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.

That is way too cheap for a decent power supply. When I do a build,
most of the PSUs that I end up looking at are $80 at a minimum but
usually spend a lot more. However, I don't know what you got for
capacity (VA or watts) for your PSU, or its efficiency (less efficient =
more heat = more thermal stress = less capacity to internal components
since the A/C outlet isn't changing its voltage).

Figure on losing 5% capacity each year with a decent PSU. That's why I
buy PSUs that are much higher in capacity than the load they will
initially need to handle. For a 400W load, I'd get a 750W PSU because
in 8 years (what I like to build for longevity) the PSU will still be
more than enough at 66% of its initial capacity. The 5% is a rough
estimate of capacity reduction, but PSUs do wane over time. Cheapies
die off much quicker. The PSU supplies the life blood to the computer.
If you do the build, you are the hematologist for the computer.

How old is your $20 PSU?

Was it a pre-built computer or did you do your own build? The PSUs that
come in pre-builts are often minimal for the standard configuration by
the model (with the same PSU used within a family of models). They
don't overbuild since they're maximizing their profit margin.

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.

4.75V is the minimum. Less voltage means supplying more current to
handle the same load, and more current puts more thermal stress on the
supply components. The computer's PSU is a switching power supply.
Unless you have an oscilloscope, you only know the average voltage, but
won't know how much ripple there is.

https://www.tomshardware.com/reviews...u,4042.html#p3

Also, rare few consumers every calibrate their multimeters. They buy
them and figure they are accurate, and think they will remain accurate.
Calibration costs money and why consumers rarely get their meters
calibrate initially or at 5-year intervals. Consumers get their meters
calibrated less than 1% as often as they check their car's tire
pressures, so consumers never get their meters calibrated. Considering
the low-end meters that consumers buy, calibration is usually more
expensive (perhaps $95) than the meter. As such, consumer-grade
non-calibrated meters are only to give an indication of voltage level,
not a precise measurement. Although you measured 4.75V, was it really
that voltage? The voltage could be worse (lower) or better (higher).

Have you noticed any of your USB devices not functioning or acting
flaky? Some USB devices don't need power from the USB port since they
have their own, like for printers, a UPS (with its USB connection for
monitoring software), and some external HDDs in cases with their own
power adapter. It would be the non-self-powered USB devices you'd have
problems with.

Computers always cost money - to buy, to maintain, to upgrade, for
Internet access, the power consumption, etc - so save up and get a
better PSU. $20 is a throw-away PSU. Presumably that wasn't an 80% off
special sale.