By 'AV' do you mean Composite? I think the RF and Composite outputs compose of a different output to what the RGB / Video port provides - so the picture from the 23-pin Video connector could well look fine and you should test it if you haven't already. Of course though that would require an RGB monitor or an RGB-Scart lead and Scart capable tv. Alternatively, an A520 modulator could be used. Alternatively, like you mentioned you could also try and fix the fault - you can start by downloading the A600 System Schematics if you fancy some bed time reading!
