|Repair of 82002A Classic Series Charger|
Message #1 Posted by Gerry Schultz on 27 Aug 2007, 4:03 p.m.
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,