Microwave oven repair - Page 1
Forum section: electronics / repair

author: jrmymllr
author_rank: Regular Contributor
timestamp: July 16, 2021, 08:23:28 pm
content: Ok, not the usual "interesting" repair item but it's an important part of my kitchen! I hate throwing things away so I keep fixing them as long as it makes financial sense.I have a Goldstar MV1610WW (if that helps at all) basic microwave from 2007. Appears to operate normally but not heating and not drawing nearly enough watts to do much. Last year the HV capacitor shorted out. I replaced that and it's worked until now.I've already performed lots of troubleshooting and read through the old but excellent repairfaq.org.-capacitor tests normal as near 1uF. on resistance, starts as low and approaches the 10M internal bleeder resistance-diode tests normal by applying ~15V through a resistor. 8.8V drop across diode one way, full voltage the other way-mag terminals test as a fraction of an ohm across both. infinite resistance from chassis to either mag terminal-transformer mag connector tests as sub-1 ohm across both terminals-transformer HV wire to chassis tests as 86 ohms-microwave uses ~100W with transformer primary disconnected and microwave "on", ~185W with primary connected and HV wire and mag connector disconnected, ~215W by adding mag connector, ~230W by adding HV wire to capacitor (so clearly each of these things are drawing power)-no strange smells or sounds-the transformer, diode, cap and mag all directly connect to each other. no fuses or thermal cutouts involvedI was hoping for something obvious but it's just not there. Any thoughts? Is it likely that the mag is bad but doesn't test bad? And before anyone says it, yes I understand these things generate very high voltages. I discharge the capacitor each time and am not making internal measurements while it's running...although I really wish I had a HV probe to check that.....

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: July 16, 2021, 08:43:37 pm
content: can you measure the resistance of the primary? The MOT that I have measures about 1.2-1.5 ohms. Check that there are not any thermal fuses anywhere on or in the transformer as well.

author: jrmymllr
author_rank: Regular Contributor
timestamp: July 16, 2021, 09:21:22 pm
content: Quote from: Alex_Baker on July 16, 2021, 08:43:37 pmPrimary resistance is 0.19 ohms. But there's a big difference in overall power consumption between having the primary connected and disconnected, so it's doing something.Also interesting is the power consumption goes up by 30W between having the mag filament connected vs. not connected (with the transformer HV disconnected), and as I understand the filament is typically 3.3V @ 10A...so that makes sense.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 16, 2021, 09:52:08 pm
content: Sudden failure or gradual?

author: jrmymllr
author_rank: Regular Contributor
timestamp: July 16, 2021, 10:07:20 pm
content: Quote from: BrokenYugo on July 16, 2021, 09:52:08 pmGood question. It was fairly sudden, but didn't seem instant. This morning I successfully heated something up that usually takes several minutes, although towards the end it wasn't boiling and bubbling like usual. Then my wife used it and noticed the item was not as hot as it should have been. Also during this time I noticed that the usual constant transformer hum noise instead sort of pulsed every 0.5 to 1 seconds. After that it essentially didn't heat at all.I've noticed for months that when the microwave hasn't been used for awhile, when cold, it cooked with quite a bit more power than when it got warmed up. I confirmed this with my Kill A Watt. Cold it was using over 1600W, then dropped a few hundred watts once the (magnetron maybe?) warmed up. I don't know if this is normal or if it always did that.Edit: you got me curious. I don't recall exactly how much the power consumption would drop from cold to warmed up, but it was easily enough to notice in cooking times and I want to say was maybe a couple hundred watts. So I put the power meter on my back-up microwave. Ran it for 3:30 heating water. Started at about 1240W. Near the end, 1190 or 1200W. So not as much.Bad mag? I'm wondering.

author: bsfeechannel
author_rank: Super Contributor
timestamp: July 16, 2021, 11:42:04 pm
content: According to the schematic, there are two points of attention you must check out. The relay contacts and the secondary switch contacts. When everything is working, it is a good idea to check the voltage at the primary of the MOT.

author: jrmymllr
author_rank: Regular Contributor
timestamp: July 17, 2021, 12:33:47 am
content: Quote from: bsfeechannel on July 16, 2021, 11:42:04 pmThat checks out. 118VAC on the primary of the MOT when microwave is supposed to be on.

author: themadhippy
author_rank: Super Contributor
timestamp: July 17, 2021, 02:03:28 am
content: Quotehas it got a low power or defrost mode?

author: bsfeechannel
author_rank: Super Contributor
timestamp: July 17, 2021, 03:47:48 am
content: Quote from: jrmymllr on July 17, 2021, 12:33:47 amIn that case, if you're not having an intermittent problem, and if you have tested everything else and it is OK, then perhaps it's time toinvestigate the magnetron.

