Hello everyone,
The problem is identified !
The serendipity discovery related below:
I was starting some more troubleshooting as you all
and others recommended.
I made a minor tweak on a capacitor, and then
turned on the record changer with motor in neutral position.
There was no regular pulsing/oscillation, all quiet
through the speakers.
Then, I engaged the 33.3 rpm speed selector, and BAM !
the pulse/oscillation came back.
I said, what the hell ?! So, I moved the speed to 45 rpm,
and the pulse/oscillation went up in frequency.
I thought, wait a minute, there's one possibility here.
That platter is made of steel, and the paint has worn off
all around the inside circumference, exposing the steel.
To confirm, I found another RCA RP-205 platter that still
has the paint on it. Tried it out, and no pulse/oscillation.
I surmise that the metal going through the motor's electrical
field must be generating enough current/voltage to be picked
up in the audio circuit. I even took out the cartridge, and the
sound was still there with the exposed steel platter.
I have saved all the suggestions for future reference for this
type of troubleshooting. Learned a lot from you guys, and
glad I still have some observational powers left !
Thanks to all,
HFF