Panasonic Genius Prestige NN-SN797S microwave

[momaka]

Not quite an electronics question, but I figure there are quite a few of you here that have done car bodywork repair, so I don't think should be okay.
....

About a month ago, I found this microwave near a trash can about. Nice stainless steel front look and overall in good shape externally. Unfortunately, the original owner must have spilled something inside and probably left it there for a while because the whole back of the microwave chamber has started to rust. I actually posted about this in the "Dumpster Finds" thread, so more info and some pictures can be seen in these posts:
https://www.badcaps.net/forum/showpo...&postcount=250
https://www.badcaps.net/forum/showpo...&postcount=257

Yesterday, I decided to cut out the back because it really is completely rusted in a lot of places. There are still some areas with heavy rust, so I'll probably end up cutting out those as well. Pics below show all of the rust damage after the back was removed.

Now, I am thinking of bolting on a new piece of steel sheet on the back. Home Depot sells 12"x24" sheets fairly cheap - around $10. The only question I have is, which kind should I get? Currently, they have regular steel sheet, galvanized steel sheet, and aluminum sheet. They all cost about the same. Any reason not to get the galvanized or aluminum sheet? I figure those would be better since they wouldn't rust as easily as regular steel and maybe I can even get away without having to buy that special microwave paint (although I probably will just to make it look right)?
Also, what thickness should I get? I'm thinking anything will do. Or is there certain thickness needed to block the microwaves from leaking outside?

[kaboom]

Re: Panasonic Genius Prestige NN-SN797S microwave

1/32" should be fine, but 1/16" may be better for strength.

Plain steel is good. Soldering/welding galvanized can be "interesting." And it may be too frustrating/elaborate with aluminum, you'd probably have to use machine screws every half inch or inch.

Make certain you get down to bare metal on the chamber! The back panel must be bonded to the rest of the chamber, unless you want leaks or re-radiation. Lap the seam an inch or so over the existing metal- look closely at the original spot welds for some ideas.

[momaka]

Re: Panasonic Genius Prestige NN-SN797S microwave

Plain steel is good. Soldering/welding galvanized can be "interesting." And it may be too frustrating/elaborate with aluminum, you'd probably have to use machine screws every half inch or inch.

In that case, I might actually go with the aluminum since I was planning on using nuts 'n' bolts anyways. It may be more elaborate, but my main concern here is rust. That is, with plain steel I'd probably need to clean and sand the steel very well before painting, and even then it's not guaranteed it won't rust through in a few years or that the paint won't chip. I'll even probably need to use primer too. With aluminum, I think the paint should last much longer and wouldn't peel.

Make certain you get down to bare metal on the chamber! The back panel must be bonded to the rest of the chamber, unless you want leaks or re-radiation. Lap the seam an inch or so over the existing metal- look closely at the original spot welds for some ideas.

Got it. Thanks for the advice! I'm also thinking of running a wire from ground and attaching it (with bolt) to the aluminum piece I'll be putting, just in case so it's grounded better.

By the way, just a curious question, what exactly do you mean when you say that galvanized steel can be "interesting"? Microwaves will have problems with the coating?

[kaboom]

Re: Panasonic Genius Prestige NN-SN797S microwave

Got it. Thanks for the advice! I'm also thinking of running a wire from ground and attaching it (with bolt) to the aluminum piece I'll be putting, just in case so it's grounded better.

A lone wire has lots of XL at 2.4GHz!

That is, with plain steel I'd probably need to clean and sand the steel very well before painting

You'd have to anyway, since you want bare metal at the point the patch attaches. With edges just touching the patch, combined with any distance (diff of potential), you may get some sparking.

A 1/4 wave at 2.45GHz is about 1 1/8 inches, and (mostly) reactive current flows along the chamber walls. There is, however, a small real part- this heats the walls.

By the way, just a curious question, what exactly do you mean when you say that galvanized steel can be "interesting"? Microwaves will have problems with the coating?

You want to avoid the zinc fumes when welding galvanized. You do not want them to ruin your day.

