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Repairing your Space Invaders
or Space Invaders Deluxe Sounds Space Invaders and Space Invaders Deluxe use a rather unique, if primitive, method for generating sounds. In the SI series, each sound has its own amplifying circuit as opposed to most systems where a sound is generated and then amplified in a common amplifier. The up-side of this is that usually only one sound goes, that gives you a pretty good clue as to what has gone wrong. Each sound has its own operational amplifier (op-amp), most likely if you don't lose all the sound completely, the corresponding op-amp is bad. SI uses LM3900's, and there are quite a few on the SI board. There are 7 basic sounds in SI, the missile shot, invader hit, explosion, saucer hit, saucer sound,bonus missile base and the thump-thump in the background. If you lose the missile sound, check the lm3900's at M4 and P4, for the explosion sound, check lm3900's at K3-4 and K5. For the Invader Hit, check K4. For the saucer hit, check M5. If you are going to work on your SI board, I would suggest you replace all the LM3900's at the same time, and use sockets so you can easily replace them if they fail again. If you are missing the thump-thump background sound, check the lm556 at H4. This is a dual timer of the 555 series and uses a bus control to set the rate of the sound. This is another common chip to fail and I would recommend replacing it as well. If you get just a high pitch for the missile shot, check the 4006 at n5, another chip that can bad. The saucer sound is generated by a 76477 at H2, this is a very versatile
chip and, configured correctly, can even sound like a train whistle! They
used to sell these at Radio Shack, but I don't know if they still do. Update: Looking for a 76477 ? They are now at BGMicro look for part # ICS76477, but they are the wrong package type so you will have to adapt. You can also get them here, but for a lot more. Another chip to check if you just lose the saucer hit sound is the 74174 at E5 and the 7417 at F5, these can sometimes only partially fail with just one line going bad. I would suggest replacing these when you fix your board. If you want to do some preventative maintenance on you SI to assure the sounds work well into the future, you will need 8 lm 3900's, 1 4006, one lm556, one 74174 and one 7417. Replace the following LM3900's, M4, P4, K5, K3-4, K4, M5, M4 and l2-3. Replace the 74174 at E5, the 7417 at F5, the lm556 at H4 and the 4006 at N5. Some More Information Sent to Me I wanted to thank you for your great Space Invaders sound trouble-shooting guide on your web-site, it really helped. I did want to give you one piece of information you may want to add to your page: Last night I was debugging a strange sound problem with my Midway S.I.. The problem was that the overall sound was VERY quiet. If I'm in the room alone and don't have the stereo playing or anything, then I can hear the game. But any ambient noise will drown out the game sounds. Even adjusting the volume had little-to-no effect on the sound volume. My first thought was that it was the POT. I pulled it and tested it. 50K-->0K, perfect. Hmmmm, I thought. I put the POT back on and went back to your troubleshooting guide. While I was re-soldering the POT back to the board, and later while testing that the traces were still in place, I noticed that one of the legs of the POT went to one of the pins on an LM3900. Gee, I thought, that must be part of the overall, post mix amplification circuit. I had a hand-full of LM3900's laying around and decided to replace it just in case. 15 minutes later I had the new chip installed and plugged the board into the game. I coined it up and... THUMP! THUMP! THUMP!!! Man, my ears were bleeding and my neighbors were WIDE awake!!! Did I mention I was doing this at 1am? 8-) So, it appears the LM3900 connected to the POT is responsible for, at least part of the final amplification of the sounds. If you have a very "quiet" S.I. and it isn't the POT, replace that guy and it should clear up. Just wanted to let you know. -Steve Copyright 2000, James R. Marous |