Simpsons mask ROM error

dbstallman

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My Simpsons board has lines in some of the character sprites. In the test mode, ROM 16L has a bad checksum.

I got the logic probe and found that while all the input lines show activity, the output on pin 15 is dead, neither high nor low. The input pin that it connects to on the corresponding LS374 at location 10K (pin 3) is also dead, while the output of the LS374 is showing activity (pin 2).

Is it likely that the mask ROM has failed? I thought those were pretty robust. Would it be possible to burn a 27C800 with the correct data and piggy back it on top to test?

I have tried piggy backing a LS374 ontop of the one at 10K, but it made no difference to the ROM's checksum or in the game's graphics.

Thanks for your help.
 
Highly likely the mask rom has died, EPROMs are robust, mask roms seem not to be at their current age. Piggy backing might work but only attach the rom with the board powered off or you may fail other checks.
 
I wanted to be sure I wouldn't damage the other chips by having the ROM piggybacked. Turns out it is a failed mask ROM. With the MAME version of .16L burned to a 27C800 the board passes the ROM tests.

Now I just have to figure out the best way to get the mask ROM off the board.

Thanks for the help.
 
If you dont have a desolder station and now you know the mask is bad the best option is to use side cutters to snip its legs off and then desolder them one by one. Then fit a socket and drop the ROM in.
 
Bingo!

Use a plunger type solder sucker... put the iron on one side and the solder sucker on the other... melt the solder and push the button to suck it all out of the holes.

Just DO NOT put excessive force on the solder pads with the iron or you will lift them from the board and cause mucho damage.
 
If you dont have a desolder station and now you know the mask is bad the best option is to use side cutters to snip its legs off and then desolder them one by one. Then fit a socket and drop the ROM in.

+1. Even with a desolder station, I still often use this method. I have no intention of keeping bad ICs. Doing this allows me to minimize the potential for damaging the PCB. When trying to fully desolder an IC, sometimes the +5 or GND legs can be a pain to desolder, due to being connected to larger heat-sinks. Sometimes, a little bit of solder can remain on one or two legs, which prevents the IC from comming off cleanly... which means you get to re-solder then de-solder those again to get the holes clean. In any case, I'm a fan of the cut-it-off-then-desolder-the-legs method.
 
+1. Even with a desolder station, I still often use this method. I have no intention of keeping bad ICs. Doing this allows me to minimize the potential for damaging the PCB. When trying to fully desolder an IC, sometimes the +5 or GND legs can be a pain to desolder, due to being connected to larger heat-sinks. Sometimes, a little bit of solder can remain on one or two legs, which prevents the IC from comming off cleanly... which means you get to re-solder then de-solder those again to get the holes clean. In any case, I'm a fan of the cut-it-off-then-desolder-the-legs method.
Agreed. I remember thinking I probably wouldn't have to cut legs again after I got a de-soldering setup, but it's still easier to cut them for me and safer for the board. One thing I need to get myself is a vice of some kind to hold the board on its side for me when trying to remove components.
 
I went with the nippy cutters option. I have a solder sucker, but have not had much success with it, although I have always tried to use the soldering iron and solder sucker on the same side of the board (quickly rolling it on as I lift the iron away).

As for a vice, I have the next best thing - my wife agreed to hold the board for me as I desoldered the 42 legs. :p

The new ROM is in place and it works beautifully!

Thanks again for the suggestions.
 
Good work, mask roms are dropping like flies on these old Konami boards!

Old?? :p "The Simpsons" is newer than any Konami PCB I've touched yet (Time Pilot, TP84, Super B-ball, Jail Break, Scramble...) Guess I'm an 8-bit snob.

EDIT: [Wait a sec, I take that back... I have a Lethal Enforcers PCB]
 
It's more that era - early 90s Konami boards, the mask roms just seem fragile these days. I am not sure if they are more susceptible to static damage from mis-handling but it always seems to be the masks right on the edge of the board or in the corners that die. If they are CMOS based then they would be far more fragile than earlier stuff.
 
Agreed. I remember thinking I probably wouldn't have to cut legs again after I got a de-soldering setup, but it's still easier to cut them for me and safer for the board. One thing I need to get myself is a vice of some kind to hold the board on its side for me when trying to remove components.

Yeah, I need to get some sort of vice setup for this too. Or a new wife who won't scoff at me when I ask her to hold a board while I desolder pins. :rolleyes: Right now I just grab two random heavy objects from around the room and wedge them up against either side of the board to hold it up.
 
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