Is a magnetron recoverable at home? - Page 1
Forum section: electronics / repair

author: gabriel_agm
author_rank: Newbie
timestamp: January 24, 2020, 05:38:13 pm
content: So, my MW oven made a strong noise on turning on and the fuse blew up leaving that known 'ampere smell' in the air.I've opened it up, discharged the HV cap and tested the main parts in separate: transformer, diodes and caps seems to be fine.The magnetron, AFAIK, should have an infinity resistance from cathode to chassis but is reading 3.6Kohms.Is that enough to tell it's gone forever and I need to buy a new one, or is there any possible maintenance?If you have time to read:The filament seems ok, 0.2 ohm, but I decided to search for dirt, bugs or anything that could cause arcing inside the non-sealed chamber at the back or waveguide at the front.No bugs or dirt that I could clean, so I didn't touched the BeO ceramic. Yet the metallic golden mesh ring that connects the antenna to the waveguide seems to have been overheated by arc, the ceramic has traces at the surface and the coils at the back have the enamel burned (see the pics).So that's the end of the line? Does it means the vaccum in the tube is gone?

author: gabriel_agm
author_rank: Newbie
timestamp: January 24, 2020, 05:39:33 pm
content: Oh, any replacement tips I should know about?

author: xavier60
author_rank: Super Contributor
timestamp: January 24, 2020, 06:05:12 pm
content: The capacitors in the filament socked can break down. Replacements are available from many places,https://www.amazon.com.au/Electronic-Microwave-Magnetron-Filament-Terminal/dp/B07N2MSGFQ

author: SeanB
author_rank: Super Contributor
timestamp: January 24, 2020, 06:07:36 pm
content: Quick answer on those cheap magnetron’s is no, the typical failure is the filament shorts to the copper block, though you do get a short circuit forming in the connections to the outside, that white plastic block contains 2 feedthrough capacitors, and they can fail. however there really are only a few variants of the magnetron, and by far the most common styles are in the cheap microwaves, meaning pretty much any salvaged microwave magnetron has a good chance of having the right combination of orientations. So long as the mounting base orientation, the fin orientation and the power connector orientation are the same so it physically fits, the new magnetron will work.Typical causes for fuse blowing are failed transformer, especially the cheaper ones wity CCA wire, and by far the most common fault is a switch failure in the door, there are 3 microswitches in the door area, and either one is faulty, or the plastic mountings are faulty. Another is dirt in the cavity of the magnetron, though that normally leaves a big pile of soot in the area, and also a failed fan or lamp can blow the fuse.

author: gabriel_agm
author_rank: Newbie
timestamp: January 24, 2020, 06:35:58 pm
content: Quote from: xavier60 on January 24, 2020, 06:05:12 pmI cutted off the coil connections to the socket/capacitor and the socket alone measures 3.6ko to chassis. The filament to chassis is measuring infinity.These seem to be cheap parts, if I can find them in local stores, and replacing to test seems a valid approach.I just need to find a way to make a junction of the thick copper wire to the socket internal terminals, as they seem welded and not soldered.

author: chris_leyson
author_rank: Super Contributor
timestamp: January 24, 2020, 06:36:41 pm
content: Microwave magnetronsDO NOTuse BeO anode insulators, it's urban myth. Read the data sheet. If they had BeO insulators then there would be a warning sticker.Replace magnetron with the same part number, anode lengths are different in order to get the proper match into the waveguide, you can't just use any old magnetron.

author: xavier60
author_rank: Super Contributor
timestamp: January 24, 2020, 06:46:45 pm
content: Quote from: gabriel_agm on January 24, 2020, 06:35:58 pmMount the new socket with screws. Some sockets are supplied with screws.Tin the ends of the chokes then position them as they were originally. Secure with a few wraps of solid wire and solder. Some extra RMA flux will help to make a good joint.

author: gabriel_agm
author_rank: Newbie
timestamp: January 24, 2020, 07:37:23 pm
content: Sometimes living in Brazil sucks. I cannot find these connectors/caps locally, from eBay or Amazon it would cost me R$10+R$50 - and from China they never make trough the customs.A new magnetron in local stores, same model/brand, costs R$80 and is promptly available.So I'm going for the lazy solution and replace it all.About chris_leyson comment, it makes sense, berillyum is expensive and if the whole part can be sold at US$20 in retail, the insulator couldn't cost over half a dollar.Galantz provides no datasheets online for that part, though.Thanks for all the comments.

