T. Ment wrote:
On Mon, 03 Jun 2019 17:00:42 +0100 (BST), rp wrote:

Replace the power supply.

It's a $20 special. I don't care about perfection.


The 5v disk connector may well come from another regulator. At
the very least measure the 5v at the motherboard multiconector
from the psu.

I won't buy another power supply if this one gets me by. My question is,
what tolerance is safe.

You can compare to a picture here.

This is just the first picture I could find, bigger than a thumbnail.

https://www.badcaps.net/forum/showthread.php?t=2265

The board doesn't have an ATX12V 2x2 power connector, with
the two yellow wires and two black wires.

That means the *CPU* runs off +5V, as well as lots of
other logic. It's natural for such an unbalanced load
situation (only an amp or two load on +12V, but
15 amps off the +5V), for the +5V to be a little
on the low side.

This is called "cross-regulation". There is only one
feedback loop in the ATX power supply for regulation.
What you'd find is, the +5V resting at its low limit,
and the +12V "higher than normal". If an ATX supply is
"non-compliant to crossloading", then the +5V could
even be too low and out-of-spec.

These are examples of ATX specs, of various generations.
The 1.1 version was back when power supplies still had a
-5V rail. The pin on the main connector is missing on
modern supplies, where the =5V pin used to be located.

# This is 20 pin, back when -5V was still on the connector.
http://web.archive.org/web/200304240...12V_PS_1_1.pdf

Table 3. DC Output Voltage Regulation
Output Range Min. Nom. Max. Unit
+12VDC (1) ±5% +11.40 +12.00 +12.60 Volts
+5VDC ±5% +4.75 +5.00 +5.25 Volts
+3.3VDC ±5% +3.14 +3.30 +3.47 Volts
-5VDC ±10% -4.50 -5.00 -5.50 Volts === pre-year-2000 PSUs
-12VDC ±10% -10.80 -12.00 -13.20 Volts
+5VSB ±5% +4.75 +5.00 +5.25 Volts

# This is 20 pin, with the -5V pin pulled from the connector.
https://web.archive.org/web/20040731...X12V_1_3dg.pdf

Table 2
Output Range Min. Nom. Max. Unit
+12VDC (1) ±5% +11.40 +12.00 +12.60 Volts
+5VDC ±5% +4.75 +5.00 +5.25 Volts
+3.3VDC (2) ±5% +3.14 +3.30 +3.47 Volts
-12VDC ±10% -10.80 -12.00 -13.20 Volts === Used for serial port RS232 chip, dont care
+5VSB ±5% +4.75 +5.00 +5.25 Volts

# This covers 24 pin versions of ATX PSU
https://web.archive.org/web/20070103...public_br2.pdf

Table 2

Output Range Min. Nom. Max. Unit
+12V1DC (1) ±5% +11.40 +12.00 +12.60 Volts \___ Allows ATX12V 2x2 to come from its own src
+12V2DC (3) ±5% +11.40 +12.00 +12.60 Volts / even though cct might only have one xfmr
+5VDC ±5% +4.75 +5.00 +5.25 Volts
+3.3VDC (2) ±5% +3.14 +3.30 +3.47 Volts
-12VDC ±10% -10.80 -12.00 -13.20 Volts
+5VSB ±5% +4.75 +5.00 +5.25 Volts

The 3.3V rail has a "private" remote sense wire bonded to
its pin. You will see two wires going into one of the
3.3V pins, and this allows "sniffing" the 3.3V right
at the load. The +5V rail doesn't have this (there were
a few supplies with separate regulation, but that's not
something you will see on a $20 supply).

In any case, the most likely thing to happen to a "+5V mobo",
is some discoloration where the pins are overheating on the
main connector. While there are four wires to carry +5V, you'll
typically find, by using a clamp-on ammeter, that two pins
carry 0.5A more than they should, and two pins carry 0.5A
less than they should. This is called "current hogging" and
it takes careful layout work to balance the pins better.

Back when they made *dual* socket Athlon motherboards,
some of those used to burn on the main connector, due to
a lack of ampacity. You're drawing 30A of current through
24A worth of pins, something like that. Back in the old
days, you could actually get some supplies with 40A
on the +5V, even if the wire couldn't really handle it.

If your device is stable, I wouldn't worry about it
too much. After all, it's run this long, and the caps
didn't burst on it, so you're in "bonus time" country
on your purchase.

*******

Modern power supplies have most of their amps on +12V.
A modern supply is one with an "80%+" efficiency rating
and active power factor correction (PFC).

Typically the +5V is only 20A, and the combined
3.3V and 5V output may not be sufficient to run
a "good" AGP video card plus your processor at the
same time. Some supplies have, say, 3.3V @ 20A, 5V @ 20A,
combined watts on the two rails less than 130W. And
your board is getting close to the limit.

Even if you bought a "1200W PSU", it could tip over, because
they still use a puny 3.3V/5V secondary converter board
for the low rails. And *all* your load is on the low rails.

I used to draw 25A on my Nforce2 board when gaming,
and the supply was good for that much. I could
measure that with my clamp-on ammeter. Eventually, the
end-connector on the video card burned off, and I had
to solder the +5V back on. Worked a treat after being
soldered in place (no more ohmic connections). These
things happen, when a connector works loose.

*******

You're doing just fine. Relax and enjoy your machine,
until that Athlon cooks out and smokes. They discolor
after a while, even if you take care of them.

Paul