Hi Brian,
You could try the same procedure that works on the PSR-S970 and Tyros models which involves holding down three Voice section buttons whilst turning on the power.
The procedure is described in section F.152 of Michael Bedesem's 'Frequently Asked Questions' document.
Here is a link to the PSR Tutorial page about this document. There is a link near the bottom of the page which opens the document...
https://psrtutorial.com/lessons/faq/PSRFAQ.htmThe equivalent Voice section buttons to hold down on an S975 would be Brass and Synth (on the top row), and E.Piano (on the 2nd row).
If you try it and it works on your S975, please let us know since it could be useful for others.
Regards
Ian