Building a replacement MTX PSU

[wyerd]

Great pics Mark. My bits and pieces have started to arrive so I'm itching to start! Just need to find some hook up wire of various colours which is harder than you'd think over here.

[wyerd]

Mark,

Can you help me out with the transformer please as I'm not sure how to wire it up? Also, how did you mount it in the box?

Thank you,
David

[1024MAK]

Details in next post below...

But first, do you have a multimeter?

Measure the resistance (Ohms, 200 ohm range) of each of the primary windings.
So the first winding is between the connections labelled 1 and 2. The second winding is between the connections labelled 5 and 6. The resistance should be about the same.

Mark

[1024MAK]

Can you help me out with the transformer please as I'm not sure how to wire it up? Also, how did you mount it in the box?

I used two M4 countersunk machine screws, two washers, and two nuts to fix the transformer to the case via two 4mm holes I drilled in the case. Before drilling the holes and before wiring up, place the transformer in the case to ensure it fits, and also check that the lid will go on
Then (assuming the test fit above was okay) mark where you want the holes drilling by using a sharp point. Then drill the holes... (see also pictures further up this thread).

Ensure you do the resistance check in the post above. Then...

Transformer wiring:

USA transformer wiring.PNG (649.75 KiB) Viewed 8725 times

Note, connect the wiring to the tag provided (this not shown in the picture for clarity) by gently twisting the stripped copper strands, threading through the hole, and then bending the wire back to form a "U" or hook shape. This improves the mechanical strength of the connection. Once soldered, cover with heatshrink

The resistor connects to the casing of the transformer at one of the mounting holes by using a "solder tag". Also not shown, at the same point where the resistor connects to the transformer case, a green (or green/yellow) earth wire should be connected, again by using a "solder tag". Ensure that the transformer case where these "solder tags" are connected is clean and free of any varnish or other non-conductive protective coating by scrapping at the metal with a screwdriver. We need a really good connection. Put the machine screw through the case, put the transformer over it, then put the two solder tags (with the resistor and the green wire already attached) on the screw, then add a washer and finally the nut.

Once you have the mains wiring finished and the fuse fitted in the IEC connector, measure the resistance between the Live/Line (L) and the Neutral(N) connections at the mains plug, or the IEC connections. It should be about half of the value you got for one winding that you took earlier.

Mark

[wyerd]

Thank you Mark. Much appreciated.

Just been cutting the switch and power connector holes out from the box. You're quite correct about the mess it makes! Best thing to to is make the mess when the other half is out and make sure it's cleared up before she's back. She'll never know then!!

[wyerd]

Yes, I do have a digital multimeter. I'll check the resistances.

[wyerd]

Details in next post below...

But first, do you have a multimeter?

Measure the resistance (Ohms, 200 ohm range) of each of the primary windings.
So the first winding is between the connections labelled 1 and 2. The second winding is between the connections labelled 5 and 6. The resistance should be about the same.

Mark

1 and 2 is 27.1 Ohms
3 and 6 is 32.5 Ohms

Is that OK?

[1024MAK]

1 and 2 is 27.1 Ohms
3 and 6 is 32.5 Ohms

Is that OK?

Assuming you mean:
5 and 6 is 32.5 Ohms

Yes .

Because the wire for one winding is slightly longer than the other, there is normally a slight difference.

Once you have wired up the wiring to the primary (input) of the transformer, measure the resistance between Live/Line and Neutral. It should be about 14 or 15 ohms. If it is at or near this value, it is a good sign that the transformer primary windings are wired in parallel.

One other IMPORTANT thing while I am thinking about it. Keep the mains wiring at least 6mm (1/4 inch) (preferably a greater distance) away from the low voltage wiring and components. Where possible, restrain or secure the wiring so that mains wires cannot touch low voltage wiring and components even if a connection breaks. Same for low voltage wiring. Try to use heatshrink or other insulating sleeve to insulate any exposed terminals (not always possible, I know).

Good luck with your construction

Mark

[wyerd]

1 and 2 is 27.1 Ohms
3 and 6 is 32.5 Ohms

Is that OK?

Assuming you mean:
5 and 6 is 32.5 Ohms

Yes .

Because the wire for one winding is slightly longer than the other, there is normally a slight difference.

Once you have wired up the wiring to the primary (input) of the transformer, measure the resistance between Live/Line and Neutral. It should be about 14 or 15 ohms. If it is at or near this value, it is a good sign that the transformer primary windings are wired in parallel.

One other IMPORTANT thing while I am thinking about it. Keep the mains wiring at least 6mm (1/4 inch) (preferably a greater distance) away from the low voltage wiring and components. Where possible, restrain or secure the wiring so that mains wires cannot touch low voltage wiring and components even if a connection breaks. Same for low voltage wiring. Try to use heatshrink or other insulating sleeve to insulate any exposed terminals (not always possible, I know).

Good luck with your construction

Mark

Oops! Yes, 5 and 6.

I have the same box you used, so I'm constructing it the same way as you, or at least attempting.

[1024MAK]

Well, if you want any specific additional pictures (I still have a small number left that I have not yet processed and uploaded), or indeed help and advice, just say so

Mark