Building a replacement MTX PSU

[1024MAK]

Interesting

But no need to panic. Some designs of meters are only specified to work correctly at mains frequencies, and sometimes part or all of the audio frequency range.

If so, the high frequency switching used by the DC-DC converter maybe confusing your digital meter.

So don't worry. It is extremely unlikely that you actually have 34.7V AC if the DC output voltage is correct (which, it is).

Mark

[wyerd]

Ok, thanks.

So I should be ok to solder on the plug and connect it up to the MTX??!!! (gulp!)

[1024MAK]

If you want to prove it, if you have a 24V 5W lamp, you can connect this to the 16V DC output. If it blows, you have a lot more than 24V. But if as expected, it glows with a yellow medium brightness (but not really bright white light), the 16V output if fine. You can of course connect the meter up as well to test the voltage.

In the UK, suitable lamps can be had from shops and garages that stock spare lamps for cars, vans and trucks.

No need for a lamp holder, either solder the wires to the existing solder terminals or hold them on with "stretched tight" insulation tape (it does depend on the type of lamp you get). Just be careful not to cause a short circuit.

Even if you don't do this, the presence of some AC voltage on the 16V DC line does not matter, as the original MTX PSU supplied 22V AC anyway.

Of course, the normal "it's not my fault if you connect up your MTX and then find it glows brighter than the Sun" warning applies

Mark

[wyerd]

I've just tried something different. Using the MTX 240v PSU connect to the step up transformer, I measured the AC voltage from pin 1 and 4 and the result on the meter was 16, so it must be re-calibrating or something.

[wyerd]

Well, I hooked it up to the MTX and REMEMOrizer, powered it up & watched it load CP/M. I was really excited by that stage, but then I noticed that the green LED wasn't lit, so I then rebooted straight into MTX Basic (by holding down the shift keys). I just about saw the ready prompt as the screen flickered horizontally. I've checked the fuses, and F3 1A is blown.

Any ideas Mark?
Thanks.

[1024MAK]

Oh dear

Right. Well first, it is highly unlikely that you have damaged your MTX.

F3 is the DC feed to the DC-DC converter.

Edited to add, just check that the fuse that blew was actually a 1A rated fuse.

Before you do anything else, disconnect the PSU from the mains. If you have replaced the 1A fuse in F3, remove it.

Now, using the resistance ranges on your meter, carry out the following measurements:-

• On the DIN plug, connect the red probe to pin 1 (do this from the pin side, that is do not remove the cover yet). Black probe to DIN pin 3 or pin 4. Or you can use any of the 0V connections / -V connections on the strip board or the DC-DC converter.
• Now inside the PSU, red probe to the side of F3 fuse holder that connects to the red wire (that goes to the DC-DC converter). Black probe again connects to 0V as detailed above.

If you get any low readings, check the wiring in the DIN plug for anything that may have shorted.
The next thing to do is to check that the wiring (red wire) and circuitry from the fuse holder to the DC-DC converter is not shorting on anything. Next do the same for the wiring on the output of the DC-DC converter (the yellow and orange wires).

Mark

[wyerd]

I don't read any resistance at all and I can't see any shorts. I have changed the screws and washers on the DC to DC converter to plastic just in case as it looked like the washer might have touched something.

Hopefully I'll be able to get a replacement fuse or two from Radio Shack when they re-open tomorrow.

[wyerd]

Replaced blown 1A fuse. Powered up PSU without connecting to MTX and green LED was on. Connected to MTX, powered up, green LED came on for a fraction of a second and no CP/M boot. The 1A fuse had blown again. Replaced and green LED is on.

Could the problems be associated with the MTX power capacitors being replaced as described here? http://primrosebank.net/computers/mtx/r ... eplace.htm

[1024MAK]

You could try fitting a 1A time delay / anti-surge fuse. I only specified the fuse at 1A to protect the diodes (which have an average current rating of 1A, but can handle a slightly higher peak current rating).

Mark

[wyerd]

You could try fitting a 1A time delay / anti-surge fuse. I only specified the fuse at 1A to protect the diodes (which have an average current rating of 1A, but can handle a slightly higher peak current rating).

Well, that did the trick!



Thank you very much for the design and for helping me out with this project. Great stuff! It was my first home made electronic project and I'm so happy that it works. Cheers!