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Old 14th Jun 2023, 12:58 pm   #1
kalee20
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Default Panasonic Microwave Oven - Magnetron replacement

This is a Panasonic microwave oven, bought new 1994, so just about vintage!

Circuitry is basically a mains transformer, a couple of diodes, and a high-voltage capacitor.

It failed due to non-heating (I'd suspected that the microwave output had been falling for some time, so it was no surprise). But it has been a good oven so I wanted to repair it.

First attempt was to get a replacement magnetron of the exact same type (used) which cost just under £25. I replaced this, powered-up with a cup of water as a load, success - replaced the cover, cooked dinner - yum - fine!

But next time, there was a loud hum and a few seconds later the mains fuse blew. Replacing the fuse, and investigating, magic smoke emerged from the replacement magnetron around the feed-through filament terminal.

Out came the magnetron, and taking a hacksaw to the crimped-on cover, I saw inside a couple of filter chokes (2.5μH) and the feed-through insulator turned out to be a twin capacitor from filament leads to mounting flange. One side was breaking down (the 'good' side measured 414pF).

With nothing to lose, I did the same to the failed magnetron - which had a 'known-good' feed-through capacitor assembly - and transplanted this into the replacement magnetron. The filter chokes were welded both ends - I reasoned that perhaps heat from the filament leads might melt a soldered joint, hence the weld. But the other end of the choke would hopefully run cooler. No doubt in a Production environment, if one lead has to be welded, they'd weld throughout so as not to have two joining methods on the bench. But I'm not set up to weld - and I even wondered if the intense welding heat had caused a latent fault on the 'failed' filter - so I soldered (using High Melting Point solder, 296°C). The voltage drop across each choke, at the magnetron's data-sheet 10A filament current, is 30mV with 2mV of this being across my new joint - negligible.

My intention is to refit the cover, soldering it all round, and replace the magnetron in the oven. It's had a thorough clean using white spirit, and a good blast with an air-line (you know what happens when mild steel is sawn in close proximity to a magnet).

However! With high voltages around (that I know about, and treat with respect) and high-power 2.45GHz radiation (which I don't, but I'm mindful of the consequences of getting it wrong), I wanted to get some opinions from the collective expertise and knowledge here - am I, like the Sorcerer's Apprentice, about to get out of my depth?
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Old 14th Jun 2023, 1:24 pm   #2
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Michael Dranfield has done this

https://youtu.be/U7oyLb_42Os
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Old 14th Jun 2023, 1:34 pm   #3
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Hmm...

While I admire your work, I doubt the wisdom of putting all that effort into fixing a 30 year old microwave oven. As you say yourself, they are potentially extremely dangerous devices for a number of reasons. I wouldn't have started down this route in the first place.
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Old 14th Jun 2023, 1:55 pm   #4
kalee20
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Thanks both - Kevin, I looked at the video, he's done basically the same as me but didn't include how he rejoined the chokes to the replaced twin feed-thru capacitor.

Paul - agree! - if I'd known it would be this much trouble, i wouldn't have even started. But I have, so I'd like to finish! I have learned a bit, I didn't even realise the high-voltage terminal gubbins was a filter capacitor till it started squirting smoke out of an 'impossible' place.

And I stripped apart the original magnetron - the bare valve, minus magnets, as a diode is passing 12mA with 25V on the anode (with filament heated 3V 10A as per the on-line data sheet I found). No magnets, no electrons moving in curly paths, no UHF oscillation... I'd have expected a much higher current so I'm confident that it has lost most of its emission.
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Old 14th Jun 2023, 4:44 pm   #5
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Hi

I have done this on £250+ commercial mags, with a micro brazing torch to braze the joint, and just re-crimped the base cover, then used foil tape over the joint to guarantee the joint is RF proof.

Personally I would invest in a cheep leakage detector to be safe, £25 ish. Worth doing once in a while on any microwave.

FYI - the diode you show across the capacitor is actually an asymmetric rectifier (short protector), it does nothing unless there is a short cct in the HT system (usually the mag), then it goes short circuit, which then causes a short circuit of the HT voltage to chassis, and blows the fuse. Without it the unit will continue to run, with a loud deep humming noise and often lead to the transformer failing.
If yours blew the fuse it may well have gone short circuit.

For test purposes you can just cut it out of the circuit, if all is well and the microwave works then fit a new one.

As already mentioned, be VERY wary of the high voltage system.

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Old 14th Jun 2023, 8:46 pm   #6
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Watch and learn.

https://youtu.be/FBeSKL9zVro
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Old 14th Jun 2023, 10:24 pm   #7
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Thanks both - Cathovisor, I'm aware of the voltages and the hazard! (Including charge retention in the high-voltage capacitor). I've had a few 240V shocks and it serves as a wake-up call - I don't want my oven to outlive me (I've already repaired the door mechanism at least twice)!!

