Another sick Jcm2000 dsl100 issue 5

Discussion in 'The Workbench' started by Jordan Elder, Feb 28, 2021.

  1. Jordan Elder

    Jordan Elder New Member

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    I was given a dsl100 issue 5 from 2002 that did not work. All I was told is that it had a bias problem.

    I bought the stable bias mod and installed it. This involved drilling out pin 5 and bypassing the bias circuit. I've replaced c38 and c39 because they were domed. also replaced c-46 with a 1kv variant because I have read it can take out the board if it goes bad.

    When I turn this thing on, it doesn't blow any fuses and you can play it, but I can hear the tubes making all kinds of noise. Popping, etc. I never saw them red plate but I didn't leave it on for more than a few minutes. Also, the moment you take it off of standby there's kind of a popping noise through the speaker and then it fades away. During the few minutes I played it I noticed the volume kept getting louder.

    The bias pins read about 170 millivolt per side with the trim pots all the way down. it was at this point I replaced r77 with a 15K resistor. I had seen on some random YouTube video it would help get that bias down more. I'm not sure if it made it worse or not. I'm pretty sure all the tubes are good. The amp plays, I just don't dare leave it on for more than a few minutes because the bias and the noises the tubes make.

    To try and save the power tubes I removed them and have been taking some measurements.

    Pin 1 0v (to ground)
    Pin 2 3.65vac (to ground)
    Pin 3 517v (about 450 with tubes)
    Pin 4 514v (no tubes)
    Pin 7 3.68vac (to ground)
    Pin 8 0v (to ground)

    My knowledge of how tube amps work is quite limited. I do understand the fundamentals and I am aware of the lethal voltages inside of these amps.

    Any help would be greatly appreciated, I know there's thousands of threads on these amps but I really couldn't find exactly what I was looking for.

    Thanks.
     
  2. JP2036

    JP2036 Well-Known Member

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  3. neikeel

    neikeel Well-Known Member

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    I do not agree.
    The Dr Tube mods work very well. You must replace the bias pots, replace the output tubes and make sure the output jacks are properly grounded.
    Then you should be good to go.
     
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  4. Jordan Elder

    Jordan Elder New Member

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    Should the new pots be an identical part or should I get some that have a greater resistance value?
     
  5. JP2036

    JP2036 Well-Known Member

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    Throwing money @ a bad board doesn't make it a "good board" it just buys you time w/a "bad board".
    When you go to sell it nobody will touch that w/a 10 foot pole unless they're naive.
    You got a $700 paper weight.
    Its the composition of the PCB itself thats the problem. You can drill & solder all day but not gonna change that.
    * flame away
     
    Last edited: Feb 28, 2021
  6. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

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    This is bullshit. do you have personal experience with the kit? If not don't spread nonesense. The kit absolutely works. It is a well thought out easy to install solution.

    I have done the Dr.Tube stable bias Kit and it works perfect, holds rock solid bias and adresses several issues the old boards have.

    The reissue boards , while addressing the material, do not address the fact that the B+ voltage and the negative bias voltage are like a mm apart..that is part of the problem, if not most of the problem, as if the traces were not that close I doubt the issue would be so evident.

    The Dr.Tube kit isolates the - bias and + high voltage traces from each other and after the the kit is installed it is IMPOSSIBLE for them to intertwine ever again. For that reason I have more faith in the kit than in the issue 20 boards.

    And op listen to Neil, replace them bias pots! Amprepairparts.com has exact replacements for like $5 a piece.
     
    Last edited: Mar 1, 2021
  7. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

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    Same part. Go to amprepairparts.com they have exact replacements.

    And make sure the 1ohm cathode resistors associated with the test points are a-ok.

    And the popping could be as simple as dirty tube sockets, clean them.
     
    Last edited: Mar 1, 2021
  8. Jordan Elder

    Jordan Elder New Member

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    Thanks for the advice. I've got some bias pots en route from that site.

    Do the heater voltages seem too high? I had read that there should only be about 6.3v on heater

    The 1ohm cathode resistors measure about 1.3-1.4 ohms. I guess it is probably because they are the five percent tolerance variants? I have a bias probe for my multimeter that I will use to dial things in more precisely.
     
