[morzh]

I suspect the tuning mechanism is the same as in 37-116. The gear itself will not slip, but there is the coupling that connects the output shaft of that whole concoction with the shaft of the cap. The cap is held by two set screws of that coupling and so is the shaft of the gear. Pretty straightforward, you just have to look what is slipping. You will have to pull out the chassis, but have to do that anyway.

Good luck and congrats, for 25 bucks this is a gift.