Probably need more background information.
Let me try here - coin comparator not working, actually 4 of them not working. Probably not a problem with all 4 comparators. Wire or power problem most likely, could be a door optic/sensor problem. Maybe just the sample coin not installed, probably not as machine was working with temperamental BV.
Temperamental BV - probably a bad BV power supply or wiring problem. I will assume the ' No sound when putting back the hopper ' is actually the BV not the hopper. Probably the BV power supply gave up the ghost and is bad.
No sound when putting back the hopper - if that is a correct statement then need more explanation.
Maybe some background information on why the coin comparator needed to be looked at and/or replaced would be helpful.
Tony