I'm sure that there are still a lot of old amps working perfectly but '
capacitor plague' was a significant problem. A company that I worked for had a recall programme to swap out boards with unreliable capacitors on. Some poor technician had the unenviable job of desoldering thousands of dodgy caps and replacing them with good ones.
Yep both of mine have been home to the factory for servicing but considering they were built in the very early 70s it's not bad going. More problematic is the preamp/control unit, the old 33 suffers from selector button woes and whilst QUAD can generally get em going again replacement is not an option.
I solved this by buying a 44 a few years ago. There is a guy in Leicester (well he was but then he moved to Littlethorpe) who deals in Vintage HiFi, mainly valve amps, and after I received some unexpected cash I gave Paul a ring and he had a 44 for sale. I was after the early Mk1as most of the leads plugged into the 33 would swap over and that was what he thought he'd got so I agreed on a price with him and arranged to go over after a couple of days (He hadn't seen it for a while so he had to have a look for it)
When I got there he'd got it out and plugged it in but it 'hummed' a bit through his set-up and it was a Mk11 (we thought) but in pristine nick with the box/packing/manual/mains lead etc. but he let me have it for the price we had agreed despite it being the later one which is worth a bit more (at the time (£300-£325 as opposed to the £225-£250 an early one fetched)
Anyway I got it home and rang QUAD to order a couple of plugs/leads for it and to enquire about getting it serviced if the hum persisted in my set-up to be told I'd have to change the power leads to the 303 but they'd get their 44 expert to ring me back once he came in. It turned out that mine wasn't a Mk11 but was the very rare Mk3 of which they only made around 3,500 and what had happened was EU electrical regs had changed and the earthing had to be altered from the original design of a 'double insulated' construction where the power amps were earthed through the mains connection on the back of the preamp to a 'bonded earth' where the signal lead earthed the poweramp, this dual earth had caused the hum but it meant that the 2 wire power leads to my 303s were the ones to use with just new plugs for the back of the control unit needed.
The good bit about this is my pre-amp is worth close on £500 as they don't come up for sale very often and more importantly for me is that they're one of the last products made in Huntingdon before IAG moved production wholly to China.