casio SK-5 missing voice 4

[wax+wire]

My SK5 only has three of the four voices working - I bought it second hand recently and have started bending/modding it, but I can't work out if this part was dead before I started on it, or after.

When I put a volt meter on the CPU MSM6283 on pins 89-91 i get a corresponding voltage (ramping up to around 3v) when a note is played, but the bass/melody envelope signal on pin 94 four channel 4 doesn't get me any sort of voltage.

What tells the CPU to generate the envelope signal?

Obviously that part of the CPU could be dead, but could it be that something isn't getting to the CPU to tell it to generate a envelope signal? 
Could I be looking in the wrong place, and should be looking at the pitch data?

I only spotted this when adding Graham's amazing Slow-AD mod, and was testing the four voices, but disconnecting the mod does nothing.  I have added a pretty extensive patch bay on the RAM/ROM chips, so there is a chance I've done something there? And I've added a LTC 1799 pitch oscillator too.

Cheers

[wax+wire]

solved:

I screwed in the bottom PCB [the one with the switches for the keys], because a few keys weren't working properly and it fixed the fourth voice.  weird!