Jack is good . Continuaty is good to ground points ,and the hot lead to C1 is good. I have a ohmn meter.

You tube post of a similar problem  " Marshall 8080 broken " will get you there if you are interested anyone. Mine is identical up to the 1 minute mark but not as loud.

Enzo. I have tried a different tube ,same deal. I will have to look at these chips again, 8 pin as i recall but I think I am missing something . Computer and amp are in two different places. Good point about the treble changing the pitch.  Thanks for help.

JM Fahey
I tried out your suggestions with a ground jumper. No difference in noise when doing this. Allthough on r2/c1 i got a big speaker crack but the fizz was still there.

Question; when looking at the schematic they have ic chip  6A and 6B. Are they refering to one side or the other of the ic 6 chip? There is only one ic 6 chip.


Enzo
Pulled tube and hiss was real quiet but the guitar volume was real low also. Switching channels does not kill hiss. More hiss the higher the gain or volume is set no matter what channel you use. Tone controls do not alter hiss but treble makes it higher pitch.

Help!
I bought this amp cheap and have been playing it for three years. It would hardly run when I got it and I replaced all the pots. Later have replaced ic line chips #3,#5,#7.(a you tuber suggested this but did not help)

It has a lot of fizz and as you turn it up on volume and gain it gets worse. I use it at home to practice but would be no good on a gig.OD 2 is real bad and not even usable. It seems that pedals in front help to quiet it down a bit. Pull the instrument cable out and it sounds like a blizzard is going on.

Also if you run the guitar into the effects loop instead of the input jack and run the blend at full effect setting, it is as quiet as a mouse. So the problem is pre effect loop which is 95 % of the amp. I am not a tech just a little handy. Thanks for viewing.

http://www.drtube.com/schematics/marshall/pc0689.pdf