Re: MK14 schematic revisions
 

I've modified the 2114 sketch to test 2111s instead, I just need to test the tester, as it were. The original sketch was nicely written and easy to read but I am bound to have made a mistake somewhere.

I've also just been notified that the issue VI PCBs have just gone in the post to me!

Re: MK14 schematic revisions
 

JLCPCB are pretty quick, I think mine arrived 7 or 8 days after I emailed the gerbers!

Re: MK14 schematic revisions
 

PCBs are still enroute, but here is another little tessera for the MK14 mosaic, a 2111 SRAM tester sketch for the Arduino UNO. Connections between the UNO and the 2111 are explained in the comment at the start of the sketch.

This was unskillfully adapted from the original 2114 tester sketch written by 'Danjovic', which was found by Philoupat83 here:-

https://github.com/Danjovic/2114-Tester

Output is viewed on the serial monitor in the Arduino IDE @9600bd. The tests are quite basic and may not catch every possible fault on a 2111 but it will catch most. It passes one of my recently acquired AM9111s and of course announces a failure if I remove the chip.

It directly manipulates the port pins of the Atmega328 in order to run the tests at a high speed, so it probably won't run on any other Arduino board.

The Arduino file extension .ino is not a supported attachment here so I have wrapped it up in a .zip file.

Re: MK14 schematic revisions
 

Still no sign of the new PCBs - where Slothie's own PCBs must have come on the stage coach, mine appear to be coming by slowcoach. To be fair, the situation is anything but normal at the moment. I'll be happy if they arrive at all.

One thing I'm still on the hunt for is suitable 16-way single sided edge connectors for the keypad edge connector and double sided 32-way edge connectors for the rear edge from which it will soon be possible to pick up all the useful signals - RS used to do two nice (but expensive) Cinch edge connectors which were exactly the right size but they have now discontinued them.

Anyone know where to find current supplies:-
Single sided 16-way, 0.1" / 2.54mm pitch, open ended, no key?
Double sided 32-way 0.1" / 2.54mm pitch, open ended, no key?

Re: MK14 schematic revisions
 

I found a 50x2 0.1" connector on eBay and cut it at position 33 to give a 32x2 and 17x2 parts, and dremmelled off the extra contact to make it 16 way. I've not since found the connectors on eBay sadly.

Re: MK14 schematic revisions
 

Yes I did the same but, the supplier is now not listing them.

Re: MK14 schematic revisions
 

I intend to fit a DIN connector on mine so it just connects to the VDU so no need for the 32 way one.

Re: MK14 schematic revisions
 

The other option is to adapt one of these https://www.ebay.co.uk/itm/Sinclair-...-/260906239646

Re: MK14 schematic revisions
 

The reason I won't do that is because my VDU is an original, so I don't want to make even the very small modifications required to get everything to connect up (e.g, clock signal, which has had to be placed on the 'wrong' side of the edge connector).

I'll therefore use a short edge connector to DIN connector cable or stripboard assembly with the vast majority of the connections going straight through, thanks to the meticulous way in which Slothie positioned the connections on the issue VI, but with the one or two anomalies sorted out on the way across, rather than on the VDU.

I fully understand why you two would just do it with a 'hard' connection, kind of wish I could do it that way myself.

What about Spectrum edge connectors, I wonder if they are still available from retro sellers? Proper ones would have a key / contact missing but I seem to remember some that I saw being sold as Spectrum connectors in the recent past were not actually keyed.

Edit: I see you're ahead of me again - not keyed, bad news for Spectrum owners but good news for MK14 owners, I'd just have to carefully cut the ends open. Good spot. I've just grabbed a couple.

Re: MK14 schematic revisions
 

You may only have to cut the one end - on my JMP board I have one end enclosed on the little adapter I made to connect up my Cassette Interface, provide Power and allow me to jumper on the single step and any other oddments...

Attachment 206440

Re: MK14 schematic revisions
 

I'll bear that in mind, thanks for pointing that out, otherwise I would have sliced out both ends without even looking. For the 16-way one I managed to find someone selling one of the aforementioned RS ones as new old stock, but unfortunately they only had one, which I bought.

I also found some of those on a site called 'AudioJumble' which seems to have a good range of connectors, but unfortunately it is closed at the moment for personal reasons.

Re: MK14 schematic revisions
 

You could correct the 'wrong' position of the XCLOCK signal by cutting short the pins on the DIN connector on the MK14 and using a short piece of wire to connect the signal to the correct side. The other 30 pins should hold the connector in place. In retrospect I probably should have routed XCLOCK to the correct pin and not worried about accidentally shorting the clock.

