Announcement

Collapse
No announcement yet.

Marshall TSL 122 positive bias voltage

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Marshall TSL 122 positive bias voltage

    Hey all.... Well, the Marshall TSL combo is back on my bench. I know, I know.. I just "fixed" it. Apparently it played very well for a few days. He loved the sound. He blew the HT fuse and smoked R63. Again. So, looking at the back of the amp, the right 2 power tubes, I have ~130-150 volts DC on pins 5 with the tubes pulled. The pot does nothing. The left pair has a proper negative voltage and the bias pot does affect the voltage. (-30--40 ish). The capacitor (C36) side of R67 (220k) I read around -42 volts... On the other side of the same damn resistor, I read +150... It starts out around +50 and climbs to 150. I've changed C36 and R67 and it stays the same... Anyone have any idea why this is behaving this way? I also changed the 5.6k resistor (R66) next to R67 and no joy.

    Here is the schematic...
    Attached Files

  • #2
    Why, but why start a new thread on the same amp?

    I could need some help locating the bias supply on this fragmented schematic. Where does CON1 from page 1 connect?
    - Own Opinions Only -

    Comment


    • #3
      Have you replaced C6 and repaired the bias board?
      I don't understand why you are looking at C36 when R63 has failed. Seems totally unrelated.
      Have you got the schematic for the bias adjust board?
      Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
      If you can't fix it, I probably can.

      Comment


      • #4
        Originally posted by Helmholtz View Post
        Why, but why start a new thread on the same amp?

        I could need some help locating the bias supply on this fragmented schematic. Where does CON1 from page 1 connect?
        Con1 connects to the bias adjust board. Hence my question about sight of the schematic as it will be badly damaged.
        Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
        If you can't fix it, I probably can.

        Comment


        • #5
          Lift one end of C6 and measure voltages again.
          - Own Opinions Only -

          Comment


          • #6
            I just lifted one leg of C6, and the positive voltage is still there.

            Comment


            • #7
              The bias board is fine. The voltages at con 1 are good. Negative on both side of the bias. And, like I said, the cap side of R67, it has the correct negative voltage. The other side of R67 has 150 Volts DC. Even after I changed it. And Jon, I noticed R63 burned and I was just checking the voltages in the amp before I installed tubes and I discovered the high positive voltage on pin 5 of the right side pair.

              Comment


              • #8
                Since "right 2 power tubes" does not tell us which exact tubes, best to disconnect the other coupling cap from PI as well (C7).
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


                Comment


                • #9
                  V5 and V6

                  Comment


                  • #10
                    And lifting the other coupling cap did nothing.

                    Comment


                    • #11
                      Originally posted by Danglin' Fury View Post
                      The bias board is fine.
                      Where's the schematic page showing the bias board?

                      - Own Opinions Only -

                      Comment


                      • #12
                        I have no idea. It wasn't in the schematic I got from the web. It's just 2 pots and the connector. I measured all the voltages at said connector and they are negative for both sides of the power tubes. All the way up to C36 and the cap side of R67. The other side of that resistor is positive, even after I have changed it.

                        Comment


                        • #13
                          Conductive power tube sockets?
                          - Own Opinions Only -

                          Comment


                          • #14
                            I don't think that's it.. I've done measurements on V7 and V8 and they measure the same as V5 and V6 and there is no visible charring. I'll look again, but... What gets me is R67. How in the hell does one side of it read a good negative bias voltage, the other side of it read +120 volts, I change it and it stays the same?
                            ​​​​

                            Comment


                            • #15
                              I found a schematic of the bias board...

                              Attached Files

                              Comment

                              Working...
                              X