The reason your machine won't accept coins is probably that the coin lockout coil is not energized. That is a solenoid under the coin mech that puts a blocking pin through the mech. You can see it if you unclip the mechanism. The solenoid should be energize (pulled in) when the machine is waiting for a coin, and it drops out when coined to stop you inserting another. It will then energize again at the end of the game ready for the next coin.
Actually the mystery of the payout control box has been investigated and is (at least partly) understood. The control for the coin lockout comes through the payout control box, which won't allow it to energize while the game is paying out. But the primary control comes from a microswitch on the back of the mechanism, which operates when the mechanism finishes its cycle after a game, and after the fingers have dropped to detect a winner. Now that microswitch might be out of adjustment, or else there may be damage inside the control box like a burned track. It's all repairable. Check the coin lockout first and verify it isn't burned out and doesn't energize. Then operate the microswitch to coin the game, hold up the solenoid on the right of the reels to allow the handle to be pulled, and pull the handle. See if anything changes. Do this until you get a winner, or hold the fan and set one up manually, and see what happens.