[MISTRAL]

My sparingly used (just for about 10 mins twice a day) 4 year old ONIDA brand 20 lts Combine Grill and Convection Microwave oven (Model No. 20CJS25... I looked up the web and found it to be actually manufactured by Galanz (Model No. D80D20ESLR-DK) on a dedicated 220v AC power supply from a Mains Board equipped with a 63Amp RCCB to trip @ 100mA leakage and then via a 20Amp rated MCB for the dedicated line. Please see fig.1


Of late, I observe that the Main Power Supply RCCB (Residual Current Circuit Breaker) trips with the oven in IDLE mode within seconds of connecting the MW to the power supply. However the 20Amp rated MCB doesn't trip at all. Some times I'm able to warm food for about a minute or two before the RCCB trips.

1. Initially I suspected this to be due to a weak HV Diode with the capacitor (Please see Fig.2), assuming that the RCCB trips after the charge accumulation in the capacitor crosses the weakened threshold of the HV Diode. I tested the Diode for fwd and reverse continuity by connecting it to a 12v dc power supply and found it to be in order (conducting in one direction and zero conduction in the other).
2. The capacitor however retained no charge even minutes (pls read moments) after disconnection from the power which though seemed a bit strange, but I guess the 10M ohm register took care of the discharging act.
3. I checked the Capacitor with the help of a digital meter at 2000k and found no sustained continuity in either direction and assume it to be ok, not shorted.
4. I checked the resistor with the multimeter for weakening and found it to be in order and showing the specified rating.
5. Next I suspected the door switches. All the three passed for their desired connectivity (NO/ NC status), burning, stickiness, carbon accumulation were missing and all three look fine.
6. Next in line I checked for defective relays. I disassembled all 6 of them from the PCB and tested them individually for desired continuity of the terminals. All turned out clean and working. I've reassembled them.
7. I targeted in vain the High Voltage Transformer (HVT) for carbon residue, frayed or brittle connections and insulations. No discolouration of insulation on the copper windings or melting of wire spools due to over heating of transformer could be seen. I have not tested the HVT for HT voltage etc... (I neither have the equipment nor expertise to test that). No carbon deposits or damaged connectors which could false trigger the RCCB could be found... All look fine and shining with the HVT.
8. I now suspect it is the Magnetron.
   However I'm not convinced, because (1) the RCCB trips even on IDLE mode. (2) I guess the Magnetron works only after switching on the device in a cooking/ warming mode. (3) In this case the RCCB trips even before I'm able to select a cooking mode leave alone starting the oven.
9. I've checked in vain for frayed wires and dirt on them which could trigger the RCCB by false earthing.
10. I've checked the PCB for burnt tracks etc, but, found no such damage.
11. I've checked for sticky micro switches on the cooking panel and found all of them clicking sharp.
12. Now I'm in a dilemma... is it the RCCB? or is it the capacitor? Or is it the Diode? Or is it the 10m ohm resistor? or is it the Magnetron? or is it the HVT? or the electronic circuit board?

The company charges 500 bucks (almost 1/7th the cost of the oven) just for the visit... additional cost for the spares (I'm told the Magnetron is likely to cost about 700-800 bucks, the capacitor about 300 bucks and the HVT not less than 1200 bucks)... Now that would total to almost 2800 bucks... add 700 bucks to it I can buy a new oven. That sounds outrageously expensive particularly the visit charge...


Would be immensely thankful if someone could please help me pinpoint the defect. Thanks in advance.

Sorry I missed this info... the internal 20amp ceramic fuse has not blown its ok.. shows continuity ...

[MISTRAL]

Alright, I went about the technique of deduction by elimination to narrow on the faulty component. So I hooked up every thing as it was in the case and went one by one with the segments.

1. First to be on check was the PCB panel with the micro switches and relays and the electronics. The HVT, Magnetron and the HV Cap and Diode, disconnected.
Observation: 1. The Panel LEDs lit up normal... no error codes
2. The timers worked
3. The cooking modes were accepted ( the glass would rotate
after 4-5 sec of selecting the cooking modes
4. As anticipated a tumbler with water did not warm
5. Best of all the RCCB or the MCB did not trip

Deduction 1: The problem may not be here, but probably may be with the High Voltage Transformer (HVT) or the Magnetron or the HV Capacitor or the HV Diode.

2. I had already checked the HVT for continuity using digi meter set at 200 ohm. The primary winding showed 2 ohm resistance. The HV terminals had abt 8ohms between them. Observed no continuity between body and the terminals except between a terminal which goes to the HV Cap and another connected to the core of the transformer and the reading at 2k and 20k settings showed about 300 ohms. I connected the input terminals to this transformer but left the High Voltage output to the Maganetron and the Capacitor open and plugged the device to the mains.

Observation: 1. The Panel LEDs lit up normal... no error codes
2. The timers worked
3. The cooking modes were accepted ( the glass would rotate
after 4-5 sec of selecting the cooking modes
4. As anticipated a tumbler with water did not warm
5. Best of all the RCCB or the MCB did not trip

Deduction 2: The problem may not be HVT, but probably may be with the the Magnetron or the HV Capacitor or the HV Diode.

