Defender CPU Repair Take 2!

Alan173

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After repairing all the boards on my Defender game I figured I would tackle my backup boardset (I like the abuse! ;)). All the boards are repaired except for my CPU and I have run into a dead end. Interesting enough there was another post from 2009 with the EXACT SAME issue I am having:

http://forums.arcade-museum.com/showthread.php?t=83845&highlight=defender+rug&page=6

Unfortunately the post had no resolution to the problem but here is a quick summary of what I have:

Game boots to a rug pattern that continues to cycle. LED's do not flash but just come on with lights 2 and 4 lit. All circuits on this CPU board check fine except the Page 0 Decoder Circuit. My 5P pin 6 is stuck high with the input from the ROM board pulsing but the input from 4K is stuck high. All of the inputs at 4K (pins 1, 2 & 3) are pulsing just like my working board but output pins 6 and 7 continue to be stuck high. Both 5P and 4K have been replaced - no change. I grounded pin 1 (A1) and it jumpstarts pins 6 and 7 to pulsing erratically...but that is as far as I can get. At 4K I have good inputs and bad outputs and have triple checked the chip is good. Totally confused???

Funny somebody had an identical problem as mine.

Any ideas from the Defender repair experts out there?

I have a strange short condition (5V to ground) I have been toying with....but don't know if I am going down the right path with that one.

Alan
 
More...

I found a completely corroded trace between the two diodes to the left of the batteries...got that fixed up. Also reflowed the header pins for the power supply connection and bumped the 5V up to almost dead on 5v. Must have been a little corrosion/crummy solder in there.

Still no change. The short I was going after is now fixed with the diode trace repair.....and now the working CPU board and this CPU board check the same.

After those corrections - no change.

So I'm back to the 4K not kicking out a pulsing signal and stuck high...:(
 
Game boots to a rug pattern that continues to cycle. LED's do not flash but just come on with lights 2 and 4 lit. All circuits on this CPU board check fine except the Page 0 Decoder Circuit. My 5P pin 6 is stuck high with the input from the ROM board pulsing but the input from 4K is stuck high. All of the inputs at 4K (pins 1, 2 & 3) are pulsing just like my working board but output pins 6 and 7 continue to be stuck high. Both 5P and 4K have been replaced - no change. I grounded pin 1 (A1) and it jumpstarts pins 6 and 7 to pulsing erratically...but that is as far as I can get. At 4K I have good inputs and bad outputs and have triple checked the chip is good. Totally confused???

I don't have schematics handy....so I don't know what's down stream from 4K pins 6 and 7....but something they feed could be pulling them high. Socket 4K (if not already) and pull legs 6 and 7 out of the socket (thus pulling them out of circuit). With the legs hanging in the air...see if they now act appropriately. If they now act right....something they feed is pulling them up (maybe the chips they feed....solder bridge to 5V...etc). If they are still stuck high....something's up with the input signals (maybe timing...I don't know what kind of chip 4K is). Now you'll know if you need to focus on before...or after 4K.

Edward
 
Dang.....

4K is a 74LS139....and I just put in my last one.....of course.....without a socket...:(.

4K feeds 5P (7432)....I'll check it again but didn't see any problems.

Great idea on lifting the legs!.....
 
darn...

If the shoe fits...

Well....I just wasn't thinking and was more worried about the traces. This board is crusty and some of the traces/pads were in crummy shape. For some reason I was thinking if I put a socket in there and had connection problems...that would be more trouble than just replacing the chip. Well......lesson learned. And I need a nice desoldering gun to alleviate the fears of using a socket all the time.

5P checks just fine. I have swapped the decoder ROM's thinking they might be causing my #2 and #4 LED's to stay lit.......still no change.

Since the inputs are good to 4K......but the outputs are wrong....the only thing I can think is the timing is off on the inputs. Could RAM cause that which does affect the LED's?
 
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Still can't find anything. Anyone ever have bad RAM cause the 2nd and 4th LED to remain lit? I guess I can swap them all out and try that. I can also swap out the CPU. I'll try that and see what happens. Running out of ideas.....:(
 
I've seen ram cause all kinds of weird issues, especially if more than one is bad.
Just be sure your power supply is good before swapping any, otherwise it may toast them too.

Are any of the ram extremely hot or cold compared to the rest ?

Have also seen the ROM which holds the self test information cause a similar issue.
(I can't recall the number off top my head)

Have also seen weak voltage cause similar issues.
In one case if I simply unplugged the I/O board it would boot.
 
Just checked all the RAM - confirmed good.

Just swapped out the CPU - no change...so CPU is confirmed working.

Both power supplies I have are rebuilt and running smooth.

The ROM board I'm testing with this CPU board is known working. Is the ROM you are talking about on the ROM board? If so....I know it is good.

I'm basically pulling the CPU out of my working game and replacing it with the back up....and that is when my 5P pin 6 stick high....and output on 4K stuck high also. Both chips replaced.

This problem has got the best of me...:(

Any other ideas?...




I've seen ram cause all kinds of weird issues, especially if more than one is bad.
Just be sure your power supply is good before swapping any, otherwise it may toast them too.

Are any of the ram extremely hot or cold compared to the rest ?

Have also seen the ROM which holds the self test information cause a similar issue.
(I can't recall the number off top my head)

Have also seen weak voltage cause similar issues.
In one case if I simply unplugged the I/O board it would boot.
 
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