Hi - DI is an IN5401 diode which gives the relay some smoothing and prevents it arcing -Easy replacement. Check the back of the board for damaged tracks these are often found when there are signs of burning. Next check R1 often open circuit and prevents 12v getting to the regulator - once this is ok check IC1 which is the voltage regulator connected to the big heatsink - you should see 12v going in and 5v coming out. Once this is all ok the only real problem areas are : The op amp IC2 , TR1 , TR2 AND TR3 apart from that its rare for anything eles to play up - Good luck and let us know how it goes - Marty