Digihome PVR 80, Vestel T810

[DAVIDTV]

I have one of this which gives the impression of poor aerial/signal.  At switch on I receive a few stations for a short while and then it appears to be low signal.  I have tried the tuners in an idential receiver and they work well.  I've also tried the tuners from that set in the faulty one and the fault persists.  I've also tried chaning the power supplies over and again the fault persists.   It would appear to be on the main board.  Can any one help?  Does anyone have the circuit diagram or know where I can obtain it?
regards,
David.