Re: I found it !. A very sorry looking MK14
 

Expanding on your own idea, do you have access to other micro toys such as an Arduino Mega? If you removed the SC/MP and connected a Mega to the CPU socket with a header you could write sketches to put any combination of address or scanned address range and RD or WD on the buses at any human-visible speed or static state you like, in order to see what is going on.

Re: I found it !. A very sorry looking MK14
 

I just thought I'd keep you updated on my progress with Micky.

First I traced the ribbon cables to the 32 pin plug, then lots of continuity testing to determine what the daughter board is doing.

It looks like the daughter board does the address decoding for :
0000-01FF : SCIOS ROM on main board ( which I'm fairly certain is the 'new' version. )
0200-05FF : 1K of battery-backed RAM on the daughter board.
0600-07FF : SCIOS ROM on daughter board ( probably the original version. )
? ?? ? : Optional RAM on main board.
? ?? ? : 8154 on main board.

The ? are because I think I've found the reason Micky was abandoned all those years ago. One of the signals for these decodes just connects to fresh air.

Don't laugh, this was my first experience with a microprocessor. I remember having the daughter board working completely ( even though the decode logic is nothing to be proud of ), but I must have done some more mods on the main board and didn't realise that a track cut left a wire floating.

Next job is to find where the floating wire should go, then fix some switches to wiggle the address lines and check the decoding.

A few more hours work, and Micky should be running again.

Cheers,

Buzby

Re: I found it !. A very sorry looking MK14
 

Good luck with the ongoing repair, we are all hoping to see it come back to life.

Re: I found it !. A very sorry looking MK14
 

The battery backed RAM is a great idea. I was going to do that on my 1.5k RAM expansion since the variant of 6116 I had was an IDT part designed to be backed up by a battery.

Re: I found it !. A very sorry looking MK14
 

My 'Maplin Z80 CPU' PCB used 6116 RAMs of any flavour and had the option to battery-back the RAM, so the principle may be applicable to any type of 6116.

Re: I found it !. A very sorry looking MK14
 

Yes I think its just some are designed to use a very low current in standby (1mA or so) whereas standard parts use 10mA or higher when not selected.

Re: I found it !. A very sorry looking MK14
 

I'm still here !.

Not had much time these last few weeks, but hope to get back on it soon.

Re: I found it !. A very sorry looking MK14
 

That will be good still looking forward to seeing it running fully - in these times lack of updates sometimes leads to concern :)

Re: I found it !. A very sorry looking MK14
 

I'm still working on Micky, there is something seriously difficult to track down.

However, I got a copy of a Slothie VI PCB, so I'm going to use that as a testbed for the components from Micky.

Re: I found it !. A very sorry looking MK14
 

Good idea to acquire one of the issue VI PCBs to use as a test bed for the ICs from your original system. Hopefully you can be a third eye on the VDU project at some point as well. There is an extra IC in the issue VI, a 74LS20 (U20) so make sure you have one of those available as well.

The IC north-west of the display with the IC 'U' number and the device number tangled up on the screen printing is either an 80L95 (as per original) or can be substituted with a 74LS365. I think Tim has tried one or two other variants in that position as well.

The two 7408s behind the display have to be 'original' 7408s and not 74(something)08. It's OK to use a 74LS08 in the U16 position, as indicated by the screen printing.

Re: I found it !. A very sorry looking MK14
 

Yes I have the 80C95 version - works just as well...

Re: I found it !. A very sorry looking MK14
 

Yes I must do something about that sometime... But until I need to make a significant change to the board I can't see myself getting more made so it might have to wait.

Re: I found it! A very sorry looking MK14.
 

Another possible way to go about troubleshooting mickie would be to connect a scrumpi style single step circuit.

Re: I found it! A very sorry looking MK14.
 

