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Thread: DSL 401

  1. #1
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    DSL 401

    The dreaded jcm 2000 ..... This is the 401 with el84s. Have a big short or current draw . Not the tubes...but only does it with the el84s in (new ones and the old set) , If i pull the heater fuse F4 , for the el84s and PI , it goes away . R34 / 35 test good.

    I measured the resistance of the secondary of the PT with nothing hooked up to it -

    HT pins = 36 ohms

    Bias pins = 10 ohms

    Fil. pins = short / 0.0 ohms

    that filament winding shouldnt measure 0 right ??


    http://www.thetubestore.com/lib/thet...-Schematic.pdf

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    Supporting Member The Dude's Avatar
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    It probably won't measure much. Consider how much less wire that winding has than the HT winding. Then consider you have a bunch of filaments in parallel with it.

    Edit: It doesn't track with, "only does it with the el84s in", but those amps are notorious for problems with BR1. It is for the DC filament supply of the V1-3. They are usually intermittent or open, but a shorted one wouldn't surprise me. They run very hot. The rectifier is supplied by the filament winding, so it does track with, "If i pull the heater fuse F4 , for the el84s and PI , it goes away.". It's worth checking anyway as it's a common problem. If you end up needing to replace it, stand the new one off the board a bit to keep heat away from the solder joints and board.

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    Last edited by The Dude; 02-09-2018 at 06:17 AM.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    With the power tubes removed check your DC voltages on the EL84 sockets. Do you have bias voltage?

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  4. #4
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    yes .

    -90 mv

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    Lest We Forget g1's Avatar
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    You should have something more like -15VDC at pin2 of the power tubes.
    Attached is the rest of the schematic.
    Attached Files Attached Files

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    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

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    sorry -- brainfart here... its -17vdc at 1 pair of sockets .. and 0 mv at the other pair . pin 2 . The four 10k res that feed those are all good .

    It looks like i have a different board- mine is labeled DL40-60-00

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    Last edited by Valvehead; 02-10-2018 at 01:20 AM.

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    i forgot i had the same model amp acting up a few years ago---found my post on it here . Ill go through all that first

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    Last edited by Valvehead; 02-10-2018 at 01:27 AM.

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    Supporting Member The Dude's Avatar
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    Here's a thread w/similar problem.

    http://music-electronics-forum.com/t26055/

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    ok guys....after a lot of testing / looking and such... i found a broken trace to V5/6---the bias feeding R84/85. I repaired that and i have -17vdc at all the sockets now. I need to piece it together and fire it up on the current limiter...will do tomorrow...its 2:46 am now......................

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  10. #10
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    Update---
    amp is up and running now , bias is great , have hv to all tubes and filiment volts ac and dc .
    no smoke or fire yet.

    Since this amp is possessed...i now have a new problem = no signal when on the orange or red channels (dirty) . The green channel works fine . The relay is clicking / switching but i dont have the dirty side of the preamp . No signal...

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  11. #11
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    Can someone tell me what kind of voltages i need to see at T9 / 12/13/14 ? Could one of those kill the ch.2 signal from getting to the relay ? Are they used to mute ?

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  12. #12
    Supporting Member Randall's Avatar
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    If you found broken traces, does that mean you had to lift or flip the board? If so, check very closely for any and all wire connections that could have broken in the process from flexing.

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    It's weird, because it WAS working fine.....

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    ill have a look , but its all pcb mount sockets..most wires are from the PT .

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    Im not finding the problem--tried replacing T9 , a j174 , it looks like the OD mute ? Or am I way off ?

    In OD mode i tried touching the grids even the phase inverter with an insulated screwdriver to see if i had any noise or buzz......nothing..

    I would think i would hear some noise down towards the end of the preamp

    checked the solder joints---even chopsticked the hell out of the board

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  15. #15
    Lest We Forget g1's Avatar
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    On page 2 of the schematic in area C-3. There are lines called ODMUTE, V2A-IN, OD1G1.
    Check that they are changing in voltage depending whether clean or dirt.
    Have you tried with the footswitch or are you using the panel switches?

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    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

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    Quote Originally Posted by g1 View Post
    On page 2 of the schematic in area C-3. There are lines called ODMUTE, V2A-IN, OD1G1.
    Check that they are changing in voltage depending whether clean or dirt.
    Have you tried with the footswitch or are you using the panel switches?
    i will check that--

    no fsw--the panel switches seem to switch the relay fine--i can hear it click --just no OD signal

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  17. #17
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    ok....

    if volts change - here we go,


    ODMUTE ---YES

    V2A-IN = YES

    OD1G1..... which leg is the gate ? looking at the transistor, flat facing me ...the left leg - yes , the middle leg = no , the right leg =yes

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  18. #18
    Supporting Member Jazz P Bass's Avatar
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    J174 P Channel Jfet: D G S

    Click image for larger version. 

Name:	fet_j174.jpg 
Views:	68 
Size:	17.0 KB 
ID:	47072

    A Jfet is normally 'closed' D to S.
    It needs the gate voltage to 'open' it.

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  19. #19
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    ok...what am i missing-------the gate connection goes nowhere on the scem..?? ?? im going to go trace it ..

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    Lifetime Member Enzo's Avatar
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    No, the schematic says the gate connects to OD1G1. Look on the other page just right of T10,11. There is OD1G1 right beneath CLNG1.

