Cassette 4-track preamps busted?

[Tavvish]

Hi there, I recently got a Yamaha mt120 4 track in semi decent condition except for a couple of things, but it works so I'm happy with my purchase and it will be a good opportunity to learn how to fix these things. Some of the switches and pots are really scratchy I should be able to fix this without too much head scratching, hopefully deoxit will do the trick I don't want to have to replace pots or switches.

my main problem is tracks 1 and 2 seem funked up, you have to be careful not to hit the channel too hard or else it distorts and the signal breaks up and sounds weird, the channels should handle the amount that I'm putting through it, most the time the db meter isn't even getting close to +3 before it starts to go crazy, sometimes the channel goes back to normal if you just turn it down to a low level but other times if you hit the channel too hard once the channel with sound like shit for awhile, you have to leave it alone for a while because you can run low amounts of audio through it again.

Can anyone give me a rough diagnoses? Could it be the dirty slider pots? Are the preamps done for? I don't have much experiences with electronics and down have an oscilloscope (yet) but I do have an iron and soldering experience.

When I get around to it I'm going to deoxit this thing, and get the heads cleaned up and everything. I still haven't done any of that since buying it. I really like this thing though I hope to get it in good condition.