I received a Joust MPU board that was pretty messed up. I replaced quite a few chips and the crystal before getting it to work right. Packed it up and shipped it off. A couple days later I got a PM that it wouldn't sync up after it arrived. Which didn't sound right, as I had my usual 4 hour burn in before shipping it. After trying a few things, I asked the owner to ship it back to me.
Sure enough the sync was way out:
This was the rug test. And it passed.
Here is the game running:
I grabbed the logic probe and started backtracking the clock signal. At first I didn't find anything. So I grapped the HP logic comparator and started rechecking all the invertors and flip flops and all of them checked out.
So I started checking the clock pulse from the crystal and when I touched the inner lead, my finger slipped and I shorted the lead to ground. And the picture cleared right up. WTF??? I pushed the reset button, the picture stayed good. I powered down and it was trashed again. When I touched the inner lead to ground using a jumper cable, it cleared up. THinking it was the crystal, I replaced it. Same issues.
After some experimenting I found that if I clipped the jumper cable across R74, it would boot up and run just fine. So I replaced R74. Picture was trashed. Jumpered, ran fine. So I replaced R74 with a piece of wire. Screen was trashed. Long story short, I could touch 3 or 4 places with the jumper cables to ground and it cleared up. Touch it with a short piece of wire, trashed. I checked the jumper cable with my DMM and found there was a small amount of capacitance that wasn't in the short piece of
wire.
Out of desperation I tried using a 22pF capacitor in a couple of places, and lo and behold, it worked. The best place was from pin 2 of the 74LS04 at 7J (the first invertor) to ground. I soldered the capacitor from pin 2 to pin 7 (ground on the chip) and this is what I saw:
(the little bit clipped on the left is normal with the little LCDs and Williams boards). If I swapped an unmodified 74LS04 in, I got the trashed screen.
The only thing I can think is that when I replaced the 74LS04, that the socket added some small fraction of capacitance that causes the first clock signal to degrade. Adding the additional capacitor seems to offset the effect, strengthening the clock signal. It is one of those fixes. Don't know why it works, just that it does.
The fix:
Weird, but it works. Time to pack it up and send it home.
ken
Sure enough the sync was way out:
This was the rug test. And it passed.
Here is the game running:
I grabbed the logic probe and started backtracking the clock signal. At first I didn't find anything. So I grapped the HP logic comparator and started rechecking all the invertors and flip flops and all of them checked out.
So I started checking the clock pulse from the crystal and when I touched the inner lead, my finger slipped and I shorted the lead to ground. And the picture cleared right up. WTF??? I pushed the reset button, the picture stayed good. I powered down and it was trashed again. When I touched the inner lead to ground using a jumper cable, it cleared up. THinking it was the crystal, I replaced it. Same issues.
After some experimenting I found that if I clipped the jumper cable across R74, it would boot up and run just fine. So I replaced R74. Picture was trashed. Jumpered, ran fine. So I replaced R74 with a piece of wire. Screen was trashed. Long story short, I could touch 3 or 4 places with the jumper cables to ground and it cleared up. Touch it with a short piece of wire, trashed. I checked the jumper cable with my DMM and found there was a small amount of capacitance that wasn't in the short piece of
wire.
Out of desperation I tried using a 22pF capacitor in a couple of places, and lo and behold, it worked. The best place was from pin 2 of the 74LS04 at 7J (the first invertor) to ground. I soldered the capacitor from pin 2 to pin 7 (ground on the chip) and this is what I saw:
(the little bit clipped on the left is normal with the little LCDs and Williams boards). If I swapped an unmodified 74LS04 in, I got the trashed screen.
The only thing I can think is that when I replaced the 74LS04, that the socket added some small fraction of capacitance that causes the first clock signal to degrade. Adding the additional capacitor seems to offset the effect, strengthening the clock signal. It is one of those fixes. Don't know why it works, just that it does.
The fix:
Weird, but it works. Time to pack it up and send it home.
ken


