Re: MK14 schematic revisions
 

I'm so sorry to hear that, and obviously the absolute priority is for you to regain your health, everything else fades to insignificance. Take care, and stay in touch as and when you can.

Re: MK14 schematic revisions
 

Indeed that is horrible for you - here is hoping for a speedy recovery and successful treatments that return you to full health soon.

Re: MK14 schematic revisions
 

Than ks guys! Progress is slow but in the right direction.

Re: MK14 schematic revisions
 

Hi all.

It's been interesting reading all the posts in this thread and it brought back some memories and as such I decided to try and resurrect my MK14. The only bit I had left was the processor chip the rest having been used to make various "computers" in the years afterwards and the PCB finally getting so messed up it ended in the bin.

Thanks for the various idea of where to get parts and I have bought the PCB plus other bits from Martin in the Czech Republic, his web site is very helpful. Most of the other chips I've managed to find in my and my friends chip store. He also had a chip checker from Bang good that shows that the old chips are still working ( well enough of them).

I'll post to let you know how it goes.

Cheers David

Re: MK14 schematic revisions
 

Hello David,

Welcome to the fold. Let us know how you get on. As a new forum user your posts will be subject to time delayed moderation, but after a few posts from you that will wear off and you will be able to post in real time.

There are several other large threads concerning the MK14 on this forum, some now closed, but any closed thread can be re-opened by a polite request to the mods if you find one which seems more directly relevant to your present situation than one which is still open.

Re: MK14 schematic revisions
 

Welcome fellow MK14 enthusiast - I have yet to build my one from Martin but, still enjoy my JS Precision one...

Re: MK14 schematic revisions
 

Hi Timbucus

Did you mean the JM Precision PCB.
I got one of the JM Precision ones as well as one from Martin - took the opportunity whilst collecting bits.

Do you know which switches I need to use the JM Precision PCB?

I have most of the chips - or at least ones that should do the trick - but have drawn a blank on the small seven segment Led display.

Cheers David (aka DeltaAlpha52 )

Re: MK14 schematic revisions
 

There are a couple of people here who have already gone down the same road as you and may possibly have bought more displays than they really needed, but if not, one dodge which you can use is to harvest the display from a contemporary basic calculator, which you can find in the usual places.

I have found that the 'Texet 880', sold in large numbers by Woolworths etc in the late seventies, has exactly the right sort of display - I have actually had a Texet 880 display connected to my original MK14 so I can say for sure that it definitely works.

I believe Tim has mentioned the exact switches which fit the JM PCB, as he had some initial difficulty locating them as well.

Edit: Yes, back in post #148 of this thread, Tim mentioned the switches...

Re: MK14 schematic revisions
 

Just checked and blimey, Texet 880s seem to have gone up a bit.

Last time I was looking they were typically between £6-£7 for a nice one and £2-£3 for a scruffy / broken one, the latter obviously being the one you would choose to buy as a parts donor.

If no-one else here has any displays they feel they can spare, hopefully they can at least tell you where they got theirs from.

Re: MK14 schematic revisions
 

I got mine from a Texet (It had a faulty key but, I have kept it so it could be restored if needed) and I also bought a Russian equivalent but, have not been able to get it to work... My JM precision has one of the large reproduction ones as it is clearer when demonstrating to people - it was from a listing by trev-ham, he was doing keypad covers and display enhancers as well but, seems to not be listing at the moment.

Re: MK14 schematic revisions
 

You may be able to contact him and ask - he has a keyboard kit for an Acorn System 1 listed currently on eBay, the large displays were available for either the System 1 or the MK14.

Re: MK14 schematic revisions
 

I've just put together the attached sketch of the pinout for the MK14's display - this is directly from an original pencil sketch which I made when the MK14 was my only computer, so no 'Paint' in those days. It should be accurate, as I used it as a guide for wiring up another display to my Karen-O PIC14 more recently.

The sketch may assist with identification of suitable substitute displays if you find them advertised along with their pinout connections, it may also prove useful when troubleshooting display problems.

Re: MK14 schematic revisions
 

Interesting about that Russian display, Tim.. It's definitely common cathode?

