Beware of the flux

While troubleshooting a Sequential Circuits Prophet 5 synthesizer I came across a quite odd phenomenon: several control voltages, generated by means of multiplexers and some dozens S&H circuits, showed a sawtooth waveform instead of a clean DC voltage. While a falling sawtooth would indicate some ground leakage, I also had some outputs showing a rising sawtooth – so there must be some stray current from a higher potential.

Visual inspection revealed a white, wax-like substance between the pads of some multiplexer IC sockets, obviously flux residue from a previous repair attempt. As both PCB sides were affected, all sockets were removed, the PCB cleaned with isopropyl alcohol. After replacing the parts all control voltage were clean again.