HP-25 Co-luminous segments

[teenix]

(04-08-2022 05:38 AM)Tom Flatterhand Wrote:  I have a Fluke scopemeter. Could it be used to make the measurements? If you write me where to measure, I could try that.

Sounds like a plan.

I'd start by testing the output from the ROM0 chip. (5061-0430)
Pin number 1 is the A segment for the display.
I haven't had to work on a Woodstock yet so I assume with the calculator opened, you can connect the battery, or maybe an external power supply set to 2.5V.

The scope earth lead should go the Battery -ve, and the actual probe to pin # 1.

With just a 7 displayed, you should see a positive pulse of about 40uS. This should repeat once at roughly 3.8mS intervals. The rest of the time, the signal should not show much. A similar signal will appear for segments b and c (ROM0 pins 3 and 4). The other segments should not show anything except some noise. (pins 2, 6, 7, 8, and 9)

All the A segments in the entire display "see" the 40uS ON pulse, but only one digit should have a path to ground to let the current flow. This is the job of the Cathode Driver, providing this path for each digit in turn. For the same LEDs to light in other digits, there must be some path that allows current flow. Just a matter of finding it.

Of course, ROM 0 could be outputting erroneous data as well - hard to say.

With one of the keys pressed, you might notice some extra signals appearing.

If this causes ROM0 to drive the A segments for the other digits, you will see that same pulse every 320uS or so, except for digit 4 which as you mention is blank.

I don't think this is happening however, as these segments are dimmer.

In the faulty state, these are probably not be on all the time, but are probably strobed on and off at some point in time during the display refresh. Having a dual channel scope will help here because you can trigger the scope on the RCD pulse appearing at the Cathode Driver IC (1820-1382) pin 16. This is a small negative going pulse of 10 or so uS and resets the cathode driver to start the display scan again every 3.8mS or so. This way you can see what is happening over the full display refresh.

You could also test the STR input pin on the Cathode Driver (pin 10). This should pulse every 320uS or so and steps the Cathode Driver to the next digit. This might be worth monitoring when one of the keys is pressed. There should be no changes to the signal.

This will give you something to start with.

The Cathode Driver IC seems to have the same HP serial number as other Woodstocks so is probably compatible with other models if a swap over is required.

cheers

Tony