i´m no synthtech and have no experience with the jx8p...
but did you try this: after power on press a key and count at which number the dead voice appears, power off/on again and repeat it, same number?
check several times...
if it´s the same number over an over again, then it might work like on an esq-1 with the filter chips.
now you got to check which chip it is, swap the suspected chip with another one, repeat above procedure, does the number of the deadvoice change?
if it does, you got it narrowed down to 2 chips, keep on swapping till you got the bad one
if it doesn´t try other candidates...
keep in mind im no expert, this is at best a sophisticated guess how to solve the problem
good luck!
hannes