PYE MF12PU 12v Converter psu.

[bigfathairyvika]

Hi All,

I'm currently modifying a PYE MF12PU , 12v up converter power supply.
It has been previously modified with additional regulators for -5v and 18v.
But, I need -5 and 20v at 600ma.

It's unregulated voltage is 25v and loaded with 600ma this drops to 20.5 which is not enough for even a low drop out regulator.

Does anyone have the specs and schematic for this power supply?

As it is a fairly simple two transistor saturation oscillator I have managed to draw a rough schematic. But , not knowing what it should be capable of I may be on a lost cause.

I have one thought that If I modify the circuit so I can vary the frequency and thus find the most efficient point, I'd be able to get a couple of extra volts.

I have a backup plan, but this involves a negative voltage generator chip, but these only just make the 30ma I need on the -5 line.
This way I can use a 7905 and have plenty of current to spare.

Thoughts?

Mark

[Jon_G4MDC]

Puzzled by this one.

I know of MF24PU which was a converter to enable 12V DC Pye equipment to be used in vehicles with a 24V electrical system. Rather than a simple regulator a switching converter should give better efficiency and could be used on either polarity +Ve or -Ve gound. I assume they made a 12V>12V version which was just to overcome the polarity issue?

Strange since most 60-70s era Pye sets were floating earth anyway. Maybe MFxx, the Europa series, were -Ve earth only. Apart from MF5AM Reporter I have not worked with them much.

As this would seem to be an inverter, transformer, rectifier couldn't you make a Voltage doubling rectifier? More chance of doing that than changing the transformer I would think.

[CambridgeWorks]

The MF12 pu was indeed designed specifically for the Europa, which was -ve earth only. It bolted on the back and the 7 pin connectors made a solid connection between the 2 when screws were tightened.
I used to use one to give an isolated 12v o/p that was then wired in series with the vehicle 12V to feed a Bulgin 3 pole connector to power my 24V Weller iron in the van.
Your suggestion Jon of a voltage doubler would be a good idea at the high frequency these operate at.
Rob

[Jon_G4MDC]

Ah so that was it... showing my ignorance of the Europa series wasn't I?

Somehow I never desired one apart from an MF5AM on 70.26MHz was a bit of fun.
Also had one on Pye Transport! Now I think on it they must have been the first to be -Ve earth only and around that time there were still plenty of +Ve earth vehicles around.

Wishing the OP luck in Voltage doubling. If you post your sketched circuit it would be interesting since I can find no gen on these on the web apart from behind a paywall.

Another thought just kicked in...if these used the same transformer for 12V and 24V version you just might find another winding with AC on it? Once version had primary 24V secondary 12V. You want the opposite. It's worth a look.

[bigfathairyvika]

My quick drawing of the schematic... which for some reason I think I've got it wrong...

PYE MF12PU

It does currently double the input , but under load drops too much.
So, I could well put a doubler before the bridge.

There will be three pin regs on the output which should take 40 ish volts.

As I do have complete unused cores available I could just wind a new one.

I tried last night varying the frequency but don't gain much.
The 3W 33R resistor linking the xfmr and bases of the TR1 and 2 does get hot, so I am probably loosing 8ish Watts there.