Power supply

[prime]

Hi all,

I've just bagged myself a boxed Memotech MTX512 on ebay, but it alas comes without a PSU.

I know that the original PSU was just a multi-tapped transformer as detailed here :http://www.sinclairzxworld.com/not-exac ... 8&start=10 however having had a quick look this morning it looks hard to get such a beast.

I have been looking at the schematic for the power circuit in the operators manual, and it strikes me that it should be possible to run the machine off a PSU that supplies regulated +5V, +12V and -12V (e.g. a Spectrum +3 PSU) by doing the following :

Remove Reg1 and connect the +12V from the PSU to where the output pin of Reg1 was connected.
Remove Reg2 and connect the +5V from the PSU to where the output pin of Reg2 was connected.
Remove D16/D18 and connect the -12V from the PSU to the commoned end of these two diodes.
Ground from the PSU would also of course have to be connected to the ground line on the board, say at J9-3/J9-4.

Does this seem to make sense, as far as I can tell this should work, and shouldn't cook anything.....

Cheers.

Phill.

[1024MAK]

Hi Phill, and welcome!

For the +5V line I suggest you also remove Q4 or FS1.
For the -12V feed, to supply the correct voltage and current for the -5V line, you may need to change the value of R60 (the 1K resistor).

Myself, if I was going to convert a Memotech to use an external dc PSU, I would do the following:-
Remove diodes D14 to D19 (1N4004 and 1N5402), REG1 (7812), Q4 (TIP2955) and REG2 (7805).
Fit wire links in place of D14, REG1 (I/P to O/P connections), D15, Q4 (emitter to collector connections) and D18.
Then use a cable to connect to a 6 pin DIN plug (to fit socket J9) with +12V dc to pin 1, +5V dc to pin 2, 0V (common) to pins 3 and 4, and -12V to pin 5.

If I get time tomorrow, I will take some measurements and work out if a new value of R60 is needed.

Mark

PS before you connect the cable to the 6 way DIN plug, use a meter on resistance to confirm that the pin allocation and circuit modifications are correct, so test DIN pin 1 to the +12V dc rail on the Memotech PCB etc.

[Dave]

Hi Mark,

given a choice, would you do the DC mods, or try to build the replacement AC PSU that you descibed in the ZX forum?

regards
Dave

[Dave]

Hi Phil,

welcome to the forum and the world of Memotech.

I have a small survey running at the moment, to see what Memotech hardware is still around, if you get a few minutes, could you maybe complete it for me please?

The survey is at:-
http://www.surveymonkey.com/s/KPR8V5K

And the results to date at:-
http://www.primrosebank.net/computers/mtx/mtxsurvey.htm

You've obviously had the board open and will have seen the main PCB, so it should be easy for you to get the details I need.

Thanks a lot for your time,
regards
Dave

[prime]

Hi Phill, and welcome!

For the +5V line I suggest you also remove Q4 or FS1.
For the -12V feed, to supply the correct voltage and current for the -5V line, you may need to change the value of R60 (the 1K resistor).

I guess the other thing I could do for the -5v would be to replace the Zenner / R60 with a 7905 -5V reg + caps.

Myself, if I was going to convert a Memotech to use an external dc PSU, I would do the following:-
Remove diodes D14 to D19 (1N4004 and 1N5402), REG1 (7812), Q4 (TIP2955) and REG2 (7805).
Fit wire links in place of D14, REG1 (I/P to O/P connections), D15, Q4 (emitter to collector connections) and D18.
Then use a cable to connect to a 6 pin DIN plug (to fit socket J9) with +12V dc to pin 1, +5V dc to pin 2, 0V (common) to pins 3 and 4, and -12V to pin 5.

And of course mark the case in some way so that it is known that you need a custom PSU, so that at a later date you don't plug in the AC PSU and cook things.....which is why I'd prolly use a different connector.

PS before you connect the cable to the 6 way DIN plug, use a meter on resistance to confirm that the pin allocation and circuit modifications are correct, so test DIN pin 1 to the +12V dc rail on the Memotech PCB etc.

Indeed always good to check that the schematic and the board match as I have come across instances where things are incorrectly labeled on the schematic PL6/7 on the Acorn Atom (side a and be of the connector wrong way round) and the external floppy on the Spectrum +3 (labeled back to front) being the examples that immediatly stand out.

BTW are you also 1024MAK on STH/Stardot ?

Cheers.

Phill.

[prime]

Hi Phil,

Phill

welcome to the forum and the world of Memotech.

