General Solenoids in Capcom machines are driven by an STP20N10L logic level N-channel MOSFET. When a solenoid problem occurs the software will report a solenoid problem in the diagnostics.
Locked on solenoid A locked on coil is often caused by a defective MOSFET driver on the driver board. With the power off, first test the resistance between pin 2 (middle) and 3 (right) of the MOSFET, disconnect the connector to the affected coil first so you only measure on the MOSFET. When the resistance is zero or close to zero the MOSFET is defective and needs to be replaced. Before powering on the machine after replacing a defective MOSFET, test the coil's resistance and check the tie back diode and it's connections because these are the main causes for a defective MOSFET. If the MOSFET seems to be good next to test is the input of the MOSFET on pin 1 (left) for a signal. When there's no pulse on pin 1 while testing the solenoid in diagnostics it could be a defective 74LS74 driving the MOSFET.
Non working solenoid A non working solenoid is often caused by a broken wire but other causes are possible. The easiest to check is the solenoid itself so check the resistance of the coil first. When the solenoid's resistance is very high or when it doesn't have any resistance at all the coil wire might be broken from the solder tab. When the solenoid seems ok check for continuity of the wires to the coil. When the wires check out fine as well the problem might be caused by a defective MOSFET driver. Check pin 1 (most left pin) of the MOSFET for a signal while the solenoid is activated in diagnostics, if there's a signal the MOSFET is probably broken, if there isn't any signal on pin 1 it's probably caused by a defective 74LS74 driving the MOSFET. After replacing a MOSFET, check the tie back diode and it's connections because in this case this is the main cause for a defective MOSFET.
Repeatedly broken MOSFET driver When a defective MOSFET is replaced but fails again after a short period of use, or even the first solenoid activation, test the coil's resistance and check the tie back diode and it's connections because these are the main causes for a defective MOSFET. This also occurs when a wrong type of MOSFET is used, a non logic level MOSFET (IRF540 for example) will not be driven correctly because the gate voltage will be insufficient to fully drive the MOSFET causing it to get too hot and fail.
STP20N10L MOSFET alternative In case of a defect STP20N10L MOSFET, this part is getting scarce but it can be replaced by the IRL540N which is readily available. Do NOT use the IRF540N instead (not logic level)! Please note that these MOSFETS are quite sensitive to static electricity.