Jul 28 2017

Update On The Melcor 390 Calculator

So, you may recall a few days ago in my thrifting adventures I snagged an old 70’s calculator that, after working for a few minutes failed in a flurry of smoke and heat. If not, the previous article will be shared below.


Well, I mentioned I would ask on the eevblog forums about what the component was. It would turn out I was right, it was a capacitor – in this case, a type known as a tantalum cap, commonly used in more compact electronics due to their relatively high capacity for space – the very device you are using to view this page certainly has several tantalum caps in it.

Now, one thing about these capacitors is they don’t do well under certain stresses – what some people think happened is that when I re-assembeled the unit, the cap, which is just kind of floating free in there, got pressed just a bit and, boom it went.

The interesting thing is apparently I got to it before it got worse – tantalums are known to just outright blow, torching boards and possibly injuring anyone holding the device. I, apparently was incredibly lucky, but since the unit was open when It began to fail, I was quickly able to rip out the battery – only minor damage to the board was done, and that was only superficial. Still, the part blew and as such the calculator doesn’t work. The unit should be fine otherwise, but that left the question of what the replacement part was?

Both the forums and a reply from Dave Jones, the man behind eevblog himself (and replies to that tweet) pointed at 10uf, 16 volts as the specs for that cap – interestingly, this is about what I thought it was, but I wanted to make sure as I could have been completely off, but it’s cool to know I was right.

That being said, I ordered the parts (well, a 10 pack, so I’ve got caps to burn if I mess up the install) and hopefully the end result will be a working calculator. If not, I’ll still be bummed out, but otherwise, well, if I save it, that will be rather awesome!

Permanent link to this article: http://www.xadara.com/update-on-the-melcor-390-calculator/

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