Re: MK14 schematic revisions
 

If you mean, should you have put the clock in the middle of all those 0V fingers on the top side - no I don't think so because that would alter one of the existing / established connections present on all other MK14s and make yours different from those, whereas, as things stand you have done a good job of making sure that all connections shared with other MK14s are identical. That is as it should be.

Yes, I could do as you suggest, I suppose the DIN connector is expendable in a way that the VDU PCB itself isn't.

There's one other problem isn't there, one of the configuration inputs on the VDU goes to INT (also an input) on the MK14 if connected one to one. I'd have to sort that out as well. Are we agreed about what we intend to do with that so we all sort it out the same way?

Re: MK14 schematic revisions
 

TBH since I don't have a Sinclair VDU I'd rather someone who does to make that decision.
I noticed on our favourite auction site there's someone selling the character generator IC (the correct CAB version) a week ago which would make a replica easier.
I'm tempted.....

Re: MK14 schematic revisions
 

I assume that even you would probably just buy one of the Czech VDU replica PCBs since that PCB doesn't really need to be improved - unless radically so, by adding onboard RAM mapped at 0200-> so it doesn't have to steal RAM from the system.

You don't actually need the char generator, the VDU will still give you a pixel graphics display without one. It obviously is needed if you want to display text.

It's just a question of deciding what line to take that VDU input to instead of MK14 'INT', and all agreeing on the same alternative.

Re: MK14 schematic revisions
 

Your right I'd probably do that, my project list is already longer than I'd like and I here the Czech VDU is a pretty good replica.

Re: MK14 schematic revisions
 

Neat circuit from a National publication to emulate the character generator that might be handy posted to the BUS thread.

Re: MK14 schematic revisions
 

Tim: better hurry up with that SCRUMPI...

Re: MK14 schematic revisions
 

It is finished and running - just working out how the latch chip works and converting 1d LIFE to it as an exercise. My set of Keys with Green and Clear covers arrived today (Two sets luckily as I am using the same for the SCRUMPI3 Keyboard rebuild...)

I have all the other components ready for the board as the 7408's came and test OK on both my quick tester and my EPROM programmer. As you pointed out I have a good stock of displays... Just waiting on replacement 74LS00 for my VDU kit but, I have one ready for the MK14 V6 which will need to be built and working first.

Is that a real one? Did they arrive? - if so they look beautiful.

Re: MK14 schematic revisions
 

Ooh black very nice!

Re: MK14 schematic revisions
 

It is, they did.

I'll get one off to you before the weekend. You have the advantage of me in one respect, I didn't order my keyswitches yet because it seemed like a lot of outlay when I had no idea if the PCBs would ever turn up. All the other parts I've ordered have a potential secondary use as spares for my original MK14, so I didn't mind ordering those.

Assuming I get everything else on the PCB I'll be able to use the Pi-Uploader and the key14 script to enter keypresses until such time as I have some keyswitches to put on it... but this means that yours is likely to be the first one fully populated.

There's one minor glitch in the screenprinting which we already knew about, where the circuit part number for U11 is mixed in with its IC type number. That will be completely hidden when the IC is in place. I couldn't summon the nerve to mess about with that.

They do look nice, I had them made in black to distinguish them from Slothie's original batch in case people eventually start collecting them. His are first editions. ;)

I'll take one to work tomorrow and start socketing it.

Re: MK14 schematic revisions
 

Good idea - always best to make visible differences. Appreciate you getting this done - I will of course build it up as quickly as possible!

Just noticed the extra decoupling caps - luckily I had a bag delivered recently so pretty sure I have it all. I even managed to get a 2Mhz crystal for my SCRUMPI so I could have the 4Mhz one back to put in this so I can use the VDU.

Re: MK14 schematic revisions
 

I am going to pimp this with some nice RED Cambion 40pin turned sockets I have as well...

Re: MK14 schematic revisions
 

I was trying to see if it might be possible to have a rainbow stripe transfer across one of the lower corners but alas, there isn't really the room.

Re: MK14 schematic revisions
 

The extra decoupling capacitors are technically optional and the value not critical so you can fit as many as you wish/have.
I noticed I didn't route NENOUT to the edge connector which is a shame because I need it for my future memory expansion/buffer board. Still nothing a mod wire can't fix! It's not a problem for the VDU fortunately so I'm not worried. I might fix the IC11 label and rework the switch footprint to accept pushbuttons like the reset button which is what the original rev 5 boards had, and I don't see me using the contact domes I originally planned to use. But that's far in the future after the Tesla PROM programmer, SCRUMPI etc etc.

Re: MK14 schematic revisions
 

Actually, don't, the worst problem with after-fit / non original keypads was always how to get the keys neatly and professionally legended. The switches you chose are ideal because you can print the key legends in whatever style or font you wish, cut them out and put them under the clear key caps.

