Thank you for your guide. I’ve ordered a logic probe and test clip to aid in my troubleshooting. I’ve also ordered a test EPROM.

I’ve got a data east Lethal Weapon 3 for my first pin purchase. Totally inop when I purchased it. I’ve got everything working at this time except the ball launch solenoid. It is locking on when I power up. I disconnected coil and installed new coil diode. For the time being I’m leaving coil disconnected. Replaced Q4 on PPB ( it was shorted on). The predrive transistors on MPU check good. The IC 7408 driving that coil circuit had been replaced by someone and a socket installed. I screwed a leg up getting the IC out and have a replacement being shipped. It’s a Chinese 7408. I hope they are legit. Also have a vintage version of 7408, SN74F08N enroute.

My questions are

1. Can I test the solenoid output on CN12 with controlling IC removed? It seems this may be a way to make sure Q27 and Q19 are not shorted. They test with a DMM identically to the other transistors in that area. I realize there will be no logic with IC removed but maybe a continuity test?

2. Reading a wiki guide online made me realize in the upper right of the MPU I have a special solenoid output area that is not used but may yet be shorted. If I determine a diode is shorted in this area can I just remove the diode or must it be replaced?

Thanks for any help you may provide.

Will do.

This guide really helped me troubleshoot my LW3 solenoid issue. Without the use of a logic probe I would have never found corrosion under my header pin connector at CN12 on the MPU

Thanks for the tutorial.

The probe is comparing voltage to ground and indicating if it is high or low relative to ground and the reference voltage. It gets the ground and reference voltage from two leads that exit the probe and you clip on test points provided on your CPU, MPU, PPB etc.

So, If you reference 5 volts and touch 12 volts with the probe it will indicate high. I am not sure what protection is provided in the probe for preventing damage to the instrument. I can say that I probed many points during my troubleshooting without regard to the voltage of the circuit and saw no ill effect on the probe.

The best use is on the pin outs on the CPU/MPU where you are trying to determine if it is sending the proper logic/signal to the PPB (high power board that is the interface between the CPU and the playfield devices). I was able to eliminate the CPU as the problem or identify it as the problem on a number of circuits by comparing the signal output with known working pins in close proximity. For example place the pin in diagnostic mode through the DMD menu and enter the "coil test" menu. Take a coil test that you know is functioning and probe that pin as you activate the coil. Observe the probe's behavior. Then, test your faulty coil and observe the probe's behavior on that pin. If they are identical you know the CPU is sending the proper signal and your issue must be downstream. Then perform the same test where those outputs enter the PPB (playfield power board) and observe that the signal is reaching the PPB. Then perform the test on the PPB output pins. You can see where I am going with this troubleshooting.

I also discovered corrosion under a header pin assembly by hearing the erratic buzzing from the probe even though the indication was correct. The tones the probe makes are very helpful. I ran alcohol under the header and the sound changed to a clear tone and reverted back to a buzzing when I taxed the header under game use and within a few moments the circuit failed again. This alerted me to the header being the issue. I unsoldered it and sure enough there was some resin corrosion ganged up around that pin's pad. Cleaned, resoldered and voila!

The CPU and PPB are primarily dealing with "taking ground" to the devices. This fact probably reduces the chances of damaging the probe. You can also probe the CPU chips and compare findings with a logic map of your chip (example 7408 IC). I would compare chip outputs with known working outputs on the same chip. Then move from the chip to the transistors and map the logic to make sure the switching was occurring as planned. You will need to google the data sheet for your IC to determine the input and output pins for each circuit and the logic of the circuit. As an example my problem was on pins 4, 5, and 6 of a 7408 chip. 4 and 5 are input pins and must both be "high" to get a valid "high" as an output on pin 6. If your IC logic is not comparing favorably with the data sheet logic map then your IC is toast. I replaced one of mine.

This process taught me how an integrated circuit and a transistor worked. Something I had never seen put to practical use and understanding.