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| Macintosh LC with Pico-PSU boots without -5v ? |
Posted by: zezba9000 on 2022-01-17 13:27:02
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? |
Posted by: Johnnya101 on 2022-01-17 13:30:47
I think it was for serial, but not totally sure. |
Posted by: cheesestraws on 2022-01-17 13:37:10
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. |
Posted by: zezba9000 on 2022-01-17 13:53:40
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. |
Posted by: Nixontheknight on 2022-01-17 14:43:26
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 |
Posted by: cheesestraws on 2022-01-17 15:04:35
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. |
Posted by: zezba9000 on 2022-01-17 16:15:22
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. |
Posted by: Nixontheknight on 2022-01-17 17:07:03
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? |
Posted by: zezba9000 on 2022-01-17 17:34:33
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. |
Posted by: joshc on 2022-01-17 23:30:57
-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? |
Posted by: cheesestraws on 2022-01-18 00:44:43
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. |
Posted by: zezba9000 on 2022-01-18 11:59:46
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. |
Posted by: cheesestraws on 2022-01-18 12:18:48
Ohhh, I had conflated options 2 and 3. OK, that makes more sense now. |
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