barryg

What was wrong with your amp the first time you sent it out, and did the amp have this problem when it came back from Marshall ? There's alot going on in this amp, but going by your post, I would reseat all the tubes (power tubes also). Try tapping on each tube with the amp on to see if makes any difference. You may have to open the amp and check all the connections, socket joints, etc. as you may have a bad connection or solder joint somewhere, which I've seen happen due to rough shipping. Check the channel switch connections and relays also. If you don't have experience with the deadly high voltage here, you'll have to send it out again. Hope this helps....barryg