ZXM General Sound - not working

[Shockwav3]

Hi,
I recently assembled a ZXM General Sound. It's using a 18MHz crystal and a Z80 rated at 20MHz, ROM is 1.05, firmware upload worked nicely, using the 18MHz variant version 1.01 and later 1.02. External power is being connected and produces nice +5V and -5V voltages for the analog part.
However when I try to use it on my Scorpion 256 Turbo+ the system hangs when attempting to upload a tune (Alienate Demo by Hooy-Program or the GS patches version of Zynaps, when no soundcard is connected it loads everything into a void and then starts the demo/game). Test 4.3 hangs for a brief moment when trying to detect a soundcard. Sympthoms are the same with a real floppy or when using a .trd image from the HDD on my SMUC2

Removing the CPU and/or ROM doesn't change things.

Any ideas what to check?

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