Vintage Synth Repair

No, no, no:

You're not supposed to solder the connector to the PCB, just the wires to
the connectors! The stock ones are only crimped.

But before you jump to this let's examine the problem a bit deeper. Since
all three oscillators are affected it is very likely that the source sits
somewhere before the summing amp of each oscillator. I'd try to see if one
of the various control voltages suddenly changes. Same for the supply
voltages.
The common control voltages are

- Bend
- Tune
- Keyboard
- Mod
- Ext

if I'm not mistaken.