The PCB is a very simple schematic. If a switch or LED does not work, it will be easy to trace down to a bad connection somewhere on the PCB.
A whole series (a row) of switches does not work.
This will be a bad connection between one of the three 'ROW' pins on the Pi connector and the middle pin of all of those problem switches. Use a multimeter to follow the schematic from connector ROW pin, through to one of the three resistors, to the middle pin of the switches.
A single switch does not work.
This is indicative of a bad connection between the upper pin of a switch and its associated COLUMN pin on the Pi connector. Look at the schematic and use a multimeter to follow the connection from GPIO connector pin, through one of the 12 resistors, to the diode of the problem switch, to the switch's top pin. Also: check between top and middle pin of the switch to see if the switch itself works. If not, email me for the fixing procedure... it is unpleasant, as yet untried, but very feasible.
A whole row of LEDs does not work. Similar to problem 1, but now look for the LEDROW pin to work from. Trace it to the IC's input pin, trace from the output pin towards the leftmost lead of the LED, and find the bad connection.
A single LED does not work. Or, a vertical column of LEDs does not work. Similar to problem 2, there must be a bad connection between a COLUMN pin on the Pi header and the rightmost lead of the problem LED. Find & fix.
PiDP-8/I: RECREATING THE PDP-8/I
CELEBRATING 50 YEARS OF PROGRAMMED DATA PROCESSING
I'd appreciate it if you first read the 'Technical Details' page a bit, and then the document under the button above and see if you can fix things. Once you understand rows&columns, it'll be much easier to proceed.
Next step is probably emailing me - not a problem at all. Most of the time, I can point you to the fix. If the problem is not fixed that easily, I would appreciate it if you ask for help on the PiDP-8 forum. Do not hesitate to do that. You know, time is limited so it helps me, and some of the guys on the forum are really helpful.
So far, out of 1000+ kits, only one is known to me as not working yet. And we intend to fix that one ;)