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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 5th Sep 2020, 9:36 pm   #21
Slothie
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Default Re: I found it !. A very sorry looking MK14

Its probable the electrolytic will need re-forming at very least if its not been used in years.
Connect an incandescent bulb (e.g.12w 12v bulb from car) in series with the cap and connect to a suitable power source (could even be the transformer/rectifier you already have). If the capacitor is still passing a significant current after a few minutes then its probable the electrolytic is toast. Leave the capacitor connected for several hours to allow the oxide film to re-form before removing the bulb. Its probably worth checking the current with a meter too - with no load there should only be a small leakage through the capacitor. With the transformer switched off and no load the voltage across the capacitor should fall fairly slowly.
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Old 6th Sep 2020, 4:01 pm   #22
Buzby123
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Default Re: I found it !. A very sorry looking MK14

Progress so far ...

Circuit boards and transformers removed from bottom of case, bottom case given a thorough scrub with foaming cleaner, and a rinse. Still stained, but stable. No flakey bits.

Refitted main transformer, checked 240v wiring, refitted combined PSU and DAC board. ( Why on earth did I put these two on the same board ?. And why did I do most of the wiring with bell wire and 4-core telephone cables ? )

Powered up. No smoke, sparks, buzzes or bangs, so it's probably OK. ( or totally dead !)

Smoothed supply is 12.8v, regulated is 4.96v, so the TO3 regulator thingy ( I can't remember the number ) is looking OK. I'll leave this running for a few hours, and move on to the case top.

First thing to tackle is the keyboard.

Question number 1, can these keytops be 'popped off' and refitted ?.

Cheers,

Buzby
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Old 6th Sep 2020, 4:54 pm   #23
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Default Re: I found it !. A very sorry looking MK14

Quote:
Originally Posted by Buzby123 View Post
First thing to tackle is the keyboard.

Question number 1, can these keytops be 'popped off' and refitted ?.

Cheers,

Buzby
Those look like a keyboard switch that RS sold many years ago. The coloured buttons do come off without damage (they fit onto a sort-of cross shaped black actuator in the main body of the switch)

Actually RS still sell a siimilar switch, but without the cap :

https://uk.rs-online.com/web/p/keybo...tches/7931712/

They also still sell the switches to solder to later verson MK14 boards in place of the keypad.
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Old 6th Sep 2020, 5:26 pm   #24
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Default Re: I found it !. A very sorry looking MK14

Thanks for the info.

I remember now, I did get them from RS. Now I can get them re-Letrasetted, but this time I'll put some lacquer on as well.

The numbering wore off while I was still using using the machine, 40 years ago, but it didn't bother me. The layout of that keyboard is burnt into my brain !.

Cheers,

Buzby
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Old 6th Sep 2020, 6:15 pm   #25
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Default Re: I found it !. A very sorry looking MK14

Here's a question with no right answer, but I'd like your views.

When I get Micky working again, should I repaint his case like new, or leave it as is ?
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Old 6th Sep 2020, 6:34 pm   #26
Slothie
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Default Re: I found it !. A very sorry looking MK14

Personally, unless after cleaning it looks really tatty then I'd leave it. A few scratches and scuffs have been earned
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Old 6th Sep 2020, 7:29 pm   #27
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Default Re: I found it !. A very sorry looking MK14

I think if you were to fit it with a repro board and use it actively again (restoring the MK14 as a standalone unit) it would be OK to paint it. If you intend it to house the original still then it should be left as is - that is what any museum would do after stabilizing the paintwork to prevent further degradation. As Slothie says the history of a piece is written in the wear it has received until the point it is preserved and ceases to be a daily used item.
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Old 6th Sep 2020, 8:11 pm   #28
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Default Re: I found it !. A very sorry looking MK14

The buttons for those switches are still stocked by digikey and their manufacturer part number is BTN MDP nn (Where nn is the 2-digit colour code).

See here.

https://www.digikey.co.uk/products/e...=1&pageSize=25

These are actually the type of switch and cap fitted to the keypad my MK14 has now, but at the time the caps were only available in red or blue so I couldn't have the command keys picked out in a third colour so I made the numeric keys red and the rest blue.

I keep thinking I should get a few slate grey ones and redo the command keys in that colour as I would have done originally. But then I think.. this has been the machine's keypad for most of its life... I shouldn't mess with it now.
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Old 6th Sep 2020, 11:11 pm   #29
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Default Re: I found it !. A very sorry looking MK14

Quote:
Originally Posted by SiriusHardware View Post
... but at the time the caps were only available in red or blue so I couldn't have the command keys picked out in a third colour ...
A third colour ?. Why did you need three ?.