I forgot to mention- unless you are certain about fully penetrating the oxide layer of that aluminum sheet, go for the plain steel.

You do not want any rectifying junctions. Or open spots.

Also, when using steel, you can solder it as well.

I hope you can visualize this:

(inside chamber)
Head of screw->flat washer->chamber sheet metal->patch sheet metal

(outside back, facing patch)
patch->flat washer->belleville or other lock washer->nut

You can probably get away with fastening every inch if you get the patch flat and lapped, even though I previously said every half inch. Under the screw heads on the inside, also have bare steel- don't let them float at different potentials. This is 2.4GHz, not DC!

The steel won't rust as long as you clean any existing rust back to bare metal and seal it good. Rust converter on 'convoluted' or hard to reach areas . But paint over it, too. "Rusty metal primer" can also be used. Then any decent finish coat or two. As long as you avoid any metallic or flake paints, you'll only have the normal solvent fumes.

Make sure you mask off your RF planes before painting. Use dielectric grease where the two surfaces lap over, both to prevent rust as well as 'shocking surprises.'


In the diagram, "A" is what you want to have.

If you get the raised edge of "B" on the inside (shown on outside in diagram), you may have sparking.

There is a bridge in "C." You'll have a hotspot if the path length is related to the wavelength- circulating currents. That's not the same as the normal currents that travel on the cavity- here it would be accidentally 'tuned' for it.

"D" may happen if you don't watch the edges of your holes, any 'swarf' may keep the two panels from lapping nicely.

Is this an "Inverter" Panasonic Genius? Or is it heavy, with, presumably, a plain ol' 'MOT,' cap and diode?

If it's got an inverter, get some security bits and pull the cover and check it. They can blow up. Be aware of how the fingers interlock so you can reassemble it properly.

When you are finished, you'll be able to run it near say, a laptop w/ WLAN, and not have it QRM anything if it's sealed up good.

[momaka]

Re: Panasonic Genius Prestige NN-SN797S microwave

Wow, thanks for the detailed write-up! Didn't think it would be that many things to consider.

That means the microwave must have been leaking quite a bit before, no? There were 2-3 1" diameter spots on that back piece that had completely rusted through.
...
And they call them stainless-steel appliances .

Is this an "Inverter" Panasonic Genius? Or is it heavy, with, presumably, a plain ol' 'MOT,' cap and diode?

Inverter, unfortunately. If it had the big traffo, I would probably have gutted it for the traffo to make one of those home-brew microwave transformer welders. Then I would have used that to patch the microwave and maybe put back the traffo in it .

If it's got an inverter, get some security bits and pull the cover and check it. They can blow up. Be aware of how the fingers interlock so you can reassemble it properly.

Already did that when I got the microwave. I didn't use hex security bits, though. Just some pliars and a small flat-head screw driver .

When I opened it, everything looked okay on the inside. I even tested the microwave with a cup of water in it before I cut the back. It heated the water in just 20 seconds to scorching hot. The back where the rust was also got very hot though. That's why I decided I need to put a new back.

You'd have to anyway, since you want bare metal at the point the patch attaches. With edges just touching the patch, combined with any distance (diff of potential), you may get some sparking.

A 1/4 wave at 2.45GHz is about 1 1/8 inches, and (mostly) reactive current flows along the chamber walls. There is, however, a small real part- this heats the walls.

Ah okay, I think I get it now. Since the microwaves are small, there can be different potentials on the surface of the metal even just an inch away and that's why it's important that there are no sharp edges or hot spots, correct?

A lone wire has lots of XL at 2.4GHz!

Of course!
Only now that you mentioned it I came to realize how funny that idea is. Maybe I should run a couple of large loops and not connect the back either. A mini 2.45 GHz antenna!

You want to avoid the zinc fumes when welding galvanized.

But I won't be welding anything though, since I don't have a welder. Or should I still stick to plain steel?

Also won't be soldering since I don't have any soldering tools for such a large job. Although come to think of it, Home Depot sells nice, large butane torches for around $30 - the kind you can use for copper pipes. Maybe worthwhile investing into one?

