Joust CPU Won't Boot

nerdygrrl

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OK, thanks to the talented and awesome local413 we were able to track my Joust board problem down to the CPU board. It looks like it starts to go through the intro sequence, but then stops.

We swapped my defunct CPU board with his working one and everything was golden so we are certain the problem lies in my CPU. In addition we replaced the Decoder RAM and CPU RAM on my board with known working chips and still got the same pattern so we can rule those out. It just doesn't seem to boot/load.

Any ideas? Does anyone know the startup sequence?

I feel like we are so close!

IMG_0530-1.jpg
 
There is also slight acid damage on the battery holder, but it's only there and didn't move throughout the pcb. I double checked that :)

No error message on the rom led, actually it doesn't light up at all with her CPU. Rom and wigit, plus ribbon cables are A-ok :) Im 99.9% sure this isn't a ram issue, as it's not throwing a code.. But more schooled Williams guys could prolly prove me wrong :)

She's got the cleanest Williams pcbs I've ever seen!
 
I was in the same predicament about 3 weeks ago, I thought from reading I would have to rebuild the power supply. On a random search here reading through threads there was a guy saying that unplugging all of the ribbon cables and plugging them back in might help. It did just that and to my surprise the Joust started working. Give it a shot, I'm not promising anything, but it can't hurt. Good luck!
 
hmmm. a sticky problem. pun intended!

umm it might be a good time to start doing some probing with a logic probe, looking for stuck bits (high or low).

page 7 & 8 of the joust instruction manual describes the inbuilt troubleshooting/diagnostics procedures. run thru those first.

then, i would refer to page 14 of the defender theory later pcbs manual for some ideas on what to start checking - you will prob need to examine each chunk of the video system to look for an addressing or decoding chip thats gone bad. a logic probe will help.

if you dont have the tools or the skills, ask yellowdog if he can do a repair for you!
 
She's got the cleanest Williams pcbs I've ever seen!

:eek:

Ok, so what are your voltages? Check them at the RAM and EPROMS.

Does the game freeze, or does it keep resetting? If the game freezes, try swapping out the CPU and see if that changes anything.

If it keeps resetting, the watch dog is probably barking and you'll need a logic probe to figure out why.

If you swapped in a different CPU board and it works, you know it's not a ROM issue.
 
Based on the picture, it looks like there is a memory addressing issue. The "random" pixels on the left indicate that the RAM test is starting and then stops. This probably indicates that there is a problem resetting the watchdog, which is addressed as a memory address that is decoded and resets a counter.

To verify that it is a watchdog issue, you will need a logic probe placed on pin 37 of the CPU chip (the !RESET line). If it is pulsing, it is getting reset by the watchdog. If it is low, there is an issue with the watchdog circuit. If it is high, then there is some other issue that is occurring. Normal operation is for !RESET to be high.

The !RESET line should also follow the reset switch. When the reset switch is pressed, the !RESET line should go low for about a second and then back high. If it doesn't, then there is an issue with the reset circuit or the watchdog circuit.

ken
 
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