Repair of 82002A Classic Series Charger


To All:

Okay, I really need some help here. I own an old HP-55 calculator that I just spent a lot of money on refurbishing and buying a new battery for it. The unit looks and works great due to Richard Anthony's efforts.

My problem is that it that I can't recharge the battery. Being an electronics guy, I know I could figure out what's going on with the charger since the battery ran the 55 just fine when I first got it about 2 months ago. I found some old schematics for the classic charger in 65 notes (V2N9P24) and the PPC Journal (V7N2P25) but they are not the same. As it turns out, my charger matches the schematic in the PPC Journal and I determined the transformer strapping for 120 VAC and I checked for continuity between the 3-pin connector and the circuit board and it's good. Also, the calculator runs fine on the charger. Also, as a precaution, I replaced the original 400uF filter cap given it was some 30 years old. Afterward, I checked its value and it was over 420uF, so it was still in good shape.

WARNING: With the charger out of its case, there are open circuits that have 120VAC exposed so if you don't know what you are doing, please don't try this.

I did a circuit board layout and matched it to the PPC Journal schematic. Q1, which feeds about 4 vdc to the calculator works fine. In testing the charger circuit, I found my problem. The calculator battery was dead, it measured about 1.1v. I hooked it up to the charger and the collector of Q3 showed about 2.2 vdc and it started to charge the battery. But, when the battery reaches around 2.2 vdc, the collector doesn't go any higher so the battery stops charging. Now, I know that Q2 and Q3 make up a constant current source of 50 mA to trickle charge the battery. Since Q3's collector voltage isn't climbing higher than the battery voltage to maintain the 50 mA current, one or both of those transistors is bad.

Okay, now the transistors are marked as follows (that these old eyes could read):



Both transistors are PNP and the "M" specs a Motorola transistor. Is this a 2N924 or is it something else? Given the age of the charger, I would not be surprised if these transistors were no longer available. So, what would be good substitutes for these two transistors that are available?

I hope someone knows since I've been googling for these numbers without success. I'm 9/10 of the way to fixing this and I hope someone can identify these parts so I can order replacements.

Thank you all for your help,



I sure you already think of this, but why not just get new charger?



The HP-55 has a very sentimental place in my heart as it is the 2nd HP calculator I have owned and I bought it when it was new. I sold my HP-45 to get the 55 and I couldn't afford the HP-65 at the time. So, instead of discarding the charger I've had for over 30 years, I would like to fix it. If I can't figure out where to get the parts and no one knows here, I may have no choice but to replace it. International Calculators and Computers want $60 for a charger and they don't have any in stock right now.

It's a simple matter to repair the charger if I can get the right parts. I've had a lot of my calculators refurbished lately (including the 55) because I want to keep these things around for a long time. I would rather repair than replace as it means there is one less HP calculator in the world.




is there not a calculator repair site that I see on site some time ago? Maybe they have parts to fix charger.


Tony's schematics of this charger are here. Although he doesn't note the part number of any of the transistors, none of them look critical. Given the minimal voltages and currents involved I'd probably just try some general purpose silicon ones like use a 2N2222 and a couple of 2N2907's.


The car charger uses an MJE800 and an 1853-0393...if i'm reading the diagram right...


Here's a link to a gigantic HP part number XREF. Sometimes HP abbreviated their part numbers on devices so the "3-338" might be something like a 1853-0338 (not sure if that's a real part number or not, but you get the idea).


To All:

Thank you for your responses. Katie, thanks for the HP parts listing. I did some searching through it but I found nothing conclusive. I should note that that part numbers were on the transistors themselves, so I'm sure their manufacturer part numbers, in this case, Motorola. Also, when I formatted my original posting, Q2 was on the first line just as a label for me, M924 was on the second and 2-274 was on the third line, which is how it is typed on the face of the transistors, but they all got put onto one line when it was posted. In my experience, the M924 is Motorola's shortened part number and the 2-274 represents the manufacturing date code. The layout for Q3 was the same. I didn't notice until later the formatting change.

Given Katie's good suggestion that there's not much being demanded of these transistors substituting generic PNPs should work out fine. My concern was that the case is sealed so there is no air flow and I didn't want either one to overheat. Also, not knowing the Hfe or other specs for those transistors, I wasn't sure if it would deliver the same current as the original parts.

So, I decided to throw caution to the wind and I went down to Radio Shack and bought some 2N3906 and a TIP42 transistors. The TIP42 is overkill for what I needed and it's larger size makes it a careful fit in the plastic case and not having its heatsink on top touch the AC transformer core, but it should stay cool. Also, the TIP42's pinout is different that the original transistor so I had to be careful.

I replaced both transistors early this morning, (my wife pointed out the tail end of the total Lunar eclipse this morning, it was beautiful!) and I found out that the original Q3 had an open base-to-collector. After installing the parts, my charger is now charging my batteries. I did some calculating and the current looks close to 50 mA, which is good. After letting it charge my old, old battery pack for a while, neither of the transistors or the battery pack are getting warm, it looks good and the battery is now properly powering the calculator by itself. It appears to be fixed!

Thank you all for your help. I hope this post will help someone else who is trying to fix their classic charger.





... make up a constant current source of 50 mA to trickle charge the battery.

50mA for a typical 500mAh classic NiCd battery pack is the nominal charging current (1/10 of the capacity for 14 hours), so be careful not to exceeded the 14 hours otherwise the batteries get damaged!

If you can't find the replacement transistors, there are two easy solutions I can think of:

1. Buy a ready-made miniature 50mA constant current source like the one in this example:

2. A 'quick&dirty' solution would to do it the same way as it is done in cheap chargers: Use an LED as pseudo constant current source (or rather'current limiter'). You have to look for an LED that draws approximately 50mA, this can either be a vintage red LED or a modern blue or white one. To limit the voltage drop over the LED, a series resistor will be required: The fully charged battery pack has a voltage of 3 x 1.4V = 4.2V, the drop over the LED is around 2V, so difference between these 6V and the unregulated supply voltage (16V according to the schematic posted above) results in a 200 Ohm resistor that will be required.
Not elegant, but it works. And it is safe, because in case something goes wrong, it will be the LED that fails and not your batteries.

Greetings, Max

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