Troubleshooting Gravitar/BW boards

MajorHavoc

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Volts (5, 10.3, 15, -15) all look good. Verified all the EPROMs and installed known good AVG, RAMs, POKEYs, and CPU.

Have 4 board sets and the VG is tripping the reset on ALL of them (grounding WDDIS stops the resets). 3 of them are tripping every second, the fourth is tripping 5 times a second.

What do I check next? Thanks!
 
It sounds like you've got a logic fault, I would start by verifying that all address and data lines are connected and functioning on the program roms.
 
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Yes, the problem is in the vector generator since the WDDIS stops the resets.

I guess I'll check all the vector EPROMs for good signals next and start digging through the schematics.
 
So I started poking around with a logic probe. POR circuit is working correctly (only resets on power-up) so that means the watchdog timer is counting all the way down and triggering the reset. WDCLR is never getting tickled.

I probed the buffer chips (244s and a 245) connected to the CPU and none of them are stuck or dead. ROM0 - ROM4 do get selected. Not ROM5 but who knows if that is a problem or just that the program doesn't access that ROM before it resets.

Haven't checked the 161s yet. I'll do that maybe piggyback the 138 that generates WDCLR.
Any idea where to check next?

Thanks!
 
Correction: WDCLR is toggling.

So does the main CPU activate VGGO and then wait for the VG to finish? And VG is likely wandering into the weeds so main CPU never restarts?
 
Thanks for directing me to this thread. I was having similar problems, reset that was cured with the Watchdog grounded. I then located a bad ROM socket, bad RAM chip and one of the SG1495s in the output had been socketed with a short. Now the board will boot as long as I start it from game mode and not in self test, when cold. Once warmed up, it will start in self test. I have gameplay, but the image is sometimes curled into a tube around the x axis, and sometimes looks like the picture below. Selftest shows pokey errors on both chips (chips and sockets are good). I've probed most of the chips on the board and nothing seems to be stuck. I've also used cold spray to see if I can identify the warm up issue, but with no success. I'm a bit over my head here, but since I started with a completely dead board, I'm feeling ahead of the game! Any ideas would be appreciated. -M

IMG_2611.jpg
 
I can borrow one. I'll look into it and report back. I'm pretty sure the output section is ok, but I'll check it.
 
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