author: gabriel_agm
author_rank: Newbie
timestamp: January 24, 2020, 08:11:32 pm
content: Done, seems to be working fine. I'm gonna check the actual power it outputs by heating some water.

author: ivaylo
author_rank: Frequent Contributor
timestamp: January 25, 2020, 05:21:52 am
content: So what was the exact fix if you don’t mind?

author: amyk
author_rank: Super Contributor
timestamp: January 25, 2020, 10:27:05 pm
content: Quote from: chris_leyson on January 24, 2020, 06:36:41 pmWas just going to mention this too, also BeO is white whereas pink insulators are alumina. BeO is far too expensive for this application.

author: james_s
author_rank: Super Contributor
timestamp: January 25, 2020, 10:35:12 pm
content: Is it possible that some early magnetrons used BeO? I'm pretty sure I've seen some that had white ceramic insulators although that doesn't mean it was BeO. Microwave ovens were very expensive back in the 1970s.

author: SeanB
author_rank: Super Contributor
timestamp: January 26, 2020, 11:31:09 am
content: Only magnetron’s I have ever met with BeO insulators were in military avionics, where you needed the power handling capacity. Also was in the power drivers and in the RF switches, as these were ionisation types, and needed a fast turn off. Still have some TO3 isolators with the circular isolator being BeO, with a plastic surround, in white ceramic, rated for withstanding around 1kV on the collector, as they are nearly 2mm thick, but are really good at heat transfer.Other than that there were some hybrid circuits with BeO substrates, which were really expensive devices, but which were really good power drivers for the size. Near identical ones were white aluminia though, and were a laser trimmed R-2R BCD encoded 16 bit signed DAC, which was prone to failure, because it sat right where sweat off you would land on the top of it, and corrode off the leads.

author: gabriel_agm
author_rank: Newbie
timestamp: April 11, 2021, 05:59:12 pm
content: Sorry, I've been offline from this topic for a while.I bought an used magnetron and replaced it.Later, disassembling the burned one I've found that, somehow food greese entered the horn cavity where the magnetron antenna is and it arced badly. The antenna is a round cap and a part of it was eroded. I guess this caused the loss of vacuum destroying the mag.Anyway, what I've learned is that an used magnetron is cheaper than my time trying to fix it ti'll I give up.

author: Gyro
author_rank: Super Contributor
timestamp: April 11, 2021, 07:20:23 pm
content: Those magnetron antenna top caps are replaceable too - they are push fitted. The actual vacuum seal is inside it, and is a very substantial copper pinch seal. It is very unlikely that the vacuum has been compromised - that doesn't mean that the anode structure hasn't suffered overheating damage of course, but it is relatively cheap to find out.https://www.eevblog.com/forum/chat/microwave-oven-pwm-frequency/msg3144818/#msg3144818

author: coppercone2
author_rank: Super Contributor
timestamp: April 11, 2021, 09:04:35 pm
content: i would not fuck wit hit, I saw my magnetron that malfunctioned in a sharpe microewave and it melted the waveguide, inside it can be totally dicked

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: April 12, 2021, 12:00:41 am
content: What he said.

author: Gyro
author_rank: Super Contributor
timestamp: April 12, 2021, 08:48:24 am
content: ... although if it is totally dicked inside, it won't have the energy available to arc anything. It is at ground potential at DC, it is purely radiating RF. It is impossible for it to output more than its rated maximum power.Yes, the waveguide needs to be clean and uncontaminated, and the oven not run empty (reflected power).

