DIY: Retro fitting a PicoPSU into a STf/fm/e machine

[thgill]

I mentioned in the other thread about having installed a PicoPSU inside my STe and thought people might want some photos. This is the second one I have done and I have the install process done pat. This is the 80 watt PicoPSU with 80 watt 12 volt brick.

 

 

I retained the original power switch and installed the 2.5mm dc power jack in place where the original AC cord plug was. This Pico plugs into an ATX 20 pin male connector that is then wired to the original 6 pin ST motherboard power connector.

 

Reason I did this was because this machine is a UK PAL machine with the original 220v power supply. I have always thought the internal power supplies on STs were there weak point.

 

 

 

 

 

 

 

 

 

[+DarkLord]

Looks like a good job there. Thanks for the pictures.

 

I've got the 120 PicoPSU here. I'm seriously thinking

about using it in my STacy. Unfortunately, the internal

setup for the STacy's power supply board is much

different than an ST.

 

So...since I've got the Ultrasatan, a fan, and a converter

for the replacement El Sheet display, and a 5v lead to the

PAK 68/3 accelerator board, I was thinking about maybe

hooking 12v off the STacy's original P/S board to power up

the Pico, and then using the Pico to drive all those items

above.

 

That way, there would actually be much less being drawn

off the original STacy P/S.

 

Whatcha think, would this work? Thanks.

[Fox-1 / mnx]

I was thinking about maybe hooking 12v off the STacy's original P/S board to power up

the Pico, and then using the Pico to drive all those items above.

 

That way, there would actually be much less being drawn off the original STacy P/S.

 

However not impossible, it's advised to use a single power supply to power all components that reside in a single unit.

[thgill]

I think the bigger problem might be overloading the 12 volt rail on the STacy power supply.

 

What's it rated for?

[krupkaj]

nice. It is a pity pico psu has not -5V. I would like to use it in my TT

[thgill]

You could always use a small 7905 voltage regulator to get the -5 volts.

[krupkaj]

good idea, but I am not sure if the -12V line is strong enough. It would be nice to measure the current which TT draws from these "-" lines,

[thgill]

7905's are usually rated up to 1 amp. I would be surprised if the TT drew more than a few hundred milliamps from the -12v rail.

[+DarkLord]

However not impossible, it's advised to use a single power supply to power all components that reside in a single unit.

 

Hmm, can you give the reasons for that? I'm not doubting it, just curious as to why (other than convenience).

 

I'd love to have the Pico do everything, but have you seen the internal connections for the STacy's P/S? Its not the

(relatively) simple connection that a standard ST is unfortunately.

[+DarkLord]

I think the bigger problem might be overloading the 12 volt rail on the STacy power supply.

 

What's it rated for?

 

Not sure, I'd have to check that. Right now, it's just powering the UltraSatan. It used to power

the 20meg MFM hard drive, so I was thinking if it could do that, it would handle the Pico since

the Pico is supposed to be low reqs.

[Fox-1 / mnx]

Hmm, can you give the reasons for that? I'm not doubting it, just curious as to why (other than convenience).

 

Convenience is indeed one reason.

 

The Atari and Pico PS' are both "switching" types, but different. They don't switch at the same frequency so there is a (small) chance one will interfere the other which may result in drifting voltages.

 

More important, a PS creates a ground relative to it's output voltage (or the other way around, doesn't matter), like, the +5V output is 5V above ground, where ground is 0V for that PS, but it's relative. It may just be the ground is in fact +2V while the +5 output is +7V. This will still give a +5V output when compared to that ground.

 

When using 2 PS' (in a single unit designed for single PS usage) you always have to connect both grounds together, so if 1 PS has ground at +2V and the other has ground at 0V there is a 2V difference. No problem for a running system as both grounds will be forced to the same level (they're are connected together) but this is only true when running. While powering on both PS' there is a small delay until the voltage output is stable but it's almost certain this delay is not the same on both PS' so during that time there is a chance your output voltages are too high. It depends on the duration of the delay and the amount of difference if it can do any harm or not.

 

In many cases it'll just do without any harm but it's up to you to decide if "many" is enough.

 

 

Not connecting both grounds is no option as it can be pretty devastating. This is (was) one of my 130XE's:

 

 

The result of relying on main power outlet earthing as the ground connection only to discover there was none at all on the meeting we attended...

 

 

I'd love to have the Pico do everything, but have you seen the internal connections for the STacy's P/S? Its not the (relatively) simple connection that a standard ST is unfortunately.

 

-Do you know what pin expects what voltage?

-Are all those voltages present when using a Pico PS?

-Do you have a header to connect to the STacy pins?

-Are the pins standard spacing?

 

If so, put a header on a small piece of vero board, make the connections on the board, plug it in.

[thgill]

I just looked up some pics of the Stacy power supply and it doesn't look like it would be that hard to replace, unless I am missing something.

 

DarkLord - have some pics of your Stacy PSU?

