Mega STe PSU replacement with Flex ATX

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Mega STe PSU replacement with Flex ATX

Postby Orion_ » Mon Apr 16, 2018 8:20 pm

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 ?
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Re: Mega STe PSU replacement with Flex ATX

Postby mzry » Tue Apr 17, 2018 6:32 am

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
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Re: Mega STe PSU replacement with Flex ATX

Postby Orion_ » Tue Apr 17, 2018 9:59 am

Thanks for the link, unfortunately he doesn't give any detail about the voltage regulator, and I don't know anything about this :/
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Re: Mega STe PSU replacement with Flex ATX

Postby frank.lukas » Tue Apr 17, 2018 11:41 am

PSU -> -12V -> 7905 -> -5V
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Re: Mega STe PSU replacement with Flex ATX

Postby Foxie » Tue Apr 17, 2018 11:53 am

Orion_ wrote: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
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Re: Mega STe PSU replacement with Flex ATX

Postby Orion_ » Tue Apr 17, 2018 6:14 pm

thank you !
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Re: Mega STe PSU replacement with Flex ATX

Postby Orion_ » Wed Apr 18, 2018 8:55 am

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
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Re: Mega STe PSU replacement with Flex ATX

Postby _KrISS_ » Wed Apr 18, 2018 9:25 am


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Re: Mega STe PSU replacement with Flex ATX

Postby Foxie » Wed Apr 18, 2018 12:31 pm

Orion_ wrote: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.

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Re: Mega STe PSU replacement with Flex ATX

Postby frank.lukas » Wed Apr 18, 2018 1:54 pm

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

7905.jpg
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Re: Mega STe PSU replacement with Flex ATX

Postby Gaiyan » Wed Apr 18, 2018 5:34 pm

_KrISS_ wrote: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.
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Re: Mega STe PSU replacement with Flex ATX

Postby rpineau » Wed Apr 18, 2018 5:43 pm

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
Last edited by rpineau on Wed Apr 18, 2018 5:48 pm, edited 2 times in total.
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Re: Mega STe PSU replacement with Flex ATX

Postby Gaiyan » Wed Apr 18, 2018 5:46 pm

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

Orion_ wrote: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.
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Re: Mega STe PSU replacement with Flex ATX

Postby Orion_ » Mon Jul 02, 2018 3:00 pm

_KrISS_ wrote: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

I didn't realize that this converter was all in one for the Mega STe, it seems like you can directly plug that AT P8 P9 in the Mega STe, that's awesome !

I finally bought a Pico PSU 80w because It was cheaper, and it seems that 80w is enough for the Mega STe.
I was a bit worried about the PSU power because I will add a ISA vga card + a second floppy drive (gotek)
but since I replaced the standard harddrive with a SD2SCSI, the original harddrive need 1.6A @5v and the SD2SCSI would need about 0.5A I guess, that leave 1A for the ISA vga card and the second gotek drive
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Re: Mega STe PSU replacement with Flex ATX

Postby Orion_ » Tue Jul 17, 2018 9:33 am

I just tried the ATX 20 Pin to AT P8 P9 adapter with 80w Pico PSU in my Mega STe, it's working like a charm, plug and play !
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Re: Mega STe PSU replacement with Flex ATX

Postby Wayne123 » Sun Jul 22, 2018 8:34 pm

Thanks for this I have a spare Pico PSU and just received a MegaSTE with a dead PSU in it, I just ordered the above linked cable from EBAY.

Thanks.
Last edited by Wayne123 on Sun Jul 22, 2018 8:39 pm, edited 1 time in total.
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Re: Mega STe PSU replacement with Flex ATX

Postby Orion_ » Sun Jul 22, 2018 8:38 pm

just compare and match the cables colors
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Re: Mega STe PSU replacement with Flex ATX

Postby Wayne123 » Sun Jul 22, 2018 8:41 pm

Thanks, I just edited my post after seeing the other thread and noticed the colors were all the same.

I should have read the other linked thread first.
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Re: Mega STe PSU replacement with Flex ATX

Postby Wayne123 » Wed Jul 25, 2018 9:11 pm

Oh well was worth a try, plugged in the adapter and only a white screen and then a hard drive cap caught fire.

Found a thread about hitting the space bar to skip the memory test and got the bee, floppy drive seems bad though.
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Re: Mega STe PSU replacement with Flex ATX

Postby Wayne123 » Sat Jul 28, 2018 8:42 pm

I made a couple of 3D printed parts to neatly hold my Pico power supply inside the PSU case, I am using the Cable from Ebay that was linked in an earlier post.

Pico PSU mount.zip
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Re: Mega STe PSU replacement with Flex ATX

Postby christos » Mon Sep 24, 2018 6:15 pm

So quick question, will the mste work without the -5V? Because I just want to check whether my MSTE's problems are due to the psu and I don't want to spend 40+ euros for that.
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Re: Mega STe PSU replacement with Flex ATX

Postby k2500x » Thu Oct 04, 2018 2:41 pm

Do the modern PSUs make these Atari systems fanless? Is it safe to use that way?


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