Kenwood KR9600 receiver

+1, on the cable.     If the problem doesn’t manifest, with any source but Phono; it’s not the power supply.     Try cleaning the selector switch, with a good contact spray.      There are a number of caps in the phono section, that could cause an issue, given the piece’s age.      Has the receiver ever had an overhaul (especially: electrolytic replacement), in the past couple decades?

I’m not trying to be funny.. OK.. Is there a TT plugged into the phono section? OR something else?

If it is a TT move it somewhere else, see if it still has the yo yo effect. (just might be a lot quieter)



In the off position and unplugged:

Flip/turn the switch a few dozen time.

THEN "tap" with a wooden pencil lightly.

Do that a few times... Clears the contacts, SOMETIMES..

Make sure there is no damage to the RCAs, in or out, clean and rough up the contact surfaces. At every point check, if something changes.

None of this stuff cost money ok...

If all else fails, on the bench it goes...and the cleaning and repair journey begins.. THEY bite.. be careful.. Discharge the caps.. AFTER you remove the cover, don’t mess around.. Now look first, then clean..
Might see a nest of  your favorite spider/Smurfs in there :-)

Regards

Check the cable from your phono.   I had the KR9600 about 48 years ago.  I believe it is a very powerful receiver at 120wpc?  I liked the tiny power level meters at the very top of the tuner dials and it lit up a nice blue.