This is a really interesting post:
Why are you re-capping it? Just curious because I talked to a tech at Mesa the other day and he said that unless there is a cap problem than the whole "recapping" craze is overblown and totally not necessary.
I recapped my Quad a few months ago (as some of you know) along with replacing a load of other parts at the same time. My Quad was a wreck when I bought it (noisy clean channel, switching problems, intermittent buzzing/humming, crackly pots, lead-1 channel not there at all, etc. etc.) so after reading up what I could about Quad related problems and studying the schematics I decided to throw a crap load of new parts at it. The main caps I replaced were the four 30uF 500V caps the two 220uF 350V and the two 220uF 63V. Trying out the repaired pre I found that all of the issues had cleared up. All channels behave perfectly, zero noise and no crackling. I'm still wrestling with it to find my tone, but that's another story.
Later on, I gave the old caps to a friend of mine who tested them for me on an ESR meter just out of interest. We found that all four of the 30uF 500V caps were measuring around 1ohm which basically means they were still in almost brand new condition. The 220uF 350V caps were not quite as good but still very usable and the 220uF 63V caps were a little worse and probably needed replacing soon anyway.
Hang on there, I'm getting to the point of all this:
The problem was obviously that I replaced a load of stuff at the same time so I have no definite idea what was the main culprit. I later tested the old LDRs that were replaced and found that one of them had a high-ish resistance and another one gave up after a few minutes of use so it's very probable that LDRs were the culprit in some of the issue areas.
I took me the most part of a day to carefully strip that pre down, note where all the wires were soldered and get to the underside of the board and start pulling components. Not a thing a wanted to do very often; infact something I only wanted to do once if I could at all help it. While it was in bits I wanted to change anything that looked like it could give trouble so I guess it was worth doing. If I hadn't replaced those caps there's a good chance that the tone paranoia would set in later on and I'd constantly be thinking "I wish I'd changed those caps" and worrying about it giving up on me during a show or something.
I guess if you're thinking that a recap is in order for whatever reason you can probably be sure that the caps are in good order (apart from the 220uF 63V), but if you've got the thing stripped down on a bench, I don't see any harm in spending a few quid or bucks or whatever on some new caps. You won't want to pull it apart again in a hurry!
