Discussion about the Korg PolySix synthesizer

Ok,

The card seems ok, 
The battery is the old style VARTA, but is not leaked (til 
yesterday...). Proabably is the second or (I hope) the third one 
(changed in 1998 maybe)

The P6 is outputting sounds, all the voices are working, and the adsr 
can be modified and the filter and resonance too (Keyboard tracking 
too).

but is not changing:

Waveform, Octave 4 8 16, pwm, sub osc mode, lfo destination and 
effect type.
(all digital paramenters?)

+

modyfing the intesity of the effects is acting on the LFO speed.

+

effect number 3 goes in "mute mode (no sound at all).


all the analog parameters are changing (I checked with a voltometer).

I checked all the traces and vias with ohmeter (full conductivity, no 
resistence.

My question:
can be the the IC31 the culprit ? What's the exact function of the 
74ls08 in that point ?
a broken ic31 can affect the parameters I described ?

can a 74hct08 can be used instead of the 74ls08 ?

Thank you

Antonio