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Old 8th Jul 2023, 4:28 pm   #21
G6Tanuki
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Default Re: Panasonic Microwave Oven - Magnetron replacement

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I seem to remember there was talk of making it mandatory to manufacture microwave oven cooking cavities from stainless steel at one point, I suppose that fell by the wayside?
It would seem an odd thing for regulation to be applied to. Adding a quid to the cost of everyone's microwave just so a tiny % of buyers could continue using their ancient appliances in 15 years time would not have gone down well with most purchasers [myself included].
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Old 8th Jul 2023, 4:41 pm   #22
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Default Re: Panasonic Microwave Oven - Magnetron replacement

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I seem to remember there was talk of making it mandatory to manufacture microwave oven cooking cavities from stainless steel at one point, I suppose that fell by the wayside?

I hadn't heard that, but for many years I've advised people looking to buy a new microwave to regard a SS interior as the #1 feature to have.

Not particularly for safety, but in my experience a rusty cooking cavity is the main reason why they are scrapped.

Domestic SS ones tend to have grill element which many people will not use.

[X'sed with G6Tanuki]
I wonder if that proposal was a Green (longevity) issue, or a safety one. I have seen >10mm holes in microwaves that started off as rust under the paint!
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Old 8th Jul 2023, 5:16 pm   #23
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Default Re: Panasonic Microwave Oven - Magnetron replacement

I believe the reasoning behind the stainless thing was there had been cases where the cavity or door bottoms had rusted through, but were still being used. The stainless ones are so much easier to keep clean too.

It obviously didn't happen so people who want rock bottom priced rust buckets and throw them away after a couple of years can stay happy

One caution often heard regarding the disassembly of the actual magnetron is that the ceramic insulators/seals contain beryllium oxide, this is generally not the case, (it is normally Alumina), however I have been told that the window aperture is made of beryllium. It is likely that this only applies to high performance devices not ones used in domestic ovens, there is a lot of contradictory information, so the best course of action is to not cut these items open.

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Old 8th Jul 2023, 7:02 pm   #24
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Default Re: Panasonic Microwave Oven - Magnetron replacement

You don't need SS to stop rust, just decent quality construction. My 80s Goldstar microwave has a painted mild steel interior and shows no sign of rust, despite seeing plenty of use and not being mollycoddled. It'll end up being scrapped because the plastics are failing (hardly surprising after 35 years in a sunny kitchen).

Maybe 35 years of baked on internal filth is acting as a rustproofer
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Old 9th Jul 2023, 2:55 pm   #25
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Default Re: Panasonic Microwave Oven - Magnetron replacement

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You don't need SS to stop rust, just decent quality construction. My 80s Goldstar microwave has a painted mild steel interior and shows no sign of rust, despite seeing plenty of use and not being mollycoddled. It'll end up being scrapped because the plastics are failing (hardly surprising after 35 years in a sunny kitchen).

Maybe 35 years of baked on internal filth is acting as a rustproofer
I have a GE badged Goldstar built Microwave oven, that I've been using for twenty years, just about every day. The interior is painted steel as well and shows no signs of corrosion!
Regarding, plastics exposed to sunlight, the trim on my fridge, handle etc, turned a nasty yellow after twenty years of exposure. They claim, Flameproof plastic has this characteristic.
Dave, USradcoll1, just an observation!
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Old 9th Jul 2023, 7:53 pm   #26
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Default Re: Panasonic Microwave Oven - Magnetron replacement

In my experience the painted steel cavities tend to corrode wherever food or moisture can accumulate, which makes sense. My parent's american-made Litton microwave was painted but had a seamless, formed flange where it met the door, and there was nowhere for food to accumulate. It only got scrapped when the mag began flashing over at the antenna.
My 27-year old panasonic is made from welded pressings but I keep it scrupulously clean (and leave the door ajar when not in use) and as such has no corrosion whatsoever even around the seams, turntable and waveguide cover.

I bought HWMBO a new microwave (a cheap Galanz-made thing) after I noticed he was using his old one with a rotten cavity. The new one lasted only 2 years and rotted out. He had got into the habit of letting stuff boil over and running into the turntable recess, and then ignoring it. He now has a stainless one.
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Old 10th Jul 2023, 11:56 pm   #27
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Default Re: Panasonic Microwave Oven - Magnetron replacement

i bought our first microwave in 1982, this Plustron. Click image for larger version

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ID:	281147We used it for at least 15 possibly 20 years. It was really solid and heavy.

In common with other early examples, a waveguide coupled the magnetron's antenna to the roof of the cooking cavity where a rotating passive antenna stirred the radiation pattern within the cavity, so no need for a turntable.

Rather than have a simple fan wafting cooling air onto the magnetron, the waveguide doubled as a ducting for a centrifugal fan allowing more efficient cooling, plus the air flow powered the rotating antenna!
(I suspect some high-end makes like my daughters built-in "Bosch" continue with this construction).

I discovered the door periphery was loaded with slabs of ferrite acting as a choke to reduce leakage.

The microprocessor controller was very flexible, allowing you to pre-programme two cooking periods at different duty cycles with a rest period in between.

It had a half inch jack socket in the cavity to plug in a temperature probe for meat, so one or both cooking periods could be temperature rather than time dependent. (It was purely a microwave, no grill or conventional oven elements).