author: coppercone2
author_rank: Super Contributor
timestamp: April 12, 2021, 04:52:54 pm
content: the problem is if you do something to clear some invisible area like using current and then its not really clear you are going to walk in on an arc welder welding your chicken best case.I have a feeling those magnetrons are not otherwise fine if they explode like that. Maybe its at the end of one of the tolerances of manufacture or something changed a little bit, because plenty dont do this for eons with the same conditions as some that do it for no good reason at all after a few months of brand new use!On a smaller microwave its not isolated by a waveguide, or maybe I should say cheaper microwave, it feeds right into the cavity, other microwaves have like a foot of waveguide in there that maybe is a slight isolation.Why do generally filthy microwaves that operate hard use for like 10 years work fine but some explode into arc welding after less then a year?What I am saying is don't try to 'clear the short with a pulse', if someone was going to attempt itThe next question is, is that a crimp or a push fit or a screw on at 2GHz 1500W?

author: Gyro
author_rank: Super Contributor
timestamp: April 12, 2021, 05:50:17 pm
content: Quote from: coppercone2 on April 12, 2021, 04:52:54 pmNope, sorry. I hardly understood a word of that.Consumer oven Magnetrons don't explode or arc weld. At worst, they short and blow the fuse.On consumer ovens, the antenna stud (see OP's photo) just sticks up into a waveguide which feeds straight into the oven cavity via a 'mica(ish)' window. The wire mesh ring compresses against the outer wall of the waveguide.I didn't say anything about clearing shorts with pulses.The antenna cap is just push fit, you can just twist and pull it off with a pair of pliers (I've done it) it's all very basic. I think the only thing that identifies them is the shape of the hole in the (triangular, square etc.) in the top of the cap. These things are very standardised for mass production.P.S. That's an image of a particularly dirty magnetron that I found on the web and with the rest of the assembly removed, but it shows how the antenna cap is fitted and the heavy crimped copper vacuum seal that is underneath it.

author: coppercone2
author_rank: Super Contributor
timestamp: April 12, 2021, 07:10:49 pm
content: mine melted the cap/antenna of the magnetron and then the bit under it about half way and blew a hole in the wave guide and kept on sparking and throwing sparks into the chassis until I turned it off, luckily that one is connected to the outlet, when i walked in on it it looked like someone positioned an arc welder electrode in the microwave.... I heard it arcing for a while before I figured out WTF was making the noisemaybe it woulda tripped a fuse eventually but it was happy to go along melting metalso to repair it you would need to replace both the magnetron and the waveguide elbow that feeds into the microwave, repair the chamber, replace the filter/screen and clean up the interior near the waveguide from I guess sputter. i call that fubar.like,, when you opened it up, you can see the molten antenna from the hole in the wave guide right as you take the top cover off lolthey defiantly weld, i could do the same thing if I stuck a over amped arc welding electrode or a gouging electrode there to blow holes/explode everything, if you asked me to replicate the damage for a movie prop or something its what I would have done

author: Gyro
author_rank: Super Contributor
timestamp: April 12, 2021, 07:35:58 pm
content: Ouch, I've never heard of one getting that bad - the wavequide is stainless steel whereas the magnetron antenna is softer and lower melting point. I've not heard of a case where it burned through both before - you must have had a habit of running it empty or cooking metal pans!Obviously such damage would be very obvious and, of course, a complete write-off as the waveguide is an integral (welded) part of the oven cavity assembly. They don'tnormallydo that.

author: coppercone2
author_rank: Super Contributor
timestamp: April 12, 2021, 07:45:18 pm
content: no, I had a small amount of chili (maybe 1/3 to 1/2 cup) for making a chili dog, and it was the smaller but not smallest sharp brand microwave. Maybe I should have put water near it or something, maybe its under the minimum size for the microwave oven, but its convenient to make chili dogs... I did find this weird gravy bowl thing though, that one side can be filled with water and the other side with something else, to balance microwave loads, if I remember to use it.. I did find the amount of damage suspicious.But I replace it with inverter panasonic one so that maybe has better failure mode, those are much nicer. With the amount of cooking Americans do with microwaves going cheap with them is a dumb idea.I used to have another one that arced near the little fiber filter thing that caps the waveguide, now I thought this one was doing the same thing, where that bit is just dirty, but it blew the fuses on next turn on as far as I can tell and when I saw the insides after taking it apart I said fuck this shit.But they don't always blow the fuse, it needs an AFIC I think, to be safe..

