I have learned more about electronics and logic in the past two weeks than I could have ever imagined, while trying to troubleshoot this Whirlwind System 11B MPU.

OK, so when I turn on the machine, I'm getting a condition where ALL of the solenoids at 1J21 are locking on -- or they don't work at all (there are similar other issues with groups of solenoids and no music). I used Leon's chips and a series of manual tests in diagnostic mode with various solenoids to confirm that the PIA chips are sending the proper signal when called for by the CPU, and all of the transistors/GATE chips are performing correctly.

The odd behavior I'm seeing in this case, doesn't correlate with MD0 through MD4 at PIA U42 (the signal outputs for the 5 solenoids), but rather at the downstream U1 74LS374 chip on the sound solenoid overlay board, where the flip-flop outputs are connecting to a clock and the blanking circuit (output control).

1.) In the case of both lock on and non-working states (using attract mode as an example) the input pins are correctly HI (disabled). The output control (pin 1), associated with the blanking signal input, probes LO which is correct.

2.) According to page 83 in the Whirlwind manual, the clock signal is comprised from a bunch of logic chip switches on a 74LS332 chip coming from signals MD5, MD6, MD7 and MCB2 off of PIA U42 on the MPU (MD0 through MD4 are the solenoids). While the clock signal appears to probe "HI" in both the working and non-working states, I observed that some of these input signals are HI in the non-working state and pulsing in the LOCKED-ON state.

3.) Here's where it gets strange. In the case of the non-working state, the input (D) pins are high and the output pins are high (makes sense). In the case of the locked-on solenoids, the input is high and the output is low -- which trips all the downstream transistors.
How can I have a HI input (D) and a LO output (Q) at each set of pins on U1, unless the chip was somehow "locked" in that state?

Am I also reading the spec right that some of these signals are actually coming not only from 1J21 (U42) on the MPU but also from U4 on the sound solenoid overlay board, with the direction controlled by pin 1 on U4 (DIR)? I'm noticing this pin (which also correlates to MCB2) is changing signals on the differing solenoid states.

I'm like 100% certain the U1/U4 chip on the sound solenoid overlay is good because this board works in another game. So, I'm left to figure out what's going on with these other signals and how they are supposed to read. I'm not getting any errors at boot up, so am I correct that it's not a ROM issue?

WOW I got this solved. Totally pissed. However, in this "fun" process I became an expert at schematics, logic chips, circuitry, flow, and could probably fix any System 11B board haha.

So this Whirlwind had two issues that I thought were related,
1.) Sound Overlay Solenoids all locked on at startup or never worked at all
2.) The music/speech didn't work, but the second test for the sounds all worked.

This board had prior battery corrosion, but it was neutralized, the traces looked OK, and all pinned out OK with continuity.
I replaced U42 (unnecessarily) -- although was on the right track.
I replaced the blanking circuit (unnecessarily) -- thinking it was a timing issue.
I replaced SRC 1 and 3 (possibly necessary in the corroded area.
I took all the boards except the MPU up to the arcade. Everything swapped into another Whirlwind worked perfectly, meaning both the solenoid and sound problems HAD to be with the MPU.
I purchased Leon's TEST ROM and everything checks out perfectly at the PIAs. WTF.

Last night I was using the sound solenoid overlay board (per the original post) to try knocking out the bizzarre lock on issue. I knew the problem was with the MPU, but I figured I'd work backwards. Per the initial post, I couldn't figure out how the U1 74LS374 chip was ending up in a "locked" state when the clock signal was HI. After reading the data sheet again, it kept talking about the clock going low to high in transition, which got me thinking this morning that maybe it's supposed to be pulsing.

MD0-4 coming off pins 3-7 on 1J21 on the MPU are the solenoids, while MD5,6 and 7 (pins 8,9,10 on 1J21) drive connect via ribbon cable to 5J2 on the sound solenoid overlay board. MD5/6/7 all control both the clock on U1 and are also somehow used at U20 to work with the sound board.

Oddly enough, when I probed all the pins on 5J2 last night, I made an annotation next to pin10 (MD7) because the probe read HI, but it was "fuzzy" sounding as if there was a subtle pulse in there. AHA!!! I checked it again just now and sure enough, it's bizzare.

OK, back to the MPU. MD7 comes from U42, pin 17, which is PULSING CLEANLY!! I then check SRC pack 1, pin 9 next in line (which I had replaced last week). CLEAN. I check 1J21 pin 10. CLEAN. I check the other end of the ribbon cable at pin 10. HI and scratchy. ARE YOU F--KING KIDDING ME?!!

The problem: A bad ribbon cable. I jumped 1J21 pin 10 to 5J2 pin 10 and now all the music speech works perfectly and the solenoids behave perfectly!!

Lesson Learned: Logic probe signals should be clean - LO, HI, PULSE. I'm not sure how I could have saved any time here. None of my other games used a 20-pin ribbon cable to borrow from. In this case the test ROM passed, no errors with the game ROM, which despite my thinking the issue was at the PIA , pointed to something else. One would have easily suspected the corroded area, and despite confirmations of a strong continuity signal across the MPU traces, I replaced parts that looked suspect (probably good to get rid of the corrosion anyways). After that I used the game manual to understand the flow to the solenoids, and noticed that 74LS374 appeared "locked" in the turn on state, which is not how a flip-flop chip should behave. I noticed that the MD7 signal seemed off, and the key to all of this was the logic probe. After noticing the LS374 clock and the music were joined at that MD7 signal, it was then finding out how I was going from strong pulse to "scratchy hi".

Will do. I've actually had this probe for over 2 years ago, and it was still new in the package. Never had to use it until this project. It was a god-send and stupid-easy to use.