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Thread: Replacement ideas for PB70 PSU that failed.

  1. #11

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    Base! Yes that was the term. I couldn't remember. I wasn't certain but I didn't think it was ground. Thank you for the link. Base, collector and emitter. When I have the time I will try to measure the terminals and find out what I can.
    @Chuck(G) - Well the PSU needs both + and - 12V and + and -5V. My simple searches haven't really found much, but I thought maybe if I could find a workable PicoPSU that has at least 2 Molex and one floppy connector and I'm not having much luck.
    Last edited by Nathanieltolb331; September 22nd, 2020 at 09:58 PM.

  2. #12
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    The -5v rail was removed from the official ATX PSU spec back in 2004, it's very unlikely you'll find a Pico PSU with that power rail.

    You'll have to make that rail yourself if you can't find a replacement supply with it. The easiest way is to hang a negative linear voltage regulator off the -12v rail to get -5v. An LM7905 (NOT 7805) will work. The more complicated method would be to use a negative charge pump circuit.

  3. #13

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    I remember. Also, the PSU uses AT connectors, so I would have to get an adapter cable to go from 20/24pin to AT. I figured when I bought one of those, I would make sure it had the -5V mod applied to it already to help. Since the machine is a 386, I want to make sure I have that -5V, right? If I remember correctly, the first couple of Sound Blaster Cards required it to work at all. The main issue with the PicoPSU is just finding one that has the connectors I want. Most of them only have 1 molex, a SATA, which I can convert and nothing else. But I would really like a floppy power connector as well. That seems to be the rub.

  4. #14
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    Quote Originally Posted by Nathanieltolb331 View Post
    I remember. Also, the PSU uses AT connectors, so I would have to get an adapter cable to go from 20/24pin to AT.
    If you don't mind it being permanent, you can chop and splice P8/P9 connectors on. It can be a pain though since most PSU vendors use aluminum or copper coated aluminum wire. Soldering aluminum is an irritating task which requires lots of heat and flux.

    Quote Originally Posted by Nathanieltolb331 View Post
    I figured when I bought one of those, I would make sure it had the -5V mod applied to it already to help.
    While such adapters exist, I haven't seen them for a number of years. There used to be some on Ebay, but I've only seen the passive adapters lately without the -5v rail present.

    Quote Originally Posted by Nathanieltolb331 View Post
    Since the machine is a 386, I want to make sure I have that -5V, right? If I remember correctly, the first couple of Sound Blaster Cards required it to work at all.
    You can try without a -5v rail, but many boards have circuitry that prevents the board from booting if any power rail is missing, even if they don't use that rail.


    Quote Originally Posted by Nathanieltolb331 View Post
    The main issue with the PicoPSU is just finding one that has the connectors I want. Most of them only have 1 molex, a SATA, which I can convert and nothing else. But I would really like a floppy power connector as well. That seems to be the rub.
    You can use Y splitters and adapters to get the connectors you want, just don't add more than a few devices to prevent overloading the PSU.

  5. #15

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    Quote Originally Posted by GiGaBiTe View Post
    If you don't mind it being permanent, you can chop and splice P8/P9 connectors on. It can be a pain though since most PSU vendors use aluminum or copper coated aluminum wire. Soldering aluminum is an irritating task which requires lots of heat and flux.
    I wouldn't mind doing that if I had the skill. I am worried that I am not skilled enough to do that. Also, I'm not certain I understand exactly what you mean by hanging a negative linear voltage regulator off of the 12V line? Are there any videos on Youtube I could watch on this which would be helpful? Like I said before, my knowledge and skill at this sort of thing is very scant.

  6. #16
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    Quote Originally Posted by Nathanieltolb331 View Post
    Also, I'm not certain I understand exactly what you mean by hanging a negative linear voltage regulator off of the 12V line?
    Off of the -12v rail, not the 12v rail. Hang a negative voltage regulator off a positive rail and bad things happen.



    Connect ground on the 7905 to ground on the power supply, any of the black ground wires.

    Connect the input pin to the -12v rail, which is pin 4 on the P8 power connector, or the blue wire.

    Connect the output pin to the -5v wire on the P9 power connector, which is pin 3, or the white wire.

    A linear regulator is extremely inefficient, as it burns the difference in power between the input and output voltages as heat. Normally -12v and -5v would be too far apart, but in this case it is usually fine because the -5v rail has a very low current draw in most cases. If you notice the regulator getting too warm, you can add a heatsink. If it's getting too hot to touch or starts burning, then you either wired it incorrectly, or there's a fault on the logic board.

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