  9. JP2036

    JP2036 Well-Known Member

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    A 1 ohm resistor measuring 1.3-1.4 ohms is {+30%} to {+40%} not 5%
    5% of 1 is {.05}
     
    Last edited: Mar 1, 2021
  10. XTRXTR

    XTRXTR Active Member

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    Is this the amp I have heard that has a drifting voltage issue? @Gunner64 is right Dr Tube has the fix correct. I think they made their own boards that are better designed than anything Marshall puts out.

    That heater is hot 7.33 VAC is ridiculously high, 6.3 VAC total is optimal for tube life, could be that was measured without tubes as well? What is your wall voltage actual? Are you on 240 220 or 120 VAC?

    You do not have a paper weight, if the iron is good you got plenty. Take your time and fix or upgrade to third party boards if necessary. In the long run your fixes and upgrades will make it more valuable as people will want these more. Ultimately its worth your own sweat and sentiment which is more valuable than an amp that sits with dust on it.
     
    Last edited: Mar 2, 2021
  11. JP2036

    JP2036 Well-Known Member

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    [​IMG]

    A main board swap is not too terribly complicated.
    Alot easier IMO than hacking up a PCB w/a "kit" plus its more Professional.
    Show people you care about your gear and you may be able to sell it someday.
    Take lots of gutshot pics for reference before you start unplugging stuff.
    There is a good chance you could swap the board and still have something wrong.
    Electronics 101.
     
  12. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

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    He already installed the Dr.Tube kit. The installation when done correctly is not hacked. Again, tell us your experience with the dsls you have repaired with or without the Dr.Tube kit.

    At this point telling him to change the main board when he already installed the kit to solve the problem is confusing to the op, and insinuates you haven't read his intire first post, or your point is just to be argumentative.
     
    Last edited: Mar 2, 2021
  13. JP2036

    JP2036 Well-Known Member

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    oh so his dsl is my problem ?

    gtfo.
     
  14. CROWEMAG

    CROWEMAG Well-Known Member

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    I wish there was a place to purchase the SBK in the states!
     
  15. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

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    No one said it was, so that post really made no sense...and I see now that you are just being argumentive.

    Good luck with your repair Jordan. I'm not going to go any deeper and have others derail your thread on my account.

    I'll just say I have used the kit and it isn't a hack job. It's a well thought out solution, and a professional looking repair. Holds rock steady bias. I have another DSL on the bench now, and it will be receiving the kit as well.
     
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  16. Jordan Elder

    Jordan Elder New Member

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    I'm well aware of the option to replace the whole pcb. It was something I considered but many people still have issues with the new board having the same problem. To be honest, it's really satisfying to fix these without buying a whole new pcb. This is only the 3rd tube amp I've worked on, the first was an 88' carvin x100b and the second was another dsl. Both have been working great for years. I know this board can be fixed.

    I really appreciate the input and advice to put in a new board. I have no doubt that would work, a least for a few years or more. I just enjoy the process and learning experience working on tube amps and I would like to go the route of replacing components. Resale value isn't a concern.

    If I can get the heater voltage down 1 volt and the bias to a suitable range, I know it won't drift.

    Thank you to everyone for the technical advice. I really appreciate it.
     
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  17. JP2036

    JP2036 Well-Known Member

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    Good luck Mr. Elder, im sure it will work out eventually.

    :cheers:
     
  18. neikeel

    neikeel Well-Known Member

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    I was very impressed with the quality of the kit and I liked the way it isolated the bias from HT. The boards and connectors hooked up and it looks very pro, far from hacked up (I could have done that myself!!). New properly lubed bias pots are essential and I personally would replace the bias resistors with 1 ohm 1% MO 2w resistors now whilst it is opened up.
    The DSL 100 is a very good amp IMO and merits a good quality repair. If you prefer a new board that is great but I have no concerns about the Dr Tube kit quality or effectiveness. YMMV.
    Getting the heater voltages right may require a variac so that the amp and your selected option matches your wall voltage.
     
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  19. Jordan Elder

    Jordan Elder New Member

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    So I installed the bias pots and turned it on. With them both clockwise all the way, I get like 34ma per tube with a bias probe. It was all good for like 5 minutes until I heard an audible noise like a pop though the speaker and the bias quickly jumped to 200ma on one tube. After I turn it off and it sits for a while I can turn it back on and get regular bias for a few minutes again.

    Any ideas? Is this something bad caps would do?
     
  20. soundboy57

    soundboy57 Well-Known Member

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    Clean and re tension your tube sockets, if you haven't done so
     

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