Alright, not much of a mystery, but have been drawing up some new PCBs over Christmas and wanted to check something on the System 100 (not M) VCF – the original schematic specified a dual 100KA for the resonance potentiometer, but I always suspected it suited a reverse audio pot better.
Here’s a picture from my 102, with the panel flipped down in front of me (so top here is bottom of the panel, just to make it confusing)
Check the orientation of the embossed numbers on the back of the pots, and compare the (upside down) resonance and HPF pots against the right-way up mixer section (RM, VCO and Ext.).
I’ve had a bunch of Curetronic 100M PCBs for ages and done nothing with them, so finally decided to start building them up as a test for Schaeffer UV-printed panels… only because I wanted to test out my version of the OpenMusicLabs clown/clone BA662, I built a single one up on stripboard.
For stripboarding I’ve switched to Veecad, which is much less pretty than DIY Layout Creator but at least allows for verifying the layout using a netlist exported from Kicad. Here’s the Veecad layout using a BA662 as the OTA:
In case you don’t have a BA662 clone or original to hand, here’s a stripboard adaptor for a BA6110. Without any other compensation it’ll be slightly quieter than the ‘662. I measured 2.68dB difference between the two, everything else being the same.
If you build the VCA, don’t be tempted to socket the transistors – I ended up frying a small pile of my clone ‘662s due to dodgy connections.