An update to this, today I took apart the Q950's PSU, it's difficult to pull apart completely due to two separate PCBs which don't come apart terribly easily; the casing opens like a clamshell but not fully.  The "crackling" was related to a poor ground wire attachment, tightened up.  I can't see any obvious signs of damage, apart from years of thick black dust, most of which I could clean out.

The main transformer MP-130B (at location T5) appears to be the culprit for giving off a louder than normal humming sound.  With everything looking OK and not "crackling", I think I'm going to try my luck and fire it up - wish me luck!