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Building a power adapter for the Atari 520ST

C

Caleb Hansberry

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Mar 23, 2014
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Colorado Springs, CO
Hi. I'm trying to build my own power adapter and hope I don't mess up.
Found a diagram online of the pinout: a DIN-7 in this configuration:
power%20connector_scheme.gif

Can I build a Molex adapter so I can just plug it into a couple 4-pin Molex plugs so I don't have to sacrifice a whole PSU? How would I wire up a negative voltage?
 
That is a good idea, but unfortunately I don't have enough money ($35) for that right now. I do have an extra ATX one I could use though. So far, I've wired all the wires correctly - +5v, +5v, ground, ground, +12v, and -12v - but it doesn't work yet.
 
You know that to make an ATX power supply activate (start up) in stand-alone mode, you need to connect the green wire (pin 14 on the 20-way output connector) to one of the black wires?

A link between pin 14 and pin 13 or pin 15 should do it.

Please be ultra sure you have your wiring to the ST correct, as PC power supplies can supply a lot of current especially from the +5V output. You don't want that going anywhere it shouldn't.
 
I've successfully made my power supply for Atari 520STm using an old unit from some Cisco equipment. Theoretically it's possible to get these voltages from AT/ATX power supply, taking 5 and 12V from Molex or main plug and -12V from the main plug. But I don't recommend using PC power supply units, especially cheap ones.
I've seen so many design defects in cheap ATX power supply units that I can't recommend them for any 8-bit computer. They may give something not satisfying before Power good signal (it looks like an amplitude-changing sawtooth on a scope). The most troublesome thing is (in e.g. Linkword, some Procomp units) voltage regulation working perfectly... over some current. If current is less, you can get 15V instead of 12. This current is quite significant for some units (200mA!).
So if you really want to use this ATX, check with voltmeter for any surprises.
 
Depending on the power requirement, an old modem power supply can furnish these voltages and look pretty much like the original. I think that Stone still might have some to offer.
 
Eh, but might the ATX work? I'll measure it in a moment. Here is where I am right now:
pc7kHSm.jpg

I have no idea what to try now though. Assume the PS is good for now, it's wired correctly to a known good ATX PSU.

When I measure the resistance on the pins in the computer side when it's not plugged in, it shows there is no continuity between the ground and 5v and 12v pins. There's 70 ohms between ground and -12v. When I press the power switch, the PSU fan turns off, leading me to believe it's being shorted. It doesn't happen if only 12v is plugged in.

Also, this computer experienced an impact on the corner, the case was shattered there. Is there any way a broken solder join could cause this behavior?
 
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Are you sure you have connected the pins properly on the Atari side since there are no numbers in your picture?

You can easily identify the ground pins since they are connected to for example the metal shielding and are not switched.

The +5 Volt is the only one switched as far as I know, but if you have connected the +5 and ground backwards you probably killed multiple parts...
 
Yes, thankfully I'm sure of the placement of ground and +5. As soon as I could I made a color-code DIN-7 adapter with the 6 used wires available on one end.

I might have solved that issue. If I wire it all up and flip the power switch on the PSU, all that happens is the computer dies. But if I then, leaving the switch on, remove and replace the wire jumping the "on" signal in the 24-pin connector, it turns on and stays on. It drives me crazy that it was something this simple, but as far as I can tell at this point, the surge from turning it on is making the PSU turn off, and I had no idea that ATX PSUs could do that.

This means we can get back to the original point of the topic though. I can build one with an ATX PSU, but it would require a second power switch for itself. Is there any more elegant solution? Also its very big.
 
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You're right, I'll consider getting one of those when I have a bit more money. I don't normally buy anything from China if I can help it, but that looks a bit more particular and difficult to find in the USA.

In other news, I found something that I'm pretty sure works well - this power supply from the junk shop:
aQVqE1L.jpg


The specs happen to line up perfectly unless I misunderstand amperage which is definitely possible. I wired it up and the Atari looks happy to me:
hpoXYyB.jpg
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TOv80XT.jpg


The plug used to be the same one as the Tandy 4860HD laptop uses, could be handy to have around. Also, Codewalr.us has a built-in system that shrinks down pictures to a more manageable size and makes them clickable, when you click them you get the full size then, even though you still just use the
 
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Interesting PSU--from an old videoconferencing box--hooks to an external TV monitor. Since comparatively few ISDN installations still exist, you have to wonder what happened to them all.
 
I do wonder. I searched but could not find any more online. It makes for quite a nice solution, all it needs besides a DIN-7 connector is a custom Atari label to be just like the original.

There is a nice little adapter on eBay right now - from a Cisco router just like MCbx suggested - on eBay right now that should also make a very elegant solution for fairly cheap. It's got the right output, is small and includes the power-in connector unlike some that just have screw terminals, and is only $15. +5v, +12v, -12V, 47W Power Supply, Cisco and Cisco 34-0874-01 ADP-30RB 5V 12V Pix Router Power Adapter. These thingys are perfect for people's Atari 520STs but seem sort of hard to find sometimes, I think.
 
The black power supply unit from auction is probably the one I used with success in my 520ST. Generally the same size and connector.
 
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