Williams Comet, displays haven't worked since day one. I originally had some strange voltage readings at the power supply board and thought that was the issue. Did a complete rebuild(caps, diodes, transistors, resistors) and am getting the correct readings with my dmm, but still no displays. I then tested +100v, -100v and +5v pins at the 4J11 connection on the display driver board and everything looked correct. Outside of that, I'm stumped as to where to look next. If someone could point me in the right direction, it'd be appreciated.

Anyone? Bueller?

Unfortunately no i don't.

Grumpy, getting 4.15v on 1J3-12. No i don't have a probe, but i can order one if needed.

Thanks grumpy. I'll order a probe.

Should have the logic probe by this afternoon.

Bumpage.

Here's what i have if I'm reading the probe correctly:
U-9 pin:
1 high
2 low
3 high
4 low
5 high
6 low
7 low
8 high
9 high
10 low
11 high

And on 1J3
Pin:
1 high and low
2 high and low
3 high and low
4 high and low
5 high and low
7 high and low
8 high and low
9 high and low
10 alternating between h/l and low
11 alternating between h/l and low
12 high

Grumpy, i may have misunderstood you. When you say check the pins at U9, are you referring to the pins of the ic, or the pins where the harness plus in? U9 only has 7 pins per side.

My error on that. I was looking at U9 on the display driver board.

U9 on the cpu had the following:
Pins 1-11 high pulse
Pins 20-23 high and low pulse

At U11 pins 7 and 14, its not giving me a voltage reading. At ZR1, I'm getting 102v in and 92v out.

Grumpy, with the black lead touching the ground strap and the red on pin 14, I'm only reading .6mv. With the red on the 5v and the black on pin 7, I'm getting just shy of 5v.

That being said, i am getting 5v at the wiring harness connection on the display board, so it looks like power is at least getting to the board.

I'm an idiot. Was taking a reading off of pin 8 instead of 14. I am getting 5 volts across pin 7 and 14.

1,3,9,11,13 all were high.
2,4,8,10,12 were all low.

No corrosion that i could see, although at one point it looks like Q81 and Q79 were fried and repaired. The 1J1 connector looks to have gotten hot and melted slightly. I have reflowed each pin and checked for continuity with the dmm.

Grumpy, at this point are you thinking it's an issue with the CPU and not the display board?

When touching the lead with the probe, both the high and low pulse lights are on. Possibly user error on my part? I took the board out yesterday to look it over and can't find any signs of corrosion or broken traces/joints. When I get home tonight, I'll re-install and test with the probe again and report back.

They are both on at the same time.

Got the board back in and tested again on 1J3 with the same results listed in the post above. I don't know if I'm accurately describing what's going on when i say it's reading high and low pulses, so i took a quick video:

Doesn't look like a bad solder on pin 1. Re-flowed solder with no results.

I'll order a 6821 and report back. Thanks again for all you help, Grumpy.

New ic installed in U5, same results as before. Tested 1J3 with the probe and got the same readings as in my previous post.

Thursday bump.

I'll check tonight. If I'm reading the schematic correctly, it looks like U9 leads to connectors 1J1 and 1J2. 1J1 looks to have gotten hot and melted some of the tabs that hold the harness on at some point. I did re-flow solder test with a DMM and have continuity from the pins to the outputs on U9, so I don't think that melted connector is the issue.

Wiring checks out. No breaks.

I know this is a long shot, but could a faulty rom set cause issues with the displays?

At this point, I think I may just cut my losses on this board and look for another working MPU. I spoke with someone at Rottendog and they are supposedly working on a system 9 board.

Resurrecting this thread with an update...
I sent the board off to K's arcade for repair. Unfortunately, he was unable to find the issue. I did a little more googling and found Edward Cheungs name mentioned for Williams board repairs, with high praise. I shot him and email and ended up sending him my board. Just got an email telling me that the board is repaired, along with a video showing it working. If anyone is looking to have a Williams board repaired, Dr Cheung is definitely the man to talk to.

I asked and he hasn't responded yet.

He said there were a couple bad ICs, but didn't go into detail. Guess I'll see when it gets here.

Just for shits and giggles, here's the preship video he sent off the board working.

The first time these displays have seen any action in a long time. Did quickly realize that one of them are bad, but i can live with that for now.

IMG_20171022_183347 (resized).jpg

No, I never did get a follow up. Looking at the board, I couldn't really tell what had been replaced. It did look like there was a bad trace coming from U5, he added a wire coming off of one of the pins.