MOSFET reverb buzz

Discussion in 'Marshall Amps' started by Ebwelds, Dec 22, 2019.

  1. Ebwelds

    Ebwelds Member

    Joined:
    Dec 7, 2019
    Messages:
    52
    Likes Received:
    60
    I got a lead 100 mosfet. The reverb tank had a broken piece so I ordered a MOD replacement. So plugged it in. And I’m getting a terrible hum when I turn the reverb knob up. It doesn’t seem to be pushing signal through the tank. If I touch the springs I can clearly hear that in the speakers. If I move the tank away from the amp the humming goes away. Like maybe some feedback from the amp when the tank gets to close. Any help would be appreciated or anything about where to look to see if I have a bad part that is blocking signal from going to the reverb tank. Bad cap or ground maybe? I noticed that it only makes noise when something is plugged into the input.
     

    Attached Files:

  2. wakjob

    wakjob Well-Known Member

    Joined:
    Dec 29, 2011
    Messages:
    5,452
    Likes Received:
    3,808
    Location:
    C-137
    Good luck...there's a LOT involved in that reverb circuit.

    Start with the simple stuff like replacing the tank cables...
    or just making sure you don't have them reversed?

    You'll have to probe around that part of IC4 and all the parts connected to it.

    Flip the board over and look for cold solder joints/reflow.
     
    Mitchell Pearrow likes this.
  3. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member VIP Member

    Joined:
    Jul 2, 2018
    Messages:
    12,571
    Likes Received:
    23,133
    Location:
    Moreno Valley CA
    Plus 1 on checking that the leads are not reversed, the reason I know this, is I reversed them on a
    AVT50H, and got the noise, other than that I have no clue, as I keep the reverb off on mine.
    Cheers Mitch
     
  4. ampeq

    ampeq Well-Known Member

    Joined:
    Sep 14, 2017
    Messages:
    1,454
    Likes Received:
    1,460
    If all seems to be good with the cir-brd, you might try a ground and / or wrap the tank with a copper shield. Good luck.
     
  5. anitoli

    anitoli Well-Known Member Gold Supporting Member

    Joined:
    Jul 9, 2011
    Messages:
    9,037
    Likes Received:
    11,888
    Location:
    Lewiston, Maine U.S.A.
    Check the supply voltages, make sure you are getting +- 15v to the IC and +- 34 to the driver transistors.
     
    Mitchell Pearrow likes this.
  6. Ebwelds

    Ebwelds Member

    Joined:
    Dec 7, 2019
    Messages:
    52
    Likes Received:
    60
    I’m getting +15.8v and -16.2. I think I was running somewhere around 34-37v for that part. ( marked in yellow )
    I measured the ground on the send and return leads. Return 0v. Send -17mV. Which makes sense because of the resister in series after that goes to 0v line. ( red marks )
    I was thinking that something may be up with ( what I think is a feedback loop) marked in blue because it doesn’t hum until something touches the ground on the input jack.
     

    Attached Files:

  7. anitoli

    anitoli Well-Known Member Gold Supporting Member

    Joined:
    Jul 9, 2011
    Messages:
    9,037
    Likes Received:
    11,888
    Location:
    Lewiston, Maine U.S.A.
    What is the model number of that amp?
     
  8. anitoli

    anitoli Well-Known Member Gold Supporting Member

    Joined:
    Jul 9, 2011
    Messages:
    9,037
    Likes Received:
    11,888
    Location:
    Lewiston, Maine U.S.A.
    On the blue loop a direct ground connection there would kill the signal, but the arrow at JS1 usually means a breakable connection, how is the input jack configured?
     
  9. Ebwelds

    Ebwelds Member

    Joined:
    Dec 7, 2019
    Messages:
    52
    Likes Received:
    60
    It’sa lead 100 mosfet 3210 from 1989 according to the inspection sticker
     
    Mitchell Pearrow likes this.
  10. Ebwelds

    Ebwelds Member

    Joined:
    Dec 7, 2019
    Messages:
    52
    Likes Received:
    60
  11. Ebwelds

    Ebwelds Member

    Joined:
    Dec 7, 2019
    Messages:
    52
    Likes Received:
    60
    So would that blue line be like a silence loop that grounds any signal when the input isn’t plugged in? That could explain how there’s no noise when there isn’t a jack plugged in. But as soon as it is it breaks that silence circuit and the hum comes through...
     
    Mitchell Pearrow likes this.
  12. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member VIP Member

    Joined:
    Jul 2, 2018
    Messages:
    12,571
    Likes Received:
    23,133
    Location:
    Moreno Valley CA
    Are you running a foot switch ?
    If nothing is plugged into the footswitch the channels will bleed together, not sure if that has anything to do with what you’re hearing.
     
  13. mickeydg5

    mickeydg5 Well-Known Member

    Joined:
    Sep 30, 2011
    Messages:
    25,679
    Likes Received:
    12,707
    Location:
    US of A
    Wondering, which MOD model tank did you buy?
     
    wakjob likes this.
  14. South Park

    South Park Well-Known Member

    Joined:
    Sep 14, 2019
    Messages:
    3,319
    Likes Received:
    2,826
    Try to find what wire to the tank is causing the hum . Disconnect one at a time
     
  15. Ebwelds

    Ebwelds Member

    Joined:
    Dec 7, 2019
    Messages:
    52
    Likes Received:
    60
    i got a direct replacement to the one I pulled out. 8DB2C1D


    I don’t believe the wire is causing the hum. Although I most definitely could be wrong about that. But I don’t believe so.
     
  16. mickeydg5

    mickeydg5 Well-Known Member

    Joined:
    Sep 30, 2011
    Messages:
    25,679
    Likes Received:
    12,707
    Location:
    US of A
    Two threads, one tank.
     
  17. mickeydg5

    mickeydg5 Well-Known Member

    Joined:
    Sep 30, 2011
    Messages:
    25,679
    Likes Received:
    12,707
    Location:
    US of A
    Does it do the same with everything mounted correctly, amplifier box closed up properly and then plug into the input with volume?
     
  18. Ebwelds

    Ebwelds Member

    Joined:
    Dec 7, 2019
    Messages:
    52
    Likes Received:
    60
    i don’t believe so. I’m getting this issue with the master volume on 0.
     
    Mitchell Pearrow likes this.
  19. Ebwelds

    Ebwelds Member

    Joined:
    Dec 7, 2019
    Messages:
    52
    Likes Received:
    60
    yes
     
    Mitchell Pearrow likes this.
  20. Emtbreid

    Emtbreid Well-Known Member

    Joined:
    Oct 22, 2018
    Messages:
    594
    Likes Received:
    631
    Location:
    East Coast, USA
    I see what you did there..
     
    Mitchell Pearrow likes this.

Share This Page