Hmmmm a bit of a problem here….. I would suggest the quickest way is to replace the chips in the offending channel; it’s doubtful if its mechanical. If the crackles vary with input gain control, then it’s the front-end chip, and that’s the most likely. Replace with the same type, or if it’s a dual, I suggest using MC33378 as an alternative to TL072. It’s best not to go for the lower noise modern equivalents as you could run into stability issues.
I’m sorry I don’t have circuits for the later 828S mixers.
As a last resort, I can probably fix it for you, but sending a mixer to Torquay is fraught with risks.