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Quoted from 4Max:I also tested the TTL involved, and that also tested ok (0.4-0.6v on every leg tested against ground)
To rule out an error with the 74LS374 chip you can measure pin 16 on it. It should be high when the solenoid is off and low when it's on.
Quoted from 4Max:Sorry, not sure how to measure high and low plus the solenoid locks on as soon as I power on the machine.
Is there any harm in leaving the TTL chip alone, replacing all the transistors, checking, and if still locks on, then replace TTL - or could that damage the transistors again?
You can measure high/low with a DMM. High is 2-5VDC and low is 0-0.8VDC.
Yes, you can change the transistors but if the problem is with the TTL (or the signal coming from the CPU) then you will have the same problem again.
You can disconnect J130 from the driver board so that the coil won't lock on. Then turn on the machine and measure the output from the TTL. If it's high then you know the problem is in the transistors and you can just replace all of them and the diode. If it's low then the solenoid is just doing what it's supposed to and the problem is with the TTL (or the signal from the CPU).
Quoted from 4Max:Oh. Thanks guys. Just edited my post with another basic question. Is it red DMM lead on ground pin of chip and black lead on pin 16?
Red lead on pin 16 and black on ground on chip or just put it under the ground braid below the boards. Reversing black and red will just give you a negative voltage reading.
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