Classics (HP-35) battery pack


Hi everybody.
I'm going to make an HP-35 battery pack from scratch, following Douglas Jones' information:
My concern is: what if I use 3 AA alkaline batteries (3x1,5V) instead of 3 NiCd cells (3x1,25V)? Is the internal circuitry of the HP-35 capable of transforming the voltage to the required level or am I going to harm some internals?
Thanks for any feedback. Chhers. Giancarlo


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Rubbish! The HP35, like all other classics, has a power converter circuit on the logic board. This takes in the battery voltage and provides the Vss (+6.2V) and Vgg (-12V) supplies for the PMOS logic chips along with some other voltage -- around 8V IIRC -- for the display. This circuit _is_ regulated (it monitors the Vss output and controls the oscillator appropriately), but one property is that the Vss output can never be less than the input. So don't ever apply more than 6V to this circuit!

However, in the classics, the 'raw' battery voltage is connected to the display driver circuit. I can't believe this will mind 4.5V, but....

Would I put a 4.5V battery in my 35? No I wouldn't. I am pretty darn sure it would be OK, but I don't like risking things when it comes to 30-year-old calculators. 3 NiCds are pretty cheap, and you can recharge them.

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