[+DarkLord]

Interesting info there, thanks. So again, just out of curiosity, how does the Pico get away with this in a PC where its actually

designed to go (plugged into the motherboards AT/ATX header) - that makes 2 different power supplies there as well. Thx.

 

I'm not sure about the wiring for the STacy's internal P?S. I'll look and see if I have any schematics, but I'm really doubting

it.

 

I do have some pics, I'll post them here in a bit. Thanks again.

[+DarkLord]

Okay here are some pics.

 

First of all, an external brick power supply brings juice into the STacy.

 

You can see the internal P/S board, and where it plugs into the motherboard. The wiring you're

seeing is where I had to move the board from its original location to install my PAK 68/3 board.

I didn't need a schematic for that because I just matched everything up 1 to 1.

 

There are 2 headers, one is 10 pins and the other is 30 pins.

 

Thanks!

 

 

 

 

 

 

So far, I've been unable to locate any pictures of the schematics. I'll keep trying though.

[Fox-1 / mnx]

If you can reach P/S pins while the system is running (I guess you can) you can just measure the voltages. Prior to that you can try to find out which ones are ground. There are probably several of them, all connected together one way or another.

[+DarkLord]

Don't know how much it applies to other ST models, but Alan H., over in my First

PAK board attempt message thread (at AtariForum) has come up with a clever

way to replace the internal STacy PSU.

 

He simply fed power from another source *back* into the 2nd floppy drive

connector, and that powers the STacy motherboard. No having to fool around

with that complex internal PSU setup/motherboard headers. You simply take

out the original internal PSU altogether.

 

He's using an ATX power supply at the moment, while he works on some other

mods, but the concept should work with the PICO as well. Anyway, when I get

all my stuff together, I'm going to try it with my STacy. Can't wait.

 

Here is how he wired the PICO floppy connector to the STacy floppy connector:

 

[+DarkLord]

Thgill - do you have to do something to a PICO to get it to work, if its not

plugged into an actual motherboard? I noticed you used an ATX connector.

 

You know how you have to ground 2 pins on an ATX power supply to "fool"

it into thinking its connected before it will work?

 

Reason I'm asking is, I've got the 120w PICO model here now, and I can't

get it to power anything up. The green light on the brick comes on, the green

light on the PICO itself comes on, but nothing will work off of it. I had hooked

it up to my STacy according to Alan H's example above and got nothing. So

then I just tried hooking up devices to the molex connnectors and even a

floppy drive straight to the PICO.

 

Nada. Nothing will come on at this point.

 

Thanks.

[+DarkLord]

Never mind, got it.

 

That's what I had to do, I had to ground/connect pin#14 to any ground. On a normal

ATX power supply, that would be the green wire and any black wire.

 

Oh, there is a 3rd connector on the jack for the power brick I got - is it supposed to

go to ground somewhere inside the ST? It's not hooked up, as is, from the factory.

 

Thanks.

 

EDIT: I went back and took another look at your original pictures - it looks like you

didn't use that 3rd prong either.

[+DarkLord]

Hmm, if you were going to put a switch in, would you put it inline with the black

or white wire on the PICO power jack? Or does it even make a difference which

one? (see pic)

 

Thanks.

 

[+DarkLord]

Just FYI, I ended up breaking the white wire for my switch. I tried the black wire

first, because according to the back info panel on the power brick, that is ground.

 

However, when I did that, with the power jack bolted into the backplane of the

STacy, the switch wouldn't work - I guess it was grounding through the metal of

the jack and the backplane. With the switch in the on or off position, when you

plugged the brick into the jack, everything would power up.

 

Everything seems to be working like you would expect with breaking the white

wire.

 

Thanks.

[+DarkLord]

Okay, after some advice from "SoundDoctor" over at AtariForum, and taking

another look at the OP's use for his switch, I did move mine from where it

was to the green wire/black wire, pin#14 combo.

 

I'm using that for the switch now, and it seems to be working correctly.

[thgill]

Sorry about not getting back to this thread.

 

Yep, wired the original switch to ground and pin 14 (green) of the atx connector to use the ATX power on feature.

[+DarkLord]

Hey, glad to see you back around! The PICO is an awesome choice, IMHO.

 

It's just too bad that I can't use it to power the STacy's LCD screen. The LCD

requires a rather odd -20v and the PICO can't give it (or at least I can't figure

out how to make the PICO give it).

 

Thanks!

[Tunnan]

Anyone know how many amp the 12v brick needs to be for atari st? :-)

[TXG/MNX]

OUCH! I was the cause of the picture below, I can still remember all the flames coming out that Atari even thru the SIO chain into a 1050 Floppy drive

 

 

Not connecting both grounds is no option as it can be pretty devastating. This is (was) one of my 130XE's:



The result of relying on main power outlet earthing as the ground connection only to discover there was none at all on the meeting we attended...

 

This machine was indeed damaged so badly that it was repairable anymore... There were parts traces where damaged aswell I think.

 

I better like other memories than these...