I get the impression from Buz that it is the nature of all the modifications on his highly customised MK14 which are proving difficult to untangle - if the machine had been certified working when put in the loft all those years ago it would just be a case of fixing it 'as found' but I think Jeff (Buz) suspects it fell out of use because it wasn't working even then, so it's more a case of getting it working the way he originally meant it to work - 30 years or more later.

Re: I found it! A very sorry looking MK14.
 

Yes, it stopped working after I removed the sprite-based VDU gubbins, but by then I'd got an Apple ][e, so I didn't bother with that manky old thing, what use would that ever be again ?.

The daughter board is a real horror. I think I might give up with that, stick Micky's bits into Slothie, then start again from a known working platform.

Regarding the display-driving 7408's, what is the reason for not using LS versions ?.

Cheers,

Buzby

Re: I found it! A very sorry looking MK14.
 

The 'plain' 7408 versions have higher output current capability or so I understand, but at the same time they also limit the current to a safe higher value so the 7-segment displays do not require current limiting resistors.

If you try to use more modern / conventional 74x08s you will probably get a rather dim display.

Re: I found it! A very sorry looking MK14.
 

The resistance of the output stage is different, and I speculate that the input capacitance helps the 74ls157s latch the segment data. I don't however know of anyone whose tried LS parts so I don't know specifically that they won't work.

Re: I found it! A very sorry looking MK14.
 

One stunt I have used from time to time when trying to troubleshoot microprocessor based systems is to fit a much lower frequency crystal or supply a lower frequency from a generator - slows everything down and makes events much easier to see. Of course that approach only works when everything on the system is derived from one main clock.

I had an interesting experience with a microprocessor controlled functional door-entry system which I eventually tried this on. The background was that this system had been in production and working perfectly for years and then it started to become apparent that new / current examples were often declaring phones to be off-hook when they actually weren't off-hook.

The sequence was:
-User presses door panel button
-Microcontroller activates relay to connect the common circuitry to the individual phone, checks for the presence of an on-hook phone.
-If the phone is present, leave the relay energised and ring the phone.
-If the phone is absent or off-hook, de-energise the relay and display 'Off' at the door panel.

Problem was that often, a phone which was present and on-hook would fail the check, and this had only recently started happening. The energise-check-fail sequence was very fast, too fast to be able to tell what was going on so that was the point at which I dropped the crystal frequency to a quarter of the usual frequency to slow everything down.

However, far from allowing me to see what the problem was, it fixed the fault! At the same time, we also discovered that raising the system supply voltage from 12V to 14V also fixed the fault. Eventually we noticed that the relays being used in current versions of the PCB were not the same make as those in older versions, so we held a 'relay race' and discovered that the changeover speed of the new relays was considerably slower than the original parts. With the original very close timing delays in the firmware the new relays weren't being given time to get across before the phone present / absent check was carried out. Once we realised that, the delay-before-check time was increased in the firmware and that problem was finally fixed, but the breakthrough came about as a direct result of running the system at a lower speed for diagnostic purposes.

Re: I found it! A very sorry looking MK14.
 

Nor do I but remember the original kit of parts contained 2 x 7408 and 1 x 74LS08, so they must have had their reasons. It would be very like Sinclair to prefer to stock only one variant so I'm surprised they didn't just supply 3 * 7408s.

Re: I found it! A very sorry looking MK14.
 

Several references to an eBay auction edited out of this thread, it doesn't affect the ongoing discussion.

Re: I found it! A very sorry looking MK14.
 

Aargh !.

Transplanted all the vital organs from Micky into Slothie, but the recipient didn't spring into life, just a faint pulse detectable.

The display is being scanned, so I presume that means the 8060 is doing something and the ROMs are not totally dead. However the digits are the 'reverse C', with lots of flickering and ghosting. Keyboard does nothing.

Too late tonight to make any attempt at diagnosis.

Cheers,

Buzby

Re: I found it! A very sorry looking MK14.
 

That is good progress - we will get that working over the weekend - check the soldering on the display as well as lots of reverse C could be a common with a dry joint - that happened to me...

