This is the post I had planned on making for awhile and just never really found the time to do until now; I previously posted some information for this project in mac622's post on starting with a partially assembled TwoBits board. https://pinside.com/pinball/forum/topic/partially-assembled-twobits-wpc-mpu In this topic, I provided a link to a Google sites spreadsheet I created which contains the complete BOM for the WPC CPU Board (A-12742-XXXXX) and includes links to Mouser Electronics for every part used in the assembly.
The primary reason I like this board enough to build one is that the component layout and schematic are a near perfect match to the original Williams board. The only thing I don't care for is the component values for all the resistors are listed under the part making it impossible to read them on an assembled board, this will make future troubleshooting of the board a little more difficult.
For this build I ordered the complete list of materials from my Google sheets page https://docs.google.com/spreadsheets/d/1C4znEqZd4K4eywhi__qDwI-3AsX6Y5qdw9Pzb0p178A/edit#gid=1430569459 substituting the 5mm LEDs for the option 3mm LEDs which are a better fit on this board; additionally I found that the 7 pin header from TE Connectivity is better fit with the IDC connector originally used by Williams than the Molex one I had used in my previous build, and based on feedback from the previous post I decided to go ahead and socket U20 even though it goes against my personal preferences.
As of this writing the Partially Assembled TwoBits board is no longer available on ebay, and I was previously informed that that it will no longer be available once supplies are exhausted. Note: The partially assembled version of this board contained an incorrect R91 resistor (2.2M ohm intstead of 22M ohms) as identified by user A12742; this caused the oscillator used to control the RTC to operate in an unstable harmonic. Please keep in mind that in order for the RTC on this board to operate properly, you must use some kind of uninterruptible power source (typically batteries), any board using NVRAM will cause the RTC to cease operating whenever the game is off, which makes this incorrect resistor value irrelevant.
The blank TwoBits board is still available at the following link for anyone interested in building this board ebay.com link » Williams Wpc Mpu Brand New Old Stock Bare Circuit Board Build It Yourself
Just for the record building this board from scratch is not for the faint of heart, while its a fun project (if you enjoy doing this type of work), it took me twelve hours to get the blank board to the same state as the partially assembled board. The majority of time is spent inventorying, sorting, bending and trimming all the components, and finding their board locations; the time you will spend actually soldering components to the board pales by comparison. Full disclosure, I'm very detail oriented and I'm certain it takes me a greater time than most to build this board as it's important to me for the component part numbers to be visible on the board after soldering, as well as for the component to be centered between through holes and all facing the same direction; it really bothers me when resistor color bands don't line up.
Close up of the finished board, seems like no matter how careful I am I always end up with one capacitor with the writing orientated the wrong direction...this is likely due to my refusal to acknowledge my need for bifocals
The BOM calls for a pair of 72 pin headers, these are used to make the 4 ribbon cable headers (J201, J202, J204, J211).
I like to build the board starting with the lowest profile parts and building upwards, on this boards the glass diodes are the shortest part and are installed first and working towards taller components, followed by the resistors, capacitors, and larger diodes, I do a quick flux cleaning as needed to help control how messy the board is, not to actually remove it all at this point.