Help With Jtm 60

Discussion in 'The Workbench' started by shaunm, Mar 10, 2018.

  1. shaunm

    shaunm New Member

    Joined:
    Feb 9, 2015
    Messages:
    13
    Likes Received:
    0
    ok so heres what happened.

    A couple of years ago, I was playing and the amp caught on fire a little bit. Sounds bad but I think it was contained to a small part of the amp, one resistor in fact.

    As I cant read the value on the circuit board, can any tell me what resistor it is?

    Here is the circuit diagram:

    http://vesselyn.com/bWFyc2hhbGwtanRtLTYwLXNjaGVtYXRpYw/

    And also Ive attached some pics.

    Thank you!
     

    Attached Files:

  2. JCarno

    JCarno Well-Known Member

    Joined:
    Jun 30, 2010
    Messages:
    1,314
    Likes Received:
    454
    Location:
    Finger Lakes
    R142 and R143 look like 100k. I have the JCM 600 board right in front of me which is the same board.
    Unfortunately I have already removed them resistors from my board because my amp died and I didn't want to invest in getting it fixed so I started stripping out different parts.
    I circled them in red.
     

    Attached Files:

  3. Gene Ballzz

    Gene Ballzz Well-Known Member

    Joined:
    Feb 19, 2013
    Messages:
    1,413
    Likes Received:
    719
    Location:
    Las Vegas, NV
    The important thing, besides identifying those resistors, is figuring out what stressed them enough to cookOnce you get the resistors removed, you "may" be lucky enough to clean the board enough to see the "R" component number. If not, you'll need to determine their number by what they connect to, through a little bit of investigative detective work.
    Best Of Luck To Ya,
    Gene
     
  4. shaunm

    shaunm New Member

    Joined:
    Feb 9, 2015
    Messages:
    13
    Likes Received:
    0
    Thank guys!

    I think I know what it was i think the tubes were on its way out and I kept on playing through the screeching noises.

    So 100k resistors it is?

    I couldnt see the writing unfortunately it was burned too deep
     
  5. JCarno

    JCarno Well-Known Member

    Joined:
    Jun 30, 2010
    Messages:
    1,314
    Likes Received:
    454
    Location:
    Finger Lakes
    Do you have me on "ignore"? I posted the "R" component number along with a pic of the location on the schematic.
     
    shaunm likes this.
  6. JCarno

    JCarno Well-Known Member

    Joined:
    Jun 30, 2010
    Messages:
    1,314
    Likes Received:
    454
    Location:
    Finger Lakes
    I don't know 100% for sure. My eyes aren't that good. But if you blow up the schematic it "looks" like 100k.
     
    shaunm likes this.
  7. shaunm

    shaunm New Member

    Joined:
    Feb 9, 2015
    Messages:
    13
    Likes Received:
    0
    thanks Jcarno!

    ok lemme see , would you say its definitely R142 and R143 thats blown judging from my pics?
     
  8. JCarno

    JCarno Well-Known Member

    Joined:
    Jun 30, 2010
    Messages:
    1,314
    Likes Received:
    454
    Location:
    Finger Lakes
    Yes! 100% R142 and R143.
    Here's a link to the particular schem: http://4tubes.com/SCHEMATICS/Music-amps/Marshall/JTM 60/jtm60e.jpg
    Again, blow it up. You'll be able to see the C134 and C135 right next to them.
    Someone else can follow their path and tell you exactly what they do. I can't.
     
  9. JCarno

    JCarno Well-Known Member

    Joined:
    Jun 30, 2010
    Messages:
    1,314
    Likes Received:
    454
    Location:
    Finger Lakes
    Here's a pic. Not very good but it's proof that they are R142 R143: jcm600 001.JPG
     
    shaunm likes this.
  10. shaunm

    shaunm New Member

    Joined:
    Feb 9, 2015
    Messages:
    13
    Likes Received:
    0
    cool! thanks!

    yeah Ive been staring at the thing for ages, i cant tell if its 100R or 100K
     
  11. JCarno

    JCarno Well-Known Member

    Joined:
    Jun 30, 2010
    Messages:
    1,314
    Likes Received:
    454
    Location:
    Finger Lakes
    Yeah, some of the "Ks" are really easy to make out. I see some 220k.
    But I can't think of what kind if resistor measurement would be designated with an R at the end of it.
    From your first pic, it still looks like you can read the color codes on the resistor. If you can see them all clearly, you can look it up on google to see what the value is.
     
    Last edited: Mar 11, 2018
  12. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    8,122
    Likes Received:
    3,179
    Location:
    On the wrong side of the tracks.
    100R is 100 ohms.

