This mod was sent to me by Brian A.
PERCUSSION/MAIN SIGNAL BALANCE MOD:
If you look at the percussion amplifier, you can see that R226 (180K) and R229 (68K) form a passive mixer of the percussion signal and main signal and feed into C214. Conveniently, 180K plus 68k is about 250K. So, I replaced those two resistors with a 250K linear pot with the wiper hooked up to C214. There is a perfect spot in the percussion amplifier enclosure to drill a hole and install a pot - and it's easily accessible when the preamplifier is installed in the organ. I ran leads from the lugs on each end of the pot and soldered them to the circuit board to by-pass R226 and R229. I unsoldered the lead of C214 where it attached to the junction of R226 and 229 and resoldered it directly to the pot wiper lug. Very clean - and now I have a 'balance' control.