Melcor 390 (1970's calculator) Blown Component repair help - Page 1
Forum section: electronics / repair

author: Kurisu
author_rank: Newbie
timestamp: July 26, 2017, 06:16:33 am
content: Hello Everyone!The past day in my thrifting adventures, I came across a Melcor 390 calculator, a nice specimen from the pocket calculator revolution of the 70's! At least, it's cool to me! Anyway, the unit worked a treat when I got it home - naturally the first thing I did was open it up and check out the board, the traces, the IC's, and all that jazz - but I noticed while it worked fine early on, once I closed it back up it started to just not power on. This got me worried so I opened it back up, checked around on the board and cleaned up a few old solder blobs that had dried up from manufacture (those probably caused no issues to begin with but you know how it is, ya see it, ya wanna clean it) and was for a few minutes confused as to what happened, thinking I had broken it.That's when the magic smoke happened - a part had blown. Now, the unit is pretty darn simple - 9 volt battery, the wires for the power, the display LED's, and 2 boards - one for the keyboard, one for the logic - this had one little red object that I *think* is a ceramic capacitor on it (I could be totally wrong of course) and that part had very clearly blown. Considering the battery was getting oddly warm I should have known something was shorting out, but it didn't click - I was too concerned, almost childlike, with the fact I had a working device not 10 seconds before hand, and once I went to seal it up and power it back on it then didn't work right. I guess that part shorted internally and next thing you know.. pop!I fear it may have damaged the IC's but at the same time, maybe not? I don't know, I'll trust what you guys think. I'm not new when it comes to electronics, but I have only a moderate knowledge and experience with many things beyond simple repairs, some understanding of the principles, and some skills with video game consoles (mostly mechanical issues though, so, eh). I still have enough compitence that, if we find out what the part that blew is and get a replacement I could get it in there without issue.All I can do now is link you all to the images of the unit. If you need anything else, don't be afraid to ask - I don't mind taking more pictures or what have you.I hope we can figure this out, but if something is probably beyond dead, then that's fine too. I just don't want to give up on this calculator, even if it was a cheap find - I'm sure some of you can relate.Thanks!

author: kripton2035
author_rank: Super Contributor
timestamp: July 26, 2017, 06:25:38 am
content: 10µF 16v tantalum capacitoreasy to replace.

author: Kurisu
author_rank: Newbie
timestamp: July 26, 2017, 06:31:31 am
content: Quote from: kripton2035 on July 26, 2017, 06:25:38 amThat's about what I was thinking, I guess I underestimated my abilities - that, and I wanted to make sure. Thanks a ton!I take it you would think the IC's are fine then, right?

author: Zucca
author_rank: Unknown
timestamp: July 26, 2017, 07:21:46 am
content: I suppose that C is just there for voltage supply filtering, if so then a shorted C will not harm the rest of the devices. The voltage there should decrease when a short like that happen...Now before you buy a new C, I will feed the calculator with a regulated power supply and starting from 0V slowly increase the voltage until 9V. While you are stepping up the voltage check the current flowing in the calculator and if some device will start to cook (use a finger, best temperature sensor we got in our stock)...Hope you will fix it.Z

author: Kurisu
author_rank: Newbie
timestamp: July 26, 2017, 07:42:09 am
content: I'm quite certain I have access to a regulated PSU at the shop, so I'll take it tomorrow and see what happens. Who knows, we may have a cap in that spec lying around, so I might have a virtually free repair.

author: Nusa
author_rank: Super Contributor
timestamp: July 26, 2017, 09:17:50 am
content: Based on where the burn marks are on those two boards, I'm guessing you managed to mechanically crush the top of the cap between the screwhead and the mated board when you reassembled the boards and case.

author: Ian.M
author_rank: Super Contributor
timestamp: July 26, 2017, 09:51:23 am
content: Solid Tantalum capacitors have a reputation for catching on fire if abused. Excessive surge current, running them too close to their voltage rating, and mechanical stress are all known to increase the risk of failure.Clean up as much as possible of the burn marks with IPA and patch in a 10uF 16V (or higher) electrolytic and see if the rest of the calculator circuit has survived. If its O.K. you can then source the correct replacement part.

author: Kurisu
author_rank: Newbie
timestamp: July 26, 2017, 05:35:36 pm
content: Quote from: Nusa on July 26, 2017, 09:17:50 amNow that you bring that up, it's quite possible and probable that I did do just that. There isn't much clearance inside the device and while I don't recall moving that part around, given how you have to layer the parts back together, it wouldn't surprise me if my finger nudged it in just such a way to kill it off.Quote from: Ian.M on July 26, 2017, 09:51:23 amWe quite certainly should have a electrolytic at the shop near enough to those specs (from high to low) to do a patch test. after all, the only way to find out for sure if it's working is to make it as proper as it can be, if only temporarily, right?