First take a really good look at that left pop that’s sticking. Make super sure that it’s put together right with no parts that could bind or pinch. Check the switch adjustments for that pop, too. Make sure everything opens and closes as it should through the range of motion of the pop. Check the diode. If that’s all good, look really closely at the wiring and see if you can find a short, especially something that might be shorting due to movement. If you can’t find anything at the left pop, follow the wires from that pop looking for potential shorts for other things in that row/column on the switch matrix.

Sorry if you covered any of all of this in your video. I couldn’t watch it with sound because it’s the middle of the night and I don’t want to wake others in my house!

The fact that you left pop doesn’t always fire makes me suspicious that your spoon and switch stack are not set up quite right. You need to look under that pop and check it out. I would raise the playfield and activate the pop while watching beneath. Maybe take a slow motion video from beneath to see what state the pop and its switches are in when it locks on. Make sure the switch is open again after the pop fires.

Also noticed on your video that when you locked the pop on, you depressed the pop skirt with two fingers, thereby putting extra an unrealistic pressure on the spoon and switch. Stick with one finger flicks at the very edge of the skirt for your pop tests, or better yet roll a ball at it for a more valid test. You want to make sure you are not causing the issue with long, hard presses that could also cause something to bind and make the pop lock on. Think of it this way: the ball activates the switch for a split second and gets immediately out of the way as it is rejected by the ring. Your finger is in there much longer and does not get out of the way as fast and can push things out of place, potentially causing the pop to bind and lock.

Special solenoids on system 9 will stay on as long as they are activated they are not one shot. There is a resistor capacitor network on each activation switch to provide a minimum pulse.

If you can’t get it to trigger with a freely rolling ball, the pop isn’t put together or adjusted right. Use a freely rolling ball for your tests.

The purpose of the resistor/cap is to take a short pulse and make it a known length, so that the solenoid can pop fully.

You really should just replace all the resistors and caps on this era special solenoids - they're almost 40 years old if original.

Any of the components in the special solenoid system can be stressed - the circuit works by putting ground (on the daisy-chained side of the switches closer to the PF in the case of pops, or the 2 switches that stick through the PF for the slings) to the other leaf of the switch, which goes back to a couple logic chips on the mpu board to drive the solenoids. Any passives in that circuit should be checked and possibly replaced. The connector pins should be resoldered or replaced that provide the ground signal back to the chips that fire it.

This firing path is completely separate from the secondary switches that provide a signal in the normal switch matrix that's used for scoring.

A logic probe is helpful in that you can probe inputs and outputs on the chips and see where the signal is locking on.