author: amyk
author_rank: Super Contributor
timestamp: July 17, 2021, 04:13:54 am
content: Gassy magnetron?

author: jrmymllr
author_rank: Regular Contributor
timestamp: July 17, 2021, 12:35:06 pm
content: Quote from: themadhippy on July 17, 2021, 02:03:28 amIt doesn't. Just a simple microwave generator circuit with a single relay to turn it on.

author: jrmymllr
author_rank: Regular Contributor
timestamp: July 17, 2021, 03:58:48 pm
content: More information.I really wanted to know if the capacitor is building voltage but without a HV probe I wasn't going to touch that. It has a built-in 10M bleeder, so I took advantage of that...I tested it by charging to 5V and observed the discharge rate, did some RC calculations, then ran the microwave, unplugged it, waited 20s then put my meter on it. Back calculating, the capacitor must have roughly 1500-2000V the instant I unplugged the power cord.I also put an inductive pickup on the filament wire and measured almost 10A. It has to be the magnetron! It bothers me however that it doesn't test bad, but I suppose there's multiple failure modes with these things.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 17, 2021, 06:03:27 pm
content: Magnetrons are magic as far as I'm concerned, but I know it is a thermionic vacuum tube of sorts, so the emissive coating on the cathode must wear out eventually.Now I wonder if one can get a little more out of a weak one with the usual tube trick of getting the heater/cathode hotter. Temporarily (with a bench supply and no HV to just cook the cathode for a while), or permanently (add a turn to the filament winding), likely an ill advised experiment.

author: floobydust
author_rank: Super Contributor
timestamp: July 17, 2021, 06:16:32 pm
content: OP I think you've done all the tests, it looks like the magnetron has very low (thermionic) emission.Usually failures involve short-circuits and something burning up, which makes it easier to figure out.You really need a HV probe to fully test the power, but you've checked all the components- the diode, transformer and capacitor.I would pull the magnetron and see if there is anything visual and to get the full part number.If it wasn't making microwaves, due to say broken magnet or melted antenna, I would expect the tube to still consume full power. After all, it is a big diode. If air got in, filament current goes high and I'm not sure if they arc inside.http://www.magsells.co.uk/magnetron-can-fail-number-waysGoldstar 56324A magnetron is made by LG 6324W1A001L is a 2M246-050GF. Amazon guy said they are shipping 2M246-01TAG. I could not find datasheets explaining the many many suffixes for the mounting plate, air flow and connectors etc.Toshiba 2M248Jis listed as an alternate part.

author: james_s
author_rank: Super Contributor
timestamp: July 17, 2021, 07:26:09 pm
content: Well you've got voltage across the primary of the HV transformer so you know all of the control circuitry and interlocks are fine, all that's left is the transformer (unlikely), capacitor, diode, wiring and magnetron. Personally this sounds to me like a faulty magnetron, they are vacuum tubes and they do wear out. They're usually not particularly expensive and it's even possible in many cases to sub one that is similar so at this point I'd try replacing it. Look on ebay and see if you can find a suitable replacement.

author: jrmymllr
author_rank: Regular Contributor
timestamp: July 18, 2021, 01:17:42 pm
content: I'm buying a new magnetron and crossing my fingers. I'll report back once I install it.

author: jrmymllr
author_rank: Regular Contributor
timestamp: July 23, 2021, 12:35:15 am
content: New magnetron fixed it. Power consumption is also much more consistent now compared with how it used to cook more vigorously until the parts warmed up.For what it's worth, I found Martin Microwave (in US) had one of the better shipped prices and guaranteed the exact replacement part.

author: james_s
author_rank: Super Contributor
timestamp: July 23, 2021, 06:05:27 am
content: Maybe worthwhile to conduct an autopsy on the dead one to see if you can see anything obviously wrong inside.

author: Gyro
author_rank: Super Contributor
timestamp: July 23, 2021, 11:04:24 am
content: Good idea, they're good fun to pull apart and the magnets are useful... If intact!https://www.eevblog.com/forum/projects/will-a-magnetron-draw-480-watts-without-radiating-energy/msg3210292/#msg3210292

author: jrmymllr
author_rank: Regular Contributor
timestamp: July 23, 2021, 02:14:49 pm
content: Quote from: james_s on July 23, 2021, 06:05:27 amOh I plan to do that. It would be a shame to toss it without getting something out of it. And the magnets as Gyro mentioned.

author: james_s
author_rank: Super Contributor
timestamp: July 23, 2021, 08:43:22 pm
content: The magnets they use are pretty nice, I have 3 or 4 of them on my refrigerator from some magnetrons I tore apart years ago. I painted them with several coats of epoxy enamel which makes them less prone to chipping, otherwise they are ceramic magnets and are quite brittle.

