Marshall valvestate Channel Switching Problem

[DaveLMI]

Hi,

I have a Marshall VS100 Combo amp. I have had several issues in the past with channel switching, usually buying a new footswitch sorts it out.
This time however the footswitch does not seem to be at fault. I have swapped the switches, checked the cable and even hardwired it to the socket, (the socket has been changed too).

This is a two button footswitch but will only switch between clean and OD1. I can manually select OD2 from the front panel of the amp, however if I do this I can't un-select using the footswitch.

I've done a bit of digging and seen the the Mosfet transistor 1 (SA872E) on the backboard controls the switching; does this only control clean to OD1 or all three channels? I'm thinking the issue is with the internal board but not sure what needs changing out?

Located North East England.

Thanks in advance,
Dave

[DaveLMI]

Hi,

I have a Marshall VS100 Combo amp. I have had several issues in the past with channel switching, usually buying a new footswitch sorts it out.
This time however the footswitch does not seem to be at fault. I have swapped the switches, checked the cable and even hardwired it to the socket, (the socket has been changed too).

This is a two button footswitch but will only switch between clean and OD1. I can manually select OD2 from the front panel of the amp, however if I do this I can't un-select using the footswitch.

I've done a bit of digging and seen the the Mosfet transistor 1 (SA872E) on the backboard controls the switching; does this only control clean to OD1 or all three channels? I'm thinking the issue is with the internal board but not sure what needs changing out?

Located North East England.

Thanks in advance,
Dave

Just as an update - I swapped out the T1 transistor and now I can switch from od1 to od2 but can't access the clean channel unless I use the front panel, turn off od2 then I can switch between clean and od1 - I feel so close to cracking this, any suggestions? I tried changing t2 as it's right next to it but that did nothing?

Thanks.

[g1]

Both front panel buttons must be 'out' when using the footswitch.
With both buttons out, what are the new symptoms?

[DaveLMI]

Both front panel buttons must be 'out' when using the footswitch.
With both buttons out, what are the new symptoms?

Yeah I've had this amp years, I know how the front panel buttons need to be set. I've put the amp back together and now I just get a realky loud hum, it's going in the bin!