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HP Forum Archive 05

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HP25 opgrade
Message #1 Posted by Paul Sorensen on 6 Mar 2001, 2:55 p.m.

Can anyone remember how to upgrade a plain HP25 to a functional HP25C ?

I seem to remember that there was done some cutting and placing a resistor and a capacitor somewhere ?

Thanks for any help

      
Re: HP25 opgrade
Message #2 Posted by Andrés C. Rodríguez (Argentina) on 7 Mar 2001, 9:38 a.m.,
in response to message #1 by Paul Sorensen

As far as I know, the upgrade is not trivial. The HP25C has CMOS memory instead of PMOS chips, and has a power "bypass" connection to keep the CMOS chips connected to the battery to keep memory contents while powered down.

Since the "wake" from power down is not exactly the same as a "cold" power on (I mean initialization routines), the firmware in ROM may not be exactly the same (I am just guessing).

            
Re: HP25 opgrade
Message #3 Posted by Paul Sorensen (Denmark) on 7 Mar 2001, 4:09 p.m.,
in response to message #2 by Andrés C. Rodríguez (Argentina)

Hi Andrés !

You are right, but then again:

The overall power consumption in the HP-25 unit is made by the LCDs. In the old days gays realized this fact and they made some changes in the circuit so they could bypass the on / off switch for power to the microprocessor.

The result was a "functional" HP-25C. From the front panel you can reset and clear program memory, so there is no need for an on / off reset.

The bypass circuitry can be achieved in many ways. I have, some 25 years ago, seen it done in a very smart and simple way - but I can not remember how it was done.

Does anybody know the trick?

After daily use for 26 years as a teacher and technical advisor my HP woodpecker and coconut in a strong part of my life.

Thanks Paul

                  
Re: HP25 "upgrade"
Message #4 Posted by Andrés C. Rodríguez (Argentina) on 8 Mar 2001, 7:28 a.m.,
in response to message #3 by Paul Sorensen (Denmark)

I'm sorry, now I understand what you meant. I suppose you should bypass the normal power switch contacts (so the internal power supply keeps powering the microprocessor, ROM, and RAM) and modify the switch to power off just the display drivers and LEDs; but I have not information about how to achieve this in a practical way. Andrés


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