Help fixing a transistor amp

[Julien]

Hey guys, i have a Bang and Olufsen amp from 1970 (Beomaster 1900). A few years ago the right channel went on it.. and now the left one when as well. I have found the schematics online but they are a bit overwhelming.

Here are the symptoms of the amp: Works fine other than the sound is not amplified. The only sound you hear from the speakers is the amplification from in the Input. As far as i can tell the bass/treble/balance don't work either.

In the schematics there is a picture that resembles this:
Input -> Tone Amp -> Power Amp -> Speaker output.

I am guessing the "tone amp" is maybe dead?

Nothing looks burned (mosfets look fine, caps have tested out fine). I can sent hte PDF of the schematic to anyone who has knowledge in this feild and is willing to help me out.

http://www.beoworld.co.uk/beomaster1900.htm

Thanks
Julien

[FredK]

Which caps have you tested? I'm no expert, but usually the power supply caps go bad over time (especially over 36 years!), and because they send unfiltered AC to the amplifier inputs, the sensitive output transistors can get damaged. However, I bet there'd be a current limiting diode protecting the output transitors. I'd check those next.

Have you verified the power supply is outputting the correct voltage?

A good forum to visit is www.diyaudio.com, maybe they'd be able to shed more light on the subject than r3vlimited.com!