Looking for help w/ a (Ms.) Pacman board...

[gonzojoey]

Hey guys. I'm looking for a little help with Ms Pacman I recently acquired. When I got it, the video output was nothing but a screen full of P's that would wave from left to right. I checked out several troubleshooting guides and didn't see anything about "P"s, but since its similar to some other symptoms I found, I figured it must be the RAM, so I picked up a set of the 4K-R IC's from Bob Roberts (2114's). Now all i get is a flashing 0 near the top of the screen...
When i look through the troubleshooting guides, it says a flashing 0 usually means something wrong w/ 4-M, which I just replaced 
I do have another working board available to me, so I did try to swap the Z80 and VRam addresser boards to see if that makes any difference, but it didn't...so I don't really know where to go from here. Visual inspection of the board around the 4-M chip is fine, no cracks or obvious damage to the runs. I haven't tried to enter into test mode (honestly i didn't realize this board had a "test" option) but i can try that tomorrow to see if it identifies any bad RAM / ROM.
Is there something obvious I'm missing? Has anyone see this sort of issue before?
Any help you can provide would be appreciated.
Thanks Guys!

[paigeoliver]

I don't have a lot of general tips to give you, however I do have a Ms. Pac hardware diagnostic daughterboard and a working boardset ready to send out and would be happy to do a board exchange or repair for $70 including shipping on my side.

[gonzojoey]

Update...
I tried with the Z80 on the main board (eliminating the ribbon cable and daughter board), but it still comes up w/ a flash zero at the top of the screen...
I guess I can try swapping the 6E-6J Proms to see if perhaps one of those is causing the issue...

If i cant get this thing working, I might take you up on that offer paigeoliver.

[mgb]

I'd say, try swapping the proms if you have them.
Are also cleaning the legs on the chips before you pop 'em in.
Those things get real grimey and that could cause bad contact.
Also a bad socket can be the case.

[orion]

Bad socket at 4M?

[mgb]

Bad socket at 4M?

Yup, definitely may be the case.
   If other repair logs show 4m as a common issue causing the flashing 0
   And you already swapped with a known good chip
Then I'd say try swapping that socket

[gonzojoey]

Well, I'm about ready to throw in the towel...
I tested all the RAM in a known working board and it's all good. I tested all the Proms and they are good too. I even went as far as testing every IC in a socket in the good board and they worked..it's gotta be either a bad socket or a bad trace somewhere on the board. Ive already replaced all the caps on the board, the ribbon cable, all the Ram chips and confirmed everything else works on the other board. At this point, I've invested more in parts and time than it would be have cost to exchange the board (I think). I guess that's all part of this hobby .

[orion]

The only other thing I know about these boards is that they regulate the power to the needed voltages on the PCB itself. Sometimes the voltage regulators can go bad and cause issues or so I hear. I would swap out that socket first though.

[orion]

I narrowed down a problem chip on my pac man the other day just by applying pressure and twisting slightly on all the row 4 chips with the power on till I got it to boot. I swapped the problem chip with another chip i had laying around and now its happy again. Try pushing and slightly twisting that 4-M and see if that does anything.

[mgb]

It's also common for resistor packs on the board to go bad.