LG microwave oven with poltergeist inside

[Ricardo Abreu]

Hi,


I am troubleshooting my microwave oven and I need your help/hints.
(Yes, I know the risks of microwave oven's troubleshooting, I know the security rules and this is not the first oven I repair with your aid. :)

Problem is everything tests OK as you can see below (I am posting all the tests and measurements I made) but still no heat.

I must also state my test and measurement gear is a regular DMM, a HV capacitor discharging resistor string and my brain cells (at least those that survived the first years of parenting).


Make: LG
Model: MG-4324B
Type: Microwave oven

Symptoms: No microwave heating (everything else is fine). No reduced performance was noticed before failing.

Troubleshoot done so far:

HV transformer:
- AC mains DOES reach the primary winding while entering cooking cycle;
- No open winding;
- Resistance of primary winding: under 1 ohm;
- Resistance of 3.3V winding: under 1 ohm;
- Resistance of HV winding: 147 ohm;
- Voltage of 3.3V winding: 3.3V open, 2.8V loaded with magnetron's filament (HV disconnected);

Magnetron:
- Resistance between cathode/filament and anode/case: above 40 Mohm;
- Filament resistance: under 1 ohm
- Voltage at the filament: 2.8V (HV disconnected);

Diode:
- Tested good, forward voltage drop: 8V;

HV capacitor, marked 1.05uF +/-3% with internal bleeding resistor:
- Capacitance: 1.027uF;
- DC resistance (after transient): slightly above 1 Mohm or 4 Mohm (depending on how the DMM probes are connected);

Connections and wires:
- Tested and revised, all OK;


Any hints besides having a poltergeist inside the oven palying tricks on me?
Please?

Ricardo

[Cydrome Leader]

That's some pretty thorough testing you've done so far, especially with
testing the filament winding open and under load. It think it's safe to
say the filament is good, and either the HV section has an issue or the
magnetron is just bad.

Testing the HV part is a little trickier. Do you have a way to safely test
the HV output from the doubler? If not, just skip this.

Next step would be swap parts, starting with the cheap ones. Swap the
cap with a new or known good one, then try the diode. If that doesn't
help, you probably have a bad magnetron, which may or may not be worth
replacing.

This has some pointers, which you've probably seen already

http://www.repairfaq.org/REPAIR/F_micfaq5.html#MICFAQ_001

It's a good doc for learning the concepts or experimenting at home, but
sort of pointless for real repair work, where speed matters. For example,
in a real shop, nobody takes the time to build a test jig for diodes. That
would cost more than just replacing it, even if that's not the problem.
Tapping into the filament with resistors seems weird too, since most
places have a current meter, and if you're confirmed the HV section is
good anyways, there's nothing left to replace but the magnetron, so
proving that the filament resistance is 4% out of spec doesn't even
accomplish anything anyways.

[Samuel M. Goldwasser]

Cydrome Leader <pres...@MUNGEpanix.com> writes:

That is thorough testing.

One other thing to check is for cracked magnets on the magnetron. That
can cause a no-heat symptom when everything lese tests good.

I've gotten original NOW/new magnetrons on eBay for under $20 so it's
worth to simply replace. Then you'd have a spare if yours turned out
to be good.

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[Phil Allison]

Ricardo Abreu wrote:

>
> I am troubleshooting my microwave oven and I need your help/hints.
> (Yes, I know the risks of microwave oven's troubleshooting, I know the security rules and this is not the first oven I repair with your aid. :)

** That sounds like pure bullshit and bravado to me.

Your first small mistake may well be the last time you breathe.

FYI:

Most microwave ovens are not worth repairing and certainly not worth risking anyone's life to repair.

I will never help anyone fool about with one of the horrible things and I think discussing such repair work should be banned on usenet.



.... Phil

[Martin Crossley]

Mine has needed two relacement magnetrons and four pairs of lamps in
32 years.
It's a Sharp Carousel with stainless steel cavity and was worth
repairing.
Over 4 times the cost of a magnetron for a new one.
(OK, new ones with painted steel cavities are only slightly dearer
than the magnetron, but they rust to death in 3 years.)
It really isn't beyond the wit of man to ensure the HV capacitor is
discharged safely when replacing the magnetron or to screw in
replacement lamps without bothering...

[Ricardo Abreu]

Hi,

Thank you for the inputs.

Answering your questions:

- no, I do not own any high voltage probe, so I will not test HV;
- I did not check for cracks in the magnetron so I will try this;

The oven is not the finest appliance on earth, but it does deserves a magnetron if that is the case. I just would like to be sure the magnetron is bad, because I am not willing to order a new magnetron just to find out the transformer is bad.

I do not own a repair shop, so it is kind of useless having expensive parts lying around as paperweights.

[Ricardo Abreu]

Had a little time to check the microwave oven again.

1) First, I checked if a short was developing in the magnetron between cathode and anode (case) when the filament gets hot, but it doesn't. If you are wondering how I did it, I disconnected HV and connected a resistor from one of the filament's terminal to case, and checked for voltage across the resistor. Repeated this for the other filament's terminal;

2) Took the magnetron off. There were no cracks in the magnets (visible, at least) as Sam suggested;

3) Connected the filament (always HV disconnected) and powered the oven with lights off. The filaments clearly glows;

4) Checked HV transformer for signs of arcing or shorts, but none visible. Also, windings are not superimposed, they are placed side by side in the same leg, which somewhat reduces chances of arcing between windings;

5) Changed the HV diode for a new one (the cheapest part), but to no avail;



Now I'm stuck, and it seems my only option (unless you have a better idea) is to start replacing parts until the thing gets alive again. HV cap is still inexpensive.

By your experience, which one fails the most: transformer or magnetron?

[Maynard A. Philbrook Jr.]

In article <a25935d9-03bf-4cfe...@googlegroups.com>,
ricardo.m...@gmail.com says...

You don't have a HV probe to test for the - voltage at the diode?

Jamie

[Ricardo Abreu]

Nope, I don't own an HV probe.

[hrho...@sbcglobal.net]

You really need to test the HV before proceeding to replace the magnetron. Do you have a tv repair place anywhere nearby? They would probably be willing to loan you a probe since they do not use them very often any more with solid state displays.

[Samuel M. Goldwasser]

Geez, the magnetron is probably less than $20 on eBay.

[Cydrome Leader]

hrho...@sbcglobal.net <hrho...@sbcglobal.net> wrote:
> You really need to test the HV before proceeding to replace the magnetron. Do you have a tv repair place anywhere nearby? They would probably be willing to loan you a probe since they do not use them very often any more with solid state displays.

A yoke tester should have the proper ranges too- if it has the correct
probe. None of the ones on ebay do.