Seeking troubleshooting tips on my Marshall JTM30

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by maddtapper, May 5, 2015.

  1. maddtapper

    maddtapper Member

    Messages:
    83
    Joined:
    Mar 6, 2006
    Location:
    Nacogdoches,TX
    I just recently acquired a DOA Marshall JTM30 210 combo. I have done a few things so far and seem to have hit a wall...

    When I got it it powered on, with no sound.
    I took the chassis out and found a blown resistor... R126 on the linked schematic... I also changed the tube sockets and output transformer while it was apart... As they were inexpensive and I don't know what this poor amp has been through... I also ordered all the radial caps to change while it's apart as well...

    The issue is one wire coming from the output transformer to a pin on the power tube socket instantly starts to arc when I take it off standby with no tubes in... This wire is right beside the resistor that was blown...The other wire feeding the other power tube socket seems fine... All resistors in this area of the board test fine...

    Also there are to caps suspiciously missing at c122& c123 respectively by the preamp tubes... Not sure why these would have been removed?

    I've attached pictures of both spots and a link to the schematic... I'm at a loss as to what to do next... Again, I got this in this condition, so the rough looking solder joints were already there... You can see where the arcing has been happening on the under side of the board pictured... I'm fairly competent at servicing and maintaining my gear, but I'm a little beyond my level of knowledge...

    Thanks in advance!


    http://www.drtube.com/schematics/marshall/jtm3-61-02.pdf


    [​IMG]


    [​IMG]


    [​IMG]
     
  2. maddtapper

    maddtapper Member

    Messages:
    83
    Joined:
    Mar 6, 2006
    Location:
    Nacogdoches,TX
    Btw I did add the jumper seen on the bottom of the tube socket when I replaced the resistor because I suspected the trace may have burned through... And the open hole visible from both the top and bottom of the board, is where the wire from the output transformer goes...
     
    Last edited: May 5, 2015
  3. garret

    garret Member

    Messages:
    889
    Joined:
    Oct 2, 2014
    Location:
    Michigan
  4. maddtapper

    maddtapper Member

    Messages:
    83
    Joined:
    Mar 6, 2006
    Location:
    Nacogdoches,TX
    Thanks! I'll give it a shot.
     
  5. maddtapper

    maddtapper Member

    Messages:
    83
    Joined:
    Mar 6, 2006
    Location:
    Nacogdoches,TX
  6. WaltC

    WaltC Member

    Messages:
    2,130
    Joined:
    Feb 21, 2006
    Location:
    Northern CA (Sacramento area)
    see all that black crap on the PC-board around the problem power tube socket, there's a good chance that it's got lots of carbon in it and is shorting B+ to ground (and lots of other places too) You've first got to clean all that off if possible, even if you've got to go down to the bare plastic and use jumper wires in place of the traces, even then there's no guarantee it'll work. Then you'll have to get hold of the proper schematic and replace the missing parts with correct replacements, and when you're all down, you'll still just have a JTM 30<G>
     
  7. maddtapper

    maddtapper Member

    Messages:
    83
    Joined:
    Mar 6, 2006
    Location:
    Nacogdoches,TX
    Thanks! Very good info! I'm in the process of cleaning it up now... And I'm not worried about the JTM30 part. I've got about 14 other amps and an AXE FX to play with... This is a toy... Lol!
     
  8. pdf64

    pdf64 Member

    Messages:
    5,143
    Joined:
    Aug 9, 2008
    Location:
    Staffordshire, UK.
    A light bulb line current limiter can be beneficial for testing / verifying repairs on amps that have big problems such as this.
    Yes, all carbon / carbonized board material needs to be removed before applying B+.
     
  9. maddtapper

    maddtapper Member

    Messages:
    83
    Joined:
    Mar 6, 2006
    Location:
    Nacogdoches,TX
    Ok, got everything cleaned up... Stuck in known good tubes... To reiterate:
    New caps (all axial caps)
    New ceramic tube sockets
    New power tranny from Hammond, lugs soldered in place.
    No more Carbon or arcing on the board...


    Now... About the 2 caps that where gone.... Any reason those would've been removed? Didn't know if that may have been a factory mod? The replacement caps where too tall, so I had to mount them laying over... But everything is connected and soldered neatly...
    I put new caps in per the schematic, (c122&c123) and it powered on. But Tubes didn't glow and blew a fuse...f101 on the schematic above... Beside the power tubes...


    I'm gonna run to radio shack and grab some fuses... These are 6.3 amp and I don't have any laying around... Maybe it was just a fluke? Gettin pretty bummed here... Ideas?
     
  10. mc5nrg

    mc5nrg Supporting Member

    Messages:
    8,159
    Joined:
    Mar 15, 2004
    Location:
    DC, Philly, Asquared, Upper Michigan
    Search for posts on the related JTM60 and also by John Phillips who was a great contributor years ago and had lots of info on this series.)
     
  11. WaltC

    WaltC Member

    Messages:
    2,130
    Joined:
    Feb 21, 2006
    Location:
    Northern CA (Sacramento area)
    build the light bulb current limiter that pdf suggested ( https://www.youtube.com/watch?v=wRFRwOnLsZI ), etc....

    pull all the tubes, replace the fuse, turn the amp on with the current limiter and if there's no short, then plug the amp straight in to the wall, turn it on with the tubes out, and measure voltages on all the pins on the tube sockets and see what's what. Post them and then we'll go from there.
     

Share This Page