Building a replacement MTX PSU

About original Memotech hardware.
User avatar
gunrock
Posts: 28
Joined: 28 Oct 2020 21:17

Re: Building a replacement MTX PSU

Post by gunrock »

Hi Mark!

I have sourced most of the parts and most have been delivered; I had to go with another transformer as it was unavailble and besides, rapidonline's postal costs to Denmark were eye-watering.

The trickiest part has been finding 6 core tinned copper cable sold by the metre (I've ordered several metres of the stranded untinned type, unfortunately). I'm not an electronics hobbyist (don't get me wrong, time-permitting I probably would be), thus having bins of M3 nuts and machine screws or diodes, for example, is fine but working out quite expensive, in my case.

I haven't added up what I've spent (would make my toes curl, most likely!) but I'm about to go through your BOM and my deliveries to check if I have them and double-check they are what is required. I'll let you know if I have queries.

Thanks for replying,
Steve
User avatar
1024MAK
Posts: 648
Joined: 24 Dec 2012 03:01
Location: Looking forward to summer, in Somerset, UK

Re: Building a replacement MTX PSU

Post by 1024MAK »

Hi Steve

Keep in mind that there is some flexibility, so for example, you don’t have to use the exact same transformer or the exact same cable. As long as the specifications of any alternatives are sufficiently similar. So the primary voltage of the transformer should be suitable for your mains electricity supply, the secondary voltage should be the same as the transformer I specified, and the power (VA) rating should be about the same, or greater than the one that I used.

It’s similar with the cable. I used tinned copper cable because it was available at the time. But stranded plain (untinned) cable would also be okay.

Anyway, I’m happy to answer any questions.

Sorry I did not reply earlier, the last couple of weeks have been rather busy, so I have not been able to check out every forum that I’m on.

Good luck with your build ;)

Mark
User avatar
gunrock
Posts: 28
Joined: 28 Oct 2020 21:17

Re: Building a replacement MTX PSU

Post by gunrock »

Actually, Mark, regarding the transformer, I managed to source that was 40VA which is obviously less than the 50VA you specified, but it was all i could find and I saw that earlier in this thread, you said the requirement was for 40VA, but 50VA was all you could source.

Is that okay or is it a bit too close to spec? :?:
User avatar
1024MAK
Posts: 648
Joined: 24 Dec 2012 03:01
Location: Looking forward to summer, in Somerset, UK

Re: Building a replacement MTX PSU

Post by 1024MAK »

A 40VA transformer is fine.

Different manufacturers make different ‘standard’ transformers and electronic suppliers stock different ranges from different manufacturers...

It was a while ago when I bought the parts for the three that I made...

Mark
Martin A
Posts: 483
Joined: 09 Nov 2013 21:03

Re: Building a replacement MTX PSU

Post by Martin A »

1024MAK wrote: 16 Nov 2020 20:15 A 40VA transformer is fine.
The original MTX PSU is 22.5VA ?
User avatar
1024MAK
Posts: 648
Joined: 24 Dec 2012 03:01
Location: Looking forward to summer, in Somerset, UK

Re: Building a replacement MTX PSU

Post by 1024MAK »

Martin A wrote: 17 Nov 2020 20:56 The original MTX PSU is 22.5VA ?
Yep, but the 1A AC rating does not translate to 1A DC. I think Memotech were a bit too close to the wire by using such a transformer. Fine with an unexpanded machine, but maybe not so good if expansions need more power. So with my design, the next size down in terms of transformers that were available, were 25VA and I wanted a bit more headroom. Hence I went for the next size up, which was 50VA. A 40VA transformer still gives plenty of headroom and allows the use of lots of expansions :D

Mark
User avatar
gunrock
Posts: 28
Joined: 28 Oct 2020 21:17

Re: Building a replacement MTX PSU

Post by gunrock »

And we are off to the races!

Here's my (slow) progress, so far.

I have started making holes in the case. I tried a different approach than scoring or scratching the shapes into the plastic. I used a sheet of A4 sticky label, cut down as required and Isketched out the holes with pencil. I did this because as you can see from the pencil marks, I make a fair few mistakes!
20210106_224734.jpg
20210106_224734.jpg (292.42 KiB) Viewed 504 times
The result is pretty good. I still need to make holes for the output cable, the fuse holder and LED, plus holes for the AC/DC circuit and step-up transformer, but it was cold in the garage (we have light snow in Denmark) and I'd had enough!
20210106_224659.jpg
20210106_224659.jpg (469.67 KiB) Viewed 504 times
Next, I tried my hand at building the circuit board, my first strip board build ever! I think it matches the images you posted, Mark and my evolving ability to read and understand electronic schematics. Would you be kind enough to take a look at the images?
20210106_224837.jpg
20210106_224837.jpg (400.58 KiB) Viewed 504 times
I have tested continuity on each strip and between adjacent strips to make sure my soldering is clean (I did visually inspect but my eyes aren't what they once were, even with a magnifying glass!).and that the drilled continuity breaks do actually break continuity.

