R-743. Text - Sharp Home Appliance Service Manual (repair manual). Page 2

1

743M -

Microwave ovens contain circuitry capable of producing very high voltage and current.  Contact with the following parts
will result in electrocution.
High voltage capacitor, High voltage transformer, Magnetron, High voltage rectifier assembly, High voltage harness.

1) Reconnect all leads removed from components

during testing.

2) Replace the outer case (cabinet).
3) Reconnect the supply.
4) Run  the oven. Check all functions.

Microwave ovens should not be run empty. To test for
the presence of microwave energy within a cavity, place
a cup of cold water on the oven turntable, close the door
and set the power to HIGH and set the microwave timer
for two (2) minutes. When the two minutes has elapsed
(timer at zero) carefully check that the water is now hot.
If the water remains cold carry out 3D checks and re-
examine the connections to the component being tested.

1) Disconnect the supply.
2) Door opened, and wedged open.
3) Discharge high voltage capacitor.

The high-voltage capacitor remains charged about
60 seconds after the oven has been switched off.
Wait for 60 seconds and then short-circuit the
connection of the high-voltage capacitor (that is,
of the connecting lead of the high-voltage recti-
fier) against the chassis with the use of an insu-
lated screwdriver.

Sharp recommend that wherever possible fault-finding
is carried out with the supply disconnected. It may in,
some cases, be necessary to connect the supply after
the outer case has been removed, in this event carry out
3D checks and then disconnect the leads to the primary
of the power transformer. Ensure that these leads
remain isolated from other components and the oven
chassis. (Use insulation tape if necessary.) When the
testing is completed carry out 3D checks and reconnect
the leads to the primary of the power transformer.

When all service work is completed, and the oven is fully assembled, the microwave power output should be checked
and a microwave leakage test carried out.

Servicing and repair work must be carried out only by
trained service engineers.

All the parts marked "*" on parts list are used at
voltages more than 250V.

Removal of the outer wrap gives access to potentials
above 250V.

All the parts marked "

∆

" on parts list may cause undue

microwave exposure, by themselves, or when they
are damaged, loosened or removed.

Service engineers may be exposed to microwave
energy if the unit is improperly used or connected.
All input and output microwave connections,
waveguides, flanges and gaskets must be se-
cured. Never operate the device without a micro-
wave energy absorbing load attached. Never
look into an open waveguide or antenna while the
device is energized.

Whilst the maximum leakage permitted in BS EN 60335-2-25 is 50Wm2 (equivalent to 5mW/cm2), it is not normal to
detect any significant leakage, and therefore any detected leakage should be investigated.

THE WIRES IN THIS MAINS LEAD ARE COLOURED IN

ACCORDANCE WITH THE FOLLOWING CODE:
GREEN-AND-YELLOW : EARTH        BLUE : NEUTRAL        BROWN : LIVE