# Restoring old SR-50 Calculator, odd Charger Voltage

I have an old Texas Instruments pocket calculator, for fun decided to get it working again. It is strictly a NiCd powered calculator, no alkaline recommended.

The charger is the 9130, says output 6 VAC, 175 mA. Tested with unloaded output measures 9 VAC, with a 20k ohm per volt VOM. Before I crack open the power supply case, is this "correct?" I don't like the sound of AC charging NiCds. Should be some kind of filtering in there that might need repair.

The SR-50 takes 3 NiCds, no circuitry in the very basic battery pack, will use new NiCds. Maybe I should get a new charger. The battery pack is required for operation of the calculator.

What is a good way to test the charger under load?

• The charging circuit in the calculator converts the AC to DC for charging the NiCads. – Dave Tweed Feb 11 '13 at 1:44

Even a single diode as a half-wave rectifier within the charging circuitry of the calculator would provide positive pulses every half-cycle, with peaks at 6 * 1.414 = 8.484 Volts approximately. A smoothing capacitor would bring this down to a charging voltage within the 5.1 Volt maximum cell voltage of 3 NiCd cells in series.