I hope you can visualize this:

(inside chamber)
Head of screw->flat washer->chamber sheet metal->patch sheet metal

(outside back, facing patch)
patch->flat washer->belleville or other lock washer->nut

You can probably get away with fastening every inch if you get the patch flat and lapped, even though I previously said every half inch. Under the screw heads on the inside, also have bare steel- don't let them float at different potentials. This is 2.4GHz, not DC!

Got it, thanks for the explanations.

The steel won't rust as long as you clean any existing rust back to bare metal and seal it good. Rust converter on 'convoluted' or hard to reach areas . But paint over it, too. "Rusty metal primer" can also be used. Then any decent finish coat or two. As long as you avoid any metallic or flake paints, you'll only have the normal solvent fumes.

So something like this for the primer is good?
http://www.homedepot.com/buy/rust-ol...y-7769822.html
I'll then cover that with specialized microwave paint. Seems to be cheap online, so I don't see why not. That's what kc8adu recommended, at least.

Make sure you mask off your RF planes before painting. Use dielectric grease where the two surfaces lap over, both to prevent rust as well as 'shocking surprises.'

So put dielectric grease on the surface where the patch laps the chamber walls? Does it have to be dielectric grease specifically, or can I use any plain old grease?
Also, I'm thinking of painting the patches after I attach them to the microwave. Any objections?

When you are finished, you'll be able to run it near say, a laptop w/ WLAN, and not have it QRM anything if it's sealed up good.

Good idea. I have a bunch of old PII laptops with PCMCIA WAN cads I don't mind sacrificing for the task.

[kaboom]

Re: Panasonic Genius Prestige NN-SN797S microwave

Wow, thanks for the detailed write-up! Didn't think it would be that many things to consider.

That means the microwave must have been leaking quite a bit before, no? There were 2-3 1" diameter spots on that back piece that had completely rusted through.
...
And they call them stainless-steel appliances .

I can't wait for the 'stainless' fad to die out. It's all about "looks and presentation," instead of substance...

Inverter, unfortunately. If it had the big traffo, I would probably have gutted it for the traffo to make one of those home-brew microwave transformer welders. Then I would have used that to patch the microwave and maybe put back the traffo in it .

Shouldn't be a problem. If you want, you can resolder all the large connections on the board. There's a rectifier and two MOSFETs on a heatsink, and some large caps, similar to what you'd find in a TV's horiz output. A wirewound resistor, etc...

Already did that when I got the microwave. I didn't use hex security bits, though. Just some pliars and a small flat-head screw driver .

Pull 'em out again and replace them with standard hardware!

When I opened it, everything looked okay on the inside. I even tested the microwave with a cup of water in it before I cut the back. It heated the water in just 20 seconds to scorching hot. The back where the rust was also got very hot though. That's why I decided I need to put a new back.

Good! The tube and inverter are working! Those inverter micros are fast, aren't they?

The I2R dissipation was higher in the back panel- rusty steel is a lousy conductor, especially when skin effect is also involved. The higher the impedance of that panel, the hotter it will be. Once you load the tube with a few pounds (multiple cups or dishes) of water, the walls and the patch should only get warm- still some current is flowing.

Any RF shield will have current flowing on it. It must, otherwise it defies the "laws of nature." That's why any decent computer case, especially OEM 'branded' ones, are steel or aluminum, with no large gaps.

The $20 special, with "free" power supply, doesn't count. While it may be the cheapest, nastiest excuse for steel in the universe, there's usually a huge gaping hole behind the bezel.

Ah okay, I think I get it now. Since the microwaves are small, there can be different potentials on the surface of the metal even just an inch away and that's why it's important that there are no sharp edges or hot spots, correct?

If the impedance is low, not so much potential difference, but nodes of circulating currents. Any 'bumps' in the impedance can give rise to voltage differences, forcing (almost) the same currents.

IOW, keep the impedance low...

