I had a very short run of the new design WPC MPU boards made a couple of months ago. As I like to make sure they work flawlessly before making any available for general sale I assembled the first one up straight away and it has been running for many weeks now in my BSD without a hitch. Here is the first one made and bench tested - ready to install in the BSD:
A few other projects were completed or at least progressed to the point where I am now waiting on sample boards or parts so I figured it was time to put together another WPC MPU. I did this and fired it up on the bench - it seemed to boot OK so I installed it in the machine. Straight away there was a problem - the "enter" switch inside the door didn't work (wasn't recognised) and a few playfield switches didn't seem to be responding?
I grabbed the schematic and metered the input chips (LM339) whilst pushing the enter switch and followed through to U15, a 74LS240 - confirming that the switch worked at the input pin to U15. I guess I was "lucky" in that I knew the fault was on the board as the machine worked perfectly with my first assembled board and its original.
What was the problem? A faulty chip? To eliminate this I replaced U15 and the few others around it for good measure - I had socketed all chips so this was easy to do. Naturally there was no change. So, pull the board out and back on the bench - looked around for any obvious signs of a mistake I may have made but all looked OK. Fired up the scope and looked at the eight Data BUSS lines that the outputs of U15 is attached to on pins 3,5,7,9 & 12,14,16,18.
They all looked OK as shown here:
When I got to pin 12 however I was greeted with this:
OK - so I figured there must be a short on this BUSS line - back to the schematic and looked at the micro for the Data BUSS pins:
Scoped them all on pins 24 through to 31 and all were perfect - in particular pin 24 as that is Data BUSS line D7 that was missing the signal at U15, P12. Went back to U15, sure enough, no signal yet it was spot on at pin 24 of the micro (U4).
OK - time to get "heavy" - these two pins were hard wired together so it was obvious I had an open track somewhere. Measuring with a multimeter on continuity (power to board OFF) I confirmed that the two pins were not connected. The Data BUSS line connects to many chips in the system and these types of faults can be hard to narrow down when you have a line shorted to power or ground (which is the normal failure mode of a known working board) but not as bad with an open track.
Eventually I came to a point a few chips above U15 where the continuity from P24 of the micro stopped!! Ah-Ha, I must be close. Careful prodding through the green solder mask narrowed the fault to just next to the connector J204 - scraping away the solder mask revealed (not much) to the naked eye and it was out with the magnifier. Still not much but the track measured open circuit right where you can see "25" printed on the board.
So I soldered a piece of jumper wire onto the track:
Checked for continuity between the micro P24 and U15, P12 and YES, connection. Put a small blob of green PCB repair goo on the "patch"
Fired up the board on the bench again and confirmed with the scope that all BUSS lines were working at U15. Back to the machine, board installed and BINGO - all working as it should.
So the fault turned out to be a microscopic break in a track from the printed circuit board factory. Most likely a minor blemish in the raw board used to make this finished one. You can pay extra for an electrical check of all tracks when having boards made but it is expensive so I don't do that. This is a board that I can't sell with this "fault" so it will remain mine.
Fault finding a new board can be an advantage BUT also a disadvantage as it has NEVER worked so the fault could literally be anything on the board. Turns out in the end (this time) to be a pretty simple thing stopping it from working.
Never overlook the obvious. Look for simple things FIRST when confronted with a fault because you can usually assume that the board has actually worked at some point.
