I then test the transistors on the driver board again, but this time I do it manually in the game diagnostics and I realize that the third test sound (solenoid #11) is NOT working. Out comes the board again. I replace the transistor and the pre-driver transistor.
Then, using Clay’s guide, I test the 7408 chip and it tests OK. I install the board, and the sound doesn’t work. In fact, I am no longer getting any sound or speech. We’re going backward here! Out comes the board again. I test the components and they test OK. So, the problem must be in the J9 connector (or so I think). But, before reworking the connector (I had already replaced the pins on the board), I decide to connect the driver board to the sound board using wires with alligator clips. I then try to manually fire each of the sound transistors. To my surprise, solenoids #9, 10, 12, and 13 work. However, #11 still does not work. When I install the connector, the sound is still not working. I then pull the connector off and then barely push it on. The sound works, but no speech. So, the connector probably is not bad, because all of the other solenoids (non-speech sound) are working just fine.
So, I’m stumped. The connectors are good. The components all test good. But I can’t get solenoid #11 to work.
Next, I decide to swap out the PIA 6821 chip that controls the solenoids. But this doesn’t help. Again, I manually connect the driver board to the sound board using jumper wires and alligator clips. This time I learn something that helps me find the problem. Again, solenoids #9, 10, 12, and 13 work fine. solenoid #11, however, doesn’t. But, I notice that when I put the alligator clip on the pin I get a sound. Then when I manually ground the transistor I get nothing. But, if I disconnect the alligator clip and then reconnect it, I get a sound as soon as I touch the alligator clip to the pin. Aha, there must be a constant signal being sent. I get out my logic probe, and sure enough, a constant signal is being sent. Since these transistors are “one shot” transistors, once the sound signal is sent, no other signal can be sent until this signal is turned off. Thus, one transistor is stuck on, meaning that since one of the five sound transistors has only one level, half of the sounds/speech cannot work. (This reduces the number of possible sounds from 2 to the fifth power to 2 to the fourth power)!
The next step was to figure out why a constant signal was being sent. I had already replaced the transistors so the next culprit was the 7408 chip. The chip tested fine when out of the game using Clay’s guide (he does mention that it is only 95% accurate). Well, Murphy’s law states that this 7408 must be one of the 5% that will test good but be bad. (This whole board was possessed anyway.) So, with the board installed in the game and running, I used my logic probe. The 7408 is a “quad” chip and it seems that one of the “quads” had shorted closed and was sending a constant signal to the transistors. The logic probe revealed this. I wasn’t sure what reading I should get on each of the legs of the chip so I just tested the readings on this 7408 and compared it with the readings on the other 7408s in the game. Sure enough, one of the “quads” had shorted and was sending a constant signal, whereas in the other chips, this was not happening. Out came the board and I replaced the 7408 chip. Success! All sound and speech were now working!
This really helped to make the game. Not only was all of the speech now working, but the game had quite a few more sounds that I had never heard. With all five of the transistors working, there were now 31 possible sounds/speech (2 to the fifth power which gives thirty-two options minus one for 00000 option). Before, with only four transistors working, there were only 15 possible sounds. So, this really helped the enjoyment of the game. Doubling the number of sounds made a huge difference. It's amazing how much sound adds to a game.
So, from this I learned that testing chips in circuit does not always work. The 7408 that was bad tested fine in circuit but was actually bad. In the future, since these chips are rather inexpensive, I’ll probably go ahead and just replace them along with the transistors just to be safe. Also, if I ever get a "possessed" board again, I may decide to just get a new one after all the time I spent on this one.
