Posts: 1,162
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Joined: Aug 2005
A little bit of info on the 'classic' series adapters. With the plug contact side up and the cable away from you, the middle pin is ground. The one on the right is the constant-current battery charging output, the one on the left is the constant voltage logic supply.
I don't know how you are naming the pins, but you should expect to see about 4V between the logic supply and the ground pin and about 17V between the charger output and the ground pin.
If you can only see a voltage between the charger output and the logic supply then the most likely cause is an open-circuit wire in the cable. Take the charger apart and investigate the cable and connections with an ohmmeter. If the battery charger output is there but the logic supply is missing then again it could be a cable problem, or it could be a component failure inside the adapter.
To get inside, unplug the adapter from the mains (obviously), take out the 4 screws and lift off the cover. On dual-mains-voltage models, the voltage selector switch will fall apart -- make sure you don't lose any of the parts (slider and _2_ contacts) or forget how to put them back.
The circuit is quite simple. There's a mains transformer, the output of which is full-wave rectified (4 discrete diodes) and smoothed. A resistor + zener form a voltage reference which is buffered by one of the transistors (the one with the heatsink stud IIRC) to provide the logic supply. The other 2 transistors form the constant current supply -- the small signal one senses the voltage across a resistor in series with the output and turns on when the current gets too high. This removes the base drive from the power transistor, reducing the output current.
In your case, check around the voltage regulator circuit (about 3 components!).
Posts: 1,788
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I have never seen a circuitry failure in the HP chargers. The cause of a missing voltage is ALMOST always a broken wire in the power plug. A lot of times you can fix the plug by carefully slicing it open across the top, repairing the break, and gluing the plug back together.
Try connecting a meter to the offending contacts and bending the wire at the strain relief near the plug and see if you can get some intermittent contact. If so you can almost always get the plug fixed.