HP-67 - 1705S02263 - Full Repair Advice Requested (was HP-67 - strange behavior)

[teenix]

(10-28-2019 10:39 AM)albertofenini Wrote:  Hi Andi,
no we didn't, this is how looks the display (this is supposed to be 0.00) when a good working CPU is loaded
The segments not working are C and F, which are driven by the RCA chip and not by the transistors.
Looking at the schematics, the four transistors (three on the back and one in front) should drive
the G segment, which in this case works, look at the next picture which is supposed to be a list of 8
Will replace the RCA and see what happens, still we need to repair this logic board.

The transistors drive the sign G segments for the mantissa and the exponent. This is because the cathode driver only has 14 outputs to drive 15 digits. The other segments for these digits are unused and in the HP-97 display there are actually no LEDs for those unused segments only the G's.

You should be able to see the C and F segments go logic high on CPU keyboard connector pins 17(F) and 20(C) if they are to be lit (say when a zero is displayed).

If not ROM 0 may be the culprit or there are open PCB tracks from pins 2(F) and 4(C) to the CPU keyboard connector.

Otherwise the fault is circuit tracks to the anode driver input pins 4(C) and 12(F), or the outputs 6(C) and 11(F) to the LED displays. If these are sound, then the anode driver would seem to be faulty.

The connections between the CPU and keyboard could also be faulty.

Not sure what problem it could cause, but the 8 x 24K SIP resistor package, which normally pulls the LED outputs from ROM 0 to logic 0, might have open circuit resistors on the C and F segments.

cheers

Tony