I had a another poke around last night.
Managed to get the panel off and tested the segments of the unit that had the digits that wouldn't work at all. Tested the segments as Chris56000 described and all segments work on the non-functioning digits and is a CC 7-segment display with the CC pins being the middle on each side of the display.
You'll see the digits aren't socketed.
To get the panel off: remove the knob of the encoder and undo the screws holding the PCB to the panel and unhook from main board, that's it
Good way to determine which pin is CC/CA and if your display is mulitplexed: all segment pins are common on each digit, so each of those pins should be connected to the same pin on the other digits (e.g. ‘a’ on digit 1 is connected to ‘a’ on digit 2, etc, etc). The ones that aren't connected to other digits are either CC or CA and that can be determined if it can be traced to any nearby transistor or resistor to a supply rail.
Once I found that I managed to trace out the following circuit
Now I made an educated guess about some things here: That the transistors were NPN, it’s the only way it could work from the connections. They both had the same markings on them.
The uC is a
Winbond W78E052C40PPin 6 of this uC is connected to part of PORT1 which is a bi-directional I/O port with internal pull-ups
Interesting way they have one pin to do 2 things. Notice that the switches S1214 & S1209 are connect to the same pin. They must switch the pins function from output with driving the digit to input when that pins doesn’t need to drive the digit.
I only traced one switch (it was getting late & cold), but if i had to guess that the when the pin changes to input, and if S1214 was pressed it would go low and probably trigger and interrupt or something, not sure how it would know which one was pressed, would have to have look beyond S1209.
Anyway going back to the problem,
If any of the transistors had gone S/C then the digit would still work (of sorts). So looks like the output of the uC might be to blame. I checked the pins on the uC’s socket and no signs of poor connection or corrosion.
Also S1214 is the pad for the ‘1’ button, which is also not working and tested D1220 (D1), which was ok, yet more evidence against the uC. So either that pin on that port has truly gone or lost pull up? I could tag a pull up onto that trace and see if it fires up, but I’ve never heard of a uC loosing its pull-up?
Anyway have a look through all the pictures below if you’re interested.