Sharp microwave does everything but heat

[ManhattanMan]

4 year old sharp microwave that overnight decided it'd do everything but
heat.

All functions normal, except heat. There is 120V applied to the HV
transformer primary, secondary measures 150 ohm, filament on both
transformer and magnetron shows negligible ohms, diode tests good on back
voltage, HV cap appears OK, magnatron does get very hot after a couple of
ten second tests...... ?????

I've had several with bad micro switches, HV cap, etc., but this one does
everything but heat for no reason.....

TIA

 

[Jerry G.]

The HV diode cannot be properly checked with an ohm meter. The
breakover voltage required is too great. If the diode is opened there
is no way to know wiithout doing a proper voltage check. For this you
will require the proper HV probe. If the diode is shorted the HV
transformer would be overloading and most likely the fuse will be
blown.

Common causes of failures as what you are describing are from the
diode being opened, transformer failure filter capacitor failure, and
or magnetron failure.

Take care for safety issues in dealing with the high voltage area of a
microwave oven.

Jerry G.

 

[Arfa Daily]

4 year old sharp microwave that overnight decided it'd do everything but
heat.

All functions normal, except heat. There is 120V applied to the HV
transformer primary, secondary measures 150 ohm, filament on both
transformer and magnetron shows negligible ohms, diode tests good on back
voltage, HV cap appears OK, magnatron does get very hot after a couple of
ten second tests...... ?????

I've had several with bad micro switches, HV cap, etc., but this one does
everything but heat for no reason.....

TIA

The HV diode cannot be properly checked with an ohm meter. The
breakover voltage required is too great. If the diode is opened there
is no way to know wiithout doing a proper voltage check. For this you
will require the proper HV probe. If the diode is shorted the HV
transformer would be overloading and most likely the fuse will be
blown.

Common causes of failures as what you are describing are from the
diode being opened, transformer failure filter capacitor failure, and
or magnetron failure.

Take care for safety issues in dealing with the high voltage area of a
microwave oven.

Jerry G.


Also bear in mind that microwave ovens should not be run with no load (if
you get it going again). A beaker of water at least, is required to be in
the oven cavity when the mag is emitting, otherwise, it can be damaged.
Nothing come adrift in the waveguide, I suppose ? Might account for the mag
getting very hot very quickly, and apparently no RF in the cooking cavity,
if the mag is driving into an abnormal load ?

Arfa

 

[Samuel M. Goldwasser]

4 year old sharp microwave that overnight decided it'd do everything but
heat.

All functions normal, except heat. There is 120V applied to the HV
transformer primary, secondary measures 150 ohm, filament on both
transformer and magnetron shows negligible ohms, diode tests good on back
voltage, HV cap appears OK, magnatron does get very hot after a couple of
ten second tests...... ?????

I've had several with bad micro switches, HV cap, etc., but this one does
everything but heat for no reason.....

My guess would be magnetron. Since it's getting hot quickly, it means it's
getting more than filament power, but not putting anything out.

But check he HV diode properly as described in another post or
at the S.E.R FAQ site below, or replace it. It will have a 6-10
V forward drop, so a VOM or DMM can't test it.

And the SAFETY issues are extremely critical!

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[ManhattanMan]

My guess would be magnetron. Since it's getting hot quickly, it
means it's getting more than filament power, but not putting anything
out.

But check he HV diode properly as described in another post or
at the S.E.R FAQ site below, or replace it. It will have a 6-10
V forward drop, so a VOM or DMM can't test it.

And the SAFETY issues are extremely critical!

Diode checked ok using the technique described w/15V applied, and the tests
were made with a saucer of water in the oven that didn't even lose it's
chill.
Oh well, we wanted a bigger model anyway....

Thanks for the replies........

 

[Jim Yanik]

. The HV diode cannot be properly checked with an ohm meter. The
breakover voltage required is too great. If the diode is opened there
is no way to know wiithout doing a proper voltage check. For this you
will require the proper HV probe. If the diode is shorted the HV
transformer would be overloading and most likely the fuse will be
blown.

Common causes of failures as what you are describing are from the
diode being opened, transformer failure filter capacitor failure, and
or magnetron failure.

I have a really old Sharp MW,and the HV cap failed(a big POP),but I
replaced the HV diode along with the cap for that reason. It still cost
only $25 for all the parts from the local appliance repair store.
That was several years ago,the MW is almost 30 yrs old now.

 

[Stacey Chuffo]

4 year old sharp microwave that overnight decided it'd do everything but
heat.

All functions normal, except heat. There is 120V applied to the HV
transformer primary, secondary measures 150 ohm, filament on both
transformer and magnetron shows negligible ohms, diode tests good on back
voltage, HV cap appears OK, magnatron does get very hot after a couple of
ten second tests...... ?????

I've had several with bad micro switches, HV cap, etc., but this one does
everything but heat for no reason.....

TIA