Repair of HP98035 clock module for HP9825

[BobVA]

Interesting series of videos from "Curious Marc" on YouTube in which he repairs (several!) HP98035 RTC modules for the HP9825.

The 98035 used a TI LED digital watch chip (!) that was repackaged into a DIP package for HP. The 98035 actually reads the seven segment outputs to determine the date/time at power up, and similarly manipulates the "set" and "read" buttons to set the clock chip when needed. (That's what you do when you don't have DS1307's available from the future!)

Much like the Woodstock calculators, the 98035 used a nicad battery as both a power source and voltage regulator for the watch chip, with predictable results: destruction of that chip when the battery eventually failed.

The series progresses from sourcing replacements for the chip and verifying they are real to reverse engineering the 98035 to diagnose a variety of other failures in the stack of them that Marc collected. (This includes a detour into doing RE of the 98035's HP "nanoprocessor" and analyzing its code.)

[Martin Hepperle]

... as "usual" - fantastic work and video!

I was lucky that in my RTC I only had to replace the battery and a broken crystal.
Then it was working again and I use it mostly for timing program execution times.
Now I switch it on regularly to re-charge the battery.