Microwave Issue

[jamesmcuk]

Hi guys, hope someone an give me a little help here. I bought a broken microwave with the intention of fixing it. The output to the HV circuit is working and after some investigation I found a HV fuse that had blown that was in a plastic carrier under the fan. Anyway I decided to order a new fuse and diode which is strapped across the case and the 2300v cap. Before I replaced these items the microwave hummed normally and it appears to work but there was no heat. Since replacing the fuse - i think the diode was OK - it now make a very loud buzzing noise when on and still no heat. I have tested the Magnatron and it tests as it should with a DMM. I have tested the Cap and it also tests as it should with the modest Cap test on my DMM. It also has no short to case and the discharge resistor is also testing out as good at 10MOhm. In short I now don't know which component Transformer, Magnatron or Cap could be bad. Any help appreciated. I have a Multi Function 1000v tester at hand could I use this to test the components further for breakdown under high voltage?

Thanks

James

[redwire]

Re: Microwave Issue

Can you give us a make and model number, that helps for the schematic.
Some microwave ovens use both a rectifier and a protection (across capacitor) diode.

If the new HV fuse is not blowing, then something has changed and gone open-circuit. So looking for shorts or breakdown is second on the list.

A microwave oven rectifier diode is usually high voltage drop ~11V due to the stack of diodes inside. So you cannot test it for open with a multimeter unless you have a power supply or a couple 9V PP3's.

When repairing things, it's best to track down and find the failed part instead of just replacing parts. One failed part can damage others and a person can get lost with what the original problem is.

I would ensure the new diode is the right part and in correctly, was the old one good. Inspecting the magnetron for evidence of arcs, as that may be what originally popped the fuse.

[jamesmcuk]

Re: Microwave Issue

Hi, Thanks for the reply. its a whirlpool jq280 which is not common here in the UK but I do have this link https://www.manualslib.com/manual/98...?page=5#manual


I agree about replacing parts but I did the diode as its was cheap and as you say something difficult to test with a DMM. I do think the old one was probably good though looking at the DMM readings.

The diode has crimped on tags that are different so its defo in correctly. I haven't yet removed the magatron so maybe thats the next step to see if there is any sign of arcing?

Thanks again

James

[Sparkey55]

Re: Microwave Issue

Check for continuity of the tube filament; could be open.

[redwire]

Re: Microwave Issue

+1

The majority of magnetrons are 3.3VAC >10A filaments, so with the transformer disconnected expect under 0.33 ohms.
I would check continuity from filament to housing and tap it, bang it a bit. What may have happened is the magnetron filament arced and later popped the HV fuse but now the filament short-circuit has cleared, vanished.

Toshiba Magnetrons for Microwave Ovens has some datasheets etc. if you know the tube number.

[RukyCon]

Re: Microwave Issue

You should also check the capacitor as i've had many microwaves stop working because of it.

Edit: realised you may have already have done that.

[jamesmcuk]

Re: Microwave Issue

Thanks for the replies guys. I have as you note Ruky checked all the items bar the secondary transformer winding on the HV circuit and they appear to test OK but thats not to say the cap which is reading correctly isn't breaking down under 2kv.

Mag is giving me 0.19 to 0.2 on filament and open circuit filament to case but I haven't tried banging on it or removing it to check for damage so I will do that today.

I have read people saying the waveguide could be faulty but I am not sure how, isn't that just a metal guide?

Thanks again.

[R_J]

Re: Microwave Issue

Check if you have high voltage, I doubt your meter can measure over 2000 volts so, you can dissconnect the magnatron high voltage lead, turn on the microwave for a few seconds, turn it off and see if the capacitor charged by checking if there is any voltage on it either with your meter or by shorting it with a screwdriver. or you can place the high voltage lead about a 1/4 inch or less from ground and see if it produces an arc when turned on.
But I suspect the magnatron is bad. you can't check a magnatron, you can only check if the filament is open or shorted to case.
wave guides don't go bad.

[momaka]

Re: Microwave Issue

You can check the high voltage with a regular multimeter if you build a proper resistor divider circuit.

Assuming the output voltage should be around 2000V, ten 100 kOhm resistors in series will divide the voltage so that each has about 200V on it. So if you measure, let's say, 226V across a resistor, then the voltage is likely somewhere around 2260V (i.e. 10x that value, more or less, assuming all resistors have close to 100 kOhm resistance.)

Now here is one important consideration when building such voltage divider circuits: power dissipation. With 200V across each resistor, the expected power dissipation, Pd, will be (200^2)/100000 = 40000/100000 = 0.4 Watts. So you will need resistors rated for at least 1/2 Watts each. And if the expected output voltage is double (i.e. 4000V), then the resistors power rating should be quadrupled - i.e. use 2W resistors.

