Of course it's not cheap to get things repaired at Quad but you do get good service. I retired my 405 a few years ago Soundray as it started banging and crashing at switch on. I was advised to cease operations forthwith for fear of a damaging cascade that might take a lot of circuitry out
I've not had it sorted yet but I have read that the "protection" circuits in this design might be a bit over keen leading to the runaway effect mentioned. I believe that there are specialist replacement kits available but they need to be fitted by someone experienced-that's not me so I'll hang on for a bit and save up.
When the system was in use I did ring Quad and a woman [definetely from Cambridge] answered. I asked "Can you sort my Tuner and she actually said "We've got old boys here who can fix anything-some of them a've been around for sixty years". I said do you need all my details and she replied "No just pack it securely and send it over"
Dave W