I did some "dead bug" rework to the existing display; basically epoxing two SMT 2N3904s to the top side of U3. I cut some traces and soldered a 20K ohm pull up between VCC and the collector of the transistors. Emitters were tied to ground. Bases were connected to the series resistors currently present in the schematic.
The goal here is to work out the kinks in the design so they can be incorporated into a FAB B board and be relatively confident the design will work out of the box.
Because the BJTs are single transistor inverters; I needed to rewire the input to U3. I wanted to reuse the existing CD4555B chip already present to keep the design simular. Turns out that by swaping A & B inputs vs the schematic; the following boolean logic become obvious:
U3A Q1 = !B*A = D5
U3A Q2 = B*!A = D6
U3A Q0 = !B*!A = D7
Remember that when the MPU is driving A5 or A6 high; the BJTs invert that to be an active low. So the boolean math makes it logical.
The result (finally):
Click here for YouTube movieHere are some pictures of the display installed in my Bally Star Trek machine:
Known issues:
Digits are too light when running in multiplexed mode (in a real machine); plan to drop the anode resistor to ~2.7k from 22k to brighten the digits. Not a good idea for non-pinball machines which aren't multiplexing the digit enables; but should be fine for more machines.
Need to "fix" the native 7 vs emulated 7 digit jumper selector given the need to re-invert D7 in native mode.