[redwire]

Re: Microwave Issue

I would say that's not right because the circuit is a half-wave voltage doubler and it does not output pure DC.

If you use a scope and HV probe, you will see 1/2 wave pulsating DC from +11V to -4,000V. If the magnetron is open-circuit, the no-load voltages go much higher than (loaded) 2kV out of the transformer. The portion of the sine-wave where the cap is charged is also not seen then.

Really good information for microwave oven repair is here: https://repairfaq.org/sam/micfaq.htm#mictstmag

snippet from Testing the Magnetron:

A magnetron with an open filament will result in no heat but no other symptoms. The bad connection may be internal (in which case the magnetron will need to be replaced) or external at the filament terminals (which may be repairable).

A magnetron with with a short between the filament/cathode and anode will likely result in a loud hum from the HV transformer and/or magnetron when the cook cycle is initiated but the main fuse will probably not blow. However, note that the actual wattage drawn from the power line will probably be much lower than under normal conditions. Although there will be a high current flowing in the HV transformer secondary through the HV capacitor (likely causing a loud hum or buzz), the real power consumed will be reduced since the current and voltage will be out of phase (due to the series capacitor) and the power factor will be low. A reading on an AC line wattmeter of 300 W compared to the normal 1,200 to 1,500 W would be reasonable.

[momaka]

Re: Microwave Issue

Thank you for the detailed info, redwire!
Learning something new every day.

[sam_sam_sam]

Re: Microwave Issue

You can also check the Magnatron output with a small cfl tube inside the microwave oven
You should only use one that is only about 2” long with the terminal end taped if the magnetron is working the cfl should glow if it does not then it is bad

One note this should not be more than 30 seconds at time and the cfl should be in center of the turn table

You need to use caution when doing this test

[desert-rat]

Re: Microwave Issue

You can check the pri and sec power trn. with the wires off. The ovens I have checked have had open pri. trn. Its cheeper to get a new oven than a power transformer .

[jamesmcuk]

Re: Microwave Issue

Once again thanks for all the replies and detailed responses. I have checked the magnatron for signs of arcing and there is none. I have tried shorting the cap after trying cook and noting happens, no spark. The cap tests out as working with my meter as I say. The only component I haven't tested is the transformer which is the bit that I think is making all the noise. I have looked at replacement caps and the diode is on the other terminal to mine does it make any difference? I guess I am going to try and test the secondary winding of the transformer.

[R_J]

Re: Microwave Issue

If you have a current clamp, disconnect the cap, diode, & magnatron and check the curent draw on the primary, it should be almost 0, If no current clamp try an incandescent lamp in series with the primary, if the lamp lights up and stays lit, you have a shorted transformer.

[jamesmcuk]

Re: Microwave Issue

I have a current clamp so will try that. However by the noise it's making I would suspect it's ok on the primary as it started to make the much louder buzzing having replaced the HV fuse. I would suspect if anything a broken secondary but I dont have a meter or other bits to test it properly. Thanks for the reply.

[R_J]

Re: Microwave Issue

If there is a short in the secondary it will show high current on the primary

[Per Hansson]

Re: Microwave Issue

Well, it's a step up transformer with roughly a 1:10 ratio I would assume.
So 2.3kV output with 230v input.
So if you feed it 24vac on the primary you should see around 240vac on the secondary...

As for the magnetron heating element that is only ca 3v with 240vac on the primary.
So that is safe to measure without a high voltage probe.
(Well, safe as in if you make a mistake and measure the wrong secondary you blow up your meter but as you are working with a microwave I better hope you already understand the risks involved!)

[sam_sam_sam]

Re: Microwave Issue

If no current clamp try an incandescent lamp in series with the primary, if the lamp lights up and stays lit, you have a shorted transformer.

Not exactly because this is high current transformer

When you power the transformer up with no load your amperage might be around 6 or 7 amp and your transformer should be humming a little bit

If when you use a incandescent light bulb in series with transformer it might start off being real bright and then reduce brightness a little bit

This is only going to tell you that you do not have an open transformer and not much else

[R_J]

Re: Microwave Issue

If you are drawing 6 - 7 amps unloaded, Thats a bit high, unloaded it will draw a couple amps.

I just went and check one that I have, it is for 110vac input, and it draws just under 2 amps unloaded.

I also checked it with a lamp in series and it does light up quite a bit, so I was wrong in post 15

Just for comparison I also check a regular isolation transformer I have and unloaded it was 300ma.