HP-25 Co-luminous segments

[teenix]

I'm not sure where the HP-34C came from, the Spice series have a different setup to the Woodstock.

I only mentioned the HP-29C manual so that you might see how the display and keyboard work which is explained in the manual and might help with a repair.

I attached a simple image of the keyboard display connections.

If you assume digit 4 is activated by the Cathode driver then any LEDs that are to be switched on for this digit will do so, but only one at a time. The LEDs are energized by the ROM 0 chip and the current flows into the Cathode Driver and then to ground. As all other digits are OFF, no LEDs in these will light during this time, unless there is a fault.

If no keys are pressed, the ACT will see all LOGIC 1's on the column inputs (1 - 5).

If, for example, the [7] key is held down continuously, when Digit 4 activates, the ROW 4 line on the keyboard is also connected to ground. Because ROW 4 and COL 3 are shorted together, the ACT will now see this LOGIC 0 in the ROW 3 line and it will generate a key code (98 in this case). The ACT will then execute code to process the 7 key.

When you release the key, the ACT will again only see LOGIC 1's on the column inputs all the time.

Now when digit 2 activates, the ROM 0 IC will output the segments (a b c and h) one at a time to display "7" and the decimal point.

Now, when you press any key on Row 4 (- 7 8 9) you see segment (a) dimly duplicated on all digits which is the fault. The trick is to find out why.

I hope this helps a bit more.

cheers

Tony