Vintage Synth Repair

Hello;

I'm currently trying to fix a Korg Micro-Preset with a few minor issues.
Some presets don't work at all; and the filter only works for the Brass
presets (and not for the synth presets, as it should). Everything else
seems to work fine.

Unfortunately the PDF of the service manual I have is hardly readable; so
firstmost I wonder if there are more legible PDF's around?

So far I've narrowed things down a bit. The Micro-Preset seems to use two
filter chips (the Korg 35 design), those are IC6 and IC7. It appears to me
as if IC6 is the brass filter and IC\ufffd7 the synth filter (without IC7
there's no sound from the synth section at all).

The presets affected are in the "Wood" category, the 2", 4" and 16" preset
namely. All those are (as far as I can see) derived from the rectangular
wave (8" and 32" use the pulse wave instead; those two work).

As I said the service manual is very hard to decipher and before I start
probing around I would like to ask for some help or comments.

Thanks and best regards,

Malte