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Old 27th May 2020, 9:14 pm   #421
SiriusHardware
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Default Re: MK14 schematic revisions

Quote:
Is that a real one? Did they arrive?
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.
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Old 27th May 2020, 9:20 pm   #422
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Default 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.
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Old 27th May 2020, 9:25 pm   #423
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I am going to pimp this with some nice RED Cambion 40pin turned sockets I have as well...
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Old 27th May 2020, 10:23 pm   #424
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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.
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Old 27th May 2020, 10:36 pm   #425
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Default 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.
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Old 27th May 2020, 11:08 pm   #426
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I might rework the switch footprint to accept pushbuttons like the reset button which is what the original rev 5 boards had...
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.
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Old 28th May 2020, 9:50 am   #427
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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.
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Old 28th May 2020, 9:07 pm   #428
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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.
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Old 28th May 2020, 10:30 pm   #429
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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!
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Old 28th May 2020, 11:34 pm   #430
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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?
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Old 29th May 2020, 12:23 am   #431
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-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'.

Did you have a particular heatsink in mind for these?
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.
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Old 29th May 2020, 9:28 am   #432
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Default 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?
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Old 29th May 2020, 10:28 am   #433
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Default Re: MK14 schematic revisions

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Originally Posted by SiriusHardware View Post
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?
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.
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Old 29th May 2020, 5:36 pm   #434
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Default 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.

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Old 29th May 2020, 5:42 pm   #435
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Default 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.
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Old 29th May 2020, 6:00 pm   #436
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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.
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!
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Old 29th May 2020, 6:03 pm   #437
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Default Re: MK14 schematic revisions

Thanks - just pulled out a NOS LM340T5 and it has 7805 in small writing on the bottom anyway!
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Old 29th May 2020, 6:13 pm   #438
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Default 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.
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Old 29th May 2020, 6:20 pm   #439
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Default Re: MK14 schematic revisions

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Putting it on the magazines was purely 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.
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?
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Old 29th May 2020, 6:27 pm   #440
Slothie
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Default 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.
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