HP Forum Archive 06

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plating on HP-35 contacts and power supply Message #1 Posted by Randy Tate on 27 Sept 2001, 4:18 p.m. I have just aquired a HP-35 when an investigator left our facility, and I help in "cleaning out" the lab. Looks like a good bit of the plating is gone from the battery contacts through just wear, time, and corrosion. How do deal with this? I don't know currently if the calculator works or not, because in addition to having a battery pack that needs rebuilding, appears there is a problem with power supply/adapter. I measure 0 volts between terminals a and b, and 0 volts between b and c, with the unit plugged into wall outlet. I also measure around 16 volts between a and c, which has me sratching my head. Any suggestions? Randy Tate randy.tate@uthct.edu http://research.uthct.edu The University of Texas Health Center at Tyler

Re: plating on HP-35 contacts and power supply Message #2 Posted by Tony Duell (UK) on 27 Sept 2001, 6:41 p.m., in response to message #1 by Randy Tate 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!).

Re: plating on HP-35 contacts and power supply Message #3 Posted by David Smith on 28 Sept 2001, 4:56 p.m., in response to message #1 by Randy Tate 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.

Re: plating on HP-35 contacts and power supply Message #4 Posted by Tony Duell (UK) on 28 Sept 2001, 6:38 p.m., in response to message #3 by David Smith Ramdy Tate e-mailed me to say that the middle pin (chassis ground) was the dead one. In which case the fault is almost certainly a broken cable/connector. It's generally possible to stick a sharp in into the wire (with the adapter unplugged from the mains, of course) to find where the break is. Most likely it's at the connector end, so you have to cut the connecotr open and repair it. If it's elsewhere in the cable then you can either shorten the cable a bit or cut out the broken part and re-join the wires (makeing sure you keep them in the same order, of course!. I have once seen a 'classic' adapter with a component failure -- the pass transistor for the logic supply was open-circuit, so that output was missing. But most of the faults are open-circuit cables, agreed.

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