No Dirty Channel on Marshall VS100 Head

[ecopc]

Hi all, I have a VS100 head, both dirty channels have packed up, the clean works fine, if you press the clean/dirty button the led changes colour but the signal stays clean, I have changed the valve in the last couple of months. Where do I start? A schematic would be a great help if anyone has one

[ecopc]

Hi all, I have a VS100 head, both dirty channels have packed up, the clean works fine, if you press the clean/dirty button the led changes colour but the signal stays clean, I have changed the valve in the last couple of months. Where do I start? A schematic would be a great help if anyone has one

I have now got the schematic, any ideas what the problem might be?