HP-97 display replacement

[Kees Bouw]

Hi all,

The time has come to start working on a very pretty HP-97 that I have, wonderful condition, probably hardly used. It has one problem: a missing display segment. Just one, the horizontal led in the middle of one of the characters. So I got a replacement display years ago but I don't know if is alright. Before I do the work of replacing the display I would like to verify that it is working correctly and that all segments light up. Is there any way to do that? I have the usual electronics equipment, including a scope, signal generator etc. Would be a shame if I replaced it and then found that it also has a missing segment, or worse. I have attached some pictures.

Yes, I know there is a big scratch on the front, but I think I can polish that away, mostly.

Kees.

   
   

[AndiGer]

I never found the pinout of a Topcat display.
Why don‘t just swap and try it in your machine before investing time trying to check the spare display? Just give it a try, it‘s no so hard to replace

[teenix]

The 97 service manual has the pin out description of the display on the circuit diagram for the keyboard PCB.

You could use a 5VDC supply, connect each column in turn (marked as C1 - C14) to -ve via a flying lead, then using another flying lead connect the +ve via a 1K resistor one at a time to the LED anodes, (marked as SA- SG)

Note that digits 1 and 13 on the 15 digit display only have a negative segment because they only ever light up as positive or negative (mantissa / exponent), all the other segments are physically missing. To make things simpler HP engineers only used 1 column to drive these negative signs hence there is no column 15. (same as on the HP-67)

Column 12 is the common for the 2 negative signs.
Mantissa sign is segment SE
Exponent sign is segment SG

I don't know if it applies to all Topcats, but removing the LED display module from its connector can be difficult because the single conductor wires are a tight fit. Too much wiggling trying to remove the ribbon cable can break the wires.

cheers

Tony

[AndiGer]

(06-30-2024 11:16 PM)teenix Wrote:  The 97 service manual has the pin out description of the display on the circuit diagram for the keyboard PCB.

Thank you very much Tony for pointing me to the service manual. Never realized :-)

Andi