Hiya mate, damn that is a deep request!! Even if you had the skills i think you need an oscilloscope rather than a humble volt meter. This is not my area of knowledge sadly (I dearly wish it was) , but for starters things like the meter being less on one fader is usually the fader itself being dirty and not letting the right voltage through, so first a thorough clean of every fader & pot, BUT is it worth it?...
last year i spent 3 weeks cleaning every channel on an old Studiomaster only to find after all that it had some faults i could not diagnose and paying someone else to fix it was really expensive.. it might be more cost effective just to buy something s/h that works... that's what I did.
BUT if you clean it thoroughly it might be 100% after that, sometines they are.... all i can say is LABEL EVERYTHING YOU REMOVE before cleaning!!! - Label every channel & particularly every bus channel, because the busses have jumpers and if you put them back in the wrong order when you re-assemble then everything will be wrong.
then after cleaning it properly and putting it back together, if some pot or fader crackles or makes some noise then replace it would be easiest i think.
sorry I cant offer more than that - my one regret in life is that i never learned circuit electronics.
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