The switches used for the reset switch are not really that nice for use as keypad switches because of the way the cap rocks about from side to side, although they are admittedly real switches, not tact switches.

I don't think we will be tending to enter much code by hand in this enlightened age so the keys will mainly be used for input to user programs, rather than for entering code.

Re: MK14 schematic revisions
 

I noticed you can get Cherry MX style key switches on AliExpress and keycaps quite cheaply. The keycaps would need re-labelling although they are available blank, but you still have the old problem of neatly labelling them. When I.can afford it I'd like to get a 3D printer and printed keycaps are an option! Any fancy keyboard, however, would likely be an external one to put in a case so it's not worth making PCB changes to the MK14. As you mentioned, bulk entry is likely to be rare with your loader and development system.

Re: MK14 schematic revisions
 

I think a keypad PCB for decent switches would be a popular project / product, given the number of original MK14s with missing, inoperative or badly made alternative keypads.

Would ideally be exactly the right size and shape, mounting holes in the right place, so that it could be mounted on standoffs directly over the original keypad ~ 3-4 mm above the main PCB.

Three build sub-options:-
-With short twelve way ribbon cable terminated in edge connector for an instant-fit get you going replacement.
-With longer twelve way ribbon cable terminated in edge connector, for when the keypad is going to be mounted on an enclosure.
-With no ribbon cable or connector: Row and column connections to the main PCB made by short descending wires which can be hidden under the keypad PCB. Leaves the keypad edge connector clear for other purposes.

My Issue VI PCB is coming along, although I was dismayed to discover that some doofus had thrown away the company's entire stock of conventional through-hole resistors a while ago. After looking through numerous tins and buckets at home - I have seen parts, some of them old friends, that I have not seen for thirty years - the only thing I'm still short of is one 2K4 resistor. Who knew that value was so rare?

I'll probably just have to fit a 2K7 for the time being.

Re: MK14 schematic revisions
 

Aside for "authenticity" I don't think any of the resistor/capacitor values are critical. A slightly higher value would be no problem. A dab of Humbrol and no-ones the wiser!

Re: MK14 schematic revisions
 

I've utilised all of the positions for decoupling capacitors, although trying to populate them all was a bit like playing 'Where's Wally?'. Every time I thought I had them all I would notice another one. (There are 14 altogether - an easy enough number to remember in this context). Hopefully it'll run like a Rolls-Royce now.

I have another couple of items for your rev 1.3 revision list, neither serious, and both on the screen printing -

-No notches on the IC outlines to show which way round the ICs should fit. I know they should all go 'notch to the left'.

-Polarity of reset capacitor C6 is not indicated by screen print. (Polarity of the input smoothing capacitor C2 -is- indicated). It's obvious enough that it goes -ve to 0V, +ve to R11.

Did you have a particular heatsink in mind for these?

Re: MK14 schematic revisions
 

If you look carefully the ICs have a little dog-leg on the package outline next to pin 1 but I agree I need to make a more conventional outline that's clearer!

The heatsink was one like this I think
https://uk.farnell.com/fischer-elekt...%20heat%20sink

I placed some plastic tape under the heatsink to prevent the solder resist scratching, and plastic washers too.

Re: MK14 schematic revisions
 

Oh I see - the little right-angle next to the square pad for pin 1. I honestly would not have noticed that. It took me a fair bit of time to work out what you meant.

The heatsink, is it compact enough to avoid everything around it including the push-on rear edge connector?

Re: MK14 schematic revisions
 

Yes, the ones I got were only just larger than the TO-220 package. The fingers on the connector are longer than they need to be so they look closer than they are in reality. I did it that way so wires could be soldered to the tops of spare fingers to feed extra signals out. It all looks snug but I printed out a layout and placed the parts on and everything seemed to be ok.

Re: MK14 schematic revisions
 

Well it has arrived - thanks guys at least Wales is not off lockdown until Monday so I have an excuse to spend the weekend building it...

Putting it on the magazines was pure accident but, I thought it would make a nice photo so fanned them out... these issues reflect three SC/MP applications can you spot them all.

Attachment 207135

Re: MK14 schematic revisions
 

Just noticed it is marked as a 7805 regulator not a LM340 as per original - they seem to be the same pinout - any reason I cannot use one, I can scavenge a 7805 otherwise.

Re: MK14 schematic revisions
 

Yes you can, I'm not sure how that happened. I used a LM340 on my previous prototype I suppose I just didn't have a schematic symbol for it and used 7805 instead. On my last board the writing was so tiny I probably didn't notice!

Re: MK14 schematic revisions
 

Thanks - just pulled out a NOS LM340T5 and it has 7805 in small writing on the bottom anyway!

