Mega STe PSU replacement with Flex ATX

[Orion_]

Hello,
I recently acquired a Mega STe, I removed the old harddrive to get a SCSI2SD device, I also acquired a VGA card to plug on the VME bus via an adapter
and I would like to add a second floppy drive (Gotek/HxC)
Adding all these things, I fear a bit that the original PSU might get a bit tired, and I don't want it to blow up, also I don't like the PSU fan noise.
So I would like to replace it with a modern PSU.
PicoPSU seems nice, but doesn't offer lots of additionnal molex connector, and it's a bit overpriced (adding the power brick)
So I found something else, a Flex ATX PSU called FSP-180-50LE
It's a 180w PSU, so I guess it more than enough, it has plenty of molex and floppy connector
I'm just trying to figure out how to match the ATX 20 pin connector, with the Mega STe PSU connector
I'm missing the -5v pin and I'm not sure about the power on pin,
Could someone tell me if the -5v pin is mandatory ? and if the power on pin is ok ?

[mzry]

This guy gives a detailed guide of how he replaced his psu with a modern one, it says he used the -12 rail to make the -5v with a voltage regulator:

http://www.mr2.net/marknias/atari_tt030 ... r%20Supply

[Orion_]

Thanks for the link, unfortunately he doesn't give any detail about the voltage regulator, and I don't know anything about this :/

[frank.lukas]

PSU -> -12V -> 7905 -> -5V

[Foxie]

Thanks for the link, unfortunately he doesn't give any detail about the voltage regulator, and I don't know anything about this :/

Screen Shot 2018-04-17 at 12.54.39 pm.png

You might need to heatsink the 7905. Don't let the heatsink touch anything else! It will be live to -12V. Pinout here: http://4.bp.blogspot.com/-opaTlI9UCDc/U ... /white.jpg

[Orion_]

thank you !

[Orion_]

I'm wondering why there is a diode between -12v and -5v, won't the current goes through the diode without conversion ?
and, is there any difference between C4/C2 and C3/C5 ? because the value is the same, but the schematic is not

[_KrISS_]

https://www.ebay.com/itm/ATX-20-PIN-to- ... SwBP9UW9d3

[Foxie]

I'm wondering why there is a diode between -12v and -5v, won't the current goes through the diode without conversion ?
and, is there any difference between C4/C2 and C3/C5 ? because the value is the same, but the schematic is not

The diode is required by the 78xx and 79xx regulators whenever the output capacitance is greater than a certain amount (10uF?). Without it, when the supply turns off the capacitor on the output will discharge backwards through the 7905 and damage it. The diode looks like it's in the wrong direction, but remember that we're dealing with -12V not +12V.

C4 is a 100uF electrolytic, while C2 is a 100nf (0.1uF) ceramic. The ceramic caps are just decoupling, they should be physically close to the 7905.

R1 is to ensure the 7905 gets a minimum load, without it the 7905 can't regulate. That's different to the 7805 which will regulate with no load. It might be that the MegaSTE will draw enough current at -5V to keep the 7905 happy, but I personally wouldn't want to chance it.

[frank.lukas]

In the original Atari TT or MSTE PSU there is also a 7905 ...

7905.jpg

[Gaiyan]

https://www.ebay.com/itm/ATX-20-PIN-to-AT-P8-P9-CONVERTER-WITH-5V-ON-OFF-SWITCH-DUMMY-LOAD-USA-MADE/251025150787?hash=item3a7243d343:g:GaIAAOSwBP9UW9d3

This is exactly the same converter from the same seller I have been using on my TT and PicoPSU for almost a year. Highly recommended.

[rpineau]

I used a PicoPSU ( picoPSU-120 ) and a small PCB with a ATX connector on it and a 7905 to generate the -5V. Works fine with my TT.
I even put a connector for the fan and reused the original PSU enclosure. I'll see if I can take a few picture if interested. I think there is a thread on the forum somewhere where I posted the Eagle files.

[edit]
Found the thread : viewtopic.php?f=15&t=30904&p=308030&hilit=PicoPSU#p309052

[Gaiyan]

I also acquired a VGA card to plug on the VME bus via an adapter

Could someone tell me if the -5v pin is mandatory ? and if the power on pin is ok ?

You almost certainly need the -5V in that case. It was required on some ISA gfx and sound cards back in the day.