Electronics > Projects, Designs, and Technical Stuff
Science of Cambridge (Sinclair) MK14 restoration
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mpk:
Salvaged this from my parents' attic recently - my brother's Science of Cambridge MK14. Clive Sinclair's first computer, and predecessor to the ZX80. It's really grungy from 30 years or so in the (non temperature-controlled) loft. I have a vague memory of messing about with it trying to get it working decades ago and blowing up the voltage regulator (which made my brother unhappy), but as this is 2018 and it runs from a 5V supply it can be powered simply enough by USB.

It is, however, fairly dead. I have to wonder if I blew up anything else while blowing up the voltage regulator, and there's very little troubleshooting documentation out there (even the original manual just says you can send it to Science of Cambridge and they'll fix it for a few quid, but I don't think that option is available any more. :) There's definitely a clock, and poking around the CPU with a scope reveals some strobing on a few pins which suggests that at least the CPU is intact, but right now I decided to unsocket all the chips (while silently thanking my brother for using sockets 40 years ago instead of just soldering'em in) and give it a clean to make it easier to see what's going on.

Unusually, I've also got the VDU card - a famously useless device that took up 512 bytes of the MK14's 640 bytes of RAM, thus leaving you a whole 128 bytes in which to write your fully-featured word processor. I guess it had hack value, at least. The jumper wires all over the back of the PCB are connecting the CPU bus to the bottom half of the MK14 edge connector as SoC for some reason didn't think that would be a worthwhile thing to do (more likely it would have involved a complete redesign of the PCB to fit the extra tracks in). The keypad is a third-party unit that's installed over the (classic Sinclair..) metal dome clicky "keyboard" and hooks into some undocumented edge connector tracks to the right.

Once it's back together.. hm, no idea where to go next. Almost all the chips are still in production, so I'm shopping for a few spares (thank you for the SC/MP, eBay) and I guess that if I end up not needing them all I'll have the parts sitting around to build another MK14 on one of the modern clone PCBs that are floating around out there!
CJay:
Ooh nice, I'm not sure you'll run it from a 500mA USB supply though, maybe a high current USB PSU...


mpk:
The manual claims that 480mA will do it, but I'm not convinced. Should maybe try something with a little more oomph, yes.
mpk:
I've been doing a lot of poking around with the 'scope, and while there are definitely signs of life coming from the CPU (there's a clock, and bus activity) I suspect it's not getting further than powering up. Starting to wonder if the PROM chips are duff (can PROMs lose their mind over time?), and beginning to wonder if there's any way to get hold of a replacement pair of PROMs. The good news is that the machine is an issue 5 board, so much improved over the earlier versions, but the bad news is that it doesn't work..

In the meantime, I guess I'll start doing some tedious continuity testing to make sure there aren't any bad traces or (more likely) knackered old IC sockets.
MK14:
My post may sound like I'm stating the obvious. So my apologies in advance, and anyway, I'm not clear on how experienced/learned in Electronics/debugging, you are.

You basically have something (MK14), which normally works just fine.

So if it is NOT working correctly, one or more of the following has occurred. Or something else!

One or more of your connections has failed (you seem to have a VERY CRUSTY example, and I'm sort of  :palm: :palm: :palm: :palm: :palm: :palm: :palm: when I look at it, sorry).
It even seems to have extensive modifications done to it, which cause me to go  :palm: :palm: :palm: multiplied by x1000. Because I DON'T know what those modifications are, and they could easily stop it from working.

One or more of your integrated circuits has failed (ebay China ICs, can be fake/faulty, so take care). Some ICs (in my experience), seem to be able to break (go bad), all by themselves, just because they are suddenly, 20/30/40+ years old. ESPECIALLY old dram chips, which fortunately, are absent, from the MK14's, at least a standard, non-expanded one.
Old PROMS in theory, should keep their contents (despite approx being 40 years old) just fine, because they are suppose to use burnt links (rumored to be something like breakable weak diodes, via excessive/bad voltage and current), but in practice they could break and/or be more modern ones (probably NOT for an ancient MK14), which are rumored to really be one time use, flash like memory (OTP).

Your power supply and/or the on board regulator is problematic/faulty. I DON'T (my opinion), like NOT having an on board regulator. Because the older ICs, need/wanted tight 5V, well regulated supplies.
The MK14, is not really designed to cope, with 5V supplies, which have 1 metre (wild estimate) leads, along with the associated, possible voltage drops and problems with switching noise, ground voltage variation etc etc.
The common, USB power supplies, are usually/often (but NOT all, best is properly isolated ones), based on some kind of mains voltage dropper. Which can mean the 5V rail, is not very accurate and/or noisy, and it can even have significant, voltages, between it and ground.

E.g. 100V at around 500 microamps (again, a wild estimate), which could (in theory), mean that when your hand is close to the circuit board, enough of that stray voltage maybe could blow the NMOS (and similar) ICs. Especially as a rule of thumb, says that relatively old technology, such as the MK14, made from very old, 1970's technology, either didn't have ESD protection (i.e. input protection diodes) and/or had very weak ones (because it was the 1970s), which don't help that much.
Although it (MK14) has a lot of TTL, which tends to cope with ESD, quite well.

Although I (would expect) to be able to fairly easily fix it myself and/or with the help of equipment (some of which I may only obtain, when needed), which you may or may not have or want (such as 2 or 4 channel, ideally digital storage scopes and/or logic analyzers, if it gets really nasty to fix, which it SHOULDN'T).
I find it very difficult to try and advice people, across forums, how to fix/debug, their microprocessor computers. Even though I can fairly easily do it, when the equipment is in front of me.

Partly, because I can rapidly get test results, within minutes. Whereas I can say, please take a trace of the  Address lines, and post a picture of it. Then, 10 days later, they come up with completely different test(s) reports, and I've forgotten what was going on, as well.

If I had it in front of me now. I'd probably make sure it had a decent on board regulator, that the decoupling capacitors, are all present and don't look like they have aged too much and/or I'd test them (very especially old electrolytic capacitors, which tend to fail, as they get older).
Then observe the address lines (and other things), starting from the lowest address bit, and see to what extent it is trying to work.
I.e. Is it almost fully working, or has it got major issues.

Sorry if my post is too long and/or I'm NOT helpful enough.
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