HP 35 adapter voltage


Hello, what should be the correct voltage at the three pins of the connector of the HP 35 adapter? Thank you for your answers in advance.


See Classic Charger Schematic for the internals. As viewed from the rear of the calculator, the left pin is battery charger supply (~16vdc open circuit), the center pin common and the right pin ~4-5vdc which powers the calculator when plugged in.


On the plug you see three pins, one with - one with + and one blank.
The - is the ground reference. The + is the current source for charging, use an amperemeter to see about 50...60 mA there. The blank one is the positive supply which is about 3.6 Volts at full load (at a 25 Ohms resistor). You can't use a voltmeter without load resistor there because the circuit within the charger needs a load to work.

There are two most common problems with these adapters. 1) broken wire at the plug, cut open the plug, resolder, and reglue. 2) Bad Electrolytic capacitor in the adapter, then excessive ripple (seen by oscilloscope with calc as a load) or chaos in the display when attempting running calc from the adapter. Both can be fixed.



It seems my adapter is broken at least by two ways: The blank pin shows correctly cca 5V DC, but on the plus pin there is upper half of the sinusoid, peak cca 15V, but the inverted lower part is missing completely, so peaks come only in one half of the period. It seems that one half of the diode bridge is faulty and moreover the capacitor is bad, too.
Would it be possible to dissasemble the adapter without causing any damage to it?


I've only worked on the UK, US (dual-voltage) and car versions of the adapter, but on those there are 4 small screws on the bottom. Take those out, the case comes apart and you can get to the PCB.

On the dual-voltage version, the voltage selector switch falls apart when you remove the PCB. Make sure you don't lose any of the parts!

You must, of course, unplug it from the mains before opening it!

When re-inserting the screws, turn them counterclockwise until they seem to engage in the original threads, then tighten them. That way, you don't re-cut the threads in the plastic. It's less likely to strip that way.


I have the original old style recharger, US model 82002A. There are no screws visible, so it's probably unrepairable without visible damage.


Dear Michael,

before trying to open the charger I'd advise you repeat the measurements on the plus pin towards the minus pin properly using an amperemeter on the plus pin (Only there !), see my post #3. Or put
a known load resistor (such as 50...75 Ohms) there and then use the scope to see the voltage on the resistor, it should correspond to I = 50...60 mA (U = I * R) and not have much ripple. Measuring the active current source at the plus pin with a scope (no load) means that the voltage on the plus pin will fluctuate as the Electrolytic is charged and discharged by the sinusoid. If you saw 120 Hertz repeat rate (double the line frequency), the rectifier diodes most likely are good anyway. Then, if you put the 25 Ohms load resistor on the blank pin (against the minus pin) and still see about 3.6 Volts DC there,
without larger dips below this value (dips at 120 Hertz), then the
Electrolytic also most likely is good. So I don't see which
problem you might have with your charger. To me, it seems to be good, at least based on the info you have posted.

best regards,


Hello Bernard, maybe my post was not correctly written, English is not my native language. The image at the oscilloscpe looks something like this: _n_n_n_n so IMHO half of the diode bridge should be faulty. Otherwise the image should look like nnnnnn. And if the capacitor would be OK, it would hold the steady voltage, since it couldn't be discharged when no load was present. So IMHO also the capacitor must be broken, otherwise the image without any load at the plus pin should look like this: _______ and so even with one diode burned. Now when the adaptor is attached to the calculator, erratic numbers appear and dissapear.


Hi Michael,

I already had the symptom of erratic (flickering) garbage numbers when the calc is operated from the charger several times (the calc does not take any key entries), and replacing the Electrolytic capacitor always has fixed it. I never found any faulty diodes.

best regards,


Al the HP35 adapters are screwed together. Some have the screws under the corners of the label. Also some used torx screws or even security torx screws (#6 if I remember corrctly). A very common failue is a bad electrolytic filter capacitor. The most common failure is broken wires in the calculator plug.

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