Is there any way without a bench supply, to test the circuit by adding power? I'd need an AC transformer that provides low voltage and amperage AC, I guess?
User avatar
1024MAK
Posts: 648
Joined: 24 Dec 2012 03:01
Location: Looking forward to summer, in Somerset, UK

Re: Building a replacement MTX PSU

Post by 1024MAK »

Looking good :D

One thing, where you have cut the tracks, just use a sharp craft or ‘box cutter’ knife to remove the ‘whisker’ of copper track that is left at the edge of where you cut it and the edge of the adjacent track.

For testing, use either a AC or DC supply with an output of between 5V (AC or DC) and 10V (AC) or 12V (DC).

If using AC, apply between the green wire and the black wire. The output between the red wire (positive) and the black wire (negative) should be DC. Connect a test load and recheck the voltage. Then power down. Without a test load, the capacitors will retain a charge for a fair amount of time.

Now do the same, but apply the input between the white wire and the black wire. repeat the test as above.

If using DC, follows the procedure above, but try both polarities. It’s best to start with positive to the black wire and negative to the green wire. There should be NO output voltage. Do the same with the white wire (negative). This test is to confirm that the diodes are in fact blocking the ‘incorrect’ polarity.

Now do the same but with positive to the green wire, and negative to the black wire. Now you should have an output voltage between the red wire and the black wire. Repeat unsung the white wire.

The test load can be a low power 12V lamp (say 3W or less) or a 680Ω ¼W resistor. Or use the 1.2kΩ resistor that you are going to be using with the LED as the power on indicator.

Mark
User avatar
gunrock
Posts: 28
Joined: 28 Oct 2020 21:17

Re: Building a replacement MTX PSU

Post by gunrock »

Hi Mark,

Thank you for your replies and your time, much appreciated.

So, I have done as much testing of the AC/DC transformer as I can, but I only have a 5v DC power supply (I've looked for a higher rated one to cannibalise, but came up short) and lockdown here in Denmark is making life safer, but more difficult to source stuff.

The tests were a bit mixed: checking that the diodes are filtering out the 'wrong' polarity resulted in 0.08V or 0.07V when testing (with a 1A fuse in place), so almost nothing but not quite. I retried this a few times and got the same results. Is this within the bounds of expected behaviour?

Next, the 'correct' polarity gave me around 4.7V/4.8V or so, which seemed OK to me. Do you concur?

And lastly, the load tests were an issue as I only had 12v 5w bulbs and my 5V input was unlikely to light them. I did however, have some DC 6V led bulbs (for dolls houses, model railways etc) and managed to get one to light up (pretty dull but lit) and without the power connected, it lit it very dull for quite some time. I used a 1Kohm resistor to drain it. Do you think this sufficient testing? I just had a thought this moment, actually, and that's to use my power supply and the LM2577 to step the voltage :geek: and test again.

Anyway, I decided to brave the near subzero garage (and my wife's dwindling patience) to finish the drilling of the case. Looking quite nice, even if I say so myself :lol:
20210107_231701.jpg
20210107_231701.jpg (838.35 KiB) Viewed 491 times
Tomorrow, retesting with better power supply, then all the tricky wiring and many more questions, I suspect.

Thanks
Steve
User avatar
1024MAK
Posts: 648
Joined: 24 Dec 2012 03:01
Location: Looking forward to summer, in Somerset, UK

Re: Building a replacement MTX PSU

Post by 1024MAK »

gunrock wrote: 08 Jan 2021 00:56 The tests were a bit mixed: checking that the diodes are filtering out the 'wrong' polarity resulted in 0.08V or 0.07V when testing (with a 1A fuse in place), so almost nothing but not quite. I retried this a few times and got the same results. Is this within the bounds of expected behaviour?
You may well get a very small voltage, plus multimeters are not perfect. So I don’t see a problem with these results. The test was primarily a quick check that the diodes are working and fitted correctly, rather than a proper reverse leakage test.
gunrock wrote: 08 Jan 2021 00:56Next, the 'correct' polarity gave me around 4.7V/4.8V or so, which seemed OK to me. Do you concur?
Yes, that’s fine.
gunrock wrote: 08 Jan 2021 00:56And lastly, the load tests were an issue as I only had 12v 5w bulbs and my 5V input was unlikely to light them. I did however, have some DC 6V led bulbs (for dolls houses, model railways etc) and managed to get one to light up (pretty dull but lit) and without the power connected, it lit it very dull for quite some time. I used a 1Kohm resistor to drain it. Do you think this sufficient testing?
Yes, this is just basic functionality testing, and what you have done is fine.
gunrock wrote: 08 Jan 2021 00:56Anyway, I decided to brave the near subzero garage (and my wife's dwindling patience) to finish the drilling of the case. Looking quite nice, even if I say so myself :lol:
Definitely looking good 8-)

Mark
Post Reply