why is the right channel on my cassette deck inaudible?

On my cassette deck Nak 680 ZX I can bias my heads for proper alignment. You might need to have it serviced if your deck does not have head adjustments.
A lot of good info has been given, check connection and clean the deck.
Be careful using q-tips some use an adhesive to attach the cotton. I get my q-tips from my local electronic store, they are made for cleaning electronics, no glue. I. Also use denatured alcohol as a cleaner. I was informed years ago that regular rubbing alcohol can leave a film on the surface it is cleaning.
Other things it could be: Volume gain adjustment is faulty or input source is faulty.
Once you have cleaned and checked all your connections and problem continues send it off to get it analyzed.
Joe

Don't use a "soldering gun", those are bad news. If you're going to do your own repairs, invest in a good pencil iron.

tried to re-align head with screws... very tight. couldn’t screw them left or right to save my life.

cleaned heads with alcohol & qtips, did nothing