Richard - can you suggest a source of UHF leakage testers? I looked on ebay without success, and £25 is a good investment to identify what I can't see.
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Old 15th Jun 2023, 1:56 pm   #8
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Default Re: Panasonic Microwave Oven - Magnetron replacement

A search on ebay for "microwave leakage" finds over 100.
I wouldn't bother with the cheep £5 item, which looks like a neon screwdriver, the results will be meaningless as there is no indication of how much it's detecting.

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Old 15th Jun 2023, 5:26 pm   #9
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Hi.
I sincerely hope Michael didn't do this to a customers microwave oven! There was once upon a time that only one microwave course was approved in the UK (Sharp) due to safety of the engineer and consumer, those attending will or should remember the warnings about repairing or modifying any internal item eg mains tx or magnetron, failure to do this was instant dismissal in any of the multinational companies.
While it's up to yourself what you do with your own property I'd just get a magnetron or purchase another oven, doing this with a third parties oven is a big can of worms.
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Old 15th Jun 2023, 7:04 pm   #10
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Default Re: Panasonic Microwave Oven - Magnetron replacement

I wouldn't have spent £25 on a magnetron new or used, when discarded ovens often turn up on the pavement waiting for a random scrap metal dealer.

I must confess I have a small stash of microwave oven parts in the garage, I sometimes feel it's a duty to take them, lest they fall into the hands of the fractal woodburning brigade.

Respectable dealers sell a range of "Universal" magnetrons, that suggests it's the relative orientation of the mounting holes, cooling fins and terminal block that's more important than the exact electrical parameters.

Some magnetrons have stud mountings rather than holes.
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Old 16th Jun 2023, 12:44 pm   #11
kalee20
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Quote:
Originally Posted by Richard View Post
A search on ebay for "microwave leakage" finds over 100.
Thanks! Dunno why it didn't when I first tried - but it does now and I've just committed £24.

Quote:
Originally Posted by Graham G3ZVT View Post
I wouldn't have spent £25 on a magnetron new or used, when discarded ovens often turn up on the pavement waiting for a random scrap metal dealer.

I must confess I have a small stash of microwave oven parts in the garage...
I'm sure that would be right! But... I don't want even a small stash of microwave oven parts in the garage or workshop, they'd be taking up space that would be better occupied by a radio needing some love and attention. It's worth £25 to me not to have that dilemma!
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Old 16th Jun 2023, 6:04 pm   #12
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Quote:
Originally Posted by murphyv310 View Post
Hi.
I sincerely hope Michael didn't do this to a customers microwave oven! There was once upon a time that only one microwave course was approved in the UK (Sharp) due to safety of the engineer and consumer..
Hi Trevor, IIRC Sharp wouldn't supply anyone with certain MWO components if they hadn't the approved MWO servicing certs, not necessarily a Sharps own delivered course.

I take it reading between the lines that Sharp suffered serious litigation at some point, I couldn't possibly comment however
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Old 18th Jun 2023, 2:55 am   #13
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Back in the day, my Panasonic microwave oven failed prematurely.

Upon opening it up, (I knew all about high voltages, my day job being working on Broadcast transmitters), I found that one of the "auto style" push on connectors for the magnetron filament was burnt black.

The other one looked like it was brazed on, but obviously the braze was a bit dodgy on the cooked connector.
I did have the capability to braze, but the thought of getting into the guts with a standard sized Oxy-acetylene torch was a bit "shudder making".

I opted to use conventional solder to attach the replacement connector, thinking, "The worst that can happen is that it will fail again".

It was still working 20 years later, but the magnetron was dying, & the Panasonic was pretty much a "Boat Anchor" so it was replaced with a more modern type.

If I had just extended food heating times & been happy with how huge the thing was, I orobably could have got another 5 years out of it!
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Old 4th Jul 2023, 11:06 am   #14
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Default Re: Panasonic Microwave Oven - Magnetron replacement

After a break, the magnetron’s filament filter box is now soldered on, and the magnetron reinstalled in the oven.

I did take the opportunity first, to measure the transformer output voltages (at 10% input, I didn’t want to fry either my meter or myself).

With 24.1 V on the mains input, I measured 0.33V on the filament winding and 204V on the main winding.

The data sheet for the magnetron specifies 3V for the filament, so that's about right: scaling up x10 gives 3.3V which will drop, on-load (the filament and main HT winding are not closely coupled to the primary, in fact there's a set of laminations to act as a partial bypass, giving a high leakage inductance).

