Microwave dismantling

[bruce]

     Our microwave crapped out on us the other day, it still works but something is burning up inside and stinks. I was going to let my son who is 9 tear it apart just for fun, its always fun tearing something apart. But I recall in the past something about pulling microwaves apart for the magnets, to be careful of a capacitor or something that may hold a charge and can bite you pretty good. Am I right, or am I thinking of something else?

     I thought before I let him do that I may check up on that subject.

                            Thanks, Bruce

[Laylow]

The last time I tore into a microwave, there was a big label that said DO NOT OPEN THIS! or something like that.  It was actually enough to stop me and I'm not the type of person that usually heeds such warnings.  Maybe it was just a big capacitor; I don't know.  Give him an old TV instead.  He can turn it into an oscilloscope!

[pyrocasto]

Bite you pretty good? It'll do more than that. lol The caps that come out usually range between 1800-2200 VAC. You just need to make sure you short it out to make sure it's discharged first. Other than that, I'm not sure of the dangers of the magnitron, as that would the the only thing in there that I can think of that could be dangerous at all, other than the cap. ;-)

[ghurd]

I let them sit a week or 2, unplugged, try turning them on a few times (still unplugged), then short them with a screwdriver just in case. Never sparked the screwdriver, but you never know...

TVs have some big potentially nasty caps too.

[ghurd]

Not positive, but what I 'think' is the magnetron is a Al tube with fins and 2 magnets... thats it. nothing frightening.

Like a pair of hard drive neos and a beer can...

G-

[Nando]

The capacitors can not charge with AC voltages.

The magnetron needs VERRRRRY high voltage to operate, 1500 to 3000 + volts DC.

Open the microwave for him then carefully short circuit the big capacitor with a low values resistor then wait a few seconds and with a bare wire wrap it around the capacitor terminals -- this way no voltage is present and no chance for the capacitor to regenerate some of its voltage ( no time to explain this effect ).

Then let him play with it, be careful with the sharp edges -- the transformer may have been what smelled BAD --

Nando

[dinges]

Bruce,

The one and only household appliance that I don't look forward to service is a microwave:

1. it has high voltage (>2000V)
   
2. at a high current rating
   
3. it has a big capacitor
   
4. it generates about 1000W of RF energy.
   
   

However, I do service them anyhow :-)

When you say you have a smell when you heat something, it may just be the mica cover plates inside the oven chamber; you should see them if you open the door. If they're black/burned, just replace them with new mica ones. Bought some from my local repairman for a few $. Cut it to size with scissors, install and your problem may me over. If it's not the mica cover plates that are burned (quick check: remove them, and start the oven) and there's no other black crud in the oven chamber (i.e., where you put the dish) (you do regularly clean your oven, right ? ;-) ), and you're not an expert (or don't think you are one), I'd leave it alone.

As far as tearing it up go: I'd let my son have a go at it, but I'd be near it when he did so. BTW, demolished (scavenged) my first tv for parts when I was about that age (with a soldering iron that was a present for my swimming diploma). Still have some of the parts :-) And TVs aren't innocent either: the tube has a high vacuum; it is also a high voltage (25kV) capacitor; DON'T probe the hole in the tube, even not a discharged one. There's no point to it anyway (like looking into a laser, it's pointless at best, dangerous at worst).

Geez, when you start to think about these things I almost get afraid to use these appliances, let alone repair :-)

Good luck,

Peter,

The Netherlands.

[jimovonz]

Every microwave I have dismantled (~100!) has a bleed resistor integrated in the cap sized such that the cap drains to almost nothing in a few seconds. I have never been stung by a microwave (except being sliced by sheet metal) I just use some decent sized pliers to pry off two of the latches on the magnetron.

[dinges]

You're right about the discharge resistors. They're a safety device, and just like other components they can become defect; even with the operator knowing there's anything wrong with the machine, because it will continue to operate normally.