Re: I found it! A very sorry looking MK14.
 

Reverse 'C' is, as we remember, 0x0F, or 00001111 on the databus. That's what is on the data bus when nothing else is driving it.

We have two proven examples of the issue VI working here already so we know the PCBs are OK, assuming yours was made from the exact same design files. I think I'm right in saying that the 2111 / 6xxx RAM type select link on the underside of the board should already be made in favour of 2111, but please do check that.

One possibility is that one of your bipolar PROMs may have gone faulty - they are allegedly prone to a variant of 'tin whisker disease' which happens whether the device is powered or not so it is quite possible for them to work one minute and not the next. There are various ways to check the PROMs, but in any case I think you have two sets there don't you - 'old' OS and 'new' OS - so try the other set.

Assuming you have some sort of device programmer, is it one which can also functionally test TTL ICs and maybe RAMs as well? If so, consider just running all the standard ICs through that to see if they are all OK.

Re: I found it! A very sorry looking MK14.
 

Try removing as much as possible and only fit the minimum to see signs of life.

You don’t need 8154 or extra ram fitted. Could probably also remove the 80L95 and maybe even one of the 74ls157 and 7408. Just look for the group of 4 and 2 digits on the display.

The 8060 is not completely dead, that usually shows a single bright digit at power on.

Its sort of scanning the display, so proms may be ok.

Might be a good idea to check the data bus for any sign of being driven high and low from different sources before swapping in any other chips. Just in case you damage both sets of proms.

Turned out to be ram chip problems on mine, possibly caused by dry joints in the address decoding. Swapping high and low ram gave different behaviour and then replacing them after waiting for delivery confirmed it was the ram.

Re: I found it! A very sorry looking MK14.
 

It does, there are 2 sets of the links on the bottom, one selects RAM type and theres a tiny trace between the 2111 pads which is hard to see with black solder mask but quite visible with green.
The other link is also pre-joined and pulls SenseA to ground so the monitor works. Break if you need SenseA.

Re: I found it! A very sorry looking MK14.
 

Just to enlarge on the point about Sense-A, the 'new' monitor with the '0000 00' prompt incorporates the firmware for the single-step feature - it still needs the extra hardware shown in the manual next to the 'single step' program in the manual, and if you do not add that hardware then Sense-A has to be taken to 0V otherwise user programs will not run. Therefore, Sense-A is connected to 0V via a fine link by default on the issue VI.

This won't be the cause of your problem, just explaining why the Sense-A link is there and why it is linked by default.

Re: I found it! A very sorry looking MK14.
 

Your photo shows 7442 at IC13. National Semiconductor used this initially in the Keyboard Kit addon but then changed to 7445 which sinks 80mA. (Did SoC ever supply 7442?)
A fault here might give the irregular scan. (But not the x0F invalid segments)

Re: I found it! A very sorry looking MK14.
 

Hello circuitryboy, nice to get a pulse from you as well, given the ongoing circumstances. I don't have my original machine to hand but the parts list (original issue II) says 7445, and my issue VI (like Buzby's) also has a 7445 fitted in that position.

Re: I found it! A very sorry looking MK14.
 

The thumbnail doesn't enlarge well. Definitely not 7445.
If it's 7447 (!) it could explain everything.

Re: I found it! A very sorry looking MK14.
 

As you know Buz populated the PCB with bits from his actual MK14 so the presence of non-canon parts there would be a bit of a mystery. The issue VI PCB has the IC type numbers screen printed on it so it would have to be a conscious decision to fit a different IC. (I've just looked at a photo I took of my issue VI PCB before I populated it, and it definitely does say 7445).

Re: I found it! A very sorry looking MK14.
 

Having downloaded the image and viewed it in a viewer I would say it is a 7442. The number is printed so close to the bottom edge that the lower bar of the '2' is lost but the middle section of the 4th character has a distinct 's' curve which the first character (7) does not, so I think it is a '2'.

