Dsl Bias Drift Repair.

Discussion in 'The Workbench' started by chiliphil1, Nov 17, 2016.

  1. chiliphil1

    chiliphil1 Well-Known Member

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    Ok guys, thanks to help I recieved on this forum I decided to go ahead and complete the stable bias mod on my friend's DSL 100.

    This amp is a 1998 and has the JCM-02-00 board. I confirmed the bias drift issue on it. The bias was set to 78mv and over about 30 minutes of running I watched it climb to over 150mv and finally pop the HT fuse. Definately an issue.

    I went onto the Dr. Tube site and ordered the kit. The service was excellent, the kit was shipped within a day and arrived in less than a week from the Netherlands to the USA. In the kit we have the board, 3 standoffs, and I ordered the hollow drill bit to complete the repairs. It is not included but sold at the same place.

    Now, for the repair.

    Ok, so first. An inside shot of the amp, see the board in the middle? The one on the bottom? Yep, that's the one we have to pull out.

    [​IMG]

    When you remove it be SURE to label all connections. The rear board has 10 and the main board has an additional 26 connections.. LABEL THEM!!!

    [​IMG]

    This is the stable bias mod kit. The little white things off to the side are the stand offs which mount it to the board.

    [​IMG]

    The first thing, after you've removed the board is to disconnect pin 5 from each power tube. If they are touching the board then the stock bias supply will get to them. You have to disconnect this pin from the board. This is where the hollow drill bit comes in. Do this to all 4 tubes.

    [​IMG]

    Now we have to connect the wires from the stable bias mod board to the pins on the power tubes where we just drilled the holes. 1 wire for each tube (specific and listed in the instructions as to which wire goes where)

    [​IMG]

    2 more wires are jumped to these resistors in the bias circuit.

    [​IMG]

    Now, we remove these 2 capacitors. Since we are no longer using the stock bias supply these are no longer needed. This is also where we are going to place the mod board.

    [​IMG]

    Now you can see the mod board installed in the lower left of the pic. It is where we just removed the 2 caps. Notice the wires going to the tube pins and resistors.(sorry for the bad pic)

    [​IMG]

    Finally, fully reassembled. You can barely see the mod board just under the big caps.

    [​IMG]

    Now, toss it back in the shell and ROCK!

    [​IMG]

    After doing it I must say that this is a VERY well thought out kit. It is extremely simple to install taking about 10 minutes of work. The removal of the board and labeling of the connectors took far far longer than the actual install of the mod. Great job @drtube. The bias is now holding at a beautiful 78mv per side with no issues.
     
    Last edited: Nov 17, 2016
  2. cudbucket

    cudbucket Active Member

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    Nice! Glad I got to finally see someone actually use that kit. I drilled out pin 5 as well but instead of the Dr. Tubes kit, I lifted one leg of C6 an C7 and flew the leads to the bias resistors etc. I have a thread on the forum about that. Then I did the Fortin mods to it. Sounds great and bias is solid as a rock. Looking for a replacement front board at the moment in case I want to go back to stock. I have a backup main board just in case.

    Enjoy the amp.

    Dave
     
  3. aryasridhar

    aryasridhar Well-Known Member

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    Good job Phil, wondering what should be the bias on the new DSL100H? I looked up and see there's a mention of 50mv per side, is that about right? I couldn't find the factory recommendation too...any info on that would be of great help please!!
     
  4. chiliphil1

    chiliphil1 Well-Known Member

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    I do not know the spec on the new one. The older ones are about 78mv per side. The JVM runs about the same, maybe a little lower. I think (could be wrong) but I think most Marshall's run about that level.
     
  5. Jazz P Bass

    Jazz P Bass Active Member

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    My hat is off to you for a well documented repair!

    That said, it would help if things where kept in perspective.

    This "mod", while it did help resolve Your Problem, is in reality a 'bandaid' for the underlying condition.
    And that is that these boards are crap.

    I have tried repairing about 5 of these and they all have different issues.
    Hum, bias drift, leaking B+ voltage, leaking heater voltage, bad ground plane, incorrect grid resistors.........
    And in the end, the boards are still crap.

    After time something else will pop up to cause grief & misery.

    My recommendation is to replace the board while they are still available.

    Again, nice job that you did.
    But.......
     
    Last edited: Nov 20, 2016
  6. mcblink

    mcblink Well-Known Member

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    :bump:


    Preparing to do this exact mod on my TSL, Just waiting for the kit to arrive from @Dr.Tube, Not really looking forward to pulling that main board (again).

    I neglected to get the 5mm hollow drill. Will probably be attempting to find one before the mod kit arrives. Seems as though it may be a slight hunt to get a hold of one.

    Are there any tips or tricks, or any pointers I should be aware of beforehand? I'm a believer in learning from others.
     
  7. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Yeah!!!! But everyone should light their older JCM2000 series amplifiers on fire and then ship them back to Marshall. I do not understand why people keep buying these things when you have JVM series, VM, 2550 and other series to purchase.

    The early JCM2000 series is just problematic. Really, really, really! That series should have actually bankrupted Marshall but hey people are kind of, well what can one say.
     
    Last edited: Jul 14, 2018
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  8. charveldan

    charveldan Well-Known Member

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    Why not just buy a whole new board ?

    Looks like a PITA ...
     
  9. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    The hell with a new board. Why not just buy a different amplifier?
     
