I've done some checks with a scope and meter using an isolation transformer.
- IC supply voltage is -16.03v (should be -17v)
- 50Hz clock input goes to -16v with respect to Vss. See scope picture.
- All three switches are internally pulled to Vdd when not pressed, pressing each shorts to 0v.
- -35v supply reads -33.8v.
- Checked ESR of 50uF - checks out GOOD.
As suspected, all points to the IC having gone to silicon heaven.