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Hi there, before I bolt up the HP 45 and replace the back label I was wondering if anyone has done the timer to quartz timer conversion as posted here:


If so, I have a few questions:

Has anyone done this conversion?
Where would you find a 784 kHz crystal?
Is the HP 55 crystal also a 784 kHz crystal?

Cheers and thanks in advance, Geoff.

It appears defective HP-55's could be a source for 784 kHz crystal. But they might be as hard to find as the crystals themselves... Here are some old threads on the subject, in case you haven't read them yet:






Thanks Gersen,

I have a 55 keyboard and battery spine with the correct quartz crystal on it's way. It is a spare which will be parted for my HP 55 restoration and now as a source for the HP45 timer.

I tried a search but it would seem your prowess at that is WAY better than mine.

Cheers, Geoff

You're welcome, Geoff!

No secret about the search: just google for site:hpmuseum.org HP-45 + crystal, in case you want only results from the museum. This search-key will return 28 results:


I am looking forward to your crystal mod pics :-)



Instead of a crystal, what about just fine-tuning the oscillator that's already there to get it close to 784 kHz. Surely you won't be using the 55 as a timer for something that requires super accuracy, will you? After all, the most inaccurate part will be the finger pushing the start and stop buttons.


I use this for timing in the aircraft as well as an HP41cx customized for the aircraft.

The HP 45 is just an excercise as I will have a spare 784 kHz crystal on it's way.

The only real use for either the HP 55 or 45 (corrected) would be while they were plugged in as this mode really drains the batteries quickly.

Cheers, Geoff

Edited: 22 Jan 2008, 8:45 p.m.