Roland SH1000 constant vibrato

I have one of these on my workbench which has vibrato constantly on. It varies with the rate pot, and the vibrato circuit does work correctly when switched in. I can’t work out what is causing it! I can see it on the scope even at the input of Pack #1 in the VCO circuit - but I have no idea where it’s coming from.

Everything else checks out in terms of the VCO and pulse board (as far as I can tell). Any suggestions gratefully received as I’m at the tearing my hair out stage!