"Danny D." wrote in message
...
On Mon, 26 Aug 2013 02:19:29 -0400, micky wrote:

Since apparently there is no current in that direction, how about
taking out the resistor and seeing what it says then?

The resistor is to prevent the diode from burning
up with too much current, I believe.

If I take out the resistor, I probably blow up the diode.

That does not sound like good advice, so I didn't read the
rest of the suggestions for fear of being led astray.



It is highly unlikely that any small dry battery would be be able to source
enough current to damage the HV rectifier diode from a microwave oven.
However, the way that you have gone about testing it is perfectly valid, and
the test result you have obtained, indicating a forward conduction with a
voltage drop of 7 volts measured across the diode, and reverse blocking with
9 volts measured across the diode, would suggest that the diode is good. The
reason that you can't normally use a 'standard' ohm meter to measure these
high voltage diodes, is self evident in the large forward voltage drop that
they produce. The internal meter battery often does not source enough
voltage to bias the diode 'on' sufficiently to obtain any meaningful
reading. If the meter has a 'diode test' setting, this most certainly will
not. The reason that these high voltage diodes behave like this, is because
internally, they are many ordinary diodes stacked in series to get the high
PIV rating. Think 10 x 1N4007 in series, which would give you a diode with a
PIV rating of around 10 kV, but with a forward voltage drop of 6 or 7 volts.

I agree with others that the problem is probably the mag. Its heater may
have gone open. That can be measured with the transformer heater winding
disconnected. Other possibilities are a high voltage fuse gone open, if it
has one, or a thermal protection switch that's gone open or failure of the
high voltage transformer control relay, or failure of the circuitry that
drives the relay.

When you start it up, and expect it to be producing microwaves, does the
transformer 'grunt' ? Current limiting for the mag is achieved magnetically
with shunts in the transformer core, and this causes it to produce an easily
audible "I'm struggling" buzz when the mag is working and drawing current.

If you do hear this, the chances are that the high voltage circuitry and the
mag are probably working, and it may be something else. Although rare, I
have on odd occasions seen crap blocking the waveguide into the oven cavity,
although that may no longer be a possibility with modern designs, as I
haven't had occasion to work on any for some time.

Aside from all this, if you are not greatly experienced working on
line-powered electronics with the covers removed, please be VERY CAREFUL !!!
Apart from the possibility of serious shock or death from accidentally
wrapping yourself around the line power circuitry, the high voltage supply
on a microwave oven is also potentially lethal. At the very least, it can
give you a very serious and painful shock, and if the mag is not drawing any
current, the high voltage capacitor can stay charged to a very high voltage
for a very long time after power is removed.

TAKE HEED !!! :-\

Arfa