Repairing a Microwave oven

[Dialtone]

Hello all:
I have a Sharp brand Microwave oven that just went out.
The main fuse (20 amp fast acting) was blown. Replacing the fuse and trying the unit resulted in a deep growling sound for 1-2 seconds and the fuse blew again (I expected it might).
Closer inspection revealed a large capacitor (9uf @ 2300 volts AC 60 cycles) that appears to be shorted when I test it with multimeter.

My question involves the magnatron and step up transformer. What is the likelyhood that either of these items could also be defective and took out the capacitor.

Any tips for a quick test for them. I have absolutely no clue how to test the microwave source.

I checked the transformer for a primary to secondary short and it looks OK. Also no opens in the primary or secondary windings.

Thanks
Dialtone

 

[williB]

did you test the Fan ?? the one that disperses the microwaves??
that is the only thing that i can think of that would make a growling noise.. i took one apart a while ago , it had a 1.1 uF cap in it rated at 2100V..

 

[Dialtone]

Well, I have heard enough 60 cycle transformer humming sounds to tell the sound was produced as a result of a short. This definately was the transformer under an abnormal load. Shorted capacitor seems to bear that out.
Dialtone

 

[awright]

Microwave Oven

You have already surmized (correctly, I think) that the capacitor is the problem, and I believe that follows the normal statistical pattern of component failure modes.

My main reason in writing is to caution you about trying to test the magnetron in any manner other than heating a glass of water in the food chamber with all doors, seals, and interlocks functioning and in good condition. Control of microwave leakage from an oven is very strongly dependent upon the condition and cleanliness of the seals around the door and around the food chamber, and trying to bring any antenna leads out of the oven could greatly increase leakage.

Additionally, I don't think it is possible to make any kind of accurate measurements inside such a small chamber because the pickup would be in such a complex, reverberant field. Measured levels would vary over a large range as you moved the pickup around the food chamber, and it would be very difficult to move the pickup without risking serious microwave leakage.

The standard method of evaluating the power output of a microwave oven is to place an accurately measured quantity of water in a thin vessel inside the food chamber and measure the temperature before and after operating the oven for a precisely known time period. (Don't leave a thermocouple, thermistor, or mercury thermometer inside the oven while it is on.) Sorry I don't have the formula for the calculation of power, but you could find it in the literature or in oven service manuals.

Or, you can figure it out from these relationships from Mark's Mechanical Engineer's Handbook, 5th edition:

One BTU is the heat that raises 1 pound of water 1 degree F and is equal to 251.996 IT calories. 1 IT calorie is equal to 1/860 of a watthour. For best accuracy, use a vessel with minimal thermal mass (i.e., a thin paper cup) and measure temperature quickly after the heating period.

There you go!

awright

 

[Nigel Goodwin]

So far the suggestions haven't been very helpful? - I actually repair microwaves, and there are a number of causes of this very common fault!.

1) Magnetron S/C.

2) HV Protection diode S/C (if mag is S/C, it blows this as well).

3) HV Rectifier S/C - see above, the two diodes are an assembly.

4) HV Capacitor S/C - very rare on Sharp ovens.

5) Mains TX shorted turns - the protection diode is designed to prevent this happening by blowing the fuse if the magnetron goes S/C.

HOWEVER! - BE VERY AWARE THAT THE VOLTAGES IN A MICROWAVE ARE ALMOST CERTAIN TO BE FATAL - TAKE EXTREME CARE AT ALL TIMES, ALWAYS UNPLUG THE OVEN AND DISCHARGE THE HV CAPACITOR.

If you have a paramedic who lives near by, you might invite him round for coffee? :lol:

Paramedics on the scene (within seconds!) have had some success reviving microwave oven shock victims, but apparently they are never quite the same afterwards!.

 

[Roboticinfo]

reply

You could have a robot do it


Just make sure he follows the three laws
:lol:

 

[eblc1388]

Closer inspection revealed a large capacitor (9uf @ 2300 volts AC 60 cycles) that appears to be shorted when I test it with multimeter.

Double and triple check the value of the HV capacitor. Never seen one with 9uF capacitance so high in a domestic oven. More likely to be around 1uF. You need to disconnect at least one terminal of the capacitor before measuring if it is shorted.

Never do any troubleshooting on microwave oven with power ON.

The following link points to a 240V Sharp model with schematics and perhaps you can have a look.

Sharp Microwave Oven R-21AT

 

[Dialtone]

Thanks all for the help:

In regards to the cap value, I missed the decimal point. Should have read .9uf

After my initial post, I found a site dedicated to microwave oven repairs and saw the admonishment about the high voltage. It was too late by then, but things were already discharged when I was poking around so I got lucky this time. Will be more careful to read before poking around next time.

Looks like only the cap is bad. Diode checks out OK and nothing else looks bad, so I will get a cap and see what happens.
Thanks for all the help.
Dialtone

 

[Nigel Goodwin]

Looks like only the cap is bad. Diode checks out OK and nothing else looks bad, so I will get a cap and see what happens.

If the capacitor reads S/C then it's obviously faulty, as I mentioned before this is uncommon on Sharp ovens (but not unknown!), but EXTREMELY common on Panasonic ones.

 

[Dialtone]

Located a suitable cap, replaced it and the unit works perfectly now.
Sipping a cup of re-warmed coffee from the micro as I type this.
Now if I can just get that stupid little interior light socket back in the hole, wife will be happy.
Again, thanks to all for the help.

Dialtone