After hauling out my early AC-motor BeoGram 4002 from a year of storage, I discovered I'm only getting sporadic sound in one channel and nothing in the other. Found out that by tapping the relay board underneath the control panel I could get constant sound in the right channel, and isolated the problem to the plastic-covered mute relay. If I remove the transparent cover and press the contacts fully down, I get wonderful sound in both channels. So! The question is, I assume I should try to replace this relay with a new part? Or is it possible that the fault lies somewhere upstream, with a faulty component not offering enough juice to fully pull the electromagnet into position? Thanks!

I'd try just cleaning the contacts of the relay first, they may be oxidized because of lack of use. Don't use anything too abrasive; the contact points usually have a thin plating, and once it's gone they will get bad again very quickly. For similar contacts, I use some rough (hard) paper dipped into isopropanol - put the sheet between the contacts, gently push them together and pull the paper out. Repeat until the paper doesn't get dirt marks anymore.

Your theory is possible as well, but I suppose if the relay operates at all, the rest of the circuitry would be OK. Do check for obvious bad solders, though.