Re: MK14 schematic revisions
 

I found a heatsink but it overspills a little bit more than the one Slothie had in mind so what I've done is to raise it off the PCB with a teflon(?) pillar maybe 5mm high - the (uncut) regulator leads only just reach through the holes in the PCB.

The space underneath will help the air to circulate on both sides of the heatsink, but also that 5mm gap between the PCB and the underside of the heatsink will allow the edge connector to slip in underneath it.

Mine is ready to put the ICs in and power up, I'm just going to apply power without the ICs in and do a lap of all the IC power pins with a meter first.

Re: MK14 schematic revisions
 

RH one has MK14 review, Middle one, SCRUMPI 3, I am defeated by the LH one however.

I didn't know Gilsoft, purveyors of fine Text Adventure Authoring Software for the ZX Spectrum etc, had ever produced an in-house magazine. How many of those did you do?

Re: MK14 schematic revisions
 

TBH the bigger the heatsink the better. A 21°/w heatsink is only just coping and gets to about 65°C on a long run without the RAM/IO. If you're powering any accessories then an additional regulator is highly recommended, or a much better heatsink arrangement.

Re: MK14 schematic revisions
 

The regulator on my original MK14 has been mounted offboard on a much bigger finned heatsink for, well, more or less forever. It still gets quite warm. When I had the VDU connected to it originally I ran everything on the one regulator but I already had it on a chunky heatsink by then.

I think with this one I'll try running it from a variable bench supply on between 7.5-8.0V, whatever the lowest input voltage consistent with good 5V regulation is, and if the regulator runs sufficiently cool that way I will make a dedicated 8V(ish) linear regulated PSU for it. I think I actually have a 7808 regulator somewhere.

Re: MK14 schematic revisions
 

Ah the LH one is a trick really as it has part two of what is on the October cover with the SCRUMPI3 the Tolinka Chess Recorder - well worth a read as its VDU system is very well described. I include it as it is also the edition with the first edition of Computing Today and the launch of the Triton... a sea change moment in home computing with the NASCOM as well - I also remember drooling over those two. Someone has found a Triton so it should form an interesting You Tube series...

https://www.youtube.com/watch?v=0cSRgJ68_tM

I think we only ever did the one magazine mainly for distributors etc...

Re: MK14 schematic revisions
 

Thanks for the link.

Regarding regulators, on a modern replica maybe one of these would be acceptable. No heatsink required.

https://uk.farnell.com/murata-power-...-5a/dp/2102101

Re: MK14 schematic revisions
 

Anyway, I expect you're all wondering whether the board works?

Re: MK14 schematic revisions
 

...Of course it does. Well done Slothie, a magnificent effort, one you can really be proud of. Starts up cleanly every time, manual reset works every time. I'm going to leave it there for now and tomorrow I'll try running some actual code on it. But looking great so far...

Re: MK14 schematic revisions
 

Well done - sir just finished mine - mad scramble to find an 74LS20 missed that extra chip - spotted it when I moved over all the working ones from my other machine to remove one possible source of problems. I also had to scavenge a power in jack from a recent binned breadboard PSU!

I have nothing on the display though so some work needed for me...

Re: MK14 schematic revisions
 

I made Link1 the 2111 type as well (and Added R16! - I used 5K though not 4.7K due to stock levels) but, I would not think it was that critical nor for R1/R11? And of course a 4Mhz crystal ready for the CPU.

Re: MK14 schematic revisions
 

Mine has a 4.00MHz crystal in at the moment although I have a 4.43MHz crystal on the way as I'm curious to see if this one can work with the uploader running at flat-out speed, or whether I will have to dial the speed down a bit like everyone else has so far.

I can't believe you have yours built already if you only got it today. I suppose aside from the ICs there aren't really too many other components on the PCB.

It's good that we both kicked off at the same time - with one of them definitely working at least we know that they both, in theory, should work.

Re: MK14 schematic revisions
 

Great work, and very reassuring. It'll be interested to see how it works with the VDU. It looks great in black :)

Re: MK14 schematic revisions
 

Well mine now works. When I was testing the bare board for power and cross shorts on the Data/Add lines I noticed that I had Zero across the reset CAP - ignored it as a fluke.

Well two minutes with the scope and a strong clock made me start at the top and PIN 7 RST was Low.... hmm checked the routes via IC11 etc - swapped the LS00 as I had a batch of 10 with some faulty gates... nope - de-soldered the cap no still short.

Button seems to be OK up and down - continuity test shows it is permanent on - de-soldered it - which is VERY hard - took it apart, can't see any reason - put it back together works fine...

duhh seems they have an orientation as it is square pins... obvious once you see the inside of the switch...

Attachment 207172