My digits 0-F were in blue, and Mem,Term,Go,Abort in grey. That's all the matrix keys done. ( Actually, the 0 was grey, 'cos I was a blue short. )

Regarding my progress, it's hit a tricky part. The three banks of LEDs around the breadboard are there to show the states of the 8154 IO pins, and the flags etc. from the CPU. All these signals are from the long edge connector.

With no MK14 board in place there should be no LEDs lit, but some are. The LEDs are driven by simple transistor circuits, but getting at them is going to be difficult.

I must have fitted the LEDs and sockets first, then put the veroboard onto the pins of the LEDs/sockets, then soldered. This means I have to desolder all the sockets to remove the board. ( I can pop out the LEDs ).

It's obvious I thought they would never need fixing in the future. Isn't hindsight wonderful !.

Cheers,

Buzby
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Old 6th Sep 2020, 11:30 pm   #30
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Default Re: I found it !. A very sorry looking MK14

One colour for 0-9, one for A-F and a third colour for the command keys.

That's just how I wanted it to be. There would be absolutely nothing wrong with all the caps being the same colour, as they are on the vast majority of keypads of all types, or the command keys being one colour and 0-F being another as they are on yours.

I would still consider redoing all of my key caps even now because at least 4-5 of the letraset legends are noticeably off centre. At the time I just wanted to get on with using it so I didn't do it as patiently as I would now, but the shoddiness of the job my teenage self did irritates me every time I look at it.

And yet, nothing dates a homebrew project to the mid-late seventies the way slightly wonky Letraset lettering does, so if I were to replace that with neater lettering done by my near 60 year old self, I would be destroying part of its authentic history.

The compromise would be to obtain a full replacement set of caps and re-letter them to my current satisfaction, but keep the original caps in a safe place so that the keypad could be restored to its genuine late seventies appearance if I ever felt the need to do that.
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Old 7th Sep 2020, 12:14 am   #31
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Default Re: I found it !. A very sorry looking MK14

I actually thought of having mine laser engraved, but then thought 'that won't look anywhere near original', so it's back to wonky Letraset !.

I'm seriously thinking of getting a repro board, not to replace mine, but to use as a known good system for testing this one. I've got enough CPUs and stuff to build the core of a repro ( with different display and no KBD ) and it would help with testing my chips. That daughter board is a nightmare as the main board won't work without it, so I won't know where any fault lies. Anything that gives me a stable starting point will help.

One thing I know will be 'wrong' when I get this working as it was 40 years ago, the display will be dim. My display driver circuit was dim from the day I built it, but it was good enough for me, so I never re-visited it. Today I would have no problem designing a replacement that performed better, but it won't be old-skool. Should I do it ?.

Cheers,

Buzby
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Old 7th Sep 2020, 8:30 am   #32
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Default Re: I found it !. A very sorry looking MK14

Since we're both talking about third-party items we made ourselves (keypad, display driver), we may be being unnecessarily fussy. If both machines (yours and mine) were complete originals it would be borderline sacrilege to replace anything but when the items in question are uniquely of our own making, so who is to say that I did not originally do a better job of lettering my keys and you did not do a better job designing your display driver? (Well, anyone who reads this, I guess...)

If it had been possible at the time to obtain (and afford) a set of professionally legended keys, I would certainly have fitted them, and if you had known then what you know now your display would be brighter.

So again the compromise, I would say, is to replace what you don't like but preserve the original driver board in such a way that the change is reversible, and maybe try, in your case, to use components and materials in the new driver which are 'right' for the period and which could be taken to be the original driver circuit by anyone who has not seen it before. You could even use bell wire and strands of telephone wire for any necessary power and signal links, to make it blend in with the rest.
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Old 7th Sep 2020, 10:47 am   #33
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Default Re: I found it !. A very sorry looking MK14

Hi,

The display driver looks like it uses a ULN2007 darlington array for the segments, and some kind of 1-transistor circuit for the anodes. The darlington has a 150R resistor array to the segments, that's not going to help !.

So it looks like I just need to change the resistor array for a start, and maybe replace the darlington with a MOSFET equivalent. Neither of these changes would need rewiring, just replace socketed chips. I'll investigate this later, I've got bigger fish fry at the moment.

Some of the 8 IO switches are a bit dodgy, I remember this from way back. I'm hoping I've still got a few spares, if not I'll have to replace them with a modern equivalent. I don't really want to do this, as I can't find any with the same toggle design. Those perfectly cylindrical covers are so 70's.

Anyway, I'm still working on replacing dead BC109s, all the while cursing my younger self for building such a servicing-unfriendly machine.

Cheers,

Buzby
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Old 7th Sep 2020, 12:08 pm   #34
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Default Re: I found it !. A very sorry looking MK14

Since your system obviously has extra RAM (over and above the standard full 640 bytes) you would have needed a hole somewhere in the memory map somewhere for that memory to drop into.

