Multiple, but not all, buttons don't register

Newbie here -- I just bought an MPC2000xl (off craigslist -- this is how all these posts start!), and it has a problem I haven't seen anyone else complain about yet. About seven of the tact switches don't register. These include the >shift< switch, which makes testing the machine rather a challenge.
So I opened it up and removed and replaced the shift switch. No help. So I put my multimeter on the switch I removed... and it seems to work great. I closed the circuit across the board manually (and using my new switch) and it still doesn't work. The darn switch only has two traces, and they both look clean as a whistle.
So here's the question: anyone have any ideas what might make seven switches (shift, 4, f1, f5, 'A', 'B', 'main menu') pretend to be dead when they are really alive? It's making me very sad.

Your issue could be any number of things.

Schematics are available here: http://www.mpc-tutor.com/akai-mpc-manuals/

Since you have a multimeter, ring out the traces on the schematic and make sure none are broken or damaged including diodes. There may be a crack in a trace or a PCB.

Also check other switches and make sure they are not broken or stuck shut.

Thanks -- I don't want to seem like I know what I'm doing, so I'll be clear that I don't know how to test diodes without risking their deaths... By 'ring out the traces' I'm guessing you mean check them to be sure they are intact (with my ohm-meter).

The schematic does show that all my "bad" switches on the main control board are on two traces, 'y4' and 'y0'. The bad switches on the upper right board don't make quite as much sense (maybe ry6 is the bad guy). I can't test the red switches' functionality (notably sw444) other than to see that they light their LEDs: does this mean anything? I think my question means that I'm having trouble reading the circuit diagram

Maybe I should give the thing a good cleaning? Thoughts, anyone?

Again, thanks for the help so far.

Update: I replaced my daughter (B) operation board (the upper right board), and my A, B, Track Mute, Next Seq button circuit is still bad. I'm figuring this means either my header (or whatever 'J500' is called) is bad or the 'cable' (bundle of wires) has a bad wire #1, or the problem is on the main operation board.

As an electronics newbie, I can't figure out how to swap out the wires in the cable. Are they soldered into the headers (or junctions, or ) or can I just pull them out?

Any help greatly appreciated.