Quoted from G-P-E:Shared lamp and switch strobes... yuck!
Strobe 9 comes out A3P3 via pin 3. Since the switch matrix is using a shift register (U5 and U7) and LS8 & LS10 work (I assume you checked) then the shift register is working. Narrows things down to only a few things.
U7, Pin 4 -- check him with logic strobe to see if pulsing.
U9, pin 11 -- should be same as U7 pin 4.
U9, pin 12. This one has a pullup to 20V so depending on your specific logic probe, you might not be able to test him with the probe. If you have an O'scope - check for pulses.
Q42 -- I would suspect him first. Can check with o'scope if you have one.
Items involved with Strobe 9:
U6 & U7 - 74HC164 shift registers. U6 works so ignore him. U7 seems to mostly work but you need to verify if output Q1 on pin 4 is active.
U9 - 7406 open collector inverter. Possible his output 6 on pin 12 is bad.
Resistors R54 330 ohm, and R42 1K ohm. Unlikely source of problem but possible.
Transistor - 12P06. The actual driver transistor. MOSFET type transistor and easily damaged. Also - the standard transistor test function does not work properly on MOSFET type transistors, especially in circuit. Often the 0.7V you see is the intrinsic diode within the MOSFET between the drain and source.
Resistor network 10K x 7 (4608x-101-103) - unlikely.
Capacitors C6 and C7 are bypass caps for U6 and U7... don't worry about these two.
Thank you so much for the advice!
A couple of things I gather from this. Btw, I made a mistake in my original post - it is not strobe 9 that has the problem, but strobe A. No big issue, I've just looked at the relevant components for strobe A instead of the ones you listed.
Strobe A comes from A3P3 pin 2. I have not checked LS 9 or 11. How do I check these?
Unfortunately I don't have a logic probe. Guess it is time to get one! I take it I will need to look at U7 pin 5, U9 pin 11 and U9 pin 12. You listed U9 pin 11 should be checked for strobe 9 but did you actually mean pin 13?
Is there any effective testing I can do on these ICs with a DMM at all?
Resistors 43 and 55 are both good.
That leaves Q43 as a potential culprit. I didn't know that a standard diode test wouldn't work with these MOSFETs. Interesting. My only experience is with other manufacturers who use transistors. Is there a better way to test it with a DMM? I will replace this one as a first step I think, then look at the ICs with a logic probe.