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PC/XT Power Supply

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lyonadmiral

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So I have here an authentic IBM PC/XT power supply, I noticed however that someone cut one of the lines, I've attached a picture so you can clearly see which one was cut.

Would there be any particular reason someone would cut this line, and if so, should I leave it as is, or should I strip and splice the line back together.

Thanks,
Daniel

IMG_0546[1].jpg
 
Last edited:
If it's the white one going to the middle, it's for -5V, which (I *think*) is only used for RS-232.
Does it put out -5V ? If it does, I'd splice it back together.
patscc
 
I don't have a multimeter, so I can't test it. :(

Let's say for argument sake, it does not properly output -5V, would the motherboard not get a "power good" signal that way and fail to start?

Thanks,
Daniel
 
The -5 was primarily used for the old 4116 DRAMs on the original 5150 first revision. There may have been some hybrid DRAMs on the IBM memory expansion boards that also used it, but I'd have to check.

The -12 is used by RS-232.
 
* 16/64K 5150 motherboard: DRAM chips plus chip U1. Lack of minus 5 volts prevents motherboard working due to DRAM not working.

* 64K-256K 5150 motherboard: Only chip U1 uses minus 5 volts. Lack of minus 5 volts to U1 results in a 131 error (cassette wrap test).

* Early version of the IBM 5.25" Diskette Drive Adapter uses minus 5 volts

* 5160 motherboard: No use.
 
Sorry for causing confusion. I completely forgot about 4116's, and for some reason MAX 232's got stuck on the brain, hence the -5V for serial.
patscc
 
I've only just now looked at the photo (to see if wire cut or broken).
The wire colours are not the normal ones that I'm used to seeing.
The white wire of your PSU corresponds with the orange wire pictured below (pin 1 of P8), and is the POWER GOOD line.
The wire needs to be fixed because most motherboards will not start without the POWER GOOD signal.

p8_p9_pinout.jpg
 
Nice catch.
Is this a IBM power supply ?
You can actually just have a floppy or similar 5/12 V load hooked up, it's enough for the switcher to operate properly.
Why would you cut the power good signal ?
I could see jumpering it to get the box to boot if there's no power good signal, but then, do you really want that power supply ?
patscc
 
I have two XT motherboards that doesn't use a POWER GOOD line, I knows that the 5160 motherboard needs a POWER GOOD line. Hey lyonadmiral, could you investigate where that PSU whas used?

The -5V earlier posts remembered me somehting: Some manufacturers stopped putting that -5V on generic AT PSU, this was a serious problems to some ISA cards, especially the Roland LAPC-I, this card NEEDS -5V, and appears to be dead if you don't use a PSU that have -5V.
 
Yeah, clone boards.

Sorry for being tedious, but the correct name is Roland LAPC-I. The Roland made two versions of LAPC, one for the Japanese NECs (Roland LAPC-N), and one for IBM PC compatibles (Roland LAPC-I). I just love that card! I miss her nowdays.
 
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