Super Pac-Man PCB kicking my butt

ELutz

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I've got a Super Pac-Man motherboard giving a RAM 5 error. Ram 5 is the 2148 at location 3K. It's good, as is the 2148 at 3L. All the address/data lines are pulsing away on both the RAMs. Here's what I've found....all ROMs are good, all Proms are good, both processors are good, all customs are good, a lot of the suspected sockets have been replaced. The sound processor at 1L isn't "turning on". The reset pin (#37) is stuck low. The schematics label this "subreset". This signal comes from the LS259 at location 2M. All of 2M's outputs are stuck low. All the inputs look good. I replaced it...no change. None of the outputs are shorted to anything "low". Other abnormalities....The main processor at 1A has pin 23 (MA15) stuck high. It should be pulsing. It's not shorted to anything "high". Everything at this processor looks good. This MA15 signal feeds the PROM at 2B. A lot of the outputs of this PROM are stuck high (none are shorted to "high")....I assume because of the bad MA15 signal. These outputs of this PROM are screwing up the LS254's at 1F, 2F, and 2K causing all of these to "isolate" their buses. Needless to say, all of this is sending me around in a big looping circle. I have replaced and/or checked a lot of these chips in the "circle". If anyone is not totally confused by this mess....any suggestions is appreciated. Anyone want to school me on a LS259. What am I missing on that 2M location. I don't quite "grasp" the datasheet on that bugger.

Big thanks!
Edward
 
That LS259 has 4 different modes it can operate in... try the TI data sheet. It's one of the better ones for explaining what the chip does.

http://focus.ti.com/lit/ds/symlink/sn74ls259b.pdf

RJ

Thanks RJ, That one is better than the one I have.

Here's the part I can't wrap my head around.....Pins 14 and 15 (not "G" and not "CLR") are high....this puts the LS259 in "memory mode". I checked a working Super Pac-Man motherboard, this is correct. Comparing my non-working board with the working board....the only pins wrong are a few outputs...pins 5, 7, 9, and 10. These four pins are high on the working board, but low on the non-working board. I replaced the 259....no change. I don't understand what's making these four outputs "wrong".

Edward
 
Thanks RJ, That one is better than the one I have.

Here's the part I can't wrap my head around.....Pins 14 and 15 (not "G" and not "CLR") are high....this puts the LS259 in "memory mode". I checked a working Super Pac-Man motherboard, this is correct. Comparing my non-working board with the working board....the only pins wrong are a few outputs...pins 5, 7, 9, and 10. These four pins are high on the working board, but low on the non-working board. I replaced the 259....no change. I don't understand what's making these four outputs "wrong".

Edward

Could be that the chip that is being fed by that signal(s) is bad. That is, the input of the attached chip is screwed up internally and is yanking the whole line low.
 
Could be that the chip that is being fed by that signal(s) is bad. That is, the input of the attached chip is screwed up internally and is yanking the whole line low.

I thought that, I lifted all the "bad" legs.......still stuck low.

Edward
 
Did you check the serial input to the chip?

It's serial in... parallel out
 
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