Defender Help!! CMOS Ram Failure

jimmywlby

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Donor 2011-2012
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I need your help!! I'm restoring a Defender and got it working from getting stuck in the set-up screen (replaced IC 5101 and 4071). Two new issues: Game works and plays fine, but when I run the CMOS test, I get the error, "CMOS RAM FAILURE. TEST MUST BE ENTERED WITH COIN DOOR OPEN." Also, even though my batteries are new and the battery holders are free from any corrosion, game always starts in set-up mode and the settings are reset. Does this have anything to do with the CMOS Failure or are they completely two separate issues? Any ideas on how to fix these problems?

Thanks,
Jim
 
Thanks for your help. I'm getting 4.46 volts to the 5101.
 
It may not be the CMOS RAM chip. Frequently one of the gates in the write protect circuit is bad. If you have a logic probe and the Defender schematics (arcarc.xmission.com has them in the Williams games folder), trace the circuit from the 3 pins on the MPU board through the gates. You may need to use a jumper cable to toggle the memory protect pin by grounding it in order to see the gates transition states. If all the gates work, then socket and replace the 5101.

ken
 
YellowDog -

Unfortunately, I'm new to the hobby and not that advanced electronically. I do, however know how to do basic electronics like soldering in chips and taking voltage reading with a multimeter. Can you point me to where the gates are in the write protect circuit? It's a bit of a shot gun approach, but I could replace those before trying another 5101, correct?

Thanks for your help!

Jim
 
The best place to start is to get a copy of the schematics. If you didn't get a set with your Defender, you can get a set from the web. A good source is the website arcarc.xmission.com. A set for the later Defender board is here: http://arcarc.xmission.com/PDF_Arca...Drawing_Set_(16P-3001-101_R-T)_April_1981.pdf

You can check for state changes with a multimeter, but you would be better off with a logic probe.

The chips that are in the write protect circuit are:
5B - a 7404 - check input pin 3 and output pin 4 - this should invert the ground signal on the memory protect header pin 1.
4J - a 7408 - input pin 4, output pin 6
6H - a 4071 - input pin 2, output pin 3 - this feeds the R/!W input to the 5101

If you jumper pin 1 to pin 3 of the write protect header you should see the output of chip 6H change state. If not check the intermediate chips to see which one is not changing state and the jumper is applied and removed. That will tell you whihc chip to try and replace.

Be very careful when desoldering chips. It is very easy to overheat them and pull the traces off the board, which is not a good thing.

ken
 
If you don't have a desoldering station then forget about desoldering the chips intact. Cut them off the board as close to the chip as possible with a very small set of sidecutters and then desolder the pins individually with a soldering iron and a pair of pliers.
 
If you don't have a desoldering station then forget about desoldering the chips intact. Cut them off the board as close to the chip as possible with a very small set of sidecutters and then desolder the pins individually with a soldering iron and a pair of pliers.

Thanks -- Good to know.
 
YellowDog -- Thanks again for the expert help! Unfortunately, I have the early version of the board, but I cross referenced your numbers to my parts catalog and came up with this:

IJ, 5E 7404 Hex Inverter
2H 7408 Quad AND
1K 4071 CMOS Quad 2-Input OR

Does that look correct?
 
YellowDog -- Thanks again for the expert help! Unfortunately, I have the early version of the board, but I cross referenced your numbers to my parts catalog and came up with this:

IJ, 5E 7404 Hex Inverter
2H 7408 Quad AND
1K 4071 CMOS Quad 2-Input OR

Does that look correct?

Those are the correct parts.

The 7404 is at location 1J (row 1, Column J). If you look carefully on the sides of the MPU board you will see coordinates that will help you locate the parts.

I think the pins are the same, but just to be safe:

1J - 7404 - input pin 1 - output pin 2
2H - 7408 - input pin 2 - output pin 3
1K - 4071 - input pin 2 - output pin 3

The following is the diagram snatched from the Defender Theory document (also a good document to have and read):

attachment.php


ken
 

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Thanks again, YellowDog! I really appreciate all of your help!
 
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