Idiot building a MK14 thread

[ScottishColin]

This is a good point. I need to hunt for a 5V power supply now.

Colin.

[SiriusHardware]

Actually not a 5V power supply, but ideally around 7V to 7.5V regulated and capable of at least 1 Amp. The onboard regulator will bring the voltage down to 5.0V but it needs an input voltage at least 2V or more higher than its output voltage.

You can use a higher input voltage than 7V but the higher you go, the hotter the regulator will run.

[ScottishColin]

Got it thanks.

Could you explain why it needs to be 2v more than the required supply?

Colin.

[SiriusHardware]

It's just the way those 78xx regulators work - in order to regulate properly, the input voltage needs to be at least ~2V higher than the voltage which is expected to come out of the regulator.

As to why ideally 7V or 7.5V and not 9V or 12V, the greater the difference between the input voltage and the output voltage, the greater the heat dissipated by the regulator itself.

You can probably get away with 9V for test purposes, but I seem to remember Slothie posted some temperature figures for the regulator when running on 7V / running on 9V - the difference was pretty significant.

Speaking of heat, when you do get the machine up and running you may be alarmed to discover that the PROMs run very hot. That is actually normal for those sorts of PROMs and for certain other devices like the ULAs used in Sinclair computers so don't worry about it.

If you find anything else running that hot, close the door and dial 999.

[Phil__G]

You could use a 5v supply but connect it to the 7805 output rather than input. All my stuff is 5v, I seldom fit 5v regulators, in fact on the 876 PIC14 the 7805 position is used for the serial input and power comes from the CH340G serial module.
But ifs to be authentic, you need the regulator and a 7.5v (ish) supply

[SiriusHardware]

For power for mine, I use an old Uniross "UNI 1200R" linear multi voltage 1.2A PSU (Ex Maplin) which has selectable output voltages of 1.5V / 3V / 4.5V / 6.0V / 7.5V / 9.0V / 12.0V. A search on that model number now may find you one or two for sale.

Maplin later replaced this linear PSU model with a very similar looking switched-mode model - for anything with old, fragile digital ICs a linear supply is to be preferred but just use whatever you can realistically find.

[SiriusHardware]

You could use a 5v supply but connect it to the 7805 output rather than input

Just to clarify, I think Phil means to use an offboard 5.0V supply instead of the onboard regulator - if left fitted, the 7805 may object to having 5V back-fed into its output.

If you do use an offboard PSU, it must be EXACTLY 5.0V regulated, not something close and certainly not 4.5V or 6.0V and its 5V +ve side would need to go to where the right-hand (output) pin of the regulator should go. It would need to be able to supply 1.0A minimum.

[Slothie]

Originally Posted by SiriusHardware

You can probably get away with 9V for test purposes, but I seem to remember Slothie posted some temperature figures for the regulator when running on 7V / running on 9V - the difference was pretty significant.

It, of course, dependent on the heat sink used, and my tests were done using the type of heat sink in the document I wrote, which from memory was a 21deg/W and should be considered the maximum heat rating usable. Its always possible to use a larger heatsink remotely and connect the regulator by wires - I think Sirius used a spacer to raise a larger heat sink up so that it didn't interfere with the edge connector and just extended the regulator leads.

Providing power directly from a stabilized 5v power supply is a perfectly good idea but I personally wouldn't risk a USB charger as their output is generally not stable unless it is a really good unit, and it can be hard to determine how good they are non destructively.

[SiriusHardware]

from memory was a 21deg/W and should be considered the maximum heat rating usable.

For maximum, substitute minimum, I think?

I sent Colin the same type of heatsink and hardware as mine has fitted. With the spacer provided, the uncut leads of the regulator only just reach into the holes provided on the PCB.

The main complication with USB supplies is that a compliant one will not supply more than the standard USB 0.5A unless requested to by the connected unit, so even if the USB PSU selected can in theory supply 2.1A it may limit its output to 0.5A unless it receives some negotiation from the connected device.

That said, there are probably all sorts of devices with USB A outlets now which supply much higher current no questions asked - it's a bit of a minefield.

I would just use the onboard regulator to provide an extra safety layer between the outside world and the MK14 and then take a little bit more time to source a 7.5V regulated supply.

[ScottishColin]

Thanks for all of the above. I'll use my heat camera to see what it all looks like temperature-wise once it's built and working.

Colin.

[Mark1960]

Originally Posted by SiriusHardware

For maximum, substitute minimum, I think?

No, you want the minimum degrees per watt for the lowest temperature, so Slothie is recomending the maximum.

[SiriusHardware]

It still looks odd to me, must be the angle I'm looking at it from.

Maybe less confusing to say that Slothie specified the smallest heatsink he could hope to get away with, and a larger one would be better.

[Realtime]

Originally Posted by ScottishColin

Thanks for all of the above. I'll use my heat camera to see what it all looks like temperature-wise once it's built and working.

Colin.

Would be great to see a photo of the MK14 thermal profile (I assume the camera outputs to a monitor or built in display).
IR cameras are very cool (if you get my drift)

[ScottishColin]

I find them very interesting. I bought it to find out where I'm leaking heat from the house but it has other fun uses.

I suppose every MK14 will be different depending on the actual chips used which are likely to be different each time.

Colin.

Originally Posted by Realtime

Would be great to see a photo of the MK14 thermal profile (I assume the camera outputs to a monitor or built in display).
IR cameras are very cool (if you get my drift)

[Slothie]

Originally Posted by SiriusHardware

For maximum, substitute minimum, I think?

I mean if its more than 21deg/W then it will overheat. A lower heat rating means it is better at dissipating heat.

[SiriusHardware]

Got it now. Me wrong, Mark right, as usual.

[ScottishColin]

Daft question coming up. I have switches for the keypad but the keypad on the PCB has more holes than the switches have legs ( see photo).

I'm assuming I'm ok to solder them in with the four legs that the switches have, but I'm wonderin what the extra holes are for on the PCB.

Ta.

Colin.

[ScottishColin]

And next question. I have the display but am wondering how others have managed to connect the pcb to the display?

Ta again.

Colin.

[SiriusHardware]

The extra contacts were to allow for the use of an alternative type of switch 'dome contacts', with a more original type of overlay laid over them.

Just fit the tact switches into the holes they obviously fit in.

For the display you have a few different options, but they were originally fitted with a short strip of semi-flexible ribbon cable with 0.1" spacing between the conductors.

A commercial solution would be this, which is what I used on my issue VI display.

https://uk.farnell.com/te-connectivi...y-1/dp/1310365

They are 20-way so you would have to cut two 'ways' off one edge to make an 18-way.

They are made by 'TE Connectivity', it might be worth doing a search for that manufacturer's part number FSN-21A-20 to see if anyone else is selling them with more favourable carriage costs.

[ScottishColin]

Thanks Sirius. Switches soldered and on the hunt for the cable.

Next question.

I have a 4mhz crystal. What's the pros and cons of fitting that vs 4.33mhz?

I have a Realview which I'd like to use but I'd also like to use it without that if possible.

Colin.