Thread: Bought an old microwave oven...

(Sorry to bother you with a dumb question.)

I bought an used microwave oven. Turns out that it's not quite in working order. I've tested it a few times now: it heats the containers/plates, but not the food. I can't even melt a piece of chocolate in it.

I just tried to heat up a burger. After four minutes in the oven the plate feels hot, but the burger is still refrigerator cold. I'm pretty sure that's not supposed to happen.

I thought I knew how microwave ovens work, but... evidently not. How can it heat the dishes without warming the food? I used the same plates in my old oven and they never got this hot.

It shouldn't...microwave ovens use radio waves (@2.4Ghz) to make water molecules vibrate. I don't see how it could heat the plate and not heat the food. I'd just toss the thing, and get a new or newer one.

That's really weird. I've had plates come out of the microwave hot... but always assumed that the food had heated the plate...

There is some residual water in most ceramics that can get heated.

So maybe that oven is spewing waves at a rather ineffective frequency? It can heat a small bit of water, but not a large bit?

I have no idea....it doesn't make sense to me.

The Magnetron in a microwave oven is connected to a wave guide which guides the waves into the oven chamber properly. Usually this wave guide is behind a mica sheet that prevents the element of the magnetron from getting dirty. My theory is that either the Magnetron is off frequency or the wave guide magnetron relationship has changed (eg someone dropped it and they are no longer aligned quite right). Now the waves are creating a hot spot at the bottom of the oven, rather than evenly bouncing about.The parts will cost more than a new one, I would buy a new cheap one since the internals are usually exactly the same as the higher priced models. You are usually paying for the extra cook functions you will never use or a slick outer covering like stainless.

Or higher wattage?

I have seen the exact same magnetron in a 1000W and 1100W microwave. The inverter was the same as well so the power input had not changed. I guess that they have upped the internal lamp wattage to light the inside, either that or watts have succumbed to marketing jargon.

I'm not trying to be difficult... But why do cooking packages have dislaimers for microwaves under 1100 watts or to use an oven rated for 1100 watts or higher, etc?

That makes sense. Thank you.

I bought this one from a refurbishing store (or whatever those are called in English). They buy broken electronics, then repair and resell them; but this one clearly wasn't fully tested. I should've simply bought a new one..

Originally Posted by Zaltys

I bought this one from a refurbishing store (or whatever those are called in English). They buy broken electronics, then repair and resell them; but this one clearly wasn't fully tested.

Sounds dangeous. I myself would never buy from such stores. How can I trust these people and be sure they know how to repair stuff? We’re not talking about toys or simple gadgets, but high-powered electrical appliances, in which a minor fault could be dangerous, even fatal, to the user. What if they don’t do a good job repairing something, don’t even test it for safety, I buy it, and it explodes when I use it in my home? No, thanks. I’d rather spend extra for a new product than risk home and life by being a cheapskate.

I like Darkhorse's explanation. If you can't return it to the store, just chuck the thing and get a new one.

Originally Posted by Neverfly

I'm not trying to be difficult... But why do cooking packages have dislaimers for microwaves under 1100 watts or to use an oven rated for 1100 watts or higher, etc?

I am no food safety expert but I would assume it is CYA. They test the package on a microwave of a certain wattage and therefore know that on their unit of 1000W the food reached the right temperature, however on less than 1000W they are not certain it will reach temperature.

The difference between 1000 and 1100 is not much and like any radio system the waves can interfere or enhance each other in different places. The engineering is designed to put the most power in the center of the unit however it is not always evenly in the Center, which why most units have the turntable to even out the cooking.

Since the magnetron and inverter were the same model numbers I assume that they were measuring it differently. Maybe they went from PEP (peak envelope power), to TPO (transmitter power output), or ERP (effective radiated power). I put it down to marketing, after all "More is better". :-)

Valid- I also considered that. That post earned you one "like" from Neverfly.
Don't spend it all in one place.


Actually, I had taken a microwave oven apart a few years ago for fun. It was old and broken... and I was being inquisitive wondering what the parts looked like.
But I also noticed that the parts seemed the same as others. I chalked it up to my own ignorance, that just looking wasn't enough to tell output, etc.

Darkhorse: What is the inverter you speak of? When I studied electronics, very long ago, a magnetron was an oscillator which was capable of generating very high frequency waves. The required fairly high voltage, perhaps 1200 volts. An inverter in current jargon seems to be a device, now solid state, which changes D.C. to an A.C. suitable for use powering appliances, using a battery as the source. jocular

Magnetrons are just what you said. There are different types but they all produce very high frequency radio waves and require a high input voltage, much higher than the 110V or 220V you get from your wall socket. Originally they were used for Radar, but now they have been commercialized and found a home in microwaves.

You are right an inverter changes DC to AC, however they are used for a lot more than converting battery power to AC, they also are used to create an AC wave that better suits your purpose. Microwaves are just one of the applications. Welding, radar, lighting, HVAC and variable frequency electric motors all use inverters as well.

