The Museum of HP Calculators

HP Forum Archive 01

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HP35 Repair
Message #1 Posted by Joe Uridil on 2 Aug 1999, 2:10 p.m.

I recently stumbled on my HP35 in the back of a drawer. I haven't used it since college and the sentimental value is, I'm sure you'll agree, awsome. I need a repair person the fix the on/off switch and whatever else. Your help is appreciated !!

Re: HP35 Repair
Message #2 Posted by Kevin Hilbiber on 3 Nov 1999, 3:04 p.m.,
in response to message #1 by Joe Uridil

I too have an hp35. what is/are the symptoms of yours? when i got mine, nothing went. there was corrosion on the machine pin sockets between boards, and when i replaced the sockets, the wall adapter worked the calculator.

however, if it works with the charger but not batteries, we are in the same boat. on mine, the switching supply doesnt work. i am hoping at some point to find out why.

for what it's worth, mine is the third one made, with dad's name where Hewlett-Packard went on all succeeding models..thats because dad designed the diffusion process that allowed lsi to build a multi function handheld calculator in the first place, and worked closely with the project throughout. so i do want to get the unit up and going again, on all cylinders so to speak. hope i shed some light on your predicament.

kevin hilbiber

Re: HP35 Repair
Message #3 Posted by Viktor Toth on 3 Nov 1999, 4:56 p.m.,
in response to message #2 by Kevin Hilbiber


That must be an impressive machine with your dad's name on it :-)

I have to correct you on one point: with HP Classics, if the machine works from the charger but not from batteries, that is NOT an indication that its switching supply failed. The switching supply is used even when you operate the calculator from the charger.

Assuming that the charger itself works (no broken wires), and your battery pack is good, the most likely cause will be those gold contacts that are supposed to connect the two side pins of the charger connector on the back of the calculator. One of these pins goes to the battery pack, the other to the calculator. When the charger is unplugged, the gold contacts connect the two pins, supplying battery power to the calculator. When you plug the charger in, the two circuits are separated; the charger supplies a charging current to the batteries through one pin, and regulated voltage to the calculator through the other.


Re: HP35 Repair
Message #4 Posted by Joe on 18 Nov 1999, 8:28 a.m.,
in response to message #2 by Kevin Hilbiber


Check the metal blade in the charging port. When the charger is unplugged the blade should short the two outside posts together. One post connects to the battery and the other to the internal circuits. When you plug in the charger/AC adapter, the batteries are disconnected and the AC adapter supplies power directly to the internal circuits. (The charger also supplies recharging power directly to the batteries, but that's irrelevent here.) It sounds like the blade isn't making a good connection.

The middle post is ground. You can measure the voltage from it to each of the outside post and the blade. You should have the same reading at all three points.


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