Differential probe isn't going to keep *you* safe...... The isolation
transformer might. (And most variac's are NOT isolation transformers)

On Tue, Nov 26, 2024 at 9:53?AM Ke-Fong Lin via groups.io
<anotherlin@...> wrote:

Hi everyone,

My HP8594E has a temperamental PSU, sometimes everything just works fine otherwise the SA just doesn't turn on.
Something is obviously wrong and needs to be debugged.

I've not opened the case yet. I've just searched a bit the internet ().
HP doesn't provide schematics but people have already attempted repair.

I may have to work on the "mains connected" (120VAC US or 220VAC Europe) side.
So I just want to be sure that I would be safe with a differential probe, and if I use a diff probe, an isolation transformer wouldn't be needed.

Seems that "curious marc" is using a variac as some form of isolation transformer:


In general, do you guys have some links or checklists for safety when working on these kind of switch mode power supply.
Thanks!


Best regards,

On Tuesday 26 November 2024 09:53:47 am Ke-Fong Lin via groups.io wrote:

I may have to work on the "mains connected" (120VAC US or 220VAC Europe) side.
So I just want to be sure that I would be safe with a differential probe, and if I use a diff probe, an isolation transformer wouldn't be needed.

An isolation transformer is always a good idea when working on mains-connected circuits. If nothing else, a couple of regular transformers connected in cascade with the second "primary" being the mains source for what you're working on will do.

Seems that "curious marc" is using a variac as some form of isolation transformer:

I haven't looked at the link, but a variac is NOT an isolation transformer.



--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, ?a critter that can
be killed but can't be tamed. ?--Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin

Don't get too enamored with tracing out and debugging your
power supply, 9 times out of 10, the problem will be simply
a case of bad electrolytic capacitors. I am not being
casual with my statistics, this is hard won personal experience.

The quickest way to "get'er'done", is to get an ESR meter, and
check each electrolytic capacitor. Look exclusively for those
that have ESR much greater than 1 ohm. When you find one, remove
it, and measure its capacitance. Most of the time, it be low
in an inverse relation to how high the ESR is.

When you find a bad cap, replace ALL identical caps in the supply.

Temperamental supplies are that way often because the electrolytic
capacitors get worse when they are cold. For instance, a becoming
very bad 100uf cap may be 25uf when hot, but only 2uf when cold.

25uf isn't great, but it may be enough to keep running, but 2uf
isn't enough to start the supply (and warm up the cap).

I do virtually all of my power supply debugging and repair with
the power off, and no schematics.

-Chuck Harris


On Tue, 26 Nov 2024 06:53:47 -0800 "Ke-Fong Lin via groups.io"
<anotherlin@...> wrote:

Hi everyone,

My HP8594E has a temperamental PSU, sometimes everything just works
fine otherwise the SA just doesn't turn on. Something is obviously
wrong and needs to be debugged.

I've not opened the case yet. I've just searched a bit the internet (
). (
) ) HP
doesn't provide schematics but people have already attempted repair.

I may have to work on the "mains connected" (120VAC US or 220VAC
Europe) side. So I just want to be sure that I would be safe with a
differential probe, and if I use a diff probe, an isolation
transformer wouldn't be needed.

Seems that "curious marc" is using a variac as some form of isolation
transformer:

In general, do you guys have some links or checklists for safety when
working on these kind of switch mode power supply. Thanks!

Best regards,

开云体育

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