The Museum of HP Calculators

HP Forum Archive 14

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HP-19C/HP-97 printer
Message #1 Posted by Tony Thimet on 12 Dec 2003, 9:13 a.m.

Hi all!

As you probably know the HP-97 and HP-19C won't work at all from the external HP power supply if there is no battery pack installed.

To get around this problem I used a regulated DC 7.5V off-the-shelf supply: Now the units work fine - however the printers won't work!

Apparently, the internal circuitry of the calculator does not allow to draw the necessary current from the external power supply but only from the installed battery pack.

Does anyone have more experience with these issues? Is there somwhere a schematic of the calculator's voltage regulation/battery charging circuitry?

Many thanks,


Re: HP-19C/HP-97 printer
Message #2 Posted by Katie on 12 Dec 2003, 10:04 a.m.,
in response to message #1 by Tony Thimet

That's correct because there's a current limiting resistor in series with the AC charger input as well as a diode. If you want to power the calculator off an external supply (and have the printer work) you need to feed power through the battery contacts -- make sure to use only 6 volts then. (Or you could bypass the resistor and diode by modifying the circuit slightly.)

There's a schematic of the 97 power supply/charging circuit in the service manual that's on the Museum CD set.

Edited: 12 Dec 2003, 10:07 a.m.

Re: HP-19C/HP-97 printer
Message #3 Posted by Stefan Hauschild on 12 Dec 2003, 11:48 a.m.,
in response to message #2 by Katie

5V would be sufficient when using the battery clips for powering the device because the battery pack contains four 1,2V cells. This avoids some heat and power loss in the resistor.

Best regards


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