TBH Tony - I would fire it up, as it is - those caps look fine.
The tops are very obviously bowed up, it’s actually visible in the ones that have lost their plastic top-caps in the pic, you can see a very distinct arc in the cross-hatch. I’d say they were all about 1.5mm up from being square. This concerns me as with computers etc it is a sign the thing is about to crap itself all over the mainboard! Obviously you know *way* more about this stuff than I ever will, but the tops of these are anything but flat. In comparison the little 220uF ones look absolutely fine based on what I saw behind the heatsinks I removed.
I’ve already taken it back out of the system, shelved it (it will actually fit in an Ikea Expidt!) and I’m really hoping there are some suitable in-stock caps out there somewhere and I can get it boxed off next week sometime. I’d be interested to hear what you & Rob think of the Vishays at RS. They are available and will fit!
If the amp has not had much use the electrolytics could all be fine. I suggest measuring them with an ESR meter before changing them. Even an in-situ measurement should give you some idea of the state of them.
It is in nice condition, but I suspect it has certainly seen some use, the caps wouldn’t be bulging if it had just sat in its box, plus there is some evidence of ‘purpling’ to the heatsinks which is apparently what happens to these with use. Annoyingly I haven’t got an ESR meter, only a multimeter (and an oscilloscope). The whole thing about the way this thing is constructed makes me want to just replace the caps once I’ve got it far enough apart to test. To get to the main PSU cap board really does involve full disembowelment and a fair bit of desoldering as part of that process.
I believe Pass uses a varistor to limit the inrush current. Point your thermometer at the disc thingy on the cap board - it should be pretty hot when the amp is on.
Yes, there are two of them in there.
I will be interested to read if the Aleph floats your boat medium/long term.
No idea at the moment. I only really want to judge it after the rebuild. My impression so far is I don’t think much of it cold, but it certainly starts to open up and get some groove after a while. So far, and I know no one will believe me, but it really wasn’t worrying the 303 much! After a good hour running it was getting there, and it clearly does some things very well indeed, but there is something very restrained about it before that point, and that didn’t entirely go away. This particular 303 (which has far larger coupling caps than normal and is a totally obsessive rebuild) just swings into Tannoys. It has the funk, sounds big and punchy, and great at low levels. I just kept wanting to turn the Pass up to get the impact I’m used to (I’ve got a sound meter handy so I know where I was). Some of this may be psychological as I just don’t trust a 25 year old notoriously hot-running amp with obviously bulging PSU caps into a pair of such valuable and exceptionally hard to replace loudspeakers!
Once rebuilt I’ll certainly get to know it properly and leave it in the system as a daily driver for a couple of months to get used to it. I’m naturally very patient with this sort of thing, so I’ll wait until I’m confident everything is exactly as it should be and then give it a fair chance.