Science Of Cambridge MK14 Replica build

[rubber_jonnie]

OK, so I've been building a cassette interface for the MK 14 :)

IMG20241013151157.jpg

And on testing it flashes the output LED as expected:)

IMG20241013152033.jpg

The output on pin 30 of the MK 14 is correct and I am seeing this:

IMG20241013151234.jpg

Which does match what the docs show for pin 30, otherwise called Flag 0:

IMG20241013151255.jpg

Trouble is the schematic was putting me off a bit due to a terminology thing:

Screenshot 2024-10-13 at 18.15.20.png

Tape out is the the left side of the schematic, and pin 3 is the output according to the schematic, but labelled 'Tape input' on the documentation. What it actually means is not the input from the tape, but the input to the tape!!! That is, pin 3 is the output from the MK14 to the cassette deck, so the 'Mic' connection. I had it the other way round, but now I've figured it out I am actually getting an output on pin 3 when I execute the save code :)

However, it is super quiet, which suggests there may be a wiring issue, so I need to go back to the PCB connections and make sure it's all wired up correctly. Progress though, which is always good.

[stephen_usher]

:D :dualthumbup:

[rubber_jonnie]

OK then, I tested it with my period correct Ferguson data recorder and...

Well it seems to work!!!

I can hear the tones clearly through the monitor speaker and I can easily adjust the levels :)

IMG20241014100126.jpg

You can see from the picture that the level slider still has some movement on it but the VU meter is at about 80% :)

Next I need to enter a program, save it and then try to reload!

And of course I've now realised why I couldn't see the signal on my scope. It doesn't read below 50mV so the signal was just too low as it's down around 1.5mV, which my multimeter can actually read.

[rubber_jonnie]

So pretty happy, I've been able to save to my 1985 data recorder :)

Also, on playback I get this signal at the input to the board, pin 3:

IMG20241014132255.jpg

Which matches the documentation :)

However it doesn't seem to be getting through to the transistor so I need to check my wiring.

[rubber_jonnie]

OK, so typical of me, the transistor was 180 degrees out as a result of using an alternative, I'm kicking myself as it's not the first time I've got that wrong...

The signal is getting way further now though, so I need to check my wiring at at the IC now. Slowly creeping closer!

[rubber_jonnie]

OK, so I think I have some wiring issues on the cassette input side so time for a good hard look at my work.

At least the save side worked so I have something to play back.

[rubber_jonnie]

OK, so I did a rewire of the cassette in side of my adapter and it looks better:

IMG20241015223728.jpg

Though it still doesn't let me load anything and I've beeped and buzzed the dam thing out and circuit traced too. Bit of a head scratcher. I did swap the IC to the save side and that seems to work, so I don't think thats an issue.

I don't seem to have any signal at the base of the transistor though, even though I do see a signal up to and including C2, but not after C2...

I also got to the bottom of the unregulated supply on pins 1-3 of the top edge connector. This is meant to allow for power IN via the edge connector, so I've hijacked pin 1 on the bottom (None of the bottom pins are connected to anything at all) and fed that regulated 5v:

IMG20241016130412.jpg

That way I have 5v regulated for any future projects.

[rubber_jonnie]

Well, despite suffering a cold, I managed to drag myself to the workbench and have yet another go at the 'load' circuit on the cassette interface.

Thankfully the 'save' circuit, which is less complex' worked first time so I have some data to play back for getting the levels right, even if not to load.

I guess 3rd time lucky was the charm as once I'd done my 3rd rewire of the loading circuit and fully verified by buzzing it out and comparing to the schematic:

IMG20241017190212.jpg

I finally got it right and it appears to work!!

This is the signal now seen at the edge connector of the MK14:

IMG20241017190132.jpg

And it matches perfectly the trace shown in the build docs:

Screenshot 2024-10-17 at 23.35.29.png

Please excuse fingers!!!

So next up (I was feeling too crappy after my repair effort to do it) will be to enter a program, save it, reload and see if it works. That's likely to be a tomorrow job now though.

[rubber_jonnie]

Feeling bolstered by the success of my 'point to point' wired cassette interface I bought a kit to do a 'hand made' PCB that I'll etch myself.

I've used the PCB shown here: Replica cassette interface as a reference and scaled the image to the correct size and stuck it to some copper clad (Single sided) and centre punched all the through holes ready for drilling:

IMG20241028085610.jpg

I'll drill the holes then cut the PCB to size and draw on the traces with an etch resist pen, then we'll see how it goes. I don't mind if it isn't perfect so long as it works. I've always wanted to etch my own PCB and this seems like the perfect project for it.

I may then use this as a basis to learn a bit of KiCad because it's such a simple circuit.

[rubber_jonnie]

Well I bit the bullet at lunchtime and went for etching!!!

Here it is prior to going for a dip:

IMG20241029090530.jpg

And after about 1 hour:

IMG20241029121125.jpg

And my test piece after the same time:

IMG20241029121130.jpg

And after about an hour and three quarters:

IMG20241029123621.jpg

and cleaned up:

IMG20241029124034.jpg

Is it perfect? No, a couple of corners didn't survive but were easily repaired, but I think it isn't a bad first effort and I feel it fits well with the MK14 aesthetic.

Next up I need to drill the holes, which I was going to do first, but then changed my mind as I was worried about the etch going too far into the holes.

Fingers crossed it'll work first go, but we'll see :)