I find the quickest way to 'buzz out' a multiplexed 7-segment display like that is to use a multimeter on its 'Diode Test' range - the voltage / available current from the probes is usually enough to light individual segments dimly if you put the negative lead on a common and the positive lead on a segment connection - assuming the display is common cathode of course. You can work through all the possible connections pretty quickly like that.

Re: MK14 schematic revisions
 

Neat trick - just tested it out and it seems it is common cathode but, several of the displays do not respond - which is the effect I was seeing on the MK14 so I suppose there must be faulty elements. I did wonder if it was my header and perhaps the board was not plated through but, studying it it uses vias under the display so it must be...

Re: MK14 schematic revisions
 

Assuming those elements which do work correspond to the pinout in #252, what do you see when you 'Diode Test' from the common of one of the non-working cells to each of the corresponding segment pins? Do you see the same forward voltage drop as on a working cell / segment, or do they appear open-circuit?

I have known LEDs, especially ones which have been subjected to overcurrent, to go 'zero emission'. They still look, electrically, like a diode but they no longer give off light. This is also a common failure mode in optocouplers, although in those cases the problem can only be surmised, not seen.

I once 'fixed' a non-working display cell on a Sinclair PFM200 frequency meter - same type of display as the MK14, as it happens - by putting a blob of fresh solder on the corresponding common connection on the edge of the display and applying heat to that connection (only) with a fine tipped iron - the hope was that the heat would travel up to the little gold wire which jumped from the pad to the actual display common and 'activate' the dead display element. To my astonishment it worked and the display is still working many years later.

You'll be familiar with the law which states that 'thou shalt not apply heat to the leads of an LED (or any semiconductor) for too long', although unfortunately just how long is 'too long' is a matter of personal judgement. If the display seems duff anyway, you might think it work the risk as you have nothing to lose.

Re: MK14 schematic revisions
 

Yes they do match the pinout in #252 but, unfortunately the commons that fail to give anything read just 1 ( N/C ), the others giving around 1.4-1.6 - strange that not a single segment lights on the common as that implies all 9 junctions are dead? Never really thought about the failure mode of a 7 segment display.

Re: MK14 schematic revisions
 

More likely, it implies that the common is open-circuit. If you think you have nothing to lose you could try applying a generous amount of heat for 30 seconds or a minute to the accessible common terminal of one of the dead displays to see if that 'does something'.

Before you do, it's also possible that despite your observation of vias going through the display PCB underneath the lens, the connection terminals along the edge of the display are not plated through (although they probably should be). If the connections are double sided (that is, there is a pad on both sides of the lower edge of the display) just try checking for continuity between the top side pads and the corresponding bottom side pads, especially those which are the common connections for the display cells which don't work.

The dud display cells may actually work if the top side pad and lower side pad are both soldered to the wire or pin passing through the connection holes.

Re: MK14 schematic revisions
 

I had thought it was continuity before and soldered the pins of my header both sides - just re-tested using the vias under the display directly and they all have continuity to the top of the PCB and also give the same effect - I will run up the iron tomorrow and try the heat trick and report back - thanks for the assist - interesting exercise and learning opportunity.

Re: MK14 schematic revisions
 

I have had a good many goes at the heat trick tonight and have managed to restore function on almost all the segments! From (where X was failed) on each iteration

OXXXOXXOX
OXXOOXOOO
OOOOOOXOO
OOOOOOOOO
OOOXO??OO - you can repeat the loop of the last two

I am also not certain of the two that are unused in SCIOS (marked as ??) but, they always tap out as working...

Neat trick however, that fourth one from the right which is the second most significant digit on the SCIOS address display seems to rapidly fail again - although another go will bring it back - temporarily I have strapped it to the 9th unused one which does work but, it is hardly ideal. I tried patch cables as well as it was acting like a loose connection but I think the capacitance of my fingers was giving it a bit of a boost and it just eventually fails...

Well worth trying and now I wonder why it works...

Re: MK14 schematic revisions
 

Answers on a postcard please...! (I'm afraid I don't know either).

It was just something I tried out of desperation to save a beloved bit of test gear. It's a shame it doesn't seem to work consistently. I was obviously quite fortunate with the one on my frequency meter.