The problem could be a lot of things and the first step is understanding how the
3, 4 or 5 inline balls are detected which results in credits.

One of the key items is the scanning disc. This is one of the motor driven disc stacks.
It scans the playfield switches for all possible combinations of 'wins'. There are also 5
relays associated with the scanning disc. These are the relays that are always 'chattering'
during a game when balls are in holes on the playfield. A 'win' is detected when 3, 4 or 5
of these relays are activated at the same time.

So the first thing to do is make sure the scanning disc is turning and these relays are
all working by putting balls in holes on the PF.
And check all connectors for corrosion and broken wires.
Good luck!
Steve