Author Topic: Restoring a (very) old pocket calculator.  (Read 1334 times)

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Offline mpk

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Restoring a (very) old pocket calculator.
« on: August 31, 2018, 09:45:06 am »
I had one of these passed to me a while ago - it belonged to a friend of the family:

It's a Canon Pocketronic, one of the very very earliest battery-powered "pocket" calculators to hit the market. I think this one dates from about 1971 going by the date codes on the ICs. It didn't have a display, but printed output using a thermal printhead onto a cartridge of paper tape (the thing you can see plugged in at the top). The display "window" is just the front of the tape cartridge.

You could put it in a very big pocket. The very substantial shielding in the back of the case houses 13 NiCd cells in various form factors, chargeable by an external mains adaptor. These are dead, dead, dead (unsurprisingly), so I need to source replacements from somewhere. They're standard sizes, but given the probably-crude charging circuit in there I don't think they'll be straight-up replaceable with NiMH.

The bad news is that it's not working. A poke around with a meter suggests things are mostly in order, but hitting a key only produces a very quiet "tick" rather than a loud click as the solenoid which drives the tape does its thing. Here's the solenoid and the main PCB:

... and for fans of old-school silicon, here's the three-chip(!) TI calculator chipset that does the work. LSI 1970 style, built on a 10um process. TMC1730,1731,1732.

I think the problem is not electrical but mechanical. The drive circuitry is producing nice neat 50ms pulses at 18V, but the thing only clicks:

The best I can figure out is that the solenoid is jammed as the bearings on the wheel that drives the tape are pretty much seized - there's a chunk of disintegrated foam padding in there which hasn't helped at all. The rubber drive surface has also hardened and stuck itself to the wheel. I'm not a mechanical type so don't feel happy taking it to bits because of the risk I'll never get it back together again, so I guess I'll seek out someone who can if getting hold of some penetrating oil doesn't free things up. I'd love to bring this back back to life, so definitely not ready to give up yet. In the meantime, here's one last picture of the underside of the main PCB - classic late-1960s single sided PCB layout design!

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Offline Ampera

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Re: Restoring a (very) old pocket calculator.
« Reply #1 on: August 31, 2018, 04:16:37 pm »
Hand drawn traces, white ceramic gold capped silicon, printer output, that's the real shit right there.
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Offline jsantoro

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Re: Restoring a (very) old pocket calculator.
« Reply #2 on: September 01, 2018, 06:20:30 pm »
I can't tell by the picture, does the armature advance a pawl or something when it is picked? What happens when you pick it by hand? If it is indeed a armature type solenoid don't oil it. THere is usually a residual of sort sort that can get gummed. I worked for IBM in the 70's on equipment that had bunches of these, there is not much to go wrong with them.  Close up pictures would help.

Offline artag

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Re: Restoring a (very) old pocket calculator.
« Reply #3 on: September 03, 2018, 06:49:14 pm »
I remember my dad's boss showing off one of these. Would have been about 1970 in the UK.

Typical calculators of the time (I worked on a bunch of Bowmars about 5 years later) would have some nicads charged with simple C/10 charging system consisting of a resistor and a crude mains power supply. NiMH, if charged at the same low level, will be fine. It's only when you get to high charge rates that careful control is required.

« Last Edit: September 03, 2018, 06:51:30 pm by artag »

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