Hello,

I have just inherited an Elka Soloist 505 synth from a deceased friend.

It's a very simple monosynth with a ladder filter but it sounds great!

It has a switch cap missing (and the switch itself seems to be broken).

Do you know if this type of switch and cap is available anywhere? (specifically the red button)

Can't help with a switch cap, but it's indeed interesting synth. I also have one, built into the Elka X-705 organ.

It has a bit unusual design for monosynth - because it actually uses top-octave divide-down circuit for an oscillator. The keyboard is connected to the array of CD4xxx series CMOS logic chips, which select the divider output corresponding to the pressed key*.
That makes it similar to some of the pre-Moog monosynths made by various organ manufacturers since '40s.
The square wave from the divider is fed through few waveshapers, one of which is quite convoluted sawtooth generator, and then into filter(s) - there are actually two ladder filters inside - the second one connected in hi-pass mode. Some presets use that to form a band-pass filter.
And the synth always switches into bandpass mode when you press the filter variations button, even if the selected preset uses lowpass only.

* keyboard is connected to the "voting circuit" outputting binary number of the highest pressed key, and that number is fed into the multiplexers which in turn select divider output.

Thank you for the info!
It's indeed a very interesting instrument.

After a deeper inspection of my unit, not only the cap is missing, the switch doesn't seem to work correctly (it works a push button but it should be an actual switch to choose between Variation and Preset). I'll have to open it and investigate what's going on...