Macintosh LC with Pico-PSU boots without -5v ?

[zezba9000]

So my LC PSU is bad (smells bad to & gets stupid hot). I will try replacing caps but if something else is wrong I'm using a Pico PSU.

So I did some Pico PSU tests to make sure computer boots fine (& it does) BUT I found out I don't need the "-5v" line to be connect...
What is the -5v line used for on the LC?

 

[Johnnya101]

I think it was for serial, but not totally sure.

 

[cheesestraws]

What is the -5v line used for on the LC?

Differential serial, which really boils down to being LocalTalk. RS232 may work without; I can't remember.

 

[zezba9000]

So in short I really want that -5v line to make sure I don't limit some hardware.

So other question is: I have a pico PSU with a weak -5v line of around 7.5v with no load.
If I power the Mac LC, the -5v line runs at -5.2v. Will this harm anything long term.

I also ordered a new Pico PSU & can just use a 7905 negative voltage regulator from the -12v line instead if thats safer.

 

[Nixontheknight]

So in short I really want that -5v line to make sure I don't limit some hardware.

So other question is: I have a pico PSU with a weak -5v line of around 7.5v with no load.
If I power the Mac LC, the -5v line runs at -5.2v. Will this harm anything long term.

I also ordered a new Pico PSU & can just use a 7905 negative voltage regulator from the -12v line instead if thats safer.

-5.2V is within the tolerance, so it should be fine

 

[cheesestraws]

So in short I really want that -5v line to make sure I don't limit some hardware.

For a proper replacement for the existing supply, you want a –5V line. For quick tests or a bodge job just to get something running, you can live without. I always have –5 personally, but then I'm a network nerd.

If I power the Mac LC, the -5v line runs at -5.2v. Will this harm anything long term.

That should be fine, I'd expect.

 

[zezba9000]

Oops this "have a pico PSU with a weak -5v line of around 7.5v with no load."
Should have been: "have a pico PSU with a weak -12v line of around 7.5v with no load."

Said -5v instead of -12v line. So week -12v line thats -7v with no load but goes to -5.2v with mac powered on in short.

 

[Nixontheknight]

Oops this "have a pico PSU with a weak -5v line of around 7.5v with no load."
Should have been: "have a pico PSU with a weak -12v line of around 7.5v with no load."

Said -5v instead of -12v line. So week -12v line thats -7v with no load but goes to -5.2v with mac powered on in short.

ok for that, you need a buck regulator to make the -12 consistent, but I don't think the LC needs -12?

 

[zezba9000]

ok for that, you need a buck regulator to make the -12 consistent, but I don't think the LC needs -12?

You use a 7905 negative voltage regulator to get -5v from -12v. Its in my earlier post.

 

[joshc]

-12V is not used by the LC. But I think you might be saying you want to derive -5V from -12V but your issue is that -12V is not stable?

 

[cheesestraws]

You use a 7905 negative voltage regulator to get -5v from -12v. Its in my earlier post.

Yeah, either use a 7905 or some kind of switching regulator there. A 7905 may not be enough if the -12V line is down at -5.2, because it has something like a 1V dropout. You might instead want an isolated DC/DC converter on one of the positive lines.

 

[zezba9000]

Seems people are confused in posts. So to be clear.

<<< Original PSU >>>
* Doesn't work but ordered caps
* Gets super hot
* Voltages wrong

<<< Off-brand Pico PSU I have in hand >>>
* Works / Tested
* -12v line reports as -7.2v with zero load
* -12v line reports as -5.2v when Mac LC is powered on

<<< Official Pico PSU (waiting for mail) >>>
* Will of course work
* Can use 7905 negative voltage regulator on -12v line producing -5v output (plus some 100uf 10v caps)

Those are my 3 options & what I'll be testing more when I get parts.

 

[cheesestraws]

Ohhh, I had conflated options 2 and 3. OK, that makes more sense now.