Good work fixing the B3.
I had a similar issue with an old G21.U. When I took it apart, the little buttons were all on the same plastic moulding, and it had snapped in a couple of places. A dab of superglue to weld the breaks, and all was well.
The buttons on my G3 (same chassis as B3) are still good. Looking inside my Zoom units reveals pretty good build quality for the money, I think.
Btw, what device do you use to measure the power consumption?