start with)

    I think this may be going over-board.
     
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  14. Sapphire Sounds

    Sapphire Sounds Member

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    Screenshot_2019-05-09-23-14-16.png

    Probably is going over-board lol, but I have a feeling my board has got that bunch of temperature sensitive resistors of the wrong value that should be corrected... When I put pictures up we will know what the best solution is in this JCM 2000 TSL 122's case, whether I must replace the full board or get moving on soldering in replacement components.. It will live on
     
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    You got 220K screen resistors? really? Are you sure about that?
    I think he means grid resistors.
    The grid is supposed to be 5.6K not the screen.

    The screen will be 1K / 5 watt for EL34.
     
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  16. Sapphire Sounds

    Sapphire Sounds Member

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    Well no I am not sure... I am the third owner and dont know if it had any work done inside

    I know we can figure out what is in it to better understand what the best option could be

    When I first brought it home I had a history of blowing the HT fuse and it killed a power tube at some point--I didnt check that out when I first bought it. Amp couldve been used with 3/4 working for months to a year if it happened with me or who knows how long. I noticed that dead power tube on the far right when the preamp tube next to the sheilded one went microphonic, so I swapped in 3 new tubes I had from my Blackstar Series One (2 power, 1 pre)

    The amp was amazing again and I used it often enough that another preamp tube started to rattle on me... so I got tubes for the first time and an account here. I had it running the blackstar 412 and its own speakers as a combo stack and miss it badly
     
    Last edited: May 10, 2019
  17. ampmadscientist

    ampmadscientist Well-Known Member

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    Not "ALL" those parts really need to be replaced.
    A lot of that is just ceremonial....

    220K grid resistors instead of 5.6K (although a mistake) would not cause the bias to drift.
    It would still operate normally.
    Which is an example of "why" you don't need to change all those parts.

    Bad bias pots "would" cause the bias to drift. Which is proven with the standby in warm-up, not operate position.
    You don't need to burn up any tubes to troubleshoot this.

    Also: one more thing.
    All resistors are temperature sensitive. I doubt that the resistors are causing bias drift.
    I think that this is just an internet rumor which ignores / misunderstands the actual cause of the drifting.
    There is no need to start changing a bunch of parts based on internet conjectures.

    Do your parts changing based on testing, not jabber from internet.
    There are a lot of people who believe that changing resistors is going to fix a DSL.
    But....not really. That is more wishful thinking than fact.
     
    Last edited: May 10, 2019
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  18. Gunner64

    Gunner64 Well-Known Member

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    Replace the bias pots and add this if needed. IMG_0820-768x521.jpg I would use this Dr. Tube bias kit before I would use the factory replacement board..that is if bias is still unstable after the pot swap
     
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  19. ampmadscientist

    ampmadscientist Well-Known Member

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    I would use the Dr. Tube bias mod too. (besides changing bias pots). I think the Dr. Tube mod is more reliable.
    I also know that a lot of those bias pots are failing.
    In 8 amps we measured, changing the bias pots fixed the problem in all but 1 amp. No shit.
     
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  20. Gunner64

    Gunner64 Well-Known Member

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    Not only more reliable but it's actually less than half the cost of a factory replacement pcb. It was an easy install in my dsl. I can't say enough good about the kit. Well thought out and works absolutley perfect.
     
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