Trying to help troubleshoot a friend's FT. Any help is appreciated.
Basically, when the game is turned on and the MPU boots, the game will go into attract mode but the knocker will fire constantly. It keeps going off repeatedly until you turn the game off or you start a game. For now it is unplugged.
I have tested the transistors at Q68 (TIP 36) and Q67 (TIP 102) and even the 2N5401 predriver transistor. All test OK. The diodes and resistors associated with these transistors are also OK. Coil resistance is 4.2 ohms which is in spec for the AE-23-800.
Where should I go from here? I considered this may be a fault with the driving 74LS374 IC, but I have never seen these fail in a way that they constantly activate the associated component. Rather, when they fail the components seem to stop working instead.
Appreciate any troubleshooting advice.