Good forums to help fix electronics?

[tjohnson]

Massively off topic for Amstrad but thought someone might know some good forums where I could seek help on fixing a double cassette deck.

The original issue I started to diagnose was the recording deck played fine but the recording would intermiitently distort on one channel.  I have access to a scope and observed the bias signal voltage on the affected channel changing very erratically.  However in my effort to diagnose I seem to have introduced a playback issue on that deck which plays back at massive amp and hugely distorted however it still records.  I think it may be in a multiplexer IC which takes the inputs for deck A and B then outputs them.  Deck A seems to play back more or less ok.  Same IC and outputs but seperate inputs.

I have the service manual and it has a very good circuit and block diagrams but with my little experience struggling to pinpoint this new fault but being a hobbist and complete amateur I'm very much trying to learn as I go along.

[Bryce]

Send me the schematics and I'll see if I can help.

Bryce.

[tjohnson]

Hi Bryce, thanks for the offer of some help.

I've attached the main schematic and a block diagram.  The original issue appeared to be a bias issue, i hooked up the scope to capacitors C130 and C230 and could see on record the bias on the right channel C230 bouncing up and down alot in voltage.  Anyway I was trying to determine the cause of this when I then noticed the playback on deck B had gone completly distorted and pushing out massive amplification where previously it had worked fine.  I've attached a sound clip with playback on deck B and then playback on deck A which still seems to works.  I was suspecting an issue in the multiplexer IC301 on in that region of the circuit as playback A still works, however I'm really clutching at straws here and could really do with some guidance on where might be suitable places to check and understanding the circuit digram, it would be really appreciated.
Trevor