HELP diagnosing guitar amp Bass pot please

[sonolink]

Hi guys,
A friend asked me if I couild fix his Hughes & Kettner Staesman Quad EL84. The amp would loose sound after a few minutes of playing. I replaced the tubes and the amp is now working ok, I've had it on for more than an hour and it sounds fine BUT the Bass knob doesn't seem to work or do much. So I wrote to H&K and they sent me the attached service manual with the preamp schem but TBH I'm a bit lost here...

Can someone please help me pointing me in the right direction towards what/where to look for?
Thanks in advance
Cheers
Sono

 

[beatnik]

Bass potentiometer seems to be dual section 500K log on schematic.

Not uncommon to see 16mm pots fail on guitar amps, particularly when the pots are PCB mounted, the carbon tracks can easily crack with the kind of abuse that combo amps have to endure during transport.

You might be able to check the pot is varying resistance while in circuit, if there is some resistance change then you probably should look elsewhere.

 

[beatnik]

It could be it's loosing bass before the tonestack, so the bass control doesn't have much to begin with. Check frequency response on the output of the preamp tubes