Building a replacement MTX PSU

About original Memotech hardware.
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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 » 05 Jun 2015 00:48

I'd thought you'd use someting clever :D ;)

Don't worry about the pic. I've been looking at strain reliefs and I needed one anyway as I'm going to be converting my Beeb's PSU to 120v and I'll be replacing the power cord with a moulded US one. I've ordered a couple as they're only 10 cents each but I'll need to order the tool from eBay.

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

Re: Building a replacement MTX PSU

Post by wyerd » 05 Jun 2015 00:50

I managed to find the same case you used.

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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 » 05 Jun 2015 01:12

You can see the inside of the cable on the left hand side at the bottom of the case, under the stripboard (with the large electrolytic capacitor and fuse) in the pictures of this post :mrgreen:

Coming up over the weekend, I have some pictures of how I cut the hole for the mains switch and of the IEC mains connector.

Mark

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1024MAK
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Re: Building a replacement MTX PSU

Post by 1024MAK » 06 Jun 2015 13:35

wyerd wrote: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
Hi David. I've looked through your list. The fuse holder you have selected is for the wrong size fuse. I suggest this one:
Mouser Part No: 576-03455LS1HX020
Manufacturer Part No: 03455LS1HX020
http://uk.mouser.com/ProductDetail/Litt ... lY4U2gc%3d

The 100nF capacitor (594-K104Z15Y5VF5TL2) is fine from an electrical standpoint, but has a lead pitch of 2.5mm. But you should be able to bend the leads to fit.

Everything else is fine :D

Mark

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

Re: Building a replacement MTX PSU

Post by wyerd » 06 Jun 2015 15:54

Many thanks Mark for checking it over and for letting me know the errors.

I look forward to seeing the latest pics you promised!

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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 » 08 Jun 2015 00:45

So, next: preparing the plastic case.

Tools I used: fixed metal rule, tool with a sharp point (for marking), battery drill, metal HSS drill bits or wood drill bits (they have to be sharp), needle files, various shape and sizes, somewhere to work where you don't get told off for making a hell of a mess :mrgreen:
Needle File Set.JPG
Needle File Set.JPG (160.28 KiB) Viewed 2303 times
As with all these, measure twice! Mark once. Very carefully line up the drill (set to a slow speed), double check that you are drilling INSIDE the marked line, and carefully drill each hole:
Cutting the hole for the IEC mains conn #1.JPG
Cutting the hole for the IEC mains conn #1.JPG (248.82 KiB) Viewed 2303 times
Once you have an outline, angle the drill slightly so that you "join the dots" :D Then carefully file the excess plastic away until you have a neat hole that is just large enough for the connector:
Cutting the hole for the IEC mains conn #2.JPG
Cutting the hole for the IEC mains conn #2.JPG (138.46 KiB) Viewed 2303 times
Of course the last picture here is a test fit!

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1024MAK
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Re: Building a replacement MTX PSU

Post by 1024MAK » 08 Jun 2015 00:48

The mains switch is done the same way, but you have to be a bit more careful, as this is a snap in type and too big a hole would be a disaster!
Cutting the hole for the mains switch #1.JPG
Cutting the hole for the mains switch #1.JPG (140.63 KiB) Viewed 2303 times
Cutting the hole for the mains switch #2.JPG
Cutting the hole for the mains switch #2.JPG (206.78 KiB) Viewed 2303 times
More tomorrow 8-)
Mark

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1024MAK
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Re: Building a replacement MTX PSU

Post by 1024MAK » 10 Jun 2015 01:08

Oops :oops: Got distracted with this
I will post some more pictures before Saturday ...

Mark

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1024MAK
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Re: Building a replacement MTX PSU

Post by 1024MAK » 12 Jun 2015 14:07

Before fitting items to the case, it is far easier to connect some of the wiring first.
Strip about 5 to 6mm of the insulation, gently twist the wire strands together using your fingers, then using round nose pliers form a "U" loop. This will give the soldered connection added mechanical strength.
Pass the stripped wire through the hole on the terminal of the item and if needed, flatten the "U" loop with smooth flat nosed pliers. Cut off any excess wire strands,then solder.
Once the joint is cool, inspect it. If happy, cut a suitable length of a suitable size heatshrink tubing. Slide over the wire and terminal and using a hot air gun (I use a gas soldering iron with the hot air "tip") shrink it down. When shrinking it, keep slowly moving the heat source, as if you try to just do one section at a time, the heatshrink will wrinkle and/or burn.

I also use heatshrink tubing to keep the wires together in tidy bundles.
Soldering the mains switch.JPG
Soldering the mains switch.JPG (277.67 KiB) Viewed 2283 times
Soldering the IEC connector.JPG
Soldering the IEC connector.JPG (141.85 KiB) Viewed 2281 times
IEC conn and switch done.JPG
IEC conn and switch done.JPG (142.88 KiB) Viewed 2281 times
Mark

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1024MAK
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Re: Building a replacement MTX PSU

Post by 1024MAK » 12 Jun 2015 17:18

Don't forget all the other holes. Here I show how to do the holes for the transformer fixing machine screws. Once you have drilled the holes, use a countersink drill bit (or similar). Go careful as it is very easy to cut too much, or too little and use a slow speed. Every so often try putting the machine screw in the hole to check if it fits nicely. The idea being that the head of the screw is flush, or almost flush with the case.

With most fuseholders, the hole they need is NOT round. Instead it is round-ish with a flat side. If you just drill a round hole that it will fit in, it will turn in the hole :-(. So drill an undersize hole, then get to love filing :roll: , while trying the fuse holder every so often until it fits through...
Transformer screw holes & fuseholder hole.JPG
Transformer screw holes & fuseholder hole.JPG (267.83 KiB) Viewed 2279 times
Don't forget to pass the wires through the fuseholder insulation boot and the plastic nut before connecting both ends of the wires :P
5A fuseholder.JPG
5A fuseholder.JPG (133.41 KiB) Viewed 2279 times
LED and resistor.JPG
LED and resistor.JPG (201.33 KiB) Viewed 2279 times
You can see the various stages of making the LED and the associated resistor connections. Using small round nose pliers, form the leads into a small coil. Note the resistor leads are far easier to make into a coil. This makes the connection more robust and easier to solder. Cut off any lead that sticks out the end of the coil, then over with heatshrink sleeving.

Mark

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