4.4.2. NN-ST677M, ST667W
SYMPTOM STEP CHECK RESULT CAUSE/CORRECTIONS
No display when oven is first plugged
in
1 Fuse pattern of D.P.C. Normal →Step2
Open Replace D.P.C. or Fuse
Pattern
2 Low voltage transforment (L.V.T.)
secondary voltage
Abnormal 0V L.V.T.
Normal →Step3
3 IC1 pin23 voltage Abnormal ZD11, Q11, ZD10, Q10
Normal=3.3V IC1, CX320, Display
No key input 1 Membrane switch continuity Abnormal Membrane switch
Normal IC1
No beep sound 1 IC1 pin 29 voltage Abnormal IC1
Normal=3.3V BZ310, Q224
Power relay A(RY2) does not turn on
even though the program had been set
and the start pad is tapped
1 IC1 pin 2 voltage while operation Abnormal IC1
Normal=3.3V →Step2
2 Collector of Q223 Abnormal Q223
Normal 0.7V RY2
No microwave oscillation at any power 1 IC1 pin 1 voltages while operation at high
power
Abnormal IC1
Normal=3.3V →Step2
2 Collector of Q220 Abnormal Q220, Q225
Normal 0.7V →Step3
3 Short circuit between collector of Q220
and emitter of Q225
Still not turn on RY1
RY1 turns on Q221, Q222
Dark or unclear display 1 Replace display and check operation Normal Display
Abnormal IC1
Missing or lighting of unnecessary
segment
1 Replace IC1 and check operation Normal IC1
Abnormal Display
H95/H97/H98 appears in window and
oven stops operation.Program High
power for 1 minute and conduct
following test quickly, unless
H95/H97/H98 appears and oven stops
1 Unplug CN702 (2 pin) connector and
measure voltage between terminals
Abnormal=0V 1. Latch Switch
2. D.P.C. /Power Relay
Normal=220~240V →Step2
2 Unplug CN701 (3 pin) connector and
measure pin3 voltage
Abnormal=0V D.P.C.
Approx. AC 3V Magnetron
4.5. Simple way of H.V. Inverter/magnetron troubleshooting
Purpose:
Simple way (3/38 seconds rule) of identifying whether it’s Magnetron, Inverter or others.
Set-up:
The unit under question is connected through the Ammeter as shown below.
Procedure:
Follow the matrix table below to identify the problem source.
Note:
Do not replace both Inverter board and Magnetron simultaneously and automatically without going through this
procedure.
Power will: Ammeter reading is: To do: Remedy:
Shut off in 38 seconds
after “Start”.
1. Between 0.5A and 1.0A. Check and repair open magnetron circuit Open magnetron wiring between Inverter
and magnetron terminal.
2. Between 1.0A and 2.0A. Check continuity of D702 in Inverter PCB.
1. D702 shorted Replace H.V.Inverter
(F606YM300BP)
2. D702 is OK Replace magnetron
Shut off in 3 seconds
after “Start”
1. Less than 0.5A Check open circuit: Latch Switch, DPC,
Power Relay and CN701
Replace defective component(s), or
correct switch, cables and connectors.
13
NN-ST657S / NN-ST677M / NN-ST667W / NN-ST657W
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