HELP repair a Yamaha T100C combo

[packman]

Hi friends, need experts aid today.
I've bought an used Yamaha T100C combo, a Soldano project
realized by Yamaha in the '90.
Great amp but... clean channell is broken! Overdrive channell works OK but when i switch to clean channell i hear a low volume distorted sound, with input level and master at the max i can hear only notes played strongly like a heavy noise gate that block all the sounds under a certain level.
Changing the valves seems not resolving.
Where i can start my tests?

This is the scheme of the amp:

http://www.blueguitar.org/new/schem/soldano/yam_t100.zip

Any advise will be higly appreciate, thanks in advance.

 

[mjf]

hello.
i would check the parts around v1 (input tube).
power off!dangerous!
try to measure the resistors,look for a bad soldering point,bad caps.clean the switches,pots......with a contact spray.
greetings.........

 

[packman]

Thanks, i'll try that.
Meanwhile after the first post i've put the guitar on the EFFECTS RETURN jack to bypass first stages and the problem remain.