Vintage Synth Repair

I don't have a QS6, but the power supply caps are the most likely cause of the problem. They are usually not difficult to replace once you get the synth opened up, and the values of the capacitors are not critical. The rule of thumb is to replace with caps of equal or greater capacity. The only way to find out if they are causing the hum is to replace them. Usually they are not expensive to buy compared to some other types of components such transformers, for example.