Only the final issue (Issue V) had that memory hole available as built. MK14 issues II to IV all had unwanted 'images' of the OS in the region 0200-07FF which first had to be removed before any new offboard memory could be mapped into that area, so some of that hackery on the underside of your MK14 PCB will be for that purpose.

In post #6 of this long running thread

https://www.vintage-radio.net/forum/...d.php?t=145663

there is an attached single page document which was sent to me by SOC (circa 1978) in response to my question to them about how to expand the memory.

The document doesn't actually describe how to add more memory, it describes the mods required to a specific revision of MK14 - possibly issue II - to remove the unwanted PROM images at 0200-07FF as a precursor to adding more memory.

Unfortunately the document describes the mods 'visually' (Cut this, link that) rather than at circuit diagram level, but if you look at yours and yours has those mods applied you can probably say those particular mods don't need to be undone in order to get the machine to work stand-alone with its onboard RAM, since all they should do is remove the unwanted PROM images.
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Old 7th Sep 2020, 2:01 pm   #35
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Default Re: I found it !. A very sorry looking MK14

Hi,

Yes, I remember doing that cut to the 'Y' track back in the day, but I think that must have been just the start of my hacking.

On my main board IC16 is totally removed, and two legs on my 2111s have been bent up and soldered. The daughter board has two 6514s, two 571s, two LS32s, a 7400, a 74LS14, and a 7493.

I can understand there would be a need for gates, but what is that last chip doing ?. It's a counter.

When I eventually get to power the full system, if it doesn't work as expected, then I'm going to have to trace the circuit on the daughter board. It's all Verowired, a real nightmare !.

It will not be soon though. I've got the IO lamps & switches, the display, and the keyboard to test and/or fix.

Cheers,

Buzby
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Old 7th Sep 2020, 3:14 pm   #36
Slothie
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Default Re: I found it !. A very sorry looking MK14

Its probable the 7493 is being used in a single step circuit; although the circuit in the V1 manual uses a MC14024 counter, later issues shows a 7493A being used:
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Old 7th Sep 2020, 3:43 pm   #37
Buzby123
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Default Re: I found it !. A very sorry looking MK14

That looks promising. The 7400 on my daughter board has only two gates used, the other pins are not connected. I also remember fitting a switch for single-stepping, but I don't ever remember using it.

I'll certainly need the combined knowledge of this forum when I get closer to running the board.

Thanks for your help,

Buzby
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Old 17th Sep 2020, 12:51 am   #38
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Default Re: I found it !. A very sorry looking MK14

I've started on waking Micky from his 40 year hibernation.

Because there is no easy access to the main board while it is in the case, I've taken it out and will work on it with no keyboard or display. I should be able to check enough signals with my scope to see if it is working.

When I was looking at the edge connector I noticed that Pin 8 was not connected to 0v, it is connected to NADS.

My board has 'MK 14 ISSUE II' printed in the copper, so I think it is an issue 2. But edge pin 8 connected to NADS is an issue 3 feature, according to the manual posted earlier.

I don't think this will make much difference to my task. I've hacked the board so much that it won't match any drawing that SOC produced !.


One question, are the PROMs likely to be OK ?

Cheers,

Buzby
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Old 17th Sep 2020, 1:23 am   #39
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Default Re: I found it !. A very sorry looking MK14

The PROMs are bipolar fusible-link PROMs and there are anecdotes to the effect that that particular type of PROM can fail due to the micro fine wire fuses re-growing and making contact again.

I certainly have come across a few cases where bipolar PROMs have died. However, there's no reason to assume they are dud without due cause.

If you have a conventional EPROM programmer you can lash up an adaptor to read a pair of 4-bit wide PROMs as though they are a common EPROM. This sketch (attached) illustrates just such an adaptor to allow a 32 by 8 bit PROM to be read by an eprom programmer which thinks it is reading a 27256.

Just make a version of this which places the MK14 PROMs side by side so that the low nibble and high nibble are read out as one byte, common up the address lines A0-A8 and limit the address range being read by the eprom programmer software to 0000 - 01FF (512 bytes).

The attached drawing shows only A0 to A4 connected because the device in the example was a 32-byte device, you will obviously need to connect A5 through A8 as well, in order to read 512 bytes of memory.

My MK14 is also issue II and I've always just taken SOC's word for it that NADs was not introduced on the edge connector until issue III. I'll have to check that when I next have my original machine down for any reason.
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Old 17th Sep 2020, 10:26 am   #40
Buzby123
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Default Re: I found it !. A very sorry looking MK14

I'd forgotten how poor these SOC drawings are !
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