I see it as an extra backup, but will NEVER EVER bet my life on it; nothing beats a screwdriver (use an old one, BTW :-) ).

Also right on the steel plate; I never got stung by a microwave, but have cut myself on the steel plate. Perhaps they should issue a warning: 'steel is sharp. do not swallow; do not touch (sharp!)' etc. ;-) Like the other warnings you see on some consumer products (when reading these warnings, my estimation for my fellow man quickly drops...). The ones I liked best were 'do not put private parts in vacuum cleaner' and 'do not iron clothes while still wearing them'. LOL!

Peter,

The Netherlands.

(however, you can't have enough warnings on a microwave nor be careful enough!) Discharging the cap & shorting it with some wire after discharge should do the trick, as long as it is unplugged.) No need to shy away from some educational fun!

[ghurd]

I love stupid warning labels!

http://rinkworks.com/said/warnings.shtml

G-

[dinges]

LOL! Downloaded the list for off-line reading.

remembered another one on a superman cape for kids: 'wearing of this cape does not allow the wearer to fly'.

Better stop it now before it gets way too off-topic.

Peter,

The Netherlands.

[bruce]

      Wow, I'm glad I asked!!! Thanks all for the help, I'll help him open it up and then I'll make sure to check that cap and make sure its safe.

            Once again, the board comes through for me.

                         Thanks all, Bruce

[richhagen]

I Was just thinking that about the TV's and their capacitors.  Could be a bit dangerous for a 9 year old, although I wasn't much older when I started tearing apart old tube type tv's, I'm not sure what my parents were thinking.  I still have parts in my junk box from then, old resistors, transistors, and even the big aluminum can caps.  Rich Hagen

[pyrocasto]

Pretty much, the case has a few copper wrapped pieces of ferrite(I think), alluminum heat sink fins, and 2 magnets. Inside the little "barrel" though I have no clue.

[tecker]

  The Cap less than 1 uf not a problem at all just don't fire it up when the cover is off

[stop4stuff]

From my experience repairing microwave ovens, the bad smell is often caused by either, the transformer burning up, the magnetron burning it's gasket, or as previously mentioned, a mica plate burning up.

Apart from the capacitor, really the only other danger is the very sharp edges of the internal metal structure... the edges can be razor sharp... i'd worry about those more than the cap.

[wooferhound]

It's hard to get electronic Goodies from a TV anymore. There are not many discrete components in them anymore. But I won't throw one away without taking it apart and getting something out of it...

[dinges]

Caps in microwaves are about 10u; at 2000V. This is not like 10u at 25V... VERY DANGEROUS STUFF. Even not being turned on, elco's can keep their charge for weeks (have one radio, that works for about 1s. on its capacitor when I turn it on, even after weeks, without it being plugged in; some kind of Sony ghettoblaster).

So I repeat: these elco's ARE DANGEROUS, unless properly discharged. Then it's just a piece of metal. If you don't believe me, believe the manufacturers, repair manuals, capacitor producers, etc.

No need to shy away from them, just take precautions.

Peter,

The Netherlands.

[cyplesma]

can the heat sinks in microwave ovens be used for heat sinks for wind genny rectifiers?

[tecker]

I looking at five sitting on the shelf got A .86 , and 4 at .65 they are going to leak down after the unit is shut down . The 2k hits about like static not as bad as a horizontal section of a 30 inch tv.

[ghurd]

From the magnetron? Most of what I saw is very thin Al.  Could be stacked for thickness.

Hard to get enough mass with Al that thin.

All heat sinks should have a lot of surface area and mass.

An Al. level from the 'everythings $1' store has some nice weight and surface area. Seems to be 2 common $1 levels, one is heavier than the other. Enough for a small mill.

G-

[dinges]

You may be right about the exact values; In fact, I think you are right.

Still, 1uF at 2000V is NOT the same as 1uF at 25V;

Notice that the higher the voltage, the smaller the capacity of a capacitor can be for the same amount of energy;

Q=1/2 C*V^2.

