Hi Simon

> when I play keys, I can hear at very low level noise like the "gate"
> opening.
> The VCA envelope controls shape the envelope of this low level noise.

if you turn all four parameters of the VCA-Envelope fully
counter-clockwise. Playing a key you should hear a klick. -> if yes ->
VCA is good and the output socket is good.

Then set
VCA-Envelope to A=0, D=0, S=10, R=10
VCF-Envelope to A=0, D=0, S=0, R=0
VCF Cutoff = 5, Resoncance = 10, Envelope amount = 10(+5)
press a key several times, you should hear a fast resonance
"Zongggg"-like sound.
-> if not -> check the VCF-cutoff-modulation input socket. -> many other
problems tooo... ;-/ (see below)
-> if yes -> turn the cutoff frequency fully clockwise and turn up noise
volume: do you hear noise
If no ->
If you are keen enough to open up the monopoly (ATTENTION: live
endangering electric power inside!) set the keyboard to hold, press four
notes (or unisono) and check the noise and VCOs signal at pin 1 of
connector 14 (the connector is at the right hand side of the VCO-pcb
between the volumes of VCO1 and VCO2). If you do not have an
socilloscope, the connect the output with an mixer and use the tip of
another cable, connected to the same mixer as signal search (don't turn
up the volume too loud, the signals might kill your
tweeters/headphones/ears).

If there is still nothing to hear I assume that the powersupply is
defect. If there is something to hear, either the cable between VCO pcb
and VCF pcb is broken, or the VCF chip SSM2044 is gone. Sometimes the
2044 is socketed, then you might check whether the chip is seated ok in
the socket. But this requires unmounting the pcb from the frontpanel,
which is not much fun.

Florian