Defender not booting

joetechbob

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I recently picked up a non-working Defender which I'm having issues getting to boot. I suspect that this is due to a ROM failure, but I wanted to check with the Defender experts.

Here's what I've found:

1) When I got the game, +5 was missing and one of the fuses was blown. Replacing the fuse restored +5. I now measure good +5, +12 and -5 at the power supply.

2) With +5 restored the game started making the Defender boot noise and displaying the "rug" pattern, which gets redrawn regularly. Also, on power up, all four ROM board LEDs momentarily light solid and then turn off (no flashing). The same happens when the CPU board reset it depressed.

3) Voltages look pretty good at the RAM chips (-5, +5 and +12), though +5 is measuring at around +4.9--not sure how sensitive the RAM chips are to +5. Didn't detect significant AC rippled in the DC voltages.

I tried re-seating the ROM decoder chips, CPU and ROM chips (including cleaning the legs if necessary) and inspected the solder joints around the header connectors--joints looked good with a visual inspection.

Having looked online I believe this is a ROM failure. Does that sound about right? My next step is to try reflowing the solder on the header connectors (even though they look good), and potentially replace the ribbon cable between the CPU and ROM boards.

Does that sound about right? Any other advice?
 
Try to recrimp the ROM cable. If you are seeing the RAM test it could be an issue reading the game start ROM after the POST is complete.

Most of the time it is not necessary to replace the ROM cable. When people remove the ROM cable, they grab the ribbon cable and pull. This pulls the wires from the IDC connectors. Recrimping will push them back down so they make good contact again.

ken
 
Have seen this fault a few times, the fault was bad TTL logic chips on the ROM board.

Have seen a fair few failures and they all seem to be with chips with the SA logo on them, your mileage may vary tho.

Mine were a 74LS32, a 74LS00 and the 74LS42 chip, across two ROM boards.
 
I now suspect that it's not a bad ribbon cable after taking measurements this morning. I measured a good connection between each board on all pins and didn't see any shorts.

Add in that the +5 fuse was popped and it almost begins to sound like maybe someone accidentally shorted between two legs on a TTL chip or maybe installed in a socketed chip backwards at one point.

How did you determine that those ICs were bad? Anything simple to look for?
 
Using a scope, could see the outputs of some gates could not be right based on the inputs.
 
I finally had a chance to dig into this some more yesterday and I was able to narrow this down to the MPU board by swapping in other boards that I'd borrowed. My audio/ROM/interface boards are all fine.

So I know this: the +5VDC fuse was popped when I got it and the MPU board is bad...The guy who had this before me had installed a CMOS battery upgrade and installed new RAM. The fuse has not blown again since I replaced it. The game was allegedly working at one point for the prior owner (a couple of years ago), but he accidentally broke the neck on the tube and hadn't fired it up recently with a working monitor.

...Anyone suspect that perhaps a connector was inserted incorrectly, a chip inserted backwards or an accidental short somewhere that fried an IC? Perhaps some of the ROM addressing circuitry is fried?
 
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