author: coppercone2
author_rank: Super Contributor
timestamp: April 12, 2021, 07:53:40 pm
content: not all waveguide is welded on, I got a piece of it out of an old microwave that had that fake wood on it (probobly old), its like real wave guide but made of shitty cheap sheet metal with a few screws to bolt it on the chassis, but it does not terminate into a connector ever, if you wanna add a N connector. but when i Look at the quality of the wave guide that you can get from a big oven, its really bad, and putting a n connector on it seems like putting a silver cufflink on a turdsteel is bad for this, i think thats why it can melt, because of the high resistances, it should be brass... they are really built bottom dollar, but I guess it would need to be substantially thick.also when I look online, the antenna damage on the magnetrons I find is small, its just a pinhole burned out, mine was melted into like a wedge with most of it gone

author: Gyro
author_rank: Super Contributor
timestamp: April 12, 2021, 08:15:43 pm
content: Chilli usually gets you one way or another! Yes, it sounds like reflected power with no place to go.Agreed, these things are, of necessity, made of non-ideal materials (they would normally be Silver plated at that frequency for a start).Here's the picture I posted in the other thread that I linked. They can't all take out the waveguide without becoming noticeable as there are so many replacement caps available for sale ( I suspect this one probably did though!)....Edit: I guess this has become academic at this stage as I think the OP gave up on the second hand Magnetron (cost of the bits in Brazil).

author: coppercone2
author_rank: Super Contributor
timestamp: April 12, 2021, 08:20:35 pm
content: I would call that 15% damage, mine was at like 70% at the tip + the hole in sheet metal next to it

author: rsjsouza
author_rank: Super Contributor
timestamp: April 12, 2021, 08:21:38 pm
content: Quote from: coppercone2 on April 12, 2021, 07:10:49 pmOurs failed that way as well - a 13 year-old GE microwave that took anything we threw at it like a champ, until near its end where arcing was clearly visible on the waveguide once I disassembled it.

author: coppercone2
author_rank: Super Contributor
timestamp: April 12, 2021, 08:33:26 pm
content: did you have a hole in it?

author: rsjsouza
author_rank: Super Contributor
timestamp: April 12, 2021, 10:34:08 pm
content: Quote from: coppercone2 on April 12, 2021, 08:33:26 pmCheck my video at 7:53https://youtu.be/uoYqH9rDwiE?t=473

author: bsfeechannel
author_rank: Super Contributor
timestamp: April 16, 2021, 01:26:01 am
content: Quote from: gabriel_agm on January 24, 2020, 07:37:23 pmYou were not even trying. Those connectors/feedthrough capacitors can be easily foundfor around 6 US bucks a popdelivered, depending on where you live in Brazil. You can even pay it in four installments, if you are really budget constrained.Anyway, at the end of last year I had a magnetronic failure of the same kind. The connector turned into a 30 ohm resistor. The magnetron stopped working, but didn't blow any fuse. I ordered a replacement and started the surgery.I used a 3.5mm drill bit to remove the rivets that held the connector in place.This produced filings that were immediately attracted by the magnets of the magnetron and needed to be removed.I also noticed some discoloration on the base of the filament support and on the internal plate of the casing, suggesting the occurrence of arching.This is the final result.To replace the rivets I used these screws for hard-disk drives. I've got lots of them.I applied solder between the nuts and the screws to prevent them from becoming loose with vibration.The magnetron is working OK ever since.

author: rsjsouza
author_rank: Super Contributor
timestamp: April 16, 2021, 12:40:44 pm
content: Pretty nice job, bsfee

author: james_s
author_rank: Super Contributor
timestamp: April 16, 2021, 11:04:42 pm
content: Another option is just sub in another magnetron from a junk oven. I did that a couple times and it worked fine, most magnetrons are pretty similar, I just found one that was about the same size with the same mounting hole pattern and it worked. Old microwave ovens are easy to get for free, they get all scuzzy looking or some other part fails and people toss them out.

author: macona
author_rank: Regular Contributor
timestamp: April 18, 2021, 06:11:08 am
content: Industrial magnetrons use BeO, Im talking the 5-10kw+ ones used in things like RF excited mercury lamps for curing adhesives and stuff. Thats probably where people thought BeO came from. Also small argon ion lasers use it for the plasma tube, its pretty much the only thing that will stand a couple kw of plasma passing down it without magnets or water cooling like the larger ion lasers.