marshall valvestate 8080 repair tips needed

[skid]

hi all i have an 8080 that has a loud hiss that turns into a growl its there all the time , with or without guitar connected , if anyone can help with any advice that would be very helpful
' i have changed caps and reworked main board , hiss is present even with the valve removed

please help

skid

 

[Nigel Goodwin]

Well the valve doesn't do much anyway, it's only used on the overdrive channel - still, at least it's connected, many of the 'so called' valve amps don't even have the valve connected.

You need to provide MUCH more information, what effect do the controls have? - for a start, does turning the main volume down completely stop the problem?. Assuming it does?, how about the channel and reverb gains?.

This will help you to find which section the fault is in!.

 

[skid]

thanks nigel for your reply, just to fill u in where im at , i have checked 15v rail its fine , all electros have been changed , when unit is first turned on cold its fine for about 20 secs then hiss starts ,on clean chanel hiss is there all the time with no guitar connected , drive channel is silent until guitar is connected then hiss , no adjustment of any pots make any difference at all , hiss even at low volume , drive ect .

i have been over the ic's and small transistors with freeze and heat , i cannot interrupt fault .


amp still works but with hiss u can hear guitar , gets a little worse on od2 but i guess thats to be expected .
im down to changing small signal transistors , maybe ones failing under load ( they all check o.k.) but having trouble cross referencing part no on signal transistors

????????
does any of this mean anything to u

cheers
skid

 

[Nigel Goodwin]

Does it hiss with the master volume turned down?, if so the fault is almost certainly in the power amp.

 

[skid]

thanks nigel i will let u know how i go

cheers
skid