As voltage doubles, with equal Capacity, energy quadruples; found this out recently, when making calcs for a homebuilt capacitor discharge spot welding machine.

Peter,

The Netherlands.

[DBGenerator]

I got bit one time while replacing a fuse in a microwave.  If they sit for a week or two the capacitors will probably be discharged.  To be sure, place large screwdriver across the terminals, then check it with a voltmeter to be sure.  Also check at the transformer.  After that, be careful of sharp edges.

On a TV, you can short it out by attaching a grounding wire to a large screw driver and poking it under the wire that goes to the tube.  Don't touch the metal part of the screwdriver, wrap a rag around the handle to be sure.  Probably best to let it sit for a week or so before and it will probably discharge itself.

[dinges]

Say WHAT?

No goodies in a modern TV?

How about power MOSFets (ideal for building dumpload regulators...)

fuses, resistors (SMD or wires), caps (elco's for your projects), power cord, some high speed diodes (Schottky), etc. etc.

If you're willing to spend the time, an old tv can quickly give you some good components; if you go all the way and completely dismantle it, you'll have lots of caps and resistors and transistors too.

Even in modern tv's, not everything is SMD. Not too much discrete IC's anymore, it's all integrated in very-special purpose ICs (i.e., a complete tv in an IC...)

Whenever I see an old telly or VCR or PC sitting on the curb, I won't pass it up :-)

(hey, if it's free, I'll have 2 please!)

Peter,

The Netherlands.

[asheets]

THis would be a good time to teach the "one hand" rule to your kid.  

Seriously, though, I caught one of my computer repair students reaching into the back of a 21 inch monitor the other day with both hands (and with his wedding ring on).  I walked over with a .5 inch craftsman screwdriver, grounded it, and discharged the capacitor.  Made a huge "pop" that scared all the novices, and took a nice chunk out of the screwdriver besides.

Now, they ALL tuck their left hands in their back pockets when the tops are off the PCs.

[dinges]

Don't they teach this at school? It is so very basic!!

Even though, I've got bitten once by a PSU from a 3D-measuring machine. Left hand in your pocket doesn't help much when your thumb is resting on chassis and your index finger is probing around... Ouch!

Peter,

The Netherlands.

[nothing to lose]

The warning label I like is "NO user servicable parts inside" and of course it is located right over top of the standard FUSE that blew.

I am not sure what that was on, but true! Remove one screw (under the label) and pop off the cover and there was a press in fuse (not even soldered in), standard easy to get anywhere fuse!

I geuss they mean the fuse itself could not be repaired ??

[nothing to lose]

Yep that's how we discharged picture tube feeds in comercail video games, same thing as a TV just different boards. Anyway I always loved doing that in the rain at a state fair or such. Rain pouring off the tent down my back, always an outside game too, never a game in the center of the tent where it was fairly dry. Ground screwdriver, slide under rubber cup, wait for POP sound and hope you didn't wet your front like the rain was wetting your back

Actaully I never wetted the front, picked up a co-worker once though who had. Hmmm, he thought he had a good ground, looking over his ground wire I found he had clamped the aligator clip over the insulation of the wire not to a bare wire, so HE was the ground.

[nothing to lose]

Well yes that does hurt, high voltage from finger to thumb, but if it's just one hand probably won't kill you.

 Real danger is when it's your finger of the right hand and high voltage passes through your arm, chest, heart, arm, on the way to the thumb of your left hand. None of it is really fun, but what passes through your heart is the deadliest and that's often on the way from hand to hand.

[asheets]

A lot of my students are hot-shot computer gurus fresh out of high school that need diplomas so they can avoid call-center jobs.  None of them think twice about reaching into a powered computer.

One student finally scared me so bad (did something really stupid when I wasn't in the room) that I banned all high-voltage work in the computer lab.  Nobody is allowed to open monitors or power supply cages on campus any more -- even just to replace fuses, cooling fans, or adjust color gun alignment.

  The undewriters were pleased.