I just bought a A-106-1 Xtreme Filter, I got it in the mail today!
Audio rate FM using four ADSR's in loop mode, the ADSR's are running in audio-rate.
ADSR1 is doing the syncronization.
ADSR2 is synched to ADSR2 and connected to a real ring mod, X input.
ADSR3 is not synched and goes into real ring mod, Y input.
ADSR4 is synched to ADSR1 and goes to CV3 (adjustable +/-10) on the A-106-1
real ring mod out is going into LP in on the A-106-1 VCF.
Resonance is up quite high, some kind of clipping is always active, either +, - or both.