Very busy with work and gigging this week - but will try and get time to take it all apart again and get those voltages as soon as possible...

Thank you for your help, Adrian.

Thought it might help if you can hear the problem so:

https://soundcloud.com/lucy-freud/marshall-5012mk2mp3

Let me know what you think!

Adrian.

The clean channel is muted when the boost channel is selected with the footswitch plugged in - I can hear a very distorted signal in the background (like the boost channel trying to work) with a clean but muted signal on top...  Hope this helps, Adrian.

Sorry - didn't mean to offend - really a beginner at this - although handy with a soldering iron!  The LED does work and does switch with the footswitch and the reverb switches in and out as it should...

Interesting - thank you for your quick reply.  I don't think it is to do with switching as without the footswitch plugged in you should be able to mix the clean and boost channels or turn one down completely to listen to the other.  My fault is consistent with or without the footswitch, and replicated with just the boost channel turned up and the normal channel off.  It also does not come and go depending on wiggling jacks, pots - or shock to the whole amp - the fault is consistent which is why I believe it to be at the component level and not mechanical...  But, I am open to all ideas!

Circuit diagram is

http://www.drtube.com/schematics/marshall/5210-iss3.gif

Hope you all can help - this amp sounds great on YouTube!

I wonder if anyone can help!?!

I have just had an old Marshall tranny amp come into my possession - it is the transistor version of the JCM800 (5210) and is meant to roar!  However, despite the clean channel having all the correct Marshall tones, the boost channel is thin and very quiet and appears to have some clean mixed into it - louder than the rather fizzy sounding distortion in the background...  I have cleaned the pots and switched jacks but no change...  I also took out all four op amps and replaced with sockets and new op amps of the correct type - that also made no change...  What would be the next move?