This amplifier isn't performing too well. It's distorting, and humming slightly.
The amplifier is nicely laid out, although I'm not too impressed with the safety earthing arrangements. The earth is taken to a point a long way from where the supply is grounded. This can exacerbate ground loop issues. This amp hums, and I also have a friend whose 405 hums quite badly. Could this be the cause?
The ground connections to the speakers are also made separately...
Removal of each amplifier board is made easy by removing the two rubber bungs thoughtfully supplied by Quad, which allow insertion of a long screwdriver to gain access to the lower screws securing each board to the large heatsink at the front. The boards fitted to this amplifier are rev. 7 boards, and are also fitted with voltage limiters, so It's suitable for driving Quad's ESL speakers.
First inspection of the amplifier show some of the Red cased electrolytic capacitors are showing signs of age, looking a bit "sweaty" and having cracked cases.
Two of the caps in my amp (C17 & C2) are bi-polar, which is a pain. C2 can be replaced by a conventional electrolytic, with the positive side to ground, but C17 must remain bi-polar... which is a pain , because they're awful and I don't stock any!
Low-and-behold, C5, a 6.3V Electrolytic is leaking and has turned the pin green!
Both of the large 10,000uF capacitors test like new, so they can stay.
I decided, in an effort to mitigate any ground-loop issues, to ground the speakers and the mains safety earth back to the same point.
So after a blanket change of the capacitors, the amplifier powers up, and tests very well, with no adverse behaviour, and there's certainly no perceivable hum! All the unpleasant distortion has gone :)
There are quite a few websites detailing modifications to this amplifier, in particular Keith Snook's site (here), I did think about implementing some of these modifications, but I'm rather taken with it in stock form, so it will stay like that!
Guilty as changed, your honour ! (There's a joke in there!)