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Marshall dsl15c lost bias

Discussion in 'Amp Tech Center' started by Burning Fingers, Nov 30, 2020.

  1. Burning Fingers

    Burning Fingers Tele-Meister

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    There I was thinking I would be having an easy day.. a few amps fixed and I was looking forward to a beer on a beautiful afternoon...until a Marshall DSL15c has visited.

    Blown mains fuse..ok replace that and test the amp...no good...check power tubes...both are destroyed internally...white powder everywhere...replace power tubes and test .

    Weak clean channel..ok replace V1 and test.

    All good for a few minutes until I smell trouble ..the new power tubes are cooking.

    Pull amp out of combo box and check plate voltages...419v on each.

    Check bias voltage with power tubes in....hmmm...49volts positive...turn the trim pots...nope only increases the positive bias voltage.

    Remove power tubes and recheck bias voltage...18 v negative on one side of the trimpots but zilch on the other side which goes to small electros.

    The bias electro caps appear to have gone to god and killed the bias voltage.

    Noticed a couple of resistors have been replaced without the board being removed ( the board is a pain to remove )..they turn out to be part of the bias circuit but I have no idea if they are the original values.

    Removed the electros and reassembled the beast and bingo bias voltage returns and can be adjusted through the expected range via the trim pots.

    Checked the electros...all 4 gone bad !

    Now it's a weeks wait for the new 10uf 100v electros to arrive by mail/courier.. I don't have any small enough in size to fit.

    I would have liked to been able to quickly check the circuit diagram rather than spending time figuring out what components are in the bias circuit.but, Marshall, being the lovely people they are, refuse to give any schematic for this (or any other ) amp so beware of any recent marshall amps that you you cannot find a schematic for on the web...and that goes for fenders as well as they have decided to be tossers regarding schematics as well.

    Ok Now it's beer o'clock !:)
     
  2. Jon Snell

    Jon Snell Tele-Holic

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    R2 and R5 = 22k 0.6W
    R3 and R6 = 15k 0.6W
    R4 and R7 = 100k 0.6W
    C11, 12, 13 and 14 = 10uF 100v 105C
    D1 and D2 = 1N4007
    C9 = 100nF 630v !
    Hope that helps to clarify what you have in there.
     
    Last edited: Nov 30, 2020
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  3. radiocaster

    radiocaster Poster Extraordinaire

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    Here. It's cathode biased though, the trim pots regulate the voltage going to the rectifier.
     

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    Last edited: Nov 30, 2020
  4. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Hmm, that doesn't look cathode biased to me. There are 1Ω resistors that could be useful for checking the cathode current, but BIAS1 and BIAS2 look like individually adjustable negative bias supplies. Note the orientation of those diodes and electrolytics.

    (As always, I could be wrong...)
     
  5. radiocaster

    radiocaster Poster Extraordinaire

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    Ah, ok, I saw "1K" instead of "1R", my eyesight is not as good...
     
  6. Burning Fingers

    Burning Fingers Tele-Meister

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    Wow ..thank you for that schematic :D
     
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  7. radiocaster

    radiocaster Poster Extraordinaire

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    It's not cathode biased though.
     
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  8. Wally

    Wally Telefied Ad Free Member

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    Interesting bias circuit in that there is a bias adjustment pot for each tube. One could run mismatched tubes if wanted.
     
  9. Wally

    Wally Telefied Ad Free Member

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    I would observe that It is advisable to know what voltages..including the bias voltage...are before installing new tubes in a situation like this when there has been a failure. Said failure was indicated by a blown fuse. I would have had the chassis out of that amp before installing new tubes.
     
  10. Burning Fingers

    Burning Fingers Tele-Meister

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    I would love to know the expected voltages for this amp but they are not shown on the schematic.
    The owner says he has never replaced power tubes and plays the amp real hard so that led me to believe that the power tube(s) had failed.
    Now I had gone over everything in the bias circuit replaced the electros and resistors and got up to -29 volts available to Grid 1 on both tubes with the tubes pulled but still 30ma current with the tubes in..I am aiming for 20ma to 22ma so I need more negative bias and it just aint there atm.
    I disconnected the coupling caps from the PI just make sure the caps were ok..yep...no positive dc leaking through those.
    Ok..so what else is going on here...hmmm 1 screen resistor has a hairline crack I can only see under a magnifier...give it a good poke and it falls apart.
    2 new 1k 5 watt screen resistors coming up when the electronics store opens tomorrow...but what took out the screen resistor ?... a shorted power tube I guess or did it get too hot from the amp being played real hard for a long time.?
    But new screen resistors will not sort out the insufficient negative bias.
    I have noted the voltages in the bias circuit on the picture below and haven't been able to replace C9 yet ( the 100nf 630v ) as my local store doesn't stock them so it is coming by post.

