Building a replacement MTX PSU

About original Memotech hardware.
wyerd
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Re: Building a replacement MTX PSU

Post by wyerd » 02 Jul 2015 22:38

1024MAK wrote: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.
I measured this as 15.1 ohms

Image

All ready for the big switch on......I have yet to connect the 6 DIN plug as I'm going to test the output of each cable beforehand. Any other tests you recommend?

So close! :o :o

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1024MAK
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Re: Building a replacement MTX PSU

Post by 1024MAK » 02 Jul 2015 23:28

Well done 8-) :D

Looks excellent :P

The resistance of the mains input / transformer primary is fine (in line with what I predicted/calculated) ;)

Now, testing. I like to use the selection of 6V, 12V and 12V light bulbs that I have.
See this post for the results I got.

But first, arrange the stripped ends of your output cable such that there is no danger of shorting anything out. For example, use a small screw terminal block (2A, 3A or 5A rating is fine). Or use a breadboard.

Before connecting to the mains, it is strongly recommended that you screw the lid on. Being safe is a wise thing to do.

Before powering up, connect you meter between the black and red wires, test on the 200 ohms range and confirm that you get a result of less than 2 ohms.

Next, connect each of your meter leads to each of the 7.5V AC outputs (wire colours green and white wires). Then power up. The meter should display a AC voltage of 16V AC or more. If this okay, that's the first test passed.

Next test each 7.5V AC output in turn (green, then white wires) to the red, then the black wires.

Now to test the 16V DC output, connect the meter between the yellow and black wires. Confirm that you have 16V DC AND the polarity is correct (yellow positive).
Before disconnecting the meter, switch to the AC range (better still, if your meter has a mV AC range). Check that you get a low reading. As this will vary between meters and meter ranges, post up your result.

That will do for the no load tests :mrgreen:

Mark

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1024MAK
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Re: Building a replacement MTX PSU

Post by 1024MAK » 02 Jul 2015 23:36

Oh, just in case someone is tempted to leave the lid off, keep in mind that mains voltages bl00dy hurt. I know :twisted:

Also, if there is a problem with the wiring, and one of the electrolytic capacitors finds itself with the wrong polarity voltage across it's terminals, it may explode and fire it's top off like a rocket. I've seen this happen to someone else's project when I was in collage. Two years later when I left, the top of the capacitor was still embedded in the ceiling :shock:

Mark
Last edited by 1024MAK on 02 Jul 2015 23:53, edited 2 times in total.

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

Re: Building a replacement MTX PSU

Post by wyerd » 02 Jul 2015 23:42

1024MAK wrote:But first, arrange the stripped ends of your output cable such that there is no danger of shorting anything out. For example, use a small screw terminal block (2A, 3A or 5A rating is fine). Or use a breadboard.
I like the idea of the terminal block. I'm sure I've one somewhere............

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

Re: Building a replacement MTX PSU

Post by wyerd » 02 Jul 2015 23:56

1024MAK wrote:Before connecting to the mains, it is strongly recommended that you screw the lid on. Being safe is a wise thing to do.

Before powering up, connect you meter between the black and red wires, test on the 200 ohms range and confirm that you get a result of less than 2 ohms.
Lid screwed on for safety.

It reads 01.0 ohms.

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 » 03 Jul 2015 00:03

1024MAK wrote:Next, connect each of your meter leads to each of the 7.5V AC outputs (wire colours green and white wires). Then power up. The meter should display a AC voltage of 16V AC or more. If this okay, that's the first test passed.
The on/off switch glows red and the LED comes on green, but nothings read on the meter. :cry:

Ahhhhhhh......AC! had the meter on DC....Doh!

It reads 19.8

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

Re: Building a replacement MTX PSU

Post by wyerd » 03 Jul 2015 00:10

1024MAK wrote:Next test each 7.5V AC output in turn (green, then white wires) to the red, then the black wires.
Green to red 9.6
Green to black 9.6
White to red 9.6
White to black 9.6

wyerd
Posts: 88
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Re: Building a replacement MTX PSU

Post by wyerd » 03 Jul 2015 00:14

1024MAK wrote:Now to test the 16V DC output, connect the meter between the yellow and black wires. Confirm that you have 16V DC AND the polarity is correct (yellow positive).
Before disconnecting the meter, switch to the AC range (better still, if your meter has a mV AC range). Check that you get a low reading. As this will vary between meters and meter ranges, post up your result.
16.02 DC
34.7 that's at the 200 VAC setting. Looks high doesn't it?

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1024MAK
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Re: Building a replacement MTX PSU

Post by 1024MAK » 03 Jul 2015 00:17

Now where's that thumbs up picture when I need it? :P

Err, maybe I don't need it just yet, 34.7V AC :shock: or are you on a mV range ?

If so 34.7mV is acceptable ;)

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 Jul 2015 00:24

Image

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