    Blown resistors are the symptom, not the cause. You need to find out what the cause is:
    or the resistors will blow again.
    You can replace the burned parts, but this does not fix the amp.

    And so, R142 143 is 100 ohm resistors.
    But I cannot see the schematic clearly.

    What happens here is:
    AC 6V is being rectified, and used as a preamp DC filament supply (schematics)
    http://www.thetubestore.com/lib/the...shall/Marshall-JTM60-60W-JTM600-Schematic.pdf

    It appears as if this circuit is a DC filament supply for some preamp tubes. If the 100 ohm resistors burn up, there is maybe high voltage entering the filament path.
    (high voltage does tend to arc into filaments)

    A. So you need to look for a preamp tube that is shorted: filament to plate, or
    B. Something on the board is shorting filament to plate supply, etc...possible arcing on board, or on tube sockets.

    Then check
    Is F101 filament fuse blown?
    Is the filament rectifier blown?
    Are the filament filter caps blown (from being over-voltage?)

    Take the preamp tubes and power tubes out.
    Turn amp ON, but leave the standby OFF.
    Measure DC voltage on preamp tube sockets, between pins 4/5 and 9. Should read about 6 volts DC between those pins.

    On power tube sockets, should read 6 volts AC between pins 2 and 7.

    Go ahead and verify that the DC volts and AC volts filament supplies is working...
    I "would" replace all those parts, and look for some reason (like a bad preamp tube)...(like an arcing socket)...
    before I operate the amp again.

    Otherwise if you don't fix the cause - the same thing will probably happen again.
     
    Last edited: Mar 11, 2018
    shaunm and JCarno like this.
  13. JCarno

    JCarno Well-Known Member

    Joined:
    Jun 30, 2010
    Messages:
    1,314
    Likes Received:
    454
    Location:
    Finger Lakes
    Thank You ams!!
     
  14. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    8,122
    Likes Received:
    3,179
    Location:
    On the wrong side of the tracks.
    A. Look for a tube:
    that is all black and burned on the inside of the glass. A shorted preamp tube will look like that.
    [​IMG] Blown tube on left - good tube on right.
    Have a look at the inside of the tubes, and see if anything is obviously cooked.

    [​IMG] See how the tube turns black on the inside?
    A new tube will be shiny silver.
    But an old fried tube turns black or dark brown inside the glass.
    If a tube looks charcoaled, it is probably spent!

    B.
    Check the speaker impedance and amp impedance setting.
    A mismatch between amp and speaker can cause the sockets to arc high voltage to filament.

    If the high voltage is arcing because of a mismatch amp to speaker, this explains how high voltage winds up in the filament supply (and burns the resistors).
     
    Last edited: Mar 11, 2018
    shaunm likes this.
  15. shaunm

    shaunm New Member

    Joined:
    Feb 9, 2015
    Messages:
    13
    Likes Received:
    0
    Thanks peeps! I will have to do the test tomor. but for now it doesnt look like any fuses are blown.

    I think I found the culprit though, a milky white power tube! that in itself would cause all this?
     
  16. JCarno

    JCarno Well-Known Member

    Joined:
    Jun 30, 2010
    Messages:
    1,314
    Likes Received:
    454
    Location:
    Finger Lakes
    Holy Crap!! I was just looking over this board and discovered something very odd.
    In your pics, you see that dark blue 22uf 450v cap(C102)?
    I just noticed that on my board the positive side was never inserted into the hole and soldered in. :confused: The leg is just laying underneath the cap barely sticking out. There is no sign of solder on the bottom where the leg should have been attached to the trace. :wtf:
    I know it wasn't anything I did because the cap still has the glue goop around it. :shrug:
     
  17. shaunm

    shaunm New Member

    Joined:
    Feb 9, 2015
    Messages:
    13
    Likes Received:
    0
    my circuit board has some wierd stuff going on too

    so! everyhing is soldered up and put back together. switches on perfectly. I have one el34b and one el34 (have to wait for a el34B) is it ok to mix them?
     
  18. JCarno

    JCarno Well-Known Member

    Joined:
    Jun 30, 2010
    Messages:
    1,314
    Likes Received:
    454
    Location:
    Finger Lakes
    How do they bias up?
     
  19. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    8,122
    Likes Received:
    3,179
    Location:
    On the wrong side of the tracks.
    "that in itself would cause all this?"

    yes
     
  20. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    8,122
    Likes Received:
    3,179
    Location:
    On the wrong side of the tracks.
    "is it ok to mix them?"

    probably out of balance.

    Did you test the bias and filament voltages?
     

Share This Page