In a Microwave, the AC line voltage is rectified and then passed to the inverter. The inverter can feed the magnetron any voltage and frequency desired. This allows the magnetron to actually cook food at 50% power. Before they used inverters the Microwaves would turn the magnetrons on and off at a 50% duty cycle. This on and off is hard on the magnetron and results in slow cook times, and uneven heating. In the 50% duty cycle you are basically heating the food, then doing nothing, then heating it again. By being able to run the magnetron at any power level you want (well anything over 30%) you can continue to cook the food continuously without over cooking it.

Originally Posted by Darkhorse

In a Microwave, the AC line voltage is rectified and then passed to the inverter. The inverter can feed the magnetron any voltage and frequency desired. This allows the magnetron to actually cook food at 50% power. Before they used inverters the Microwaves would turn the magnetrons on and off at a 50% duty cycle. This on and off is hard on the magnetron and results in slow cook times, and uneven heating. In the 50% duty cycle you are basically heating the food, then doing nothing, then heating it again. By being able to run the magnetron at any power level you want (well anything over 30%) you can continue to cook the food continuously without over cooking it.

Thank you for this! History: Large G.E. microwave oven, built 1976, was still operating when we sold it last year! 36 years of service, big cavity, 1.6 cu. ft., made up some of the finest roast turkey we ever ate! Now, I particularly like the way it handled it's "power settings": it pulsed the magnetron on and off, no doubt about that, but did so in rapid-fire succession. That is, "Medium-High" pulsed on and of at about a 1-second rate, "Medium" perhaps 1/3 slower, etc. No doubt, as our old farmhouse with it's ancient wiring, dimmed and brightened to the cyclic load of that magnetron!

Now, every other microwave oven we have had, perhaps 5 or 6, unless I am all wrong, also pulsed the magnetron on and off, though their scheme was thus: "50%" power level had magnetron "on" about 8-10 seconds, then "off" a like time interval. Several exhibited slight dimming of the read-out upon tube turn-on. These things I am nearly certain, did not operate the tube at reduced output level, as you explained. Our newest is a 2002 model. Did inverter-equipped models come about after that?
jocular

Originally Posted by Darkhorse

Originally Posted by Neverfly

I'm not trying to be difficult... But why do cooking packages have dislaimers for microwaves under 1100 watts or to use an oven rated for 1100 watts or higher, etc?

I am no food safety expert but I would assume it is CYA. They test the package on a microwave of a certain wattage and therefore know that on their unit of 1000W the food reached the right temperature, however on less than 1000W they are not certain it will reach temperature.

The difference between 1000 and 1100 is not much and like any radio system the waves can interfere or enhance each other in different places. The engineering is designed to put the most power in the center of the unit however it is not always evenly in the Center, which why most units have the turntable to even out the cooking.

Since the magnetron and inverter were the same model numbers I assume that they were measuring it differently. Maybe they went from PEP (peak envelope power), to TPO (transmitter power output), or ERP (effective radiated power). I put it down to marketing, after all "More is better". :-)

I have a 1200 watt microwave and it does cook things a lot faster than lower wattage microwaves. Sometimes you need that power to get things cooked during a commercial break.

Don't get me wrong I like powerful things. :-) The one I was dealing with however had no difference in parts. However thinking on it more, it may have less to do with the actual part and more to do with the supplied electrical profile from the inverter. Makes me wonder just how high those magnetrons could go. Maybe my idea for a microwave death ray is not so far fetched ....

Originally Posted by Crimson Sunbird

Originally Posted by Zaltys

I bought this one from a refurbishing store (or whatever those are called in English). They buy broken electronics, then repair and resell them; but this one clearly wasn't fully tested.

Sounds dangeous. I myself would never buy from such stores. How can I trust these people and be sure they know how to repair stuff? We’re not talking about toys or simple gadgets, but high-powered electrical appliances, in which a minor fault could be dangerous, even fatal, to the user. What if they don’t do a good job repairing something, don’t even test it for safety, I buy it, and it explodes when I use it in my home? No, thanks. I’d rather spend extra for a new product than risk home and life by being a cheapskate.

um..there is no guarentee that stuff won't happen when you buy a new product..the only difference is with a new product you can sue someone..

and with used..test it before you leave the store.
and they are usually cheap enough so if it only worked a few times, you would still get your moneys worth out of it..
i say usually because i have experience working for a used appliances store..the guy thought he was doing everyone a favor by making used fridges 200 cheaper than a new one..(DUDE no wonder they aint selling!..grr..he wont listen..)
but he was anal about making sure they worked right..

In my experience, used electronics are rarely worth buying unless they are practically free or come with an excellent warranty.

I remember back in college I once bought a used window air conditiner. It ran for exactly one day and then broke. Shortly afterwards, I saw a brand new one at the store for just a bit more than I had paid for the used one. Damn, I was pissed off.