Dsl100 Ht Fuse Continuously Blowing

Discussion in 'The Workbench' started by rawker999, Apr 7, 2019.

  1. ampmadscientist

    ampmadscientist Well-Known Member

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    Was the output transformer disconnected when you tested it?
    If so, pin 5 on the output socket would be shorting to pin 4 on the board, which means the board is bad.
    This was the 7th board we tested, and the only one that was (seems to be) shorted.

    The clue was that when you replaced the bias pots, the bias was reading -27 volts (was it really reading that?) I believe you, I'm just trying to confirm this.
    The bias should read about -45 to -47 DC on pin 5.
    If it really was -27, that means that the bias voltage is shorted on the board and the voltage is being loaded down by that short circuit.
    (that's what we didn't expect) Shorted to pin 4 I'm expecting.

    Was the bias reading -27 volts on pin 5 "before" you turned the standby to operate position? Very important need to know this.

    This is important because it gives a way to judge if a board is bad or not. It's going to help trouble shoot a lot of other DSLs.
    And unfortunately, it will also mean that the board you have was actually bad.
    But it didn't show any sign of being bad until the bias pots were in place / working. (the - 27 was the signal that the board was bad / bias was loaded down.)
     
    Last edited: Apr 20, 2019
  2. rawker999

    rawker999 Member

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    Yes. W3-W5 disconnected. Speaker cable going to a cab. Not a single tube in the amp. Power on and standby in operate. Pin 5 on all power tubes read approx -27 to -28. I just turned it all back on for the hell of it. V5-V6 still read -28 and V7-V8 smell like burning and read “OL”. Same with standby off or in operate mode. The fuse has not blown.
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    OK so the board was bad. Pin 5 is probably shorted (arced / burned) to pin 4 on the board.
    That is extremely useful information which can help a lot of others.
    The only thing that you can do is replace the board or install the Dr tube bias board modification.
    The OT is shorted, as we already knew.
    And therefore the amp is probably not worth fixing, compared to buying another working amp.
    It's going to cost at least $350 in parts to replace the OT and the board.

    But do us a favor
    Unplug power from amp.
    Set meter to ohms.
    Measure between pin 5 and pin 4 of all output sockets on the board.
    How many ohms does it read between pin 5 and 4 of the output tube sockets?
    Please pass those readings on to the forum?
     
    Last edited: Apr 20, 2019
  4. rawker999

    rawker999 Member

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    The Dr Tube kit is only 45EUR and a new output transfer is $80 so I may give that a shot before buying a new head. Not sure.

    From pin 4 to 5 with no power on, multimeter set to ohms reads OL across the board.
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    What ohm meter are you using?
    Is the range of ohms adjustable or is it auto-range?
     
  6. rawker999

    rawker999 Member

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    I tried again at the 600k range and this is what I get.

    V5-V6 read about 235 and continue to increase over 300.

    V6 reads a steady 0.8
    V7 reads a steady 14.8
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    And V7 is the one that arced?
    And all the tubes are out of the amp? (measure it that way?)

    "V5-V6 read about 235 and continue to increase over 300."

    That reading would be normal, you are reading / seeing capacitors charging up.

    "V6 reads a steady 0.8
    V7 reads a steady 14.8"


    That reading would be a short circuit between screen grid pin 4 and control grid pin 5.
    The short circuit is reading 800 ohms between the 2 pins at V6 socket.
     
    Last edited: Apr 20, 2019
  8. rawker999

    rawker999 Member

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    No tubes anywhere near this amp.
    V7 is the one that lit on fire, yes.
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Apparently there is a carbon spot burned on the board, under the tube socket.
    That's why Dr Tube cuts pin 5 off the circuit track / board with a hollow point drill.
     
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  10. rawker999

    rawker999 Member

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    Sounds about right there is indeed a burnt circle that I can kinda see through the tube socket opening. This is all way more of an adventure than I thought i would be going on when I opened up my amp haha.
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    a burnt circle that I can kinda see through the tube socket opening.

    Damn.
     
  12. rawker999

    rawker999 Member

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    I’ll see if can get a picture
     
  13. rawker999

    rawker999 Member

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    Decided to give dr tube a try. Are they still in business? The website won’t let you add anything to your cart. It just jumps you back to the home page if you click add to cart.
     
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    Dr. tube is a forum member. You can send a PM to him.
     
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  15. Gunner64

    Gunner64 Well-Known Member

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    I found ordering on the website difficult. I ordered from valvetubeguitaramps.com.
     
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  16. Sapphire Sounds

    Sapphire Sounds Member

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    https://valvetubeguitaramps.com/product/marshall-jcm2000-bias-mod-kit-tsl-series/

    Good luck with your DSL I am going to be taking my amp apart to see the board name/revision number my TSL 122 has. The combo amps using the old boards have them upside down so all the heat goes up to it from the tubes underneath with no top vents....
     
  17. Adrian R

    Adrian R Well-Known Member

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    I forgot to mention... It is very important you replace the fuse not only with the correct current rating, but also whether or not it's a slo-blow or quick acting fuse! 'T' stands for timed blow....and is totally required in this position. If you put in a quick acting fuse it will always blow instantly regardless of the amp.
     
  18. Fender

    Fender Active Member

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    I sometimes think that marshall should do once and for all a recall of every JCM2000 DSL with the faulty PCB and replace them with a new DSL HR. Those conductive PCBs are nothing other than a disgrace :/ and responsible for 90% of the bad reliability reputation attached to modern marshalls...
     
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  19. ampmadscientist

    ampmadscientist Well-Known Member

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    It was a very embarrassing streak of design mistakes...from exploding hum balance controls, to circuit boards with defective insulation, defect bias adjust design, to programmable controller chips that crapped out, etc.
     
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  20. Gunner64

    Gunner64 Well-Known Member

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    I agree..but doing so probably would have doomed the company.
     
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