Replacement PSU for MTX512

[schombi]

Maybe it´s just my lack of knowledge in electronics, but why is the MTX PSU so complicated and why is there a need for three voltage regulators inside the MTX? Other computers of that age require a single voltage only. I also wonder what can be done if the original PSU breaks? I guess there´s no simple off-the-shelf replacement like there is e.g. for the Spectrum (of course, the Spectrum uses DC)?

[Martin A]

The Simple answer is that the MTX "PSU" is a multi voltage transformer.

The AC it supplies is converted to DC on the main board.

It needs more than one regulator as the Video ram chips need 12v, 5v and -5v

I expect Dave to pop up shortly with the links to the schematics etc. ....

[Dave]

Popping up as requested

http://www.primrosebank.net/computers/m ... mtxpsu.htm

Regards
Dave

[schombi]

Thanks to both of you. Yes, it does look complicated and the question remains: Would would be the easiest way to replace an MTX PSU with off-the-shelf parts or do the original PSUs never break or then always get repaired?

[Dave]

It's not easy - otherwise people would have done it.

Your question is actually answered, though probably not to your satisfaction, at the top of the page that I posted the link to . . .

The options are :

"Obtain a like-for-like replacement for the original PSU
Providing AC voltage outputs and requiring no modifications to the MTX computer board
Totally interchangeable between different MTX computers

Provide an alternative method of providing the DC voltages required by the MTX system board
Modifying the MTX computer board as necessary to accept DC inputs of +12V, +5V and -5V (-12V optional).
Making the modified MTX only suitable for use with the redesigned PSU"

As I said, neither is easy, otherwise it would have been done already.

Mark is working on a replacement for the MTX PSU (transformer) but this is a "work in progress"

regards
Dave

[1024MAK]

THE MTX is not unusual in needing multiple DC supplies. The bulk of the circuitry only needs a +5V DC "logic" supply. It is the 1 bit x 16k 4116 DRAM (on the MTX used for the video RAM), cassette tape processing circuits and (if fitted) the RS232 serial board that need the +12V, -5V and -V supplies.

Other retro computers that use 4116 DRAM, need multiple DC supplies. For example, the ZX Spectrum 16/48k. The difference being that the Sinclair used an internal switch mode power supply as well as a 7805 voltage regulator inside the computer.

Memotech instead opted for a multi-tapped transformer. Internally, the supply circuits are conventional. A 7812 for the +12V. A zener shunt regulator for the -5V and a 7805 plus current boost transistor for the +5V (this circuit is even shown in some manufacturer 78XX series data sheets).

Each manufacturer came up with a different solution.

Commodore had the power supply internal to the computer in its PET computer range. The PET needing +5V, +12V and -5V to supply the 4116 DRAM.

Even Acorn, with the BBC A/B machines needed multiple DC supplies. The BBC main board uses +5V (as expected), but some of the circuitry (audio and RS423) uses -5V. There is also a +12V supply for external floppy drives.

I am working on a replacement PSU. I am making progress, but keep getting distracted by work, real life and having to repair things (at least my repair of the electronic control for my central heating system appears to be holding ).

Mark

[schombi]

Thanks for explaining to me, Mark.

The replacement your´re working on sounds good to me. I´d be a customer of yours, definitely.