3. I had already checked the Magnetron for continuity using digi meter set at 200 ohm. The filament showed 7 ohm resistance. The HV terminals had no continuity between them or between any of the terminals and the body at 20K, 2M, 20M, 200M or even at lower settings. I connected the HV terminals of the HVT to the Magnetron but left the HV Cap connections open.

Observation: 1. The Panel LEDs lit up normal... no error codes
2. The timers worked
3. The cooking modes were accepted ( the glass would rotate
after 4-5 sec of selecting the cooking modes
4. However a tumbler with water did not warm
5. Best of all the RCCB or the MCB did not trip

Deduction 3: Probably the Magnetron also seems to be OK, but dunno how correct I am, because the tumbler of water DID NOT WARM UP even with the Magnetron now connected.

4. This time I connected the HV capacitor (Please See Attached Pict) too but left the HV Diode unconnected. I had already checked the HV cap with my multimeter with the 10M ohm shunt resistor and the diode removed and had found that though there was no continuity between the terminals of the HV Cap, the low charge between the two terminals dropped rather abruptly than gradually. I had also checked the HV Diode and found it functioning as it should fwd and rev direction after connecting it in series with the Digi Meter to a 12V DC supply. The 10M ohm Resistor read 10m ohms on the 200M ohm scale of the Digi meter. Keeping my fingers crossed I connected the MW to the power supply. waited for 5 minutes on stand by mode and later started the oven as in the earlier tests.

Observation: 1. The Panel LEDs lit up normal... no error codes
2. The timers worked
3. The cooking modes were accepted ( the glass would rotate
after 4-5 sec of selecting the cooking modes
5. As was during the earlier tests, the convection and grill
modes produced heat
4. But, this time too a tumbler with water did not warm
5. Best of all unlike when the problem was first noticed when
the RCCB or the MCB would trip even on standby mode,they
did not trip now

Deduction 4: Already beginning to feel that the capacitor seems ok, now left is the Diode. However, when the oven did not warm the tumbler of water even a bit I'm still unable to discount the Magnetron.

ANOTHER IMPORTANT OBSERVATION: AFTER THE LAST OF THE TESTS ABOVE, I TRIED DISCHARGING THE HV CAPACITOR MOMENTS AFTER SWITCHING OFF THE MICROWAVE, BY SHORTING THE LEADS OF THE HV CAP, BUT FAILED TO NOTICE ANY HV SPARK OR CRACKLE. PROBABLY ITS BECAUSE OF THE 10M ohm SHUNT RESISTOR BUT NOT SO CONVINCED THAT THE DISCHARGE CAN BE SO SWIFT. SO NOW I AM WONDERING IF AT ALL THE HV TRANSFORMER IS PRODUCING ANY HIGH VOLTAGE.

Question:

1.Is there a way to know if the HVT is ok with out holding a well insulated screwdriver near the HT terminal to see if there is high voltage spark bridge as the TV techies of the yester years did? Though I have a screw driver with a 5kv insulation I dont want to attempt that.

2. I wish someone could tell me which of thse seems most likely to be at fault, the magnetron, the HVT or the HV Cap.

3. I wish some one could enlighten me if the magnetron works only when the capacitor is connected.

Shall be grateful if some one could throw some light... Thanks in advance.

Edit: I forgot to mention, I checked the thermostat and found it in order too contacts closed in normal condition...

[MISTRAL]

No matter how strange this may sound... with no responses or any ideas to get ahead with coming by, I decided to check out the HVT. after seeing videos on the net about how dangerous it could be trying to fathom the HV strength from the HVT, I thought of a round about way to see if the transformer was doing its work.

I took out the HVT once again and fed the transformer about 1/20 th the rated ac input which is about 12DC pulses @ 3amp (since I did not have a standalone step down transformer to feed 12v ac from my auxiliary 12v/3Amp DC power supply) by intermittently touching one of the input terminals while the other terminal of the 12V Dc was clamped to the other input terminal of the HVT. I had clamped the multimeter terminals to the high voltage secondary at 750 v ac range. I got a 200v step up deflection every time I touched the input terminal. Though a crude assessment but a positive indication that the HVT was working.

Now remained the HV Capacitor and its HV Diode. since the RCCB and the MCB would not trip with the capacitor connected, I decided to go ahead and connect the High Voltage Diode (quite sure by now, that this was the culprit). The microwave that would not heat, with just the capacitor connected, was now working again and warming the water in the tumbler. I repeated the warming process a few more time... It was heating now every time like before. I put the oven in stand by and the MCB and the RCCB does not trip now... its working like new again...

Dunno what caused the malfunction, but the experience has brought me more insight about the working of Microwave ovens and the dangers stored inside it... I strongly feel that a connector here or a connector there must have loosened due to variation in temperatures inside the oven causing it to malfunction. I hope my account of experiences will help others like me who may not be professionally connected with electricity and maintenance, but who by virtue of their interest and experience in electronics and electricals can do enough to maintain their own appliances and gadgets. It looks like as it has happened many times in the past with other appliances, I have been saved from spending at least 500 bucks on account of a techies visit, if not anything for spares.

Though I am an architect by profession, I have a passion for electronics and electricals from an early childhood age and this background spanning over 3and a half decades helped me a lot in many a diagnosis and test. Lookes like it has come to my help again. I agree its rather a bit too early to rejoice, I'm keeping my fingers crossed and hope the situation holds. Will post again if there is a change.

Thanks everyone for your patience, support and for stopping by.