author: eblc1388
author_rank: Frequent Contributor
timestamp: July 24, 2021, 06:46:48 am
content: Take great care not to cut or smash the ceramic parts on a magnetron as they contain beryllium oxide, which is highly toxic.

author: Gyro
author_rank: Super Contributor
timestamp: July 24, 2021, 10:31:44 am
content: Quote from: eblc1388 on July 24, 2021, 06:46:48 amThis comes upeverytime magnetrons are mentioned. The ceramic parts are Alumina, not Beryllium oxide. Any device containing Beryllium would have safety labels, but it was never used in consumer magnetrons.EDIT: Whilst on the subject, it's probably worth noting the best way of opening up a magnetron...- First pull off the sheet metal antenna cap from the top of the magnetron, this is easily done with a pair of pliers.- Underneath you will find the copper pinch vacuum seal. Saw through the base of this. This both releases the vacuum (very small hiss) and also detaches the thick copper anode connection that comes up from the main body.- Now the hard part. saw through the Copper anode cylinder itself. There is normally a small groove where the copper anode meets the steel end piece which holds the top insulator. You can use this as a saw guide to minimise the amount of anode cylinder that you have to cut through. Hold the anode cylinder (not the bottom insulator) gently in a vise (it distorts fairly easily) rotating slowly as the saw just breaks through. Don't try to cut straight across the whole thing as you will damage the internal structure.- Once free, lift to top portion slowly, unthreading the (aforementioned) anode connection that goes up to the pinch seal.- Marvel at the beautiful un-oxidised copper internals while they are still fresh and look for the failure.

author: chris_leyson
author_rank: Super Contributor
timestamp: November 25, 2023, 10:45:11 am
content: Thanks Gyro for your comments on magnetrons and Beryllium oxide. I think most people who say the anode insulator for the coupling electrode is Beryllium oxide are just repeating what people have posted on the internet and haven't bothered to read the data sheet. Some magnetrons have a pink insulator for some reason and maybe the pink ceramic has better RF properties.Also, if the insulator was Beryllium oxide there would be a warning on the data sheet.

author: SeanB
author_rank: Super Contributor
timestamp: November 25, 2023, 11:25:02 am
content: Pink is merely they added impure aluminia to the oxide sintered to create the seal, as it will turn to a glass internally, making for a better seal.Common for a magnetron in use for 15 years to go low emission off the filament, and to evaporate off tungsten as well, coating the internal copper anode with it, making a higher resistance thin film, and thus make it not want to start oscillating reliably, along with the magnets becoming weaker with hundreds of thermal cycles on them. So it draws current, has heater current about right, but it just is a very high power dissipation diode, as the electrons do not swirl around and excite the cavities well, or one or more of the internal straps, there to join the alternate cavities to reinforce the correct oscillation mode, have cracked loose, and thus the magnetron oscillated way off frequency, and thus the cavities do not resonate to build up the power to couple to the output. I tend to keep old good ones, as they often are useful as donors, seeing as typical failure on the cheap ones is the actual metalwork rusting through, or the CCA wire wound transformer going short between turns. Last few magnetrons that failed, blowing fuses, the RF feedthrough to the filament and negative supply had broken down internally.

author: Gyro
author_rank: Super Contributor
timestamp: November 25, 2023, 01:28:07 pm
content: Cracking of the magnets, leading to degraded magnetic field, and loosening of the press fit contact between the cooling fins and copper anode, can really screw up the internal structure of a magnetron.Here's a failed panasonic one that I took apart a while back. It's difficult to maintain correct resonance when the copper anode cavity dividers are melting down into nice shiny blobs!https://www.eevblog.com/forum/projects/will-a-magnetron-draw-480-watts-without-radiating-energy/msg3210292/#msg3210292

author: CaptDon
author_rank: Super Contributor
timestamp: November 25, 2023, 08:05:58 pm
content: To investigate the 'cooking' power output we measured the temperature rise of 1 liter of water after 1 minute. Also, strangely enough magnetrons for microwave ovens tend to work better when they get slightly gassy and fail completely about 1 month later. A slightly gassy magnetron usually has a very high body temperature and may trip the thermal cutout attached to the mag, it may melt any plastic air directors attached to it and it may crack one of the magnets causing very little current to be drawn which seems contrary as one would perhaps expect to current to rise as the electrons travel directly to the anode without circulation. Maybe the current is low because no useful R.F. power is being produced. Running the oven empty or unloaded seems like a bad idea as the energy produced but not absorbed in the oven cavity is reflected as high VSWR and the energy is returned to the magnetron to be passed on as anode heat but in reality doesn't seem to reduce the life expectancy? That does seem odd.