Re: Repair fo HP67 AC Adapter Message #3 Posted by Tony Duell (UK) on 8 Oct 2002, 7:52 p.m., in response to message #1 by Gene R. DiResta
I've hopefully sent you an ascii-art schematic by private e-mail. It's also available from the CUCPS website (do a google search!) and if Dave (Hicks) wants it I can e-mail it to him also to put up here somewhere.
The charger consists of 2 somewhat independant supplies -- a constant voltage one to run the calculator logic and a constant current one to charge the NiCd pack. These run off the same unregulated transformer/rectifier/smooting capacitor circuit.
The first thing to do is to check for continuity between the pins on the socket and the approprate points on the PCB inside. Many problems are due to breaks in the output cable, often near the calculator socket. Then, with the charger assembled and plugged in, measure the voltage between the middle pin (common -ve) and the 2 outside pins (one at a time). One should read 4.2V or so (constant voltage logic supply), the other should read about 16V (the open-circuit voltage of the constant current supply). From that you can tell if any of the power transistors are open, for example. Another common fault is that the 400uF smoothing capacitor is dried up. In this case, the constant current supply's open-circuit voltage will test very low (perhaps 10V). Or you can check the capacitor on its own, out of circuit.
Don't forget there is live mains on the charger PCB (at the primary side of the transformer), so take great care if you work on the PCB when the unit is powered up.
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