HP55 power supply


I have a couple of calculator from my dad. There are no batteries but the power supplies don't work either. Do these need a battery in place for the a/c adapters to work??

Thanks, Larry


Hi, Larry;

I have an HP55 and the User's Manual does not explicitly state that you should not put the unit to work without the batteries inside the batteries compartment. It's clear that you should not insert the AC adaptor with the calculator switched to ON (may damage the unit) or use it with the plug partially inserted (may damage batteries and AC adapter). This is because the AC adapter delivers two voltage levels: aprox. 5Vcc, stabilized (internal calculator circuitry while charging) and 12Vcc not regulated (charger circuit). If you insert the adapter plug while the calculator is ON, chances are the internal circuitry will receive the 12Vcc instead of expected 5Vcc.

Mine was given by a friend that was using it without batteries for the last ten years, no damage at all. I accept the fact that the AC adapter already delivers the correct voltage to the circuits, so the battery is disconnected when you plug the charger/adapter while working with it. You can make a simple test: gently fasten the golden flat-spring "jumper" that connects both inlet connectors for the AC adapter in the calculator. The calculator will not turn to ON, batteries are disconnected while charged is plugged in. No need for the batteries while charging. That's why the User's Manual does not explicitly asks not doing it. Neither the opposite.

Just think of the fact the batteries contacts may fail wihle charging; should this damage the unit? My 82143A was damaged because I forgot to put the battery pack back inside before connecting the AC adapter. I do not agree with this procedure when you have an equipment that allows batteries to be easily removed and are connected with surface connectors, not pressure-clamp or soldered type.

Hope this helps.

Luiz C. Vieira - Brazil


luiz is right.

also, you said that you have a couple. if your other is an hp 65, 67, 97, 21 - 29, or 31 - 38 (except the 35) then NEVER CONNECT IT TO THE AC ADAPTER WITHOUT A GOOD RECHARGABLE BATTERY PACK IN PLACE AND WELL CONNECTED!!! doing so can fry it's little brain. this may not happen the first time, or the tenth, but it will happen and you dont know how many times it has happened in the past.

this warning does not apply to the 67cx :-)


The HP55 should work correctly (and without risk of damage) from the AC adapter without a battery pack installed. When the adapter is connected, the battery is disconnected from the rest of the calculator, and the adapter provided 2 separate outputs -- a constant voltage one to run the calculator and a constant current one to charge the battery.
I can e-mail you an Ascii-art schematic of the 'classic' series adapter if you are sure that's what the problem is. But why do you suspect the adapter rather than a fault in the calculator?


Hi Tony, its a 50-50 toss up. I have been told that the adapters are notorious for their lack of ruggedness. Hence, the suspicion. Does that schematic show voltages at the pins? If so, your emailing it to me would be appreciated.



I can't remember if I included the voltages or not. However, with the connector 'holes upwards' and the cable going away from you (i.e. with the connector in the same position that it would be in if plugged into a calculator that's in use) :
The middle pin is ground. The pin o nthe left is the constant voltage logic supply and should read 4.2V or so. The one on the right is the battery charger supply and will read about 16V off-load. Only the logic supply is important to getting a 55 to power up.


... is easy if you know the trick. The trick is that there are 12 testpoints on the logic board of a 35, 45 or 55, going down the RH edge of the PCB (looking at the component side of the PCB). From the 'top' (display end of the calculator) they are :
Vss , Gnd, Phi_1, Phi_2, Vgg, IS, IA, Sync, WS, Init, Carry, Data
Start by checking the voltage on Vss (should be +6.2V) and Vgg (-12) wrt ground (or the middle pin of the AC adapter) with the adapter connected and the machine turned on. If both voltages are missing, check the AC adapter, power switch, PSU circuit on the logic board, etc. If one of them is missing/incorrect, check the PSU circuitry on the logic board.
When you've got that right, check the clocks Phi_1, and Phi_2 with a 'scope or logic analyser. If one or both is missing, check the clock buffer chip, the anode driver chip (on the keyboard/display PCB) and the oscillator 'tank'.
Got power and clocks? Now check the IA and IS lines with a 'scope. They should show irregular digital puse trains (IA is the instruction (ROM) address from the C&T chip, IS is the Instruction data from the ROMs)
If they're wrong, suspect the C&T or ROM chips.


The 35/45/55/70/80 work just fine without a battery pack. The 65 and 67 are best not connected to the charger without a battery because the battery terminal is connected directly to the card reader circuit... without a battery there is a slight chance it may be damaged... even if the machine is off. The '21-29 are VERY easily damaged if conencted to the charger without a GOOD battery pack installed and making GOOD contact.

It is VERY common for the HP35 style charger to develop bad connections in the calculator plug. If you are careful, you can slice off the top of the plug and fix the connection and then reglue the top down.

Another problem is a bad filter capacitor in the charger. Usually shows up a dim, flickering digits.


Is there a diagram somewhere that shows how the a/c adapter pins should read on a voltmeter??

Thanks, Larry



looking at the pins form the connector, you see:

14.2Vcc | o o | 5.2Vcc
GND |_ o _|
\__ __/
| |

Measures were made with a common voltmeter (not such a good precision instrument, yet somewhat reliable) and AC outlet (measured with same instrument) measured as 222Vac. The 14.2Vcc will be reduced to 12.8Vcc when connected to charge, i.e., voltages measured without current consumption.

Hope this helps.

Luiz C. Vieira


If the filter cap is bad, the voltages can look pretty normal and can even look good on a scope unless a load is connected.


The adapters seem to be putting out a/c instead of dc. I measured 0 volts for dc for either output and 17.4/4.6 in a/c mode. Is that right?




you should read DC instead of AC . There is something wrong.

For the records: can you have a look at the AC connectors side and check for their # ID? You should read:

    H E W L E T T - P A C K A R D

if there is another # ID instead of 82002A you have a differnet AC adapter type. Otherwise, there are problems in yours and it seems bad rectifier bridge, filter capacitor is probably blown out and voltage regulator is dammaged, too.

I hope you have a different AC adapter and you have not tried them in your units. Anyway, transformer is fine and electronics may be repaired with new components.

Success. I'm waiting for good news.

Luiz C. Vieira - Brazil


It does say 82002A. Too bad.

It looks well-sealed. Can it be serviced internally?

Thanks, Larry


Hi, Larry;

mine has four screws, two at each corner at the AC connectors side and two coser to the three-wires cord. Have you checked if yours have them too?

Tony Duell seems to have the dircuit diagram for these adapter/chargers. I think it's a good idea contacting him by his e-mail address, ard@p850ug1.demon.co.uk , as posted in this very thread.

I can open mine and figure its schematics out, if you want to. Be my guest.

Best regards.

Luiz C. Vieira - Brazil


Some of the units require a very small security torx bit to open (which can be hard to find), others have normal screws.


Mine have Phillips head screws.

You guys have been incredibly helpful. Honestly, I cannot find the time or the will to rip into these adapters and start troubleshooting them at this time. Tony, I will keep your email address for later use, if that is ok? Luiz, please don't open yours up for the purpose of drawing me a schematic. When the time comes I will be back. Meanwhile, they are confirmed dead, to be resurrected later.

Thanks again, Lar

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