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  10. mcblink

    mcblink Well-Known Member

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    For what it's worth, I do have a few other amps, I don't need to buy a new one yet, secondly, I HAVE replaced the board, it's brand new with very little play time on it. It worked great for a while, but the drift problem returned and persists. I am not the only TSL owner to have replaced the main PCB and still have persistent runaway bias problems.

    While I do advocate enjoying a nice fire, I think I'd rather fix this thing as a project/something to do with my free time rather than just burn it. It was my introduction to tube amps, and the very reason I joined this forum. Seems like a good candidate amp to tear into and learn some things with, being as that it's not exactly a huge loss of anything should I fuck it up royally. In the meantime, I can still lay down some hot noodles in a bowl on any of my other amplifiers that still work.

    I don't really see the folly in my line of reasoning here.



    Thanks for the tips anyway.
     
  11. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Wait a minute. Are you saying you are screwed with your DSL or not?

    I AM SAYING EARLY JCM2000 AMPLIFIERS ARE PROBLEMATIC. WHY DO PEOPLE KEEP PURCHASING THEM?


    All I can say is ignorance is bliss.
     
    Last edited: Jul 14, 2018
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  12. mcblink

    mcblink Well-Known Member

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    I purchased this TSL amp in 2011-12. Also sometime afterward that same year, I puchased a JCM DSL401. Both amps had problems, although not the same problems. Therefore, I don't disagree with the idea that JCM2000 amplifiers are fucked. I'm not sure I agree either, but I surely don't disagree with you there.

    Around 2 years ago, I finally got around to putting the new PCB in the TSL, new tubes, biased to 78mV. Worked great for a while, several weeks, even. One day I'm playing, I hear a funny sound, tubes redplating and so forth, get the meter out and sure as shit.....amp sits. Been sitting ever since.

    The past few months I've been jammin with a new group of dudes, I kinda just sporadically decided to get this fuckin turd running again, with the intention of either using it or offloading it, either one is fine with me. Makes no difference since it ain't my main amp anyway. I'm no fuckin amp tech. But I can't deny that there is an opportunity for me to tinker with and learn about some things here.

    I have a Vietnamese DSL also, and it's never given me the slightest amount of shit at all, but that's beside the point. So no, I'm not fucked with my DSL, yet I'm also not fucked with my fucked up TSL...and none of these will be my last amps, so....

    Here's what I've learned so far:

    JCM2000 amps, '03-'04 and before, are fucked. I'm okay with this.

    People who keep buying them are idiots. Also okay with this. Mostly...... I just want to be clear that while some people may continue to buy them, I am not one of them. The last JCM2000 series amplifier I bought was over six years ago, and I've learned my lesson. Haven't bought one since. But that doesn't mean that I want a dead TSL sitting around, especially when I can repair it.

    So that's what I plan to do regardless of whether or not they are turds or whether or not the people who keep buying them are mouth breathers or not....
     
    Last edited: Jul 13, 2018
  13. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Fine. But two years ago makes you one of them. 2000 was eighteen years ago. Newer models do not count except those asian models may get you sooner or later.

    Make a stand. Do you have one of them or do you not?
     
    Last edited: Jul 14, 2018
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  14. mcblink

    mcblink Well-Known Member

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    That's some high quality bait man. Really. I almost took it too.

    :D

    ...that being said, I didn't buy that amp 2 years ago, I decided to finally getting around to repairing the turd after 4 years of being useless.
     
  15. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    So 2014, that makes you special? :)
     
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  16. mcblink

    mcblink Well-Known Member

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    Yes.

    That makes me special.
     
  17. ampmadscientist

    ampmadscientist Well-Known Member

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    On the wrong side of the tracks.
    Because the modification might be better than a new board.
     
  18. charveldan

    charveldan Well-Known Member

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    I would never even buy an amp like that.
     
  19. mcblink

    mcblink Well-Known Member

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    This mod actually will be in conjunction with a new board.

    The mod is relatively cheap.

    As it stands, I have a boat anchor with a Marshall label on it.

    OR

    For less than 100 bucks and a little of my time, I can make it work again, AND learn some new shit.

    Why the fuck wouldn't I do this?!

    In my opinion, a person either has to be destitute or completely ignorant to do otherwise...

    Not that there's anything wrong with that lol

    But that's not why I posted here in the first place.

    Are there any actual tips or tricks from somebody who may have done this a time or two before?? Stuff where someone might say, "y'know, if I had to do this again, I think maybe I'd do it this way or that way..."

    I mean, I AM doing this one way or another. I'm going to do it with or without any tips from anyone here, so ultimately it doesn't really matter.
     
  20. 6StringStewie

    6StringStewie Active Member

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    This happened to me with my TSL. I had replaced the main pcb and a couple of months ago, I found I was experiencing runaway bias again. I installed the Dr. Tube mod (got mine from Valve Tube Guitar Amps) and the amp is stable again. I don't know if it's actually happening or not, but my amp feels like it is not as hot as before.

    Another feature of the bias kit that I don't see talked about is the power reduction feature. It is taken away from the main pcb and is handled on the bias kit board.

    I bought my TSL to learn how to work on tube amps until I could afford to buy a new amp. I ended up loving it. Some of these amps had problems and some didn't. If you are willing to put the time and effort into restoring a bad unit, you are going to be rewarded with a great sound amp.
     
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