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Just posted at the HP museum my HP calc restorations.
One of the restorations involved an HP 45 with an 'easter egg' in the form of a timing display. It seems the HP 45 came out before HP had access to Quartz crystals for time control.
The engineers decided to program a rom sequence and include the complimentary wiring for testing for a proposed timer in the next version called the HP 55. The sequence and programming resulted in a non-documented timing sequence that had all the correct display of an LED stop watch but on average a loss of 7 seconds per minute. Remember they were not producing the HP 45 with an accurate circuit but just for practice as it were. The access to the time display was extremely awkward as it was not supposed to be accessable by the general user. It required one to press the RCL key, then simultaneously press the CHS+7+8 keys.
Well it turns out that a simple inclusion of a 780 kHz crystal in series with a choke removed is all that is required to turn the 7 second loss per minute into a quartz accuracy.
Well I did it! After restoring the calc and securing a 780 kHz crystal my HP 45 is now as accurate as my best LED wristwatch. The access to the display has also been simplified, now all I have to do is press the RCL key and then press the enter key and voila!
If you are interested I have a few links below (the last link has HP 01 content so not entirely off topic!)
Here is the HP45 during restoration:
http://www.hpmuseum.org/cgi-sys/cgiwrap ... 014#131014
Here is the original article on the HP 45 quartz conversion:
http://www.hpmuseum.org/45crys.htm
and here are the pictures of the collection:
http://www.hpmuseum.org/cgi-sys/cgiwrap ... 145#132145
here is a shot of the display when the RCL and modified ENTER key are pushed 00 hours: 09 minutes: 18 seconds: 18 hundreds of a second.(hours.minutes_seconds______hundreds).
![Image](http://i45.photobucket.com/albums/f96/geoff_q/completed.jpg)
One of the restorations involved an HP 45 with an 'easter egg' in the form of a timing display. It seems the HP 45 came out before HP had access to Quartz crystals for time control.
The engineers decided to program a rom sequence and include the complimentary wiring for testing for a proposed timer in the next version called the HP 55. The sequence and programming resulted in a non-documented timing sequence that had all the correct display of an LED stop watch but on average a loss of 7 seconds per minute. Remember they were not producing the HP 45 with an accurate circuit but just for practice as it were. The access to the time display was extremely awkward as it was not supposed to be accessable by the general user. It required one to press the RCL key, then simultaneously press the CHS+7+8 keys.
Well it turns out that a simple inclusion of a 780 kHz crystal in series with a choke removed is all that is required to turn the 7 second loss per minute into a quartz accuracy.
Well I did it! After restoring the calc and securing a 780 kHz crystal my HP 45 is now as accurate as my best LED wristwatch. The access to the display has also been simplified, now all I have to do is press the RCL key and then press the enter key and voila!
If you are interested I have a few links below (the last link has HP 01 content so not entirely off topic!)
Here is the HP45 during restoration:
http://www.hpmuseum.org/cgi-sys/cgiwrap ... 014#131014
Here is the original article on the HP 45 quartz conversion:
http://www.hpmuseum.org/45crys.htm
and here are the pictures of the collection:
http://www.hpmuseum.org/cgi-sys/cgiwrap ... 145#132145
here is a shot of the display when the RCL and modified ENTER key are pushed 00 hours: 09 minutes: 18 seconds: 18 hundreds of a second.(hours.minutes_seconds______hundreds).
![Image](http://i45.photobucket.com/albums/f96/geoff_q/completed.jpg)