Terry asked me to share this story. I picked up a cherry looking Whirlwind a month ago, and had yet to play a game. The right sling shot locked on at startup, and once I solved that, the REAL problems surfaced.
I have literally spent the past 4 weeks troubleshooting two seemingly unrelated issues with the MPU, because a.) I want to learn board repair b.) I got a good deal and didn't want to spend $3-400 on board repair or taking the easy way out with a Rottendog. It turns out that had I sent my MPU in for repair, it would have been sent back to me.
During this exhaustive and frustrating process, spending seemingly hours in the garage, I became an expert at schematics, logic chips, circuitry, flow, and could probably fix any System 11B board haha.
Here were the two issues with the game.
1.) Sound Overlay Solenoids (3 lightning flash lamps, topper blower and playfield spinner discs) all locked on at startup or never worked at all. Which of these occurred was seemingly random.
2.) The music/speech didn't work, but the second test for the sounds all worked.
The System 11B MPU board had prior battery corrosion, but it was neutralized (and I reneutralized it). The traces looked OK, and all pinned out OK with continuity. With my initially limited understanding of schematics (never had to deal with much beyond blown TIP102/TIP36C), I posted a tech thread here and got little response, so I took my college physics knowledge and went to work.
I created a neat molex connector that went from an arcade switching power supply to 1J17 so I could work on the bench.
I replaced and socketed U42 (unnecessarily) -- although was on the right track. It was near the corrision area and goes to the sound solenoid overlay board.
I replaced SRC 1 and 3 (possibly unnecessary, but in the corroded area so this was easy and worth it).
I took all the boards except the MPU up to the arcade. Everything swapped into another Whirlwind and worked perfectly, meaning both the solenoid group lock on and sound problems HAD to be with the MPU.
OK, time to open the Elenco logic probe which I've had in the drawer for two years and never needed. This thing is stupid easy to use (and never even saw this thread before). Read the back of the package and find the ground and +5 test points to clamp onto. Now's where this thread gets interesting.
I replaced the blanking circuit at U43 and Q50 -- thinking the behavior was some bizarre timing issue. The blanking circuit would begin HI, instead of LO then HI after the CPU booted. This was a good fix, but unrelated.
I purchased Leon's TEST ROM and everything checks out perfectly at the PIAs out to the pins. WTF. Now I was getting pissed. How was this possible?
Last night, I was using the sound solenoid overlay board to try knock out the bizzarre solenoid issue. Again, they either didn't work at all, or locked on when I turned on the machine. I knew the problem was with the MPU, but I figured I'd work backwards. The Whirlwind manual provides a nice flow chart on how it the solenoids are powered. All the transistors were good, taking me back to the 74LS374 flip-flop logic chip, U1, immediately downstream of the input ribbon cable. I pinned out U1 on the sound solenoid overlay with the logic probe with the solenoids in the non-working state and wrote them down. I did the same with the solenoids locked on (pulling the 50V connector to avoid blowing a fuse). I then did the same with the ribbon cable input at J2. I couldn't figure out how the U1 74LS374 chip was ending up in a "locked" state with the clock signal HI. The outputs on U1 for the solenoids were either all HI (off) or all LO (on) and wouldn't change. Even more bizzarre, NOTHING changed on the input pins from J2 in either the locked-on or working case. So I googled 74LS374 and pulled up the data sheet to see what this chip is supposed to do. Stumped, I went to bed.
I re-read it again this morning, and it kept talking about the clock (pin 11) going low to high in transition to latch the input and output to LO or HI. This got me thinking this morning that maybe the clock pin was supposed to be pulsing and not HI.
Looking at the schematics from the MPU to the Sound Solenoid Overlay board... MD0 thru 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) all connecting via ribbon cable to 5J2 on the sound solenoid overlay board. According to the schematic, MD5/6/7 (whatever they are) all control both the clock signal on U1 and are also somehow used at U20 to work with the sound board. AHA!!!
Oddly enough, when I probed all the pins on ribbon cable input J2 last night, I made an annotation next to pin10 (MD7) because the probe read HI, but it was "fuzzy/scratchy" sounding as if there was a subtle pulse in there. The "low" LED kept glowing very faintly every second or so. AHA!!! I checked it again just now and sure enough, same thing.
OK, back to the MPU. 1J21 pin 10 for 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 J2 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. Visibly and audibly. 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".