Building a replacement MTX PSU

About original Memotech hardware.
gunrock
Posts: 13
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
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1024MAK
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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
gunrock
Posts: 13
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? :?:
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1024MAK
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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
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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 ?
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1024MAK
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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
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