Jul 25 2017

I Got A Vintage 70’s Calculator, And Then It Blew Up!

Not the typical article here on Xadara, but these types of posts will become more common as time passes. I love random gadgets of yesteryear – be it old adding machines and typewriters, to slightly more modern things like computers and other consumer devices of the 70’s, 80’s and 90’s, there is a charm to old things that just isn’t found in what’s out there currently, at least for me.

Today, in my thrifting adventures I found an early to mid 70’s calculator, specifically a Melcor 390. We’re talking those red bubble LED segmented displays, 9 volt power sources and proper integrated circuits doing the math, all with a style that’s just plain awesome. Oh yes, this little machine looked like it would fit right in on my Commodore desk – it worked fine, and (for a calculator) was oddly fun to use – sure in usage it’s nothing special, but the idea of using a 40 some odd year old device from the early era of electronic calculators was just awesome.

See, the 70’s was really when such became possible to develop – the integrated circuit was a new thing, and the logic to be able to do complex math calculations was finally able to be put into something you could carry around easily. They were expensive machines at first, but then (thanks to companies like Commodore) they went down in price, and of course today you can pick up proper calculators for next to nothing… but back then these little wonders were a modern miracle, and in just a few years following their introduction we would be thrown into the computer revolution!

The traces are very classic 70’s in their design, which makes sense as this is a 70’s machine.

Anyway, back to this particular unit, it worked fine. I opened it up, checked out the IC’s on it and the board layout (which was a beautifully simple early 1970’s surface trace design) and put it back together. It worked fine up until I had it sealed back up and had put the screws in; that’s when it gave me no power.

I feared I had done something wrong; I was seriously worried I had killed it. I opened it back up and checked and cleaned up where I felt there may have been some loose solder possibly causing some issues, but nope. Nothing.


You can see the display and the IC’s here.

Then, when I sat it on my desk, I smelled something funny, then saw the magic smoke escape. Anyone who has fried anything electrical knows that smell and that smoke – the “magic smoke” as they call it had escaped!

Basically, whatever this red component is – I think it’s a ceramic capacitor, but I’m not sure – had failed… and had failed spectacularly! Thankfully it didn’t destroy all it’s information when it popped, but I guess with the unit sitting around in storage for decades, it finally had worn out and after having some power go through it, well, it had worked for a time but during that time was dying – capacitors do that, hence why it’s always good to fire up electronics at least once a year, to keep things in a more “active” state – it helps, especially with capacitors.


The edge connector where the keypad integrates into the machine. Note the scorch mark on the right side with the damaged part visible.

In any case, now the question is – what was the part that failed, and then the minor annoyance of sourcing a replacement (easy enough once I know the part) and then the random chance that this part failing also killed the IC’s. I doubt it, but with this 1970’s chip design, even as simple (relatively) as this board is, I’m wary of 9 volts being tossed around randomly – voltage like that unregulated could do quite a bit of damage to an integrated circuit.


The part that blown. I’m thinking this is a ceramic capacitor… right?

I’ll probably hit up a site I’m not yet on, but need to get active with, http://www.eevblog.com/ . That site is certainly a place I’d trust for some accurate information and quality help with this – hell, I’ll probably link this article to them to help TL:DR the process. That being said, eh, the little thing was only 99 cents so it isn’t like I broke the bank in getting it, but I wish it hadn’t blown up on me like that!

I even more so hope it IS fixable because if not, that would be quite the bummer – I already have enough irreparable stuff I need to part out or get rid of as is, but that’s the life of a technology enthusiast like me.

Permanent link to this article: http://www.xadara.com/i-got-a-vintage-70s-calculator-and-then-it-blew-up/

Leave a Reply

%d bloggers like this: