Defender MPU Repair

YellowDog

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Help. I am trying to bring a Defender MPU back from the dead and I am stuck.

It first started with power issues. I replaced the power and video headers (they were really corroded) and the fliter capacitors. Once the power was stable the next issue was the watchdog line not being reset and the CPU reset line pulsing:

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After replacing 4 7474s and the 74LS139 I got it to this point:

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After some fiddling around I found one of the 74153 was bad and replaced it. The RAM test seemed to indicate a couple of bad RAMs so I replaced them and it now boots, sort of:

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It seems to pass the POST and then hangs. Also the video is spaced out and there is the tear at the bottom.

If anybody has seen this before and can point me in the right direction, that would be a big help. I have check most of the components in the video circuit with an HP Logic Comparator and they all seem fine, so I am stumped at the moment.

ken
 

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Check your decoder ROMS, replace with a good pair

Check your decoder ROMS, replace with a good pair


Help. I am trying to bring a Defender MPU back from the dead and I am stuck.

It first started with power issues. I replaced the power and video headers (they were really corroded) and the fliter capacitors. Once the power was stable the next issue was the watchdog line not being reset and the CPU reset line pulsing:

attachment.php


After replacing 4 7474s and the 74LS139 I got it to this point:

attachment.php


After some fiddling around I found one of the 74153 was bad and replaced it. The RAM test seemed to indicate a couple of bad RAMs so I replaced them and it now boots, sort of:

attachment.php


attachment.php


It seems to pass the POST and then hangs. Also the video is spaced out and there is the tear at the bottom.

If anybody has seen this before and can point me in the right direction, that would be a big help. I have check most of the components in the video circuit with an HP Logic Comparator and they all seem fine, so I am stumped at the moment.

ken
 
Thanks. I'll try that. I know I tried replacing them at one point, now I'm not sure if I did it before or after I got the RAM test....I think I need to get my RAM replaced, I am forgetting everything lately ;)

ken
 
Update: I pulled the decoder chips and replaced them with a known working set. No change. Then I happened to notice something when I was replacing the good chips. Somebody had swapped the #2 & #3 chips positions on the board I was working on. They had them backwards. I am very careful in swapping chips and had not even noticed they were reversed. DOH! I kept swapping chip 2 for chip 2 and chip 3 for chip 3 in the wrong spots. That cleared up the video except for the tear at the bottom. :banghead:

Now I am having trouble with the clock. I have replaced all of the chips and the crystal and it works when it is first powered up, but if I push the reset button, it starts going into pulsing the RESET line on the CPU. Now the weird thing is that if I touch the clock line that goes between pin 8 & 11 of the 74LS7404 (Chip 7P) with the logic probe, it settles down and works right and starts the rug test. :confused:

I think I will put this board to the side for a while and take up a new hobby, alcoholism. It sounds a lot easier and you get some fun before the headache....:alcoholic:

ken
 
Ken,

I belive you are seeing the effect of capacitance on one leg of the timing circuit. It is late, and I do not have one nearby, but I have had to solder an extra 22mfd capacitor on several older Defender boards to make them stabilize. The capacitor goes from the outside leg of the crystal to ground.

Hope this helps,
Christian


Update: I pulled the decoder chips and replaced them with a known working set. No change. Then I happened to notice something when I was replacing the good chips. Somebody had swapped the #2 & #3 chips positions on the board I was working on. They had them backwards. I am very careful in swapping chips and had not even noticed they were reversed. DOH! I kept swapping chip 2 for chip 2 and chip 3 for chip 3 in the wrong spots. That cleared up the video except for the tear at the bottom. :banghead:

Now I am having trouble with the clock. I have replaced all of the chips and the crystal and it works when it is first powered up, but if I push the reset button, it starts going into pulsing the RESET line on the CPU. Now the weird thing is that if I touch the clock line that goes between pin 8 & 11 of the 74LS7404 (Chip 7P) with the logic probe, it settles down and works right and starts the rug test. :confused:

I think I will put this board to the side for a while and take up a new hobby, alcoholism. It sounds a lot easier and you get some fun before the headache....:alcoholic:

ken
 
Ken,

I belive you are seeing the effect of capacitance on one leg of the timing circuit. It is late, and I do not have one nearby, but I have had to solder an extra 22mfd capacitor on several older Defender boards to make them stabilize. The capacitor goes from the outside leg of the crystal to ground.

Hope this helps,
Christian

Thanks. I've seen that mod on several Defender boards. I just looked at the schematics and in the original Defenders vs Stargates, the Stargates have an extra 22pf cap.

Stargate:

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Defender:

attachment.php


The Defenders that I have seen look like they have a factory mod soldering a 22pF cap from Chip 7P to the ground leg of cap C88.

I'll see if I can get a 22pF cap from Frys in the next couple of days and try it.

Thanks.

ken

PS: Now I see why Defender always kicks my ass, it uses a 120MHz crystal :D.
 

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