When there are selector faults on something like a tac20, especially ones that are intermentant, how often are these actually caused by weak magnets?

Usually a bad selector board.

I've replaced many selector boards and the problem went away. I'm sure that the board is usually the problem and I feel that on a tac20 the selector board is probably the most sensitive device when it comes to power surges. But I've also replaced selector boards,Checked magnet placement, relearned the hoistway,the problem seemed to go away, but after a week or so there will still be a few selector faults with possible causes that could be related to weak magnets. I would like to know what percentage of selector faults are caused by weak magnets in comparison to bad selector boards as well as bad cpu's. And, is there any advice on testing the strength of a magnet without having to replace.

PM sent.

I have heard of "weak magnets" causing problems. But in all the selector related issues I have worked on personally, none have come down to a bad magnet. Selector boards, sensors, guides and magnet placement but no magnets yet.

2nd the selector boards and guides. Have changed a lot of them.

I should add on this subject since the original question mentioned TAC 20. If you continue to get intermittent BP related selector faults despite checking magnets, replacing tape guides and boards, try hard wiring the Binary Position inputs back to the 188e. Its a well known problem that they are not reliable through LON. You need full IMS, a 5 pin red MTA, an 8 pin screw connector for the 188e and a couple of spare travel cable wires. Time well spent!