Seriously though, my first step would be to connect a wire from a genuine earth like a water pipe or your electrical system ground/earth to the chassis of the synth. After that, not sure but Dave will know.

There's some hum from the left speaker, which is probably mains interference. I did see a small 60Hz signal on the output of the op amp, but that was when using the old power adaptor. I haven't tried it with the replacement. Something may have been damaged, or perhaps I did something by accident while opening the keyboard, doing my investigation or when putting it back together.

Here's the schematic from Yamaha's service manual, fascinating!

Ron

The difference in leg length is inversely proportional to the square of the distance you are from the prime meridian, but only for leap years that are also Fibonacci numbers when the month has an R in it.

Of course, if you are well grounded or earthed, as you might say, then it is irrelevant. 

Take care my almost royal friend,

Ron

   Fine ... I was wondering why my left foot seemed further away from my head than the right foot .... take care, my friend, Dave

  We all use shorthand descriptions, especially when we know the other person(s) we are talking to. When I don't know the other person's experience, then I confess to tending to over explain, on the basis that being a bit over the top is probably safer than leaving someone to have an accident or create a new problem.

As to colonialist - imperialist paradigms, with the dubious exceptions of some small islands, I don't really feel that has applied to the UK in my lifetime, and certainly never figured in my thinking. I just try to help people to the best of my ability.

Best wishes, Dave

 Sorry if I misunderstood your comment, but you said ", placing my scope across the speaker terminals ", which I interpreted literally.

With the class AB and B outputs, which were common in the late 1960s and early 1960s, when one speaker terminal was often connected to ground, this would have been the same thing.

So far as I remember, the term anti-phase is common with respect to linear amplifiers and op-amps, when both the incoming (non-inverted) and the anti-phase (inverted) signal passes through the amplifier. Perhaps referring to anti-phase is more common on my side of the Atlantic?

 Best wishes, Dave