Fender Frontman 212R-Clean channel sounds "dirty"

[EronSilva]

I just bought a Fender Frontman 212R. The PO (previous owner) duped me by saying that the fuzzy sound on the clean channel was due to a blown speaker. Upon testing at home, I found the speakers are fine, the trouble is inside: the clean channel does not sound clean at all - the sound is fuzzy, like a distortion channel. The drive channel sounds "normal", that is, distorted.
What could make the amp's clean channel sound like that? What should I look into first? (I confess I am not very experienced with electronics. Built some kits with my sons (they worked after some heavy fiddling and consults to the seller, but they did). "Fixing" is something else - and professionals here in Israel are all sharks, so please try to help me!
Aharon

 

[chris661]

Hmmmm...

Are there any components that look damaged on the circuit board? Any burnt/blackened bits, bulging capacitors...
Anyway, don't worry just yet - a fault inside the amplifier will almost certainly be cheaper than a replacement speaker, so you'll save money!
Does the sound change as you alter the volume?

 

[Nigel Goodwin]

There's a 'preamp out' socket, and a 'power amp in' socket, so first thing to do is find out if it's the preamp or power amp that's at fault, the sockets make that easy.

It 'might' even be dirty switch contacts on the 'power amp in' socket, I've seen that cause distortion on some gear.

 

[EronSilva]

Thank you Chris and Nigel for your quick and insightful replies.
Nigel, my very first tought was that it might have a damaged switch. This amp however has two separate channels, and activating the second channel only added more distortion!
It is almost midnight right now, but tomorrow I will start by testing the preamp out/ power amp in, and check for evidences of damage on the components.
Thank you, and "shavua tov" (a good week)!
Aharon David

 

[EronSilva]

The problem seems indeed to be in the preamp.
Nothing seems evidently burned or "puffed up" (capacitors". The only thing I have noticed is the mids pot (R12), which has been replaced with a non-fender model - different from all others, this pot does not have the outside metal "feet" (like the one in the picture attached).
I have read in another thread in diyaudio (which I of course am unable to find again...) that Fender uses these "feet" for local grounding. Do you guys (and girls) think this could be the cause of the bad (distorted) sound I hear?
Aharon - crazy to plug and *$%@@#* play!

 

[WarrenT]

check to make sure the pot is the correct rating and that its grounded
you can get the schematic here:
http://support.fender.com/schematics/guitar_amplifiers/FM212R_schematic.pdf

if the distortion you are hearing is like an overdriven amp then it might be likely
that someone changed the pot to make it sound like that.