R-291KM - Sharp Home Appliance Service Manual (repair manual). Page 17

This unit uses CMOS LSI in the integral part of the circuits. 
When handling these parts, the following precautions should 
be strictly followed. CMOS LSI have extremely high impedance 
at its input and output terminals. For this reason, it is easily in -
fluenced by the surrounding high voltage power source, static 
electricity charge in clothes, etc., and sometimes it is not fully 
protected by the built-in protection circuit.

In order to protect CMOS LSI.

1) When storing and transporting, thoroughly wrap them in 

aluminium foil. Also wrap PW boards containing them in alu -
minium foil.

2) When soldering, ground the technician as shown in the fig -

ure and use grounded soldering iron and work table.

approx. 1M ohm

We describe the procedures to permit servicing of the touch 
control panel of the microwave oven and the precautions you 
must take when doing so. To perform the servicing, power to the 
touch control panel is available either from the power line of the 
oven itself or from an external power source.

1) Servicing the touch control panel with power supply of the oven:

CAUTION: THE HIGH VOLTAGE TRANSFORMER OF THE MI -

CROWAVE OVEN IS STILL LIVE DURING SERVIC -
ING AND PRESENTS A HAZARD. 

 Therefore, before checking the performance of the touch 

control panel, 

(1) Disconnect the power supply cord and then remove 

outer case.

(2) Open the door and block it open.

 Discharge high voltage capacitor.

(3) Disconnect the leads to the primary of the power transformer.

(4) Ensure that these leads remain isolated from other com -

ponents and oven chassis by using insulation tape.

After that procedure, re-connect the power supply cord.

(1) After checking the performance of the touch control 

panel,

(2) Disconnect the power supply cord.

 Open the door and block it open.

(3) Re-connect the leads to the primary of the power transformer.

(4) Re-install the outer case (cabinet).

(5) Re-connect the power supply cord after the outer case 

is installed.

(6) Run the oven and check all functions.

 

a)  On some models, the power supply cord between

  the touch control panel and the oven itself is so short
  that the two can't be separated. For those models,
  check and repair all the controls (sensor-related ones
  included) of the touch control panel while keeping it 
  connected to the oven.

 

b)  On some models, the power supply cord between the

touch control panel and the oven proper is so long
enough that they may be separated from each other.
For those models, therefore, it is possible to check
and repair the controls of the touch control panel while
keeping it apart from the oven proper; in this case you
must short both ends of the door sensing switch (on
PWB) of the touch control panel with a jumper, which  
brings about an operational state that is equivalent to
the oven door being closed. As for the sensor-related 
controls of the touch control panel, checking them is
possible if the dummy resistor(s) with resistance equal
to that of the controls are used.

Disconnect the touch control panel completely from the 

oven proper, and short both ends of the door sensing switch 
(on PWB) of the touch control panel, which brings about 
an operational state that is equivalent to the oven door be-
ing closed. Connect an external power source to the power 
input terminal of the touch control panel, then it is possible 
to check and repair the controls of the touch control panel; 
it is also possible to check the sensor-related controls of the 
touch control panel by using the dummy resistor(s).

Tools required to service the touch control panel assembly.

1) Soldering iron: 60W

(It is recommended to use a soldering iron with a grounding terminal.) 

2) Oscilloscope: Single beam, frequency range: DC - 10MHz 

type or more advanced model.

3) Others: Hand tools

1) Before turning on the power source of the control unit, re-

move the aluminium foil applied for preventing static elec -
tricity.

2) Connect the connector of the key unit to the control unit be -

ing sure that the lead wires are not twisted.

3) After aluminium foil is removed, be careful that abnormal 

voltage due to static electricity etc. is not applied to the input 
or output terminals.

4) Attach connectors, electrolytic capacitors, etc. to PWB, 

making sure that all connections are tight.

5) Be sure to use specified components where high precision 

is required.

R-291(K)M

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