Re: I found it! A very sorry looking MK14.
 

Hello all,

Sorry, not had a chance today to even power it up.

Yes, the chip is a 7442. Not sure if that was original from SoC, or if I put it there. Some of the other chips are definatley not SoC, 'cos they've got RS part numbers !.

When the chips were in Micky there was my off-board display, with transistors and FND500's I think, so maybe the on-board chips were not so important then.

Regarding the display, is it the correct one, and have I wired it correctly ?.

The PCBs, both original and replica, have two empty holes at the left end of the display connector. ( I used these forty years ago to supply power to my off-board display. )

At least with this PCB I'm not debugging the wiring, just the chips.

Hopefully I'll get some time tomorrow.

Cheers,

Buzby

Re: I found it! A very sorry looking MK14.
 

That looks like the correct display and the connections on the display look correct.

Connections on the main pcb should be to the rightmost 16 pins, two on the left with no connection.

Re: I found it! A very sorry looking MK14.
 

This has been posted before but here it is again for convenience. If you have a DMM with a 'diode test' feature you can use that to quickly test out the display, it will provide just enough enough power to light each display segment (one at a time) dimly.

That display certainly 'looks right', very similar to my original MK14 display.

Re: I found it! A very sorry looking MK14.
 

I'd forgotten you'd had extra circuit/big display - 7442 would be fine for that.
The R->L scan is not 'linear'. Varying brightness and some segment h activity.
From the right, are digits 3 and 7 completely off?
Does swapping IC9&IC10 or IC14&15 change the display?

Re: I found it! A very sorry looking MK14.
 

According to the data sheet the 7442 has normal totem-pole outputs which have a short-circuit current of 18-55mA as apposed to the 7445's open collector outputs with 80mA sink capacity. This would imply that the digits will be limited to this level, which would cause the digits to vary in brightness depending on how many segments are lit. The 2 chips are pin compatible, but the design really needs the open collectot outputs of the 7445. That said, it should work to some degree and so the symptoms you are experiencing are probably something else. Do the chip legs just need a clean?

Re: I found it! A very sorry looking MK14.
 

I can see why, when you were driving extended / external display hardware, you changed that IC from an open collector type to a TTL-output type but for the 'classic' MK14 display setup you probably need to change it to a 7445 - note this was an 'original' 7445 and not 74xx45.

That still won't be the root cause of the problem though.

Re: I found it! A very sorry looking MK14.
 

Well, I've spent the last few hours checking the address decoding.

I removed ROMs, RAMs, and CPU. Put wires from a breadboard micro to drive A11-A8 and WDS/RDS.

Wrote code to wiggle pins cyclicly at a few hundred Hz, and prodded my scope where the decodes should come out. They all looked OK. ( I didn't actually check the addresses, just that the CE pins wiggled up and down nicely. )

So it looks like ROMs and/or ROMs are faulty. I've got four 2111s, and swapping them in all combinations made no difference at all. Using 'old' ROMs gives a different wrong display.

Tomorrow I'll rig up a RAM tester, and a ROM dumper.

I'm assuming those of you with VI boards just plugged all the bits in and it worked straight off ?.

Cheers,

Buzby

Re: I found it! A very sorry looking MK14.
 

Yes, that was certainly my experience, I put the parts in and it 'just worked'. My board and Tim's both came from a single batch of 5 that I had made by uploading the unmodified project files from Slothie's post early on in the 'vdu' thread to JLCPCB.

It's impossible to know whether whoever had your board made carried out any changes but why would they when they know, from reading the relevant thread here, that the boards work exactly as designed. If you are spectacularly unlucky you could have an etch fault or some other manufacturing fault on your PCB.

To read out the PROMs, if you have an EPROM programmer make up a jig which places the two PROMs side by side as D0-D3 and D4-D7, commons up the A0-A8 pins, _CE pins and power pins on both ICs, maps them onto the matching pins on the footprint of a 2716 EPROM and drop the 'jig' into your EPROM programmer. On the programmer, choose 2716 and then limit the address range to 0000-01FF and read the contents out of the '2716'.

