Casio CZ/ VZ/ FZ - Pro Series

Hi, I have a Casio CZ 1 with some sort of problem with the audio output. 
  After doing a search and not finding something similar on the past 
posts I think that I need to ask for your help :)

Both line out and phones out share the problem. The output is fixed at a 
very very low volume. It sometimes changes, but by his own desires, from 
very very low to very low, or very very very low, and it ignores the 
volume set in the audio pot. Yesterday, trying to find the problem, 
there was about a second where the volume suddenly was restored to the 
correct value, but after powering the synthesizer off and on, it was 
again at the low value. Audio jacks are OK, audio cables are OK, 
internal cables from the synthesizer are OK too. I already restored the 
defaults and the battery outputs 3.45v, so I think it's OK too. 
Schematics and tips would be welcome.

Thanks in advance! :)

Manu