Building a replacement MTX PSU

About original Memotech hardware.
wyerd
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Joined: 13 May 2013 23:16
Location: Upstate New York. USA.

Re: Building a replacement MTX PSU

Post by wyerd » 01 Jun 2015 02:40

Mark, I'm having problems locating a suitable 110v-120v transformer with the dual 7.5v output. The best place I've found is Digikey.com, but not the required output. Here's their list of transformers http://www.digikey.com/product-search/e ... ers/786735

Is there a suitable replacement I could use and would it require further circuit changes?

Thanks

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1024MAK
Posts: 569
Joined: 24 Dec 2012 03:01
Location: Looking forward to summer, in Somerset, UK

Re: Building a replacement MTX PSU

Post by 1024MAK » 01 Jun 2015 11:28

wyerd wrote:Mark, I'm having problems locating a suitable 110v-120v transformer with the dual 7.5v output. The best place I've found is Digikey.com, but not the required output. Here's their list of transformers http://www.digikey.com/product-search/e ... ers/786735

Is there a suitable replacement I could use and would it require further circuit changes?
Any one from this list should be okay. This one (Triad Magnetics VPS16-2700, Digi-Key Part Number 237-1261-ND) is what I recommend you get. Wire the primary 115V windings in parallel (see the datasheet from the Digi-Key web site) and the secondary windings in series, but use one of the connections used to join the two secondary windings as the centre tap.

In my original design, I selected a 50VA type because the requirement was for a 40VA, but at the time the 50VA size was the nearest that I could find at a sensible price. So a 43VA type will be okay :-).

The secondary voltage of dual 7.5V windings was selected in order to provide enough voltage for the 5V regulation circuit in the MTX, but without providing too much voltage (as any extra voltage is dissipated as heat). However, a secondary voltage of dual 8V windings is okay. The half a volt difference will not have any significant effect. Depending on the actual voltage of your mains supply, a secondary voltage of 7V may be okay, but it leaves rather too little margin if the mains voltage dips for any reason.

Don't forget, when running on 115V, the mains fuse, F1 needs to be a 2 Amp Time-Delay (Slow Blow) type. And if you use a neon illuminated rocker switch, you need a 115V type ;-). Apart from this, no further circuit changes are required.

Everyone, please note: I have just noticed a mistake on the schematic, there are two F2's and no F1! :oops:. F1 is the left hand fuse, the mains voltage fuse. I will also add the wire size to the next version of the schematic. All mains voltage wire is the stranded equipment wire 16/0.2m. The connection from the secondary windings to the 5A fuse, F2, is the stranded equipment wire 24/0.2mm. The secondary side wiring going directly to the MTX 6 pin DIN plug is the output cable wires. The remaining secondary wiring (to the LED, the strip board circuit and the LM2577 step-up DC-DC converter module is all stranded equipment wire 7/0.2mm.

I hope this helps :mrgreen:

Mark

wyerd
Posts: 88
Joined: 13 May 2013 23:16
Location: Upstate New York. USA.

Re: Building a replacement MTX PSU

Post by wyerd » 01 Jun 2015 16:20

Thank you Mark. It helps a lot!

What I'll do is modify your spreadsheet so that it lists the parts from US stockists to help anyone who might need the information in future. If that's OK with you of course :-)

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1024MAK
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Location: Looking forward to summer, in Somerset, UK

Re: Building a replacement MTX PSU

Post by 1024MAK » 01 Jun 2015 19:13

wyerd wrote:What I'll do is modify your spreadsheet so that it lists the parts from US stockists to help anyone who might need the information in future. If that's OK with you of course :-)
Please do ;)

Mark

wyerd
Posts: 88
Joined: 13 May 2013 23:16
Location: Upstate New York. USA.

Re: Building a replacement MTX PSU

Post by wyerd » 02 Jun 2015 17:48

1024MAK wrote:Construction Details
Diodes:
Two 1N4001 or 1N4002 or a higher voltage rating.
Capacitors:
The small blue capacitor is a 100nF ceramic multilayer type, 5mm spacing.
The large capacitor is a 2200uF 16V electrolytic.
Fuse:
PCB fuse clips, 5mm pins, suitable for a 20mm type fuse.
1A "Fast" (F) or "Quick Blow" (QB) 20mm x 5mm glass fuse

More tomorrow (sorry it's slow, I'm a bit time poor...)