Cheers.

I have a small survey running at the moment, to see what Memotech hardware is still around, if you get a few minutes, could you maybe complete it for me please?

Sure once I have the machine, I've bought it and paid for it just waiting for it to arrive.....

Cheers.

Phill.

[1024MAK]

Hi Mark,

given a choice, would you do the DC mods, or try to build the replacement AC PSU that you descibed in the ZX forum?

regards
Dave

Well, I have two original Memotech PSU's (transformers), but four Memotech computers, two of which work, and two of which have faults.
I think I only have one spare ATX PSU (condition unknown) plus two faulty ATX PSU's. I do have some Sinclair +2A and +3 PSU's but these are needed for the Speccy computers.
So I would most likely buy a metal case and build the replacement AC PSU (using two transformers).
Transformers are very reliable (but large and heavy). As long as a transformer is protected with a fuse, so it is protected in the event of a short circuit, a transformer will easily outlive the computer.

Many years ago (in the 1980's), I got a dK'Tronics keyboard for my 48k Spectrum. As I added various interfaces, I noticed that the Sinclair PSU got rather hot (not helped by me leaving it on 24 hours a day ). So I build a higher rated PSU. This still works (but is rather large).

I originally investigated the MTX power supply arrangements because at the time I had just got my second Memotech computer and that had come with a PSU, unlike the first Memotech computer which came with nothing!

The components I selected for the replacement AC PSU were the best match that I could find from the three suppliers that I checked with. The transformers are rated higher than what may be needed, because there is not much choice of power ratings (in VA units) at the voltage rating that is needed. If wanted, I could take some measurements, then I could see if lower powered transformers could be used.

Mark

[1024MAK]

I guess the other thing I could do for the -5v would be to replace the Zenner / R60 with a 7905 -5V reg + caps.

You could, but I expect the current used on this supply to be low. A 1A rated voltage regulator may be a bit overkill!
I think the existing resister should be okay, but until I have tested the voltage and current, I cannot say for certain.

And of course mark the case in some way so that it is known that you need a custom PSU, so that at a later date you don't plug in the AC PSU and cook things.....which is why I'd prolly use a different connector.

Of course! If you intend getting more Memotech computers, doing either (or both!) would be wise

BTW are you also 1024MAK on STH/Stardot ?

Yep! Also the same on the QL Forum, Atari-Forum, WoS Forum and as you are probably aware, the ZX80/ZX81 Sinclair ZX World forum

Mark

[Dave]

came with nothing!

The components I selected for the replacement AC PSU were the best match that I could find from the three suppliers that I checked with. The transformers are rated higher than what may be needed, because there is not much choice of power ratings (in VA units) at the voltage rating that is needed. If wanted, I could take some measurements, then I could see if lower powered transformers could be used.

Mark

When you get some time, I think that would be good. It sounds like a more "user friendly" mod to do the external transformer than to mess with the PCB. As you say, it would also take away the risk of plugging the wrong voltages into the MTX if had not replaced the PSU connector on the MTX

regards
Dave

[1024MAK]

Okay, found time to do some measurements on one of my MTX's.
The MTX is a "standard" MTX500 with no modifications or extras.
These results are all from a faulty machine (I just get a continuous tone on the sound and no display) as it does not have a keyboard attached, but the power supply section is working (the +12V, +5V and -5V lines are all correct).

Input voltage to +12V regulator (7812) = +17.1V
Input voltage to +5V regulator circuit (7805 & boost transistor) = +11.2V
Current to +5V regulator circuit (7805 & boost transistor) = 932mA
Input voltage to R60 (1k resistor) = -14.35V
Zener voltage = -5.1V
So current through R60 = 9.25mA

What does this mean?
If you use an external DC PSU and feed -12V to R60, the current through R60 will reduce to 6.9mA approx.
This may be okay, you would need to test the voltage on the -5V line and confirm that it is between -5V and -5.1V.
If it is "lower" than -5V, remove the 1k resistor in position R60 and fit a 750 ohm resistor. The current should then be 9.2mA approx. and the voltage should then be okay.

It is not easy to test the +12V line current, I suspect the current used on this line is not that much. If so, this will not make that much difference.

I will do some further tests when I get a chance.

Doing a quick rough calculation, I think the 9V 50VA transformer may be able to be replaced with a 9V 20VA transformer. This depends on the +12V line not using much current and the +5V line not needing any more than 1A. So you would not be able to add any expansion cards.

Mark