Columbus, IN
Submitted for your approval
Dead WPC-89 CPU from a Twilight Zone
P/N A-12742-50020
I bought a WPC-89 test rig last week, and it came with 2 non-booting CPU boards.
First test, booting the board on the bench, by providing +5 and +12 to J210
Result:
D19 and D21 lit, solid
D20 is out
D20 is the diagnostic LED
As a reminder, the expected behavior of a 'good' booting CPU is as follows:
When the game is powered up, D21 lights to indicate +5 is present. D19 lights for 1 second and goes out. D20 starts flashing and stays flashing.
Should D20 blink once, it indicates a problem with the CPU ROM. Usually, it's bad, but it's also possible that there's a bad socket, trace, etc
Should D20 blink twice, the game RAM is faulty, or, again, traces, etc
Should D20 blink thrice, there's a problem with the ASIC, or again, traces, etc.
At this point, I like to swap the game ROM, ASIC and 6809 with known working chips to verify they are not the problem. I tried the chips in a working MPU, and it continued to be a working MPU, so I reinstalled them.
Using my logic probe, I then checked the address lines and data lines of the 6809 to see if any were 'stuck'. Sometimes, a fried IC on the CPU board can take the entire board down. In this case, data line D6 was stuck low, but all other data lines were pulsing.
Then, I decided to look closely at prior work - because I often find prior repair work causes a LOT of the problems that come across my bench.
Sometimes, the problem is diagnosed correctly, but not repaired correctly.
Sometimes, the problem is *not* diagnosed correctly, and I now have more than 1 problem to fix (ie, broken traces, etc)
U20, the ULN2803, had been socketed. Upon closer analysis, I saw that a trace was lifted on the component side of the board, and the wrong socket had been installed. Technically, the wrong socket wasn't part of the problem - someone had used a 20 pin socket and cut the leads off the leftmost pins of each row of the socket off.
I removed the improper socket, and replaced it using a good quality 18 pin machine pin socket, buzzed out the traces to the schematic, and ran a short jumper to repair the lifted trace.
U5 had been replaced and socketed. I checked this one to the schematic, and it was fine.
So, I started looking closer. Flexed the board to see if that made any difference. Nope.
And then, I noticed a very small crack in the ASIC socket in the bottom right corner of the ASIC socket, between pin 74 and 75. I found that if I squeezed across the socket both ways - pulling top and bottom in, and left and right in, the board would boot! And, when I released my squeeze, the crack got bigger!
photo 1-348.JPGphoto 2-541.JPG
There's only 1 way to fix this - replace the ASIC socket. I've seen people do all sorts of creative fixes over the years involving zip ties, etc.
The ASIC socket is not for the novice solderer - it's 84 pins, and let's just say if you damage even one trace, you can have a very fun time chasing it out.
An experienced solderer with a good vacuum desoldering station (Hakko 808, 470, etc) can pull one of these without lifting traces in a reasonable amount of time.
photo 3-132.JPG
Note when reinstalling the PLCC socket that there is an arrow that indicates pin #1 - the pins of the socket will insert in the board with the socket turned 90, 180 or 270 degrees from where it should be. The arrow should point to the right side of the CPU board!
Next, replacing the socket, I powered it up with a PC power supply.The LEDs functioned as expected for a booting CPU, D21 lit solid, D19 lit for 1 second, and D20 continually flashing.
Next, I threw it on the test fixture to test out the switch matrix etc and make sure my work was solid.
The game booted up in French, so I snipped the language jumpers and tested the board out.
Finally, I pulled the battery holder, and installed a set of DIP switches for setting the default language. While I've all my games in English, it's possible I could use this board to repair a game that is desired in a non-English language, or I could use it in an Addams Family Values, where game config settings are done with DIP switches.
I will probably install an AnyPin NVRAM on this board. I am fresh out, so I'll order one for it soon. I also need to clean the board.
How was the PLCC socket on the ASIC damaged? Who knows. There are *proper* tools to replace the ASIC, and reseating it is a common troubleshooting step in game repair. But, if you don't have the right tool to reseat it, don't do it! Don't try to use a screwdriver to pry it out. You could damage the chip or the socket. The chip isn't super expensive, but the socket is a PITA to replace without the proper tools.
Here's a photo of the 2 ASIC removal tools I own.
The tool on the right is pretty common, and is generic in the fact that it can be used to pull most any socketed PLCC chip of any size.
The tool on the left is a special PLCC extractor made by AMP to pull 84 pin PLCCs. It is P/N 821590-1. Not sure what the new cost is, but I got mine by watching eBay and making an offer on one. I think I paid around $20 shipped. The one on the right can be had from Radio Shack or other places. I keep a few on hand for ROM customers, so if you need one, LMK.
(Also posted and blogged at http://blog.myhomegameroom.com/?p=169)
