Help with troubleshooting Acoustic brand G20-110 amp please

[Tootsie]

Hi there gents (and ladies). I've had this little amp (Acoustic G20-110 solid state) for about 25 yrs with no issues, it's been a great practice amp. Of late it has developed a problem. About 5-10 mins after I have turned it on, it loses volume rather dramatically, and the sound is all fuzzy and crackly. The sound is as good as ever until this happens. Any ideas please? Thanks in advance.

[J M Fahey]

Hi Tootsie.
Being what it is, a small (although excellent) practice amp, there's practically no info about it.
Did you by any chance get a schematic when you bought it?
I used to be the official Acoustic service manager in Argentina, but the company went down the drain in the early 80's and its management had been crummy in the last years.
They only made 50/100/200/300 W amplifiers, and those 20W ones were a last ditch effort.
Is yours one of the brown units, or a classic black one?
How's your troubleshooting skills?
Because here the only sensible way is to inject some clean signal and follow it along its path and where it stops or garbles start measuring DC voltages around that stage; but you must be able to do that and previously  sketch a schematic, even a basic one.
Post a few clean pictures, both above and below boards.
If it were made in the last 10 years, it would be a very generic design, with a TDA something power chip and a 1 or 2 IC preamp; but yours is discrete all the way, if I remember correctly.
If any of the technicians which post here (including me) had it on his bench would certainly be able to repair it, no schematic, but without it I find it hard to explain you what to measure.
Maybe there is some Acoustic lovers board out there, do some Googling.
Good luck.

[Tootsie]

Thanks for replying Mr Fahey. It is a 'brown unit' and alas I bought it 2nd hand and it came with no schematic. Everything is on the one circuit board.