Carlsbro Glx documentation - pre amp problem

[tedsorvino]

Hi. I'm using an old Carlsbro Glx 80 for about 20 years now.
It's well maintained but suddenly the preamp section on both channels went really weak (I checked the other sections - power amp in, effect send- and they work fine) .
I can't figure out what could have gone wrong since all the mechanical parts work ok....Leads, pots, etc. .
I 'm not sure how it is constructed so I'm looking for any relevant or specific documentation (service maual, schematics etc.) of any model of the Carlsbro GLX series- especially something close to this one.
Thanks in advance