Jethro_uk wrote:
On Sat, 15 May 2021 09:05:15 +0100, Brian Gaff \(Sofa\) wrote:

But assuming its not some computer controlled device, there is very
little to go wrong, so it might be a simple repair.
Brian

It might be. But as I say 15 years in ????


Also, there aren't many things I am afraid to fiddle with (happily work
on gas, petrol and electricity) but ****ing up a microwave is not a risk
I like to run.

I have some first hand experience with how many
joules are stored in those old microwave oven oil-filled caps.

You don't want to be standing near it, if the HV arcs
over inside the unit. Lost hearing in one ear for
ten minutes.

I'm willing to take the cover off, with its torx security
screws for a "look-see", but touching is out of the question.

And don't be discharging the cap with a screwdriver either.
It's like a gun going off. You want some sort of resistor for
discharging.

http://microwaveovenparts.weebly.com/

"For the high voltage capacitor in a microwave oven, use a
100K ohm resistor rated at least 5 kilovolts and several watts
for your discharge widget, with a clip lead to the chassis.
As a practical matter, a single resistor like this will be hard
to find. So, make one up from a series string of 10 to 20 1/2 W
or 1 W normal resistor.

The reason for specifying the resistor in this way is for
voltage hold-off. Common resistors only are rated for 200 to 500 V,
but there may be as much as 5 kV on the HV cap. You don't want
the HV zapping across the terminals of the resistor. Special
high voltage resistors are available but they are expensive
and not readily available from common electronics distributors".

By that they mean:

--- 10K --- 10K --- 10K --- 10K --- 10K --- 10K ---10K --- 10K --- 10K --- 10K ---

which equals 100K total resistance and 10*resistor_power_rating .
I'd probably aim for a higher value, as that's still likely
to be "noisy enough" if it is energized. The circuit has a
bleeder, but enthusiasts assume the bleeder is open circuit,
before working on such things, and that the capacitor is still
fully armed. The oven itself happens to use 100K ohms, according
to one schematic.

I've worked with higher voltage than that, and where my
resistors were soldered together, I coated with corona dope,
because there was a bit of corona without the coating in place.
You want the resistors soldered, to at least avoid your
contraption "accidentally falling apart while electrified" :-)
Don't just twist the legs on ten resistors like that, when one
croc clip is running at 5kV above ground.

If you're clever, you can probably make Perspex handles for the
"contacts" that touch the capacitor. To avoid getting too close
to the work.

I made a Tesla coil once, but that wasn't nearly as exciting
as Microwave ovens. I could hardly draw any purple sparks out
of mine, and maybe an inch-long discharge was all I got from
the project. Tesla coils are safer, because the high frequency
current travels on the outside of your skin and not through
nerves. The Microwave is DC and your entire body is the
conductor for those purposes.

There's more to know on this topic, and obviously because
Chris is still posting, he knows all this. Me showing
the diagram above, is to at least make you think twice before
sticking a screwdriver shaft across the oil-filled cap terminals.
Using the discharge apparatus is still mondo-dangerous. One
of the reasons I'm still alive today, is keeping a fresh
supply of adrenalin while working. If you're the type that
"gets sleepy" while working on stuff, then just leave the
cover on your microwave.

The microwave discharge that caused me to lose hearing for
ten minutes, that was a flaw in the oven, and *not* me
reaching inside. The conformal coating failed somewhere,
and that's where it arced across. The microwave was one
at work, used to heat microwave popcorn. We learn two things
here. One is, microwave cap is powerful enough, to knock
out your hearing. Two is, microwave popcorn (salt-loaded
clouds of vapor), are a test of the conformal coating,
and too many bags of such popcorn products going
through a microwave, could lead to an "acoustic surprise".

Paul