With a load of a mug of water, everything powered-up as it should, and the water heated up. The leakage monitor showed slight leakage but of no concern (around 1mW / cm²). As the monitor came with no certificate of calibration (!) I checked this against another (much newer) microwave oven, which I would hope would be ‘safe’. Both gave indications of leakage, although very much of a muchness, and both well under the limit 5mW/cm² (neither showed more than 1.6mW/cm²).

The cover is now back on, the oven is recommissioned, and last night’s broccoli and asparagus were successfully cooked by absorption of radiation at 2.45GHz.

The old, clapped-out magnetron – I stripped it down and removed the magnets, leaving just a directly-heated diode with an anode that’s fancy on the inside. No magnets – no electrons moving in curly paths, so no UHF oscillation possible. Applying 3V to the filament leads, 30V to the anode, it pulled just 5.5mA. Raising filament voltage to 5V (drawing 13A!) the anode current increased to 12mA so it certainly is very low on emission.

Quote:
Originally Posted by paulsherwin View Post
Hmm...

I doubt the wisdom of putting all that effort into fixing a 30 year old microwave oven ... I wouldn't have started down this route in the first place.
Well, most of our thermionic technology based repairs, as on this Forum, probably don't make economic sense – the satisfaction comes in returning to use an item that would otherwise be scrapped. This one is no different, and it did start as a relatively simple repair that got more complex – but having given good service for 28 years, the oven’s now set for another spell of life. And I’ve learned a few things along the way – every day is a school day! All I’m left now thinking – if I’d replaced the mains fuse with an audiophool-grade gold-in-quartz-tube, would the oscillation be more spacious and coherent, and would the food have tasted better?
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Old 4th Jul 2023, 6:51 pm   #15
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Well done Peter! Hopefully it will give good service for a few more years, I wouldn't put any money on any modern oven lasting as long as it has already. I must admit though, a Microwave oven isn't something I would even try to 'restore'.

Quote:
Originally Posted by kalee20
The old clapped-out magnetron – I stripped it down and removed the magnets, leaving just a directly-heated diode with an anode that’s fancy on the inside. No magnets – no electrons moving in curly paths, so no UHF oscillation possible. Applying 3V to the filament leads, 30V to the anode, it pulled just 5.5mA. Raising filament voltage to 5V (drawing 13A!) the anode current increased to 12mA so it certainly is very low on emission.
I had always wondered if a magnetron with the magnets removed would make a usable HV rectifier, that however is only curiosity, there is really no advantage to it over solid-state diodes, and plenty of disadvantages.

As for the bêtises alimentaires, ideas for improving the flavour of food prepared in a modified microwave oven I'll leave that for others to judge.
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Old 4th Jul 2023, 7:31 pm   #16
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Congratulations on a job well done.

Microwave leakage is only going to be a problem with an intact domestic oven if you're daft enough to stand with your face pressed against the window watching the food cook. Even standing a couple of feet away will dilute any radiation below harmful levels.
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Old 7th Jul 2023, 6:13 pm   #17
kalee20
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Agree Paul, and I always look through the window in the oven door from a couple of feet away!

The reason I wanted to test it with a leakage tester, was to be sure that the old oven had leakage comparable with a new one, that the window shield (or any other shielding) was doing its job, and it was, as you say, intact.

With measurements as described, I'm confident that it is intact. But I did my due diligence, and it's informed confidence.
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Old 7th Jul 2023, 9:57 pm   #18
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Default Re: Panasonic Microwave Oven - Magnetron replacement

Good job.

-Regards longevity, it's annoyed me on several occasions that they typically make the chassis out of mild steel with a cheap paint job, and the oven rusts at the seams beyond cosmetic or hygienic limits whilst the guts are still serviceable. (The way of things.)

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Old 7th Jul 2023, 10:28 pm   #19
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Default Re: Panasonic Microwave Oven - Magnetron replacement

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?
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Old 8th Jul 2023, 3:01 pm   #20
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Default Re: Panasonic Microwave Oven - Magnetron replacement

I wonder if that was yet another of those things that legislators felt would be a Good Idea, but fell by the wayside under manufacturer lobbying. There's always money and resources at hand to demean safety initiatives if it would incur corporate expenditure- ask Ralph Nader.... On the other hand, many people have a fashion/decor-driven replacement cycle for domestic appliances, so the majority of microwave ovens probably only have to last a few years, it's only outliers like us who soldier on with stuff for ever and a day (speaking as a regular user of a painted mild steel 1988 model!).

Kalee20's endeavours prompted me to open up a scrap magnetron here, it looks as though there has been an attempt at broad-band choke construction.
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