One other issue we have been made aware of somewhere in these threads, and this is not just relevant to the issue VI - some versions of 74LS157 don't work in the basic MK14, apparently even in original machines, so if those RS brand ICs you mentioned happen to include the 74LS157s then maybe try a pair of other-brand 74LS157s. I didn't bring this up before because it seemed you were using parts from a once working MK14 but since you seem to have several non-original parts in there this may be worth looking at.

Originally all the chips supplied in MK14 kits were National Semiconductor (DM or MM prefix) but a few other brands were eventually substituted for some of the chips - notably Texas (SN prefix).

In my issue VI I have a mixture of DM and SN prefix types, the 74LS157s are SN (Texas) types in mine.

Elsewhere in these threads there are posted Arduino Uno sketches for (a) reading out the contents of a 512-byte ROM and (b) doing basic functional tests on 2111 RAMs. If you don't already have a specific way to test the PROMS and RAM I'll try to track those posts down for you.

Re: I found it! A very sorry looking MK14.
 

Looking again at the image buz posted earlier it looks as though the 157s have the original NS 'Wave' logo on them, so they are likely to be DM prefix - should be OK.

Re: I found it! A very sorry looking MK14.
 

I used Texas SN74LS157's in my original prototype which worked correctly.

Re: I found it! A very sorry looking MK14.
 

Arduino Uno based 2111 tester in post #403 of this thread:-

https://vintage-radio.net/forum/show...145663&page=21

I still haven't fathomed out Slothie's trick of posting a link to the exact post on the page.

Re: I found it! A very sorry looking MK14.
 

I think the trick is to click on the number at the top right of the post, opens into a new window with only that post, then copy the internet address of the post.

https://vintage-radio.net/forum/show...&postcount=403

Re: I found it! A very sorry looking MK14.
 

Right click on the number at the top right of the post and select "copy link address"

Re: I found it! A very sorry looking MK14.
 

Got that. :)

Re: I found it! A very sorry looking MK14.
 

Have you 'scoped the A3-A0 inputs to IC12 and all o/ps from IC13? If these look OK then the KYBD routine in either Monitor is executing correctly.

Re: I found it! A very sorry looking MK14.
 

It gets better, or worse, depending on your view !.

I didn't give much attention to the display driving chips, as all segs and digits were active at sometime during testing.

Today I received a TS200 chip tester, and it showed my 7442 is bad. ( It also showed my other 7442's are bad as well. How unlucky is that ?. )

So, without 100% confidence in the display mechanism, I am going to have to get another 7442 or 7445 before anymore testing.

Cheers,

Buzby

Re: I found it! A very sorry looking MK14.
 

How do you know if the TS200 is working correctly?

Re: I found it! A very sorry looking MK14.
 

That's a good question !.

I've got three 7442's, all test as BAD, and all behave same in Vicky.

I tested a few other of my stock of chips, simple gates mostly, and found a faulty 7404. Other 7404s test OK, so I know the TS200 can tell the difference in simple gates.

It does seem strange that three 7442s all tested bad, so I've not binned them yet !.

Cheers,

Buzby

Re: I found it! A very sorry looking MK14.
 

Issue...VIcky. I think I see what you did there.

My partner has your habit of giving names to gadgets. I have to keep myself constantly updated otherwise I have no idea who or what she's talking about when she asks where 'Serena' is.

I too find it suspicious that your chip tester is failing your entire stock of 7442s and to be honest the job of that chip is to connect the cathode of each display cell to 0V in turn, which it did seem to be doing (the displays were being scanned).

At the same time, the segment-driving hardware should be outputting a pattern specific to each display cell as the 7442 / 7445 steps through them, and that's the bit which does not appear to be happening.

You were always going to have to fit a 7445 eventually, so you might as well get that over with.