Of course!
Only now that you mentioned it I came to realize how funny that idea is. Maybe I should run a couple of large loops and not connect the back either. A mini 2.45 GHz antenna!

Not if ya like your eyes...

But I won't be welding anything though, since I don't have a welder. Or should I still stick to plain steel?

I only mentioned that since galvanized was one of your 'possibilities.'
The plain steel will be fine, with a good paint job.

Also won't be soldering since I don't have any soldering tools for such a large job. Although come to think of it, Home Depot sells nice, large butane torches for around $30 - the kind you can use for copper pipes. Maybe worthwhile investing into one?

I don't know what other things you'd ever use it for, but if you were to use a torch, you'd have to keep the flame away from that plastic fan holder gizmo, at the very least.

Once you've got the back cleaned up, use some magnets to stick the new panel in place temporarily. Then you can mark and drill. Remove panel and deburr. Check the fit, deburr again as required.

The only place that may be a challenge is where the fan intake is. It looks like you'll have to remove the fan to drill/secure. Be sure to arrange things so the fan can go back where it belongs!

Got it, thanks for the explanations.

No problem, I'll take this over any vistrash disaster.

So something like this for the primer is good?
http://www.homedepot.com/buy/rust-ol...y-7769822.html
I'll then cover that with specialized microwave paint. Seems to be cheap online, so I don't see why not. That's what kc8adu recommended, at least.

Exactly what I thought of. You can get that in a can, too.

So put dielectric grease on the surface where the patch laps the chamber walls? Does it have to be dielectric grease specifically, or can I use any plain old grease?
Also, I'm thinking of painting the patches after I attach them to the microwave. Any objections?

Where the two surfaces meet is all you have to worry about. You're only sealing them to each other, keeping the air out.

No problem painting afterwards, no guessing how far back to go with it, or ending up with gaps that you'd have to paint afterwards, anyway.

As long as it's lapped over and free of burrs, you should be fine. If anything, all you'd have to do is take it easy with the door. While less convenient, you might try pushing the release button in while closing the door- it may, depending on how 'banged up' the unit was, keep the hinges from developing slop.

Good idea. I have a bunch of old PII laptops with PCMCIA WAN cads I don't mind sacrificing for the task.

A few milliwatts won't fry anything. If your stuff survived with those holes you originally had, go for it. Another choice is a 2.4GHz cordless phone. Those are sensitive enough to pick up the normal, or 'allowed' leakage from a micro. You should be able to talk on the phone near the micro, with the base an easy 50-100 feet away. You may get a little splatter, but it shouldn't lose the base (and the call.) Just understand that these 'dirty' methods aren't calibrated per se.

Have a look here:
http://repairfaq.cis.upenn.edu/sam/micfaq.htm


Edit-
Not to ruin your fun, but if you can't get ahead of that rust on the bottom, you may want to quit while you're still ahead. If you file and grind those edges and it then seems like it'll fall apart, save your sanity and stop.

On one level, anything can be fixed. But you have to determine what's practical to fix.

[momaka]

Re: Panasonic Genius Prestige NN-SN797S microwave

Well, as it turns out, I found someone interested in the parts in the microwave, so I guess I won't be repairing it. The back was quite rusty anyways - much more than I thought originally. So I would have had to do a lot of metal bending and cutting to make it work. It's probably still do-able, but with school, I don't have all that much time.

Anyways, I included a few final pics of the whole thing stripped down just for fun.

[momaka]

Re: Panasonic Genius Prestige NN-SN797S microwave

... and also, here are all of the parts that are in this microwave. Got a main board and timer (comes in the whole front panel), inverter, wires, door switches, magnetron, and a few other misc parts (turntable motor, input power filter board, and fan).

I need to start salvaging more microwaves. There seems to be a lot of good parts in there for fun projects .
I found out that if I turn the shaft on that turntable motor and put my fingers across the terminals, it gives me a funny feeling .

[goontron]

Re: Panasonic Genius Prestige NN-SN797S microwave

...
I found out that if I turn the shaft on that turntable motor and put my fingers across the terminals, it gives me a funny feeling .

could that be 100mv or so.

