[AndiiT]

Hi, From memory (and it's been a while since I replaced belts on a 3V29/30) the best way to approach this is to remove the two screws which secure the assembly to the chassis then release one of the circlips on the gear assembly pin to allow it to be withdrawn then replace the loading belt.

Upon re - fitting the assembly to the chassis you need to ensure that the tape guides are held back towards their rest position, I think you may only have to do this with the right hand guide however as I said it's been some time since I performed the procedure and no longer have a machine to hand to check.

The buttons not working are most likely due to a failed cassette lamp, there's a small white spring loaded plunger at the front left side of the mechanism under the cassette housing - pressing this plunger should light the lamp (in normal use the cassette would perform this action)

Regards

Andrew