Neil's News

HP-35 Repair

4 March 2018

The calculator that sits on my desk and that I've been using for decades just died. It's an HP-35 RPN calculator which is older than I am, and which I'm quite fond of.

The calculator wouldn't turn on or show any form of life. A quick check of the power supply showed 18v on one line, and 1v on the other line. That sounds plausible enough, so I figured there was probably something wrong in the calculator itself. A full disassembly showed no obvious problems. The amount of gold in this device is quite remarkable.

Around this time my friend Chris found documentation indicating that the 1v line on the power supply should actually be providing 4v. Time to disassemble the power supply.

Well, that doesn't look good. There's a serious scorch mark on the PCB. Not surprisingly the resistor on the flip side of the board tests at infinite resistance.

Unsoldering the offending resistor was easier that expected, since all the solder had evapourated away from one of its legs. A replacement 470Ω resistor was found that appeared to have a considerably larger current rating.

After installing the new resistor, the power supply started producing the desired 4v. It turns out that this 4v is used to run the calculator, whereas the 18v line (which was running fine) is used to charge the calculator's batteries. Of course since my calculator was manufactured in 1972, the batteries are long gone.

Reassembling everything was pretty easy, though I needed to consult a reference photograph to figure out where all the keys went.

Good as new. Hope this calculator lasts many more years and that my daughter will use it one day. For now, it's back on my desk; next to the slide rule I use to thwack people who tease me about my ancient calculator.

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