Amp Turns On But Tubes Wont Light Up..

Discussion in 'The Workbench' started by rhoadsfan, Jan 9, 2017.

  1. rhoadsfan

    rhoadsfan Member

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    hey all, been a while since ive been on here but i have run into a problem so im in need of the plethera of knowledge in here...
    my jcm800 head turns on, as in the switch lights up and you will get zapped if you touch the wrong thing so there is power going to it but it seems to stop at the tube socket..
    i havnt used this head for a few years since i built my own .. back then it had blown a tube..i had a couple of old spares so i put them in and they didnt light up...at 1st i just assumed the tubes were dead so i got a new set, power and pre amp tubes, still nothing..
    since then ive looked at just about everything.. i found a couple dodgy solder joints on the pc board and fixed them but to no avail...i thought maybe the PT but i figure if it was blown then the amp wouldnt power up at all??????
    also when i test it with power and the heater wires disconnected from the tube socket i am getting the correct 6.3 vac across them but as soon as i hook them up to the tube socket it has bugger all voltage..
    i had 1 new power valve socket so i replaced V5 and still nothing.. obviously it could be another socket and i have ordered more.. the 1k5w resistor on V5 was shot and reading in mega ohms so i replaced that as well.. the resistor on V4 was fine..
    im wondering if there is something with the OT that could cause this??? how would i test for that? also, on a jcm 800 if one valve socket goes does that stop all from working??
    any help is greatly appreciated...cheers
     
  2. ricksconnected

    ricksconnected Well-Known Member

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  3. rhoadsfan

    rhoadsfan Member

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    is some special kind of techtalk?? haha..
     
  4. ricksconnected

    ricksconnected Well-Known Member

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    means I'm following the thread.
     
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  5. johan.b

    johan.b Well-Known Member

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    The 800s have fuses for the heaters on the pcb. They are small and look like resistors. Might say "picofuse" on them. Is at the power switch end of the pcb
    j
     
  6. rhoadsfan

    rhoadsfan Member

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    do you mean the diodes perhaps?? otherwise there is no fuse on the pcb.. perhaps there is on some jcm models or the combo`s but there isnt on this model.. its a lead series 2204 head.. there is a few resistors, 5 diodes, bias trimpot and a couple of electrolytic caps at the switch end but no fuse.. the only fuses are on the back for the mains and the HT fuse
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    (.) He's lurking... It's the man of mystery dot.

    "when i test it with power and the heater wires disconnected from the tube socket i am getting the correct 6.3 vac across them but as soon as i hook them up to the tube socket it has bugger all voltage.."

    What voltage do you get when the heaters are hooked up?

    Can you post pictures of the board?
    True, the pico fuses may be blown....
     
    Last edited: Jan 10, 2017
  8. SirArcsAlot

    SirArcsAlot Active Member

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    I had a friend with a Superbass circuit that had the exact same issue. Heaters wouldn't light. I fixed it, but for the life of me I can't remember how. I vaguely remember maybe wiggling one if the preamp tubes? Try that.
     
  9. Micky

    Micky Well-Known Member VIP Member

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    A 2204 without a fuse?
     
  10. rhoadsfan

    rhoadsfan Member

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    no probs.. my multimeter reads nothing at the heater wires when its hooked up. if i disconnect the wires it will give the correct reading according to the schematic.. here is the board..
     

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  11. rhoadsfan

    rhoadsfan Member

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    tried that a few times...lol
     
  12. rhoadsfan

    rhoadsfan Member

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    indeed it is.. i can see the empty spot on the board for f1 and f2 but there is and never has been anything in them and i think i might know why.
    when i built my clone i had a major bitch of a time with the switches becuase every schematic i could find was for the jmp style on off switches with the 2 metal switches and seperate pilot light.. the marshall i had has a rocker switch with the pilot light in it and the black standby switch.. the 2 sets of switches are wired differently which is where my problems came from with the clone.. also im pretty sure from memory all the schematics had the pilot light running through the standby switch whereas mine has the pilot light in the power switch
    is this picofuse inside the rocker or standby switch perhaps?? i have been suspecting there is a problem with the power switch, its old and has been soldered too many times i think and if the fuse is inside that its more than possible its blown, i just cant see it without busting the switch.. as you can see in the picture above of the pcb right at the switch end there is slots for F1 and F2 but there is nothing in them, never has been and i bought this head in 94 or something like that.. i gigged with it for over 10 years so if its impossible that a 2204 could operate without this picofuse then it must be hidden in one of the switches becuase it isnt anywhere else in the amp..
     
  13. Gunner64

    Gunner64 Well-Known Member

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    If you have voltage with the heater leads off the sockets chances are its not a fuse. Sounds like it craps out under the full load of the heater string..I'd look for bad connections at the tube sockets, and on related connections on the board. One of the heater wires may be broken internally and only make contact in a certain position..An Original electrolytic capacitor may have failed or is at that point. Could be anything in the heater circuit..But a fuse I doubt. No fuses in the switches on my 2204.
     
  14. johan.b

    johan.b Well-Known Member

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    Did you read the heater voltage at the transformer or at the sockets?
     
  15. rhoadsfan

    rhoadsfan Member

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    sorry for the late reply, it was 39 degrees (about 102.2f )here yesterday so i didnt do much...lol
    thats pretty much been my thoughts, but finding exactly what it is is the problem..lol..i am already waiting on new valve sockets to arrive so when they do i`ll know if it was that and i`ll go through all the connecting wires one at a time..
    i did have a old dead rocker switch laying around so i pulled it apart and there was no fuse in it..
    given that the heater wires read correctly when disconnected, i would think that means the PT is fine..
    but i`ll get the new valve sockets and probly get some new wire and see how that goes..
    is it possible the OT could cause this?? i would assume the valves would still light if the OT was shot but i obviously wouldnt get any output?
     
  16. rhoadsfan

    rhoadsfan Member

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    both.. direct from the transformer it is 6.3vac.. indicating the PT seems to be ok.. when connected to the valve socket it reads 0.00.. so something is stopping it when contact is made with the heater pins on the valve socket.
     
  17. Gunner64

    Gunner64 Well-Known Member

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    You did work on the sockets before? Double check your work if so..something isn't adding up as one heaters connected to the other then the other ect. On down the line...seems almost like it shorts out when connected.
     

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  18. rhoadsfan

    rhoadsfan Member

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    exactly.. yes i have changed 1 of the valve sockets with a new 1.. i only had the one.. i ordered more and as soon as they arrive ill put them in and check everything else at the same time...
    the problem was already there before i changed the socket so it isnt from my work, not that im saying im great or anything..im not... but the problem existed before i touched it so i feel confident it wasnt me...lol..
    the new sockets should get here early next week so ill rip into it then and then post how it goes... cheers for the help so far..
     
  19. rhoadsfan

    rhoadsfan Member

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    PROBLEM SOLVED.. as i think i mentioned in an earlier post i had suspicions about the power/rocker switch.. it didnt click anymore when switched on and off.. i have checked every wire and connection on the amp, put in new valve sockets and valves and still nothing.. today i went to jaycar and grabbed a new rocker switch.. 4.95.. got home, put it in and halleluja, the tubes light up.. had it plugged in and running for around 6 hours now, all is fine and not one single snap, crackle or pop.. so after all that it came down to a 5 dollar switch.. i noticed in the old switch there is a little resistor or perhaps its a picofuse but in anycase it obviously blew.. cheers to everyone that offered advice..
     
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  20. myersbw

    myersbw Active Member

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    ...usually solves the problem of "the lights are on but nobody's home"...except when it comes to people! ;)
     

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