    dsl15c bias.jpg
     
  11. Jon Snell

    Jon Snell Tele-Holic

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    With the output valves out of the chassis, the negative voltages should be as described and adjustable. If not, work backwards with the schematic in the bias circuit.
    Check the final stage 22n coupling capacitors C31/2
     
    Last edited: Dec 7, 2020
  12. Wally

    Wally Telefied Ad Free Member

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    If all is correct in that circuit and -29vdc is as negative as the pots will take the voltage, then one would look at the resistances R3/R4 and/or R4/R7 to change that voltage.
     
  13. 2L man

    2L man Tele-Meister

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    If you change R4 and R7 to 82k you get higher negative bias voltage. Another way is to change R2 and R5 bigger or install perhaps 10k resistors between ground and trimmer.
     
    Last edited: Dec 8, 2020
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  14. Burning Fingers

    Burning Fingers Tele-Meister

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    Ah ha..finally have it sorted...after replacing every component in the bias circuit plus both screen resistors ( one of which had crumbled apart ).
    The culprit was either D1,D2 or C9 all of which which I replaced together.
    Now I can get the current through the tubes down to 20 ma and the tubes are happy and the amp is sounding good .
    A BIG THANKS to all who helped me with this. :)
     
  15. tubelectron

    tubelectron Tele-Afflicted

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    Yes - many of the JCM900 series I saw coming in suffered the same problem of bias : diodes, filtering caps, and even coupling caps going bad...

    -tbln.
     
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  16. Jon Snell

    Jon Snell Tele-Holic

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    Excellent news. Always good to see a result.
    Keep safe out there.
     
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  17. Burning Fingers

    Burning Fingers Tele-Meister

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    Yep..too many amps have turned up recently with premature component failures...Having the name Fender or Marshall or Laney on an amp means little in the way of quality these days.
     
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  18. NTC

    NTC Tele-Meister

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    I guess the big question is why the bias circuit would go bad? Perhaps a filter cap shorting that then takes out the diodes, with screen resistors as collateral damage?
     
  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    IMO, the two weakest parts in the circuit are the tube and the cap. Either one of those could take out the rest.

    If a diode failed, the cap, tube, and screen resistor likely would fail.

    Hard to say what went south first.

    Curious that the main fuse blew but the HT secondary 315mA fuse did not blow. Perhaps, since the 315mA fuse did not blow, one could speculate a diode went first as the diodes are rated for 1A???

    It wouldn't be the first time a part failed... and a fuse was protected.;)
     
  20. Burning Fingers

    Burning Fingers Tele-Meister

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    It is one of life's mysteries as to what initiated the complete collapse of the bias circuitry and the screen resistors and power tubes.

    It was not the only bias gone rogue problem in the past 10 days... a cathode biased Laney VC30 was cooking tubes and blowing the fuse.
    The plate voltage was within the El84's range and I replaced the cathode resistor and cap..I could not find a faulty component anywhere and did some calculations from the voltages listed on the schematic that showed that the amp was designed to run the EL84 quartet at about 90%....that's pushing it a bit too hard in my opinion as El84's are the tubes I see red plating more than any other type of tubes.

    This amp was running them at 123% ! :eek:

    The amp is designed for a wall voltage of 230 V but here we get closer to 250 V ... could that have caused the tubes to be run that much hotter?...anyway my solution was to increase the cathode resistor from 56 ohms 7 watts to 100 ohms 10 watts ...that brought the current down to 78% and no more power tube cooking and fuse blowing.

    A tube amp is not always just a tube amp...some are poorly made and/or designed PITAs.

    No more amp repairs for me until mid January..I need to take a break from the soldering iron and multi-meter and put some more time into building my home studio.:)
     
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