R-1900M (serv.man7). Test Procedures - Sharp Home Appliance Service Manual (repair manual). Page 6

R1900M

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7) If the display fails to clear when the STOP/CLEAR pad is depressed, first verify the flat ribbon cable is making good contact, verify that the door

sensing switch (stop switch) operates properly; that is the contacts are closed when the door is closed and open when the door is open. If the
door sensing switch (stop switch) is good, disconnect the flat ribbon cable that connects the key unit to the control unit and make sure the door
sensing switch is closed (either close the door or short the door sensing switch connector). Use the Key unit matrix indicated on the control
panel schematic and place a jumper wire between the pins that correspond to the STOP/CLEAR pad making momentary contact. If the control
unit responds by clearing with a beep the key unit is faulty and must be replaced. If the control unit does not respond, it is faulty and must be
replaced. If a specific pad does not respond, the above method may be used (after clearing the control unit) to determine if the control unit or
key pad is at fault.

8) For the door key unit, check the open/short and/or connection of the switch unit and the harness, too.

8. If the Key unit is defective.

1) Disconnect the power supply cord, and then remove outer case and rear cover.

2) Open the door and block it open.

3) Discharge two high voltage capacitors.

4) Replace the Key unit.

5) Reconnect all leads removed from components during testing.

6) Re-install the rear cover and the outer case (cabinet).

7) Reconnect the power supply cord after the rear cover and the outer case are installed.

8) Run the oven and check all functions.

The following symptoms indicate a defective control unit. Before replacing the control unit, perform the Key units test (Procedure O) to determine if
control unit is faulty.

Connect the power supply cord. And check for followings.

1. In connection with pads.

1) When touching the pads, a certain group of pads do not produce a signal.

2) When touching the pads, no pads produce a signal.

2. In connection with display

1) At a certain digit, all or some segments do not light up.

2) At a certain digit, brightness is low.

3) Only one indicator does not light.

4) The corresponding segments of all digits do not light up; or they continue to light up.

5) Wrong figure appears.

6) A certain group of indicators do not light up.

7) The figure of all digits flicker.

3. Other possible problems caused by defective control unit.

1) Buzzer does not sound or continues to sound.

2) Cooking is not possible.

4. If the protectors F1 and/or F2 on the control unit PWB are open, the control unit is defective. To check, carry out following procedures.

i)

Disconnect the power supply cord, and then remove outer case and rear cover.

ii) Open the door and block it open.

iii) Discharge two high voltage capacitors.

iv) Remove the control unit from the control panel, referring to “CONTROL PANEL ASSEMBLY AND CONTROL UNIT REMOVAL”.

v) Check the continuity of the protectors F1 and F2 on the control unit

1. Check the out put voltage of power unit at CN21 as follows.

i)

Disconnect the power supply cord, and then remove outer case and rear cover.

ii) Open the door and block it open.

iii) Discharge two high voltage capacitors.

iv) Disconnect the leads to the primary of the power transformer.

v) Ensure that these leads remain isolated from other components and oven chassis by using insulation tape.

vi) Check the voltage between Pin No1 and No2 of CN21.

vii) The output voltage should be approx. 24V. 

viii) If not so, the power unit is defective.