The odd thing that I remember, that in the store it was displayed next to another make with a manual rotary "ding" timer, that was more expensive.
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Old 26th Oct 2023, 11:53 am   #28
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Default Re: Panasonic Microwave Oven - Magnetron replacement

I did saw open the original magnetron - I didn't notice a brief hiss of air as the saw bit through the copper anode, but then the interior space isn't massive.

I'm full of admiration for the copper work, all done on a production basis! What a way to generate 800W of RF.
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Old 26th Oct 2023, 4:36 pm   #29
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Yes, note the two copper strapping-rings - these cause the individual cavities in the block to oscillate in phase rather than randomly - I remember reading about the discovery of this effect back in WWII and how the straps stopped what was then known as 'mode jumping' which caused unpredictable frequency-shifts and reduced output power.
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Old 26th Oct 2023, 5:57 pm   #30
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Thanks!

Do you have any idea why the partitions forming the cavities have a notch cut out, in different positions for each partition?
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Old 26th Oct 2023, 6:04 pm   #31
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Default Re: Panasonic Microwave Oven - Magnetron replacement

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Thanks!

Do you have any idea why the partitions forming the cavities have a notch cut out, in different positions for each partition?
Sadly no... I guess that in the 70+ years since the cavity magnetron was 'tamed' by Randall&Boot and the Air Ministry there has been a lot of subsequent research into optimising them for different applications.

For a Microwave-heating device, optimum efficiency at converting power-in to heating-microwaves-out is important, frequency-stability less so.

For RADAR, though in-to-out power equations are important, the tight frequency-stability of that output-power might be just as important, if only to stop your RADAR from wandering and flattening the other RADARs in the squadron you're flying with.
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Old 26th Oct 2023, 7:29 pm   #32
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Default Re: Panasonic Microwave Oven - Magnetron replacement

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Do you have any idea why the partitions forming the cavities have a notch cut out, in different positions for each partition?
In a plain cavity magnetron there are several slightly different modes which it can enter into and it can transition rather erratically between them. Expensive military radar magnetrons were fitted with wire loops linking cavity to cavity. This acted to stabilise one mode and make the thing much more useful. This was called 'strapping'

In el cheapo mass production maggies, the wire strapping would be too expensive, so the cavity to cavity (well the stem gap) wall being notched looks like a cheap alternative. Having notches not line up spoils coupling over more than one jump.

David
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Old 26th Oct 2023, 9:08 pm   #33
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Default Re: Panasonic Microwave Oven - Magnetron replacement

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I'm full of admiration for the copper work, all done on a production basis! What a way to generate 800W of RF.
Me too- amongst RF devices generally, magnetrons come across as a particular height of sorcery!

Re. the strapping/cavity slots, presumably the need to keep oven magnetrons on mode and 2.45GHz (or should that be 2.45KMc/s on a vintage forum....) gets ever more pertinent with the proliferation of use of this section of spectrum.
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Old 26th Oct 2023, 10:21 pm   #34
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Thanks all! David's explanation makes sense, to inhibit unwanted modes of oscillation.

I do wonder how accurately the frequency is set - there's no way of tuning it after assembly and evacuation as far as I can see, so it's got to be set by dimensional tolerances. There's only a small window in the spectrum set aside for cooking...
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Old 27th Oct 2023, 2:11 am   #35
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Default Re: Panasonic Microwave Oven - Magnetron replacement

In the photo earlier, you can see the two strapping rings.

Drift? Quite a lot.

I have a spectrum analyser which goes high enough in frequency and when a neighbour is using their microwave cooker I can watch it warm up, and also see the slow frequency reciprocation at the speed of the turntable rotating. They intentionally run at a frequency at which water is absorbant and is scheduled as an ISM, Industrial, Scientific, Medical band originally created for heating devices at 2.4GHz.

There was a paper mill in Inverkeithing which used a monster 2.4GHz heater to fast dry the paper they made. This was easily seen by any spectrum analyser with a couple of inches of unscreened wire at its input. So there was a big microwave heater (not very well enclosed!) on the North side of the Forth, and a spectrum analyser factory nearby on the South side of the Forth.


The ISM bands have been treated as the muck heaps of the RF spectrum and a lot of comms type services that nobody wanted in their back yard got dumped there on the assumption that no-one would complain. The 27MHz ISM band got CB radio.

Wifi and Bluetooth were to some extent seen as nuisances in the beginning, but the chouce of the water absorbant 2.4GHz band was deliberate. Absorption by atmospheric humidity moderates the range of these services, allowing more re-use of frequencies over moderate distances without interference.

David
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Old 27th Oct 2023, 9:07 am   #36
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Default Re: Panasonic Microwave Oven - Magnetron replacement

To show the strapping across the cavities better this is a photo of a more powerful pulsed magnetron( 2.5Mw peak )
The cathode is in the centre, at the bottom is a wire leading to the aerial, the tuner is driven by the silver ring on the side an below that are the water cooling inlet and outlet connections.
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Old 27th Oct 2023, 10:18 am   #37
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Years ago we had a client with a giant microwave that was used to heat rubber for an extruder and periodically we would get the controller in for repair with a CMOS memory checksum fail and the memory full of rubbish. I did wonder if there wass something a little amiss with the microwave part of the outfit.
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