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R27STMA

9 – 1

R27STMA

Service Manual

 

The touch control section consists of the following units as shown in
the touch control panel circuit.

(1) Switch Unit      (2) Control Unit       

The principal functions of these units and their related signals are
explained below.

The switch unit is composed of a matrix, signals generated in the LSI
are sent to the switch unit through P52, P54, P55 and P56. When a
button is pressed, a signal is completed through the switch unit and
passed back to the LSI through AN2, AN3 and AN4 to perform the
function that was requested. When the jog dial is rotated, the encoder
converts the signal from the power source circuit into the pulse signal,
and the pulse signal is sent to the LSI through P51 and P46.

Control unit consists of LSI, reset circuit, indicator circuit, power
source circuit, relay circuit, buzzer circuit and synchronizing signal cir-
cuit. 

1) Reset Circuit

This circuit generates a signal which resets the LSI to the initial
state when power is supplied.

2) Indicator Circuit

This circuit consists of 4-digits, 12-segments and 3-common elec-
trodes using a Liquid Crystal Display.

3) Power Source Circuit

This circuit generates voltage necessary in the control unit from the
AC line voltage. In addition, the synchronizing signal is available in
order to compose a basic standard time in the clock circuit.

4) Relay Circuit

To drive the magnetron, fan motor, turntable motor and light the
oven lamp.

5) Buzzer Circuit

The buzzer is responsive to signals from the LSI to emit audible
sounds (switch unit touch sound and completion sound).

6) Synchronizing Signal Circuit

The power source synchronizing signal is available in order to com-
pose a basic standard time in the clock circuit. It accompanies a
very small error because it works on commercial frequency.

7) Door sensing Switch

A switch to “tell” the LSI if the door is open or closed.

8) Back Light Circuit

A circuit to drive the back light (Light emitting diodes LD1 - LD4).

9) Energy Save Mode Circuit

A circuit to drive the touch control transformer. If the oven has not
been used for 3 minutes or more, the contacts of the relay RY-6 will
be opened and the control unit will not be energized.

This unit uses CMOS LSI in the integral part of the circuits. When han-
dling these parts, the following precautions should be strictly followed.
CMOS LSI have extremely high impedance at its input and output ter-
minals. For this reason, it is easily influenced 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 aluminium foil.

2) When soldering, ground the technician as shown in the figure and

use grounded soldering iron and work table.

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 exter-
nal power source.

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

CAUTION: THE HIGH VOLTAGE TRANSFORMER OF THE MICRO-

WAVE OVEN IS STILL LIVE DURING SERVICING 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.

3) Discharge high voltage capacitor.

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

5) Ensure that these leads remain isolated from other components

and oven chassis by using insulation tape.

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

After checking the performance of the touch control panel,

1) Disconnect the power supply cord.

2) 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 

Symbol

Voltage

Application

VSS

-5V

LSI(IC1) 

approx. 1M ohm

R27STMA

9 – 2

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 opera-
tional 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 resis-
tance equal to that of the controls are used.

2. Servicing the touch control panel with power supply from an exter-

nal power source:

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 being closed. Connect an exter-
nal 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 termi-
nal.)

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, remove the

aluminium foil applied for preventing static electricity.

2) Connect the connector of the key unit to the control unit being 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 ter-
minals.

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.