Mark
For F3, the BoM describes it as a slow blow. Can you please confirm Mark? Thanks.

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1024MAK
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Location: Looking forward to summer, in Somerset, UK

Re: Building a replacement MTX PSU

Post by 1024MAK » 03 Jun 2015 01:58

Whoops! :o :shock: :oops: :roll: :lol: The parts list / BOM is wrong.
Fuse 3 (F3) should be a 1A "Fast" (F) or "Quick Blow" (QB) 20mm x 5mm glass fuse, NOT a Time Delay/Slow Blow. I must have still been thinking about that F1 :x (I was putting the spreadsheet together as I was checking parts on the Rapid web site, I searched for 1A 20mm fuses as including time or fast or slow blow will not show all valid types. One set of terms is used mainly in the US and the other mainly in Europe (it depends on which certificating organisation is used). But all these fuse types are available in the UK...

Sorry about that. Please flag up anything else that does not make sense :mrgreen:

I will amend the parts list / BOM later this week.

Mark

wyerd
Posts: 88
Joined: 13 May 2013 23:16
Location: Upstate New York. USA.

Re: Building a replacement MTX PSU

Post by wyerd » 03 Jun 2015 04:17

No problem!

Can you check over my shopping cart when you have a minute as I want to make sure that I've ordered the correct items and with it connecting to mains voltage I don't want it to go bang! I wish there was a Rapid over here as I've had to get bits the parts from different places, but the bulk is from Mouser.

http://www.mouser.com/ProjectManager/Pr ... 9517318b8e

Getting the cables is difficult as they only deal in bulk, especially the 6 core. I'm going to have to order it from China via eBay.

I might need help wiring up the transformer once everything's arrived as it talks about series and parallel connections. The last time I did anything like that was over 35 years ago back at school!!

Thanks,
David.

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1024MAK
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Re: Building a replacement MTX PSU

Post by 1024MAK » 03 Jun 2015 11:15

Sure thing, don't have time at the moment, but I will look through it later.

Funny that you talk about having to get parts from different suppliers. I often use Rapid because they have a good general range and for the most part, their prices are similar or cheaper than Farnell or RS (the two big suppliers in the UK), but sometimes have to use Farnell, CPC or RS for some parts like IC's (chips) for example. There are also some smaller suppliers I sometimes use if the size of the order is not big.

Buying cable and wire is often a problem. Some suppliers offer cut-cable by mail order. Also we have Maplin which has shops that sell cut-cable. Maplin started off as a mail order electronic parts supplier for hobbyists. But has since expanded by opening shops in most cities in the UK. They now mostly sell electronic products, but still sell a limited range of components (a bit like Tandy/Radio Shack, we lost Tandy in the UK years ago). Maplin are expensive for small electronic parts, but it is very handy to be able to go to one of their stores to buy project cases and cut-cable :D

Because hobby electronics is my other hobby, I actually have rather too many parts including some reels of various wire and cable. At current rate of use, most of the reels look like lasting for a large part of my lifetime :lol:

Shout out if you do need help with the transformer ;-)

Mark

wyerd
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Joined: 13 May 2013 23:16
Location: Upstate New York. USA.

Re: Building a replacement MTX PSU

Post by wyerd » 04 Jun 2015 22:39

Thank you.

How does the cable that is connected to the 6 pin DIN socket exit the box? I expect through a hole, but what strain relief have you used? Can you post a close-up shot?

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1024MAK
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Re: Building a replacement MTX PSU

Post by 1024MAK » 04 Jun 2015 23:57

I'm on a train at the moment. I will post a picture on Saturday. Can't do much tomorrow (Friday) as I have to go to London for a meeting.
It is best to use a cable gland, but at the moment I have just drilled a hole, covered the cable with heat shrink tubing and used a cable tie either side of the the wall of the case.
The PVC cable should be covered, as PVC and ABS plastic (that the case is made from) react if in close contact over time.

If a metal box is used, a cable gland is a must.

Mark

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