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    i just found it while you posted---sorry...

    so we need voltage from T2 yes ? and im looking for 24v or bit lower ?

    sorry , trying to get these transistors--so these are sending signal to ground when they are switched in?... now i see T10 called ODmute

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    Im lost. Heres what Im getting , testing the j174's with the amp fired up and switching channels -

    T9 (j174) gate has 20v clean or OD
    clean -- D+S = 47K
    OD D+S = 47K


    T2 (mpsa13) HAS 28V , CATH OF D18 HAS 20V CLEAN / 0V IN OD

    T10 (j174) GATE HAS CLEAN = 0V / OD 19V
    CLEAN D+S = 43 OHMS
    OD D+S = 300K


    T11 (j174) GATE HAS CLEAN 0V / OD = 18V
    CLEAN D+S = SHORT
    OD D+S = SHORT

    shouldnt T11 open with that 18v ??

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  23. #23
    Stray Cap DrGonz78's Avatar
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    Just a quick question... Have you cleaned the JS-1 foot switch jack? I figured you probably did but just had to ask.

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    Quote Originally Posted by DrGonz78 View Post
    Just a quick question... Have you cleaned the JS-1 foot switch jack? I figured you probably did but just had to ask.
    yes--ill hit it again to be sure

    so if T10 is getting its gate voltage...it should "open" , ? because i have 1 leg going to ground still --is that it maybe ?

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    Lifetime Member Enzo's Avatar
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    T10 seems to be working. With zero v on the gate the JFET reads 43 ohms - a low resistance. Put 19v on the gate for OD, and resistance goes up to 300k.

    T11 is controlled by the same gate signal, and so I expect it to behave the same as T10. Looks bad.

    T9 always had gate voltage? Then go back to OD!G! source and find out why. Remember to plug something into the input jack, because it has a mute contact involved with this. Empty input jack and R100 is grounded at the top, and that is the 47k you are reading, I'd wager. meanwhile I'd expect T9 to still vary once that is corrected. it looks like it isn't. SO I suspect those two transistors at this point.

    The MPSA13 is bipolar and doesn;t test this way.

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    T9 still has 20v at gate with or without something in the input jack.

    i dont know what switches the gate volts on/off at T9 . T2 bad?

    T2 = E = switches 25v to 9mv
    B = switches 25v to 0.6v
    C = switches 26v to 27v

    i have swapped out T9 and T11

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    "T9 still has 20v at gate with or without something in the input jack.
    I don't know what switches the gate volts on/off at T9"

    Either the Ft Sw Ring normally closed contact (going open) or the Fr Pnl Sw (going open) will remove the +20V from the gate of T9.

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  28. #28
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    I inserted a cable in the fsw jack- it does switch and the T9 gate volts go to 0.2v when in od mode......still no sound though--also tried pressing down on the contacts

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  29. #29
    Lest We Forget g1's Avatar
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    You had a fet that looked bad, T11. You swapped it with T9.
    Do the resistance checks on both again. When the voltage is at the gate the D & S should not be shorted.
    If one of them is shorted all the time, remove it and see what the amp does with it removed.

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    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

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    Quote Originally Posted by g1 View Post
    You had a fet that looked bad, T11. You swapped it with T9.
    Do the resistance checks on both again. When the voltage is at the gate the D & S should not be shorted.
    If one of them is shorted all the time, remove it and see what the amp does with it removed.
    ive swapped out T 9 / 10 / 11 at this point with a J175 (that should work as a 174 yes? ) = no change

    with 20v at gate of T9 or T11 -- D+S is open on T9 / T11

    before i swapped T10 out i tested like above and D+S was 1.8k so i tried swapping it = nothing....

    if i pull T11 out do i need to leave t9/10 in ...or can i pull more than 1 ?

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    I powered it up tonight , removing 1 transistor , out of the amp 1 at a time for signal , then replacing it....next one , replacing it , etcc T9 - 10 - 11

    Nothing--no OD channel .

    Im so close......and so frustrated .

    tested each of the 3 J174 , out of the amp and on the bench for resistance and Diode test---all 3 are identical .

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  32. #32
    Stray Cap DrGonz78's Avatar
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    Did you suss out that T11 issue and do the voltages make any sense now? So we have no signal on OD channels. Perhaps you have all the channel switching circuit running fine now. Have you tried to replace V2 to make sure it is not bad? Just a quick thing to check or at least to make sure you have checked it.

    Edit: Or do I have that backwards and V1B is the tube used to generate the Overdrive in the amp? Perhaps it is V1 since I see ODMUTE at that tube.

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  33. #33
    Lest We Forget g1's Avatar
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    Agree with Dr. Gonz that the fet switching cirucuits seem to be working properly. Time for signal tracing.
    You hear the relay click, but are it's switch contacts closing and passing signal?

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    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

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    FOUND THE PROBLEM !!!

    What a F$!$@@#! n relief............ !!

    i went back thru the preamp area with my meter and magnifier , triple checking connections---finally found a bad one....looks like someone replaced a 100k in there previously , and it was just barely on a bent up trace....so i soldered and Bingo---------------amp works perfect now !!

    I appreciate everyones time and knowledge and I definitely learned a little about the J174 !!

    THANKS GUYS .

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