the search relays powering isn't really random
as the search wipers spin, they are checking to see if balls are in holes in the playfield. If the hole switch is closed, the appropriate search relay is powered. If three+ adjacent search relays power at the same time, the search index coil is powered to stop the wipers on those rivets and the wipers are held there for the duration of the payout.
the info on manual pages 162 and 163 tells you which holes are being checked at each of the 50 wiper positions. Note the wipers are spinning clockwise, so the chart is traversed from the bottom up.
on big time the search wipers spin all the time, and although the search index coil is prevented from powering until the selector lock trip relay trips (4th ball is shot - or 5th ball if you have before 5th lit), search relays start chattering as soon as the first ball lands in a hole.
later bingos locked the search wipers and you had to push a button to release them to scan for a win. The early games preferred the noise and wear-n-tear.
the easy way to test basic function is shoot balls and dump them in holes to light 3+ adjacent numbers, verify the selector lock trip relay tripped, and if the search index coil doesn't power, hold down 3 adjacent search relay armature plates yourself. if the search index coil powers then, the problem is likely cruddy search disc rivets (search relays not powering when they should). If search index coil doesn't power, it's time to debug the circuit thru the search relay switches with a voltmeter and/or jumper wire.