Hp91 with very dim display

[teenix]

(08-02-2020 11:05 AM)James Linder Wrote:  I recently acquired a very nice 91 TopCat with a display so dim that I have to be in a dark room to see what’s on the display. I found no loose connections inside, no corrosion issues, and there were no obvious burnt components on the PCB’s. What are the likely causes for this problem? I’m using a known good, fully-charged battery pack and hooking up a known good charger changes nothing. Just guessing I thought maybe replacing one or both pairs of identical chips on the display board with the 24 wire ribbon cable going to the display might’ve my next move. Any help would be appreciated. I’m capable of de-soldering and soldering and have spare 97 display and all other 97 parts to use for repairing the 91. Thanks for your help!

It looks like the only common part of the display driver circuit is what looks like a constant current source built from a transistor, 2 resistors and 2 diodes. Either of these could be faulty or have a poor connection and is limiting the current to the display segments. Both anode drivers on their pin 14's which are common, connect to the collector of PNP transistor Q1.

cheers

Tony