| Electronics > Projects, Designs, and Technical Stuff |
| Science of Cambridge (Sinclair) MK14 restoration |
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| MK14:
My apologies. :palm: I realized afterwards, that I had not given you the best of advice. It dawned on me that people who made these MK14 computers by hand, must have had a reliable way of identifying, which PROM was the high nibble and which was the low nibble. Scratching my head, as it is around 40 years, since I made a MK14. I vaguely remembered either a slightly different number, on the right of the IC part number, or a coloured spot or something. But I was not sure, as it was too long ago. I've checked with the manuals, and apparently, there is a WHITE SPOT, on the PROM, which belongs in IC socket IC2 (which is in the corner). Obviously, the other one, goes in the other position. Please see the very end of the manual, where the schematics and parts lists, are located. Source: http://s400081762.websitehome.co.uk/manuals/mk14ManualV1.pdf |
| mpk:
Any white spot on one of the PROMs is long gone, although I had noticed a vague smear on one which might have been 39 year-old glue residue so thought that might be IC2. However, I now have a working MK14! Okay, it's not the same one I've been debugging - my Issue V replica PCB and PROMs arrived so I populated it today with the various spares I'd been accumulating, powered it up and it worked first time (I know, deeply suspicious). I've tested all the chips from the old machine in the new one, and from that I've learned that the PROMs are, indeed. duff. The machine interprets enough garbage from them at startup to throw some junk onto the LED before looping back and doing it again. This must be the repeating pattern I saw in the traces I took a few days ago. The lack of display of that mess on the old machine might well be explained by the broken lines on the display flex. So, new PROMs installed (lucky I got two sets), and now I just need to repair the display flex and get hold of a crystal to replace the one I "borrowed" to get the new machine working and we can take it from there. Today's experiment has shown that the rest of the ICs are working just fine, so if the machine doesn't work after this we're probably looking at a worn IC socket or some subtle PCB damage. |
| MK14:
Congratulations. You have made really good progress :) I think the MK14 clone/copy/modern-PCB version, is looking really good (apart from the keyboard/keypad), but that is just because it hasn't been labeled yet. That's why I wanted to find out more about the "suspicious" repeating scope trace(s) you were apparently getting. So, as you have just explained, it was getting to at least one of the causes of your malfunctioning original MK14. I guess the failure of the PROM(s), could be through what Gyro suggested ("P.S. I remember that old bipolar PROMs had a failure mode where fused junctions tended to regrow (dendritic)"), or some other reason. I vaguely imagine in my mind, the weaknesses of the structures which allow bits to be stored in it, may have meant they don't always survive, approximately 40 years of aging. Sadly, the PROMS can be harder and more costly to replace than some of the more common components. But anyway, you were already able to source some PROMS, so no problem for you. Especially if they need programming, as that can be tricky, these days. |
| mpk:
It works! Took a bit of effort to get there but here's a shot of a cleaned up, fully functional Issue V MK14 complete with external keypad. It turned out to be a couple of problems working together, but having identified that the PROMs were duff and replaced them I at least had a starting point to work from. The data bus on the machine looked pretty healthy when powered, but only the lower half of it. D4-D7 were full of junk. This pointed me towards the high nibble side of the board, where a bit more poking around with a scope showed the data lines on that side's PROM looking suspiciously quiet. This looked to be because CS was held high. Tracing the line back to where it came from (CS on the low-nibble PROM) everything looked fine, which meant it had to be the socket the PROM was plugged into not making decent contact any more. Replaced the IC socket and the machine came *mostly* to life but with a couple of display quirks - one digit was missing and the decimal point was stuck on on every digit. Traced the DP back and found another dodgy IC socket - replaced that and all that remained was the missing digit. Poking at the display flex it turned out that the relevant wire had snapped - whole thing was just worn out even thought I'd stripped it back and reterminated it - a small wiggle broke another 2 or 3 lines so I sighed, reached for the desolder wick again and replaced the flex with a more resilient but less flexible (and less prototypically accurate!) SIL header. So that's it - now back in full working order with *almost* all the original parts still present. I'm trying to decide if I feel brave enough to tackle the VDU card next. Thanks for all the advice! |
| MK14:
You have done a great job. It is looking really good now :) Just like (I remember) a MK14 should look like. I number of things which I thought would be a good idea, but I did not mention them, you have done anyway. So, I'm pleased about that (there must be some relevant saying, but I would feel too boastful to mention, such as "great minds think alike"). Especially, (apparently) changing all the electrolytic capacitors (I hinted about them, but did not go as far as saying, definitely change them because they are so old, because opinions differ about it). I also had the idea of using the new PCB MK14 to test/help fix the old one. Putting on that small/sensible heatsink on the voltage regulator. I did similar, a very long time ago, but it needs care. As, I would occasionally accidentally knock the top of the heatsink, potentially eventually metal fatiguing the voltage regulators leads. It is a pity, some of your IC sockets, have gone bad and needed replacing. I presume it was the (somewhat) poor storage, that did it. But, after around 40 years, even if well stored, things can still oxidize and go bad. I'm glad you were able to diagnose the digital issues, with the digital oscilloscope. You seem to have made sensible progress, with the checks, conclusions and repairs. This is where the age of the MK14, actually helps a lot. All the data and address bits come out on the board (unlike most microcontrollers), the PCB is not multi-layer, and there are no abnormally complicated chips on it. Such as custom ICs (ASICs), FPGAa, ULAs, Gate arrays, Pals, specialist sound or graphics/video chips, etc etc. Which can make things much trickier to diagnose/fix and find replacement chips. All of the MK14 is just standard (off the shelf, around 40 years ago) logic chips, Microprocessor, Memory and PROM. It would be nice to get the video (card) thing working. Although the 128 bytes of free available memory, you mentioned earlier as regards when you use the VDU, is a total joke these days. It is still a fun and interesting project to get working. The old ZX80/81 only had as standard 1K of ram, which was shared with the screen memory. Yet there was a very wide range of software produced for it. So I guess even 128 bytes, could be used to do something. Ironically, machine code can do quite well, even with only 128 bytes. These days, of high speed (digital) HDMI, in full 4K, is getting well beyond what a hobbyist constructor could easily do with some pieces of TTL logic. But I refrain from saying it is impossible to do, because there can always be someone out there who does it. |
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