https://www.flickr.com/photos/kriswood/5498051786

This (bottom unit)? It was made by a division of Koss that is defunct and no longer supporting its products, in a mix of Singapore, Taiwan, China, Japan and Korea - quite a range. I am speculating given that this is an R/C unit, which typically requires special internal controls. By the way, replacement remotes are readily available from the likes of Amazon and others.

Very likely a solid-state volume control - nothing you can get to or clean. Possibly a stepper volume control which uses a small motor and a conventional VC. Those you can get to and clean.

The on/off switch is likely a latching reed relay, about the size of a large squared-off jelly bean. You would have to replace this outright. Can you say "PITA"?

Do not clean the slide controls with conventional contact cleaner. That will require a cleaner formulated to those types of controls.

It may comes to pass that with sufficient exercise, the controls will clean themselves.

If not, you will have to troubleshoot on a component level, that is part-by-part. Given its purpose, I suggest you live with its quirks as from your description, it eventually does work. By the time you are done with it, unless you are exceedingly lucky or there is a directly visible problem, it will likely not work.

Peter Wieck
Melrose Park, PA