How do I fix oscillation sound after increasing volume nob past 70%?

@dman03could very well be a bad capacitor since it starts up at 70% volume. Sure, it could also be the speaker :-) I'd do some straight forward trouble shooting. Disassemble your cassette player an disconnect the speaker. For now just temporarily connect a small speaker to where the original is. If that eliminates the issue, then you know it is the speaker that is causing this. It would need to be replaced.

If a speaker exchange does not resolve it, you know that the issue will be one of the components on the main circuit board. I'd be looking at the amplifier circuitr y and see if there are any odd/missing or otherwise damaged components. Take some well-focused pictures of your board (both sides) and post those with your Question. That way we can see what you see and hopefully help you further, if needed. Bilder zu einer vorhandenen Frage hinzufügen

Use the Panasonic RQ 337 service manual to work on it.