Defender Troubleshooting

Vraz

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My Defender (later system board with CPU @ 2I) unexpectedly froze after being powered on for several hours last week. The video displayed a frozen attract mode screen (and it would not respond to any buttons). After I power cycled the cabinet, it came up with a blank screen-- no sound or diagnostics. Checked the power supply and have 5.15v at the supply, 4.85v at the chips. Have converted to 4164s so +12v & -5v are not needed (and +12 looks fine at the power supply as well).

I located the "CPU Board Logic Probe Troubleshooting Guide" and started working through it. The first failure case was chip 5P is not pulsing on pin 6. According to the flowchart, this indicates a problem with either 5P or 3D (I have not lifted 5P-6 to isolate yet). I looked at the Defender "Theory of Operation" manual to better understand what this failure might mean and that is when confusion started.

Am I blind, or does the Theory of Operation manual have major accuracy problems? I seem to be finding lots of errors. For example, figure 3-7 Video RAM Controls labels both a 7410 and 7432 as 5P, shows pin 1 of 7432 4R as an output (its actually an input on the datasheet), duplicates the same gate and pins in two different places (there are two different 4R gates using pins 4,5,6 though one has mixed up pins), etc.

Has anyone come up with annotated list of corrections for this document or can someone point me to a correct schematic? Given these old schematics were largely manually created, a few errors are expected, but this is way beyond what I have seen before. Am currently spending more time trying to decode the schematic snippits in the Operation manual than actually troubleshooting the board.

Thanks!
 
>> There are two Theory of Operations manuals. ... There was also a later once published that covered the later Defender MPU boards.

Yes, that is the document I was referring to which appears to be full of labeling errors (wrong chip identifiers, wrong pin numbers, etc). On the upside, I found a high resolution TIFF schematic which seems better. Already found one error in it, but at first glance, it seems much better than the Operations manual.
 
The Defender schematics are better in that they were kept reasonably up to date. Whereas the theory docs were already out of date by the time the game shipped.

I'd probably redo the power supply headers on both the MPU and the PS boards. You shouldn't drop that much voltage between the PS and the RAM.

ken
 
>> The Defender schematics are better in that they were kept reasonably up to date. Whereas the theory docs were already out of date by the time the game shipped.

I guess so. On the upside, the Logic Probe Troubleshooting Guide seems pretty good.

>> I'd probably redo the power supply headers on both the MPU and the PS boards. You shouldn't drop that much voltage between the PS and the RAM.

And you would almost be a mind reader. Once I tracked down the real 5P (from the TIFF schematic), I found it was an OR gate with two inputs E7 & PS0. E7 was pulsing (as it should), but PS0 had no signal at all. PS0 is generated on the ROM board and I quickly realized that board had no power. I was careless when originally checking power and only checked the RAM on the CPU board.

Not sure how it happened so dramatically, but the 4P2 connector on the power supply was suddenly not providing +5V to the ROM board. I would have expected a progressive voltage drop over time causing intermittent failures, but having a power connector pin completely fail during game use was new to me. For now I just cleaned the pins with some sandpaper and it fired back up. Per your suggestion, I will order some new connectors for the ROM and PS boards and redo them this fall (I actually replaced the CPU connector back when I did the 4164 mod).

Thanks!
 
Whenever there is more than a couple tenths drop in voltage from the PS board to any of the other boards, I immediately suspect the power connectors. That is why you have to measure the voltage at points on the board rather than just at the connectors. They get oxidized or burned and start increasing the resistance and that causes them to get hotter and speeds the oxidation. Round and round it goes until they fail and they can fail quite rapidly. Or the other thing that happens is that they get hot enough to melt the solder and stop makeing contact between the pins and the boards, Or the solder joints at the connectors get micro-cracks in them ans stop being able to carry enough current. It's a b*tch getting old for boards too!

ken
 
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