Vintage Synth Repair

Leaving the bass synth aside (which seems to function perfectly), 
that leaves the four oscillators of the synth section and the the 
four of the string section. Indeed, these are built using identically 
structured modules, the two types differ only in the wave shapes they 
generate. The synth section has the LFO,envelope and filtering 
blocks, and the string section merely has an amplitude envelope.
Here is where the problems arise: The 'hollow' button, which a 
universal 'controller' shall we say, is intended to merely subtract, 
as far as I can hear, the saw wave osc. from a given 'voice' ... say 
for example the violin. Whenever this 'hollow' function is engaged, 
it results in the generation of this G note regardless of the note 
played. Moreover, There is a related problem in that on the two 'full-
range' voices of each type - that is, viola and violin (for the 
strings), and the two highest 'registers' (for the synth), this very 
note does not play at all. 
Problem number two is similar, but a bit trickier. Although in all 
voices the low Eb plays fine, whenever one of two situations occurs 
this note sounds as well. Situation one is when the release slider 
for the string synth is set at all right of hard left. Situation two 
is whenever 3 or more notes are played! Needless to say, this is a 
bit limiting.