Operation Defender

mecha

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this thread is for Defender repairs for Smart Bomb, BonanaKing, and rogerxy. out of all the Defender boards I had on hand, mine from home was the only one I knew of that worked 100%; I logic probed the entire thing, and that made doing these repairs a lot easier as I was spinning my tires for what seemed like forever trying to figure them out.

BonanaKing
game: Williams Defender, R8570 (Later Series)
symptom: lines in screen, intermittent boot, entire screen flashing colors

fix:
going off memory, a lot of perusing the Defender Theory of Operation revealed a couple of chips in the color ram circuit that are probably never really considered, 74LS257. replacing these (and I don't even know why I had replacements on hand) corrected the flashing color issue. I had to chase around for awhile in the ram circuit to figure out the line issue. I learned the hard way that when shift registers fail, it affects the write enable/memory control deeper in the circuit. so I probably had some 7410 chips I didn't need to replace before I figured out the 2 shift registers nearest to the edge of the board were bad. after that, the graphic issues went away and the game consistently worked again.
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rogerxy
game: Williams Defender, R8570 (Later Series)
symptom: completely dead (initially), blank screen (later), stuck in bookkeeping (even later)

fix:
plugging this board in, I was greeted to a black screen, as in, the game was dead. after some probing around on the CPU I found that it had no clock at all. this was a battery leakage board, and it swallowed the entire crystal area up. I meticulously had to pull each part out around the crystal and clean the corrosion off. after that, the rom LEDs flashed all 4 lights twice signifying that the game was booting properly. the next issue was I had no video output at all. Defender Theory of Operation, I was able to narrow it down to a few areas -- using my logic probe findings from my board, it wound up being the 7400 and 7408 chips were bad. now I have a picture, but the image is doubled vertically one on top of the other; pressing the decoder nearest to the headers it cleared up; I then replaced the CPU and decoder sockets. now the game is stuck in bookkeeping. it turns out the 4071 is a high failure part on Defenders for some reason, as I had 3 of these (including the one at work) that needed them all replaced. a side issue with the rom board pretty much falling apart, I just subbed that out with an extra I had and the game works again.

Smart Bomb sent me a Defender and Stargate a very long time ago, the Stargate of which I made a thread about and still haven't arrived at a solution for. the Defender was similar to BonanaKing's where it would boot when it felt like it. I think it was a bad shift register thing too. I was able to clear up whatever graphical issues it had, but now it infinite rugs and the LEDs on the rom board flash around like christmas lights. the amazing part was it worked at one point, and when I went to plug it back in the next day to test again, it hasn't booted properly since. I also had to disable the watchdog on it. instead I arranged to send replacement boards that I know work, and I'll revisit these another time.

I plugged all 3 sets in Sunday and they all still work (please understand my paranoia; they're Defenders) as well as the Stargate. I would like to get these sent out this week. I presently have no cell phone cause I had to mail it back, and the replacement will come sometime this week I hope. so I'll be PMing everyone.
 
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