Hi Jake, yes, I have the circuit diagrams for the 1228; it’s a delightful simple mixer, it sounds good and there really hasn’t been very much better in the years since then.
I will be sending the circuit diagrams by email direct to you.
‘B-check’ is just a simple switched function in the monitor section so that should be no problem.
As for cleaning, use WD40 or one of the artificial greases…. don’t use switch cleaner or the pots might easily fall apart!
I can’t help with the lamps; I would suggest altering it to take white LEDs.
The monitor pots were always the ones that disintegrated on these mixers.
All the best with the project…… it’s surprising how many of these old mixers are still around…… like me really.