I made a repair to my Whirlwind to night that I decided I would document in case someone else runs across this problem. If you want to follow along, get out the CPU schematic for just about any System 11 game.

Symptoms:
The ramp down coil would energize as soon as the flipper relay would engage. It smoked the ramp down coil (special coils #6), shorted out Q79, and blew fuse F3 on the auxiliary driver board. This coil is one of the 6 special coils that are in the game.

After replacing the coil and Q79, as soon as the flipper relay would engage, the coil would still lock on. Lucky for me I have a few re-setable breakers for just this kind of situation, so I only went through 1 fuse.

Operation:
The way this circuit operates, when the flipper coil is energized, it will drop one input of U50 (NOR gate) to low. When the CPU fires the coil, it goes through the buffer at U49, and will pulse the other input at the NOR gate low. When both signal are low at the NOR gate, the output will go high, and trigger Q78, and that triggers Q79 that allows the special coil 6 to ground, and fires it.

Solution:
U49 and U50 both test out good. When I put the game into coil test mode, I got a good strong low alternating signal at the buffer (U49) input that originated at U54 pin 39 (a 6821). In my case the output at U49 was only showing about 2 volts at the high side. I dawned on me that with only 2 volts at the buffer output, the NOR gate was assuming the signal was low, and turning on the coil. This is why it locked on when the flipper relay would activate.

Looking at the wiring diagram, the +5 volt high signal originates downstream of the U49 buffer. The voltage has to pass through SR20, and is tempered by a small 0.1 microfarad cap. When I measured the resistance of SR20 while still in the board, it was only reading about 2k ohm, instead of the 4.7k ohms it is spec'd out at.

So I thought I had a bad series resistor. I pulled the resistor, and checked it out of the board... No dice. It was measuring a nice 4.7k ohms. So that just left the Cap. With the series resistor out of the board, I measured the resistance across the cap. I was getting 200 ohms... That should not happen. A cap should never show resistance. It should test out as an open circuit. I then went to pull the cap from the board, it fell apart before I even got it completely unsoldered. So it was definitely a failed cap causing my issue.

I have a system 9 donor board on hand that I pulled a cap from, and put everything back together. The machine now works great again!