I bought this nice little mono synth on eBay in 2019. It was advertised as working and sure enough it was. SOme of the key tops were missing, so I managed to get some silver ones meant for EMS synths.
First I added a sub oscillator. The sound of this synth is nice but a bit thin. This mod will give a lower register. I used a 4013 driven from the existing 4013. Kevin Athertons website has details on how to wire this.
I also changed out the ceramic capacitors in the filter for some polystyrene ones. To be honest, I think I preferred it before with the 'non musical' ceramics.
I also tried to increase the volume of the noise source as it's a bit lame on this synth.
I also added an LFO rate LED plus a triangle ramp wave shape. The triangle was just adding a switch across the two terminals of the LFO speed pot. The LED connections are as below.
I had a bit of a disaster when trying to remove the control knobs. Some have allen screws! I just thought they were stuck, so I tried to pull one off and pulled the shaft of the multi switch through. It was a bit of a nightmare dissembling the multi switch and fixing it.