Error Code [61]

[Badbaud]

Here is how the circuit works.
U5, U6, and U7 compromise a 24 bit parallel in serial out shift register.
Each input has a 1K pullup resistor to reference +Vb as high to the inputs of the 4021's.
The 1 Meg (1,000,000 Ohm) resistor in series with each input is basically a voltage level shifter to protect the 4021's inputs from getting the full +Vb voltage into a input pin.
When a button is pressed it grounds to chassis ground (Vb GND) and the related input on a 4021 goes low.
The input results of 1 and 0 bits, 1 being input off, 0 being input on, are a serial stream of bits that exits pin 3 of U5 and is buffered by U12 which also flips the 1's and 0's to 0's and 1's.
The serial stream exiting pin 6 of U12 is flipped again (0>1, 1>0) by the open collector buffer U21.
U21 drives the LED inside U14 and when the LED drive is low the LED turns on then turns on the logic gate inside U14 to produce  a low output.
U21 and U14  convert the serial stream to 5VDC logic levels and uses logic ground as reference.
Effectively converting the +Vb dirty voltage logic to the clean +5VDC logic level which the MPU can use to read the serial stream and determine if a input is on or off.
The plus of this design is inputs can be read and processed faster, the minus is a high voltage spike can make it through the 1 Meg resistor and blow a 4021 input or cause the 1 Meg resistor to change value and not process a viable input signal properly.

[Stayouttadabunker]

I totally agree with you BB.
I'm thinking it very well may be a voltage spike that got thru and blew out something along those lines.

Trying to determine what particular component went bad is the hard part for me because I don't know how to actually test a PDIP device.
Simply testing an onboard resistor, diode, capacitor, or a trace is simple enough with a multi-meter.
Checking for abnormal or non-existent logic signals with a probe tester isn't easy though because I don't know which legs to touch and what I should be looking for with flashing LEDs.
Same thing goes for my BK oscilloscope...I'll be checking out Randy Fromm videos till I'm dead. 

This board I believe is buffered rather by an 8-pin DIP at U14, which is located just above the 4021 at U6.
U12 on this board is waayy over on the left in the reel chips near the main caps but I get what you're saying.

I'm hopeful your tester can help us pinpoint this unusual problem?
I've never had a bad Self Test switch circuit before now.
Are you ready for me to send this off to ya? Less typing! 

[Badbaud]

Sounds good, sending PM.

[Stayouttadabunker]

Just a huge shout out to Badbaud over at K-LAR in Las Vegas as in*** Thanks*** for repairing my S+ MPU!
The board works perfectly now!

[Badbaud]

Glad to help.