Repair fo HP67 AC Adapter



#2

Greetings all,

I am looking for a circuit diagram or repair procedure for the HP67 AC adapter. Can anyone provide technical information, etc.?

Thank you for your help


#3

95 percent of all charger failures are due to a broken wire in the plug cable end at the calculator. They break where the conect to the calculator pin sleeve. You can open the charger case and verify that the connection to the plug is baad with an ohmeter.

Fixing the problem involves carefully slicing off the top of the plug rubber, fixing the broken wire, and gluing the top back on. Some people can do this so well it is hard to tell it was ever opened... I don't tend to be one of them.

#4

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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