I think he means the motorized "Spinning disc"

the "solenoid expander boards" are tiny boards that drive the flashing pop bumpers

I am not sure how they are related ..... is the connector keyed correctly on these tiny boards?

This suggests there should be a lamp socket near it and needs a good bulb http://techniek.flipperwinkel.nl/ballyss/rep/index3.htm#expand

Is the ground braid properly attached in the head?

The schematic shows the fuse for the spinning disc on the transformer assembly

Did you place the wire correctly into the connector after re-pinning , is the rejection key in the correct place ?

Bally Fireball Classic sol ex board (resized).jpg