HP67 AC adapter logic


Hi everybody.
Can someone enlighten me as to how a 67's AC adapter interfaces with the calculator? Looking down into the plug of the AC adapter (calculator end) with the cord pointing away from me I get 5.2 VDC from the left to center pin, and 16.3 VDC from the right to center pin. The outside pins measure 11VDC right to left. I get no AC readings anywhere. Does the smaller voltage run the machine and one of the bigger voltages charge the battery?
Thanks in advance, Hal


The "bigger" voltage comes from a current source to charge the battery. You may want to measure its current to ground (not the voltage). Expect something around 50 mA, but in any case, less than 100 mA. "Ground" is the middle pin.
The smaller voltage runs the calculator. Beware that many of those old chargers have a bad electrolytic capacitor which may cause excess ripple and calculator malfunction. You need an oscilloscope to check that. Ripple under full load should not exceed a few 100 millivolts. Full load can be simulated with a 25 Ohms / 0.5 Watts resistor. Voltage at 144 mA load current should be around 3.6 volts.
Beware that the higher voltage from the current source might damage the calculator if no battery is inserted (or the contacts are bad). There was/is a dispute among HP users on the risk level. My own experiments indicate that somehow the HP67 Card Reader IC (which is supposed to get damaged) clamps the voltage to a harmless level but gets quite warm, so prolonged plug-in without battery might cause damage. The HP65 card reader IC didn't have that clamp and the manual explicitely warns about operating it on the charger without battery inserted. If, however, you switch the calculator ON and then insert the charger into the wall outlet, the current source is too weak to develop dangerous voltages (if it is intact). This may be a safe way to operate HP67 without battery from a charger - but never switch it off, just pull the charger from the wall outlet. If you switch it off with the charger still in the outlet, the potentially dangerous higher voltage will appear on the card reader IC.


Thanks Bernhard for the info.
I got to looking at it a bit more last night and noticed the little leaf contact in the receptacle that opens up when the adapter is plugged in and disconnects the battery circuit from the calculator. Are you saying that there is another path from the battery circuit to the card reader IC other than through the leaf contact in the receptacle?
Thanks, Hal


Here are two more for your reading pleasure:

Classic Charger Schematic

Why 65/67 card readers can be damaged without a battery

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