[momaka]

Re: Panasonic Genius Prestige NN-SN797S microwave

^ It's a 120V, 60 Hz, 4W motor with a high gear ratio on the output so you don't need to turn the shaft all that quickly to get 120V AC on the output .

[kc8adu]

Re: Panasonic Genius Prestige NN-SN797S microwave

nasty arcing damage on the antenna.did it still heat?

[goontron]

Re: Panasonic Genius Prestige NN-SN797S microwave

that's a magnetron not an antenna and telling from the 72 microwaves i have got me hands on there is good chance that the magnetron still works fine.

[edit]the arcing was more than likely caused by failure to sustain itself from back bombardment once RF was being produced[edit]

[ben7]

Re: Panasonic Genius Prestige NN-SN797S microwave

LOL I have accidentally shocked myself with the 120v geared motors before

BTW, some micro's have big laminated power transformers, they can be used to make arcs from the ~2000V output :P The inverter ones are much less robust, if you can get them to work without tripping their overcurrent protection... (The iron core ones need a ballast too, or else they will eventually overheat, if your breaker doesn't trip - drawing arcs if basically like shorting out the secondary)

WARNING: The output from the HV transformer is lethal, ~2000v at half an amp!

[joshnz]

Re: Panasonic Genius Prestige NN-SN797S microwave

LOL I have accidentally shocked myself with the 120v geared motors before

BTW, some micro's have big laminated power transformers, they can be used to make arcs from the ~2000V output :P The inverter ones are much less robust, if you can get them to work without tripping their overcurrent protection... (The iron core ones need a ballast too, or else they will eventually overheat, if your breaker doesn't trip - drawing arcs if basically like shorting out the secondary)

WARNING: The output from the HV transformer is lethal, ~2000v at half an amp!

I melt glass with my mot I have it going through full wave rectification at the moment i'm trying to find caps for it.

[goontron]

Re: Panasonic Genius Prestige NN-SN797S microwave

meh
i use my MOT to cook old MOBO's,pop caps,and burn old CMOS batterys.

[momaka]

Re: Panasonic Genius Prestige NN-SN797S microwave

nasty arcing damage on the antenna.did it still heat?

Yes, quite well too. The mica cover had lots of burn spots on it. Perhaps it's possible that the magnetron has been arcing to it. When I did the testing, I had the mica cover removed.

that's a magnetron not an antenna and telling from the 72 microwaves i have got me hands on there is good chance that the magnetron still works fine.

LOL I have accidentally shocked myself with the 120v geared motors before

That's what I did too yesterday multiple times. Kind of funny and it doesn't hurt that much at all. I tried measuring the voltage across the terminals today as I spun the motor. Depending on the speed I turn it with, I can easily get to 130 VAC. Hook up a bridge rectifier and a HV cap, and I got a HV charger .

[ben7]

Re: Panasonic Genius Prestige NN-SN797S microwave

Yes, quite well too. The mica cover had lots of burn spots on it. Perhaps it's possible that the magnetron has been arcing to it. When I did the testing, I had the mica cover removed.

You mean the white ceramic looking material that is part of the waveguide of the magnetron? That likely isn't mica - it is probably beryllium!

[goontron]

Re: Panasonic Genius Prestige NN-SN797S microwave

ben7 i think momaka means the metalic looking removable cover that is inside the microwave and yes the white ceramic looking stuff on the magnatron is beryllium so dont break it momaka it can damage your health.

[momaka]

Re: Panasonic Genius Prestige NN-SN797S microwave

ben7 i think momaka means the metalic looking removable cover that is inside the microwave

exactly

and yes the white ceramic looking stuff on the magnatron is beryllium so dont break it momaka it can damage your health.

Yeah, I watched a bunch of videos on YouTube before, and I remember one of them specifically mentioned to be careful NOT to break the ceramic rings on the magnetron because of the health risks.

Also, I no longer have the magnetron and inverter from that microwave.