Will a magnetron draw 480 Watts without radiating energy? - Page 1
Forum section: electronics / projects

author: AndersJ
author_rank: Frequent Contributor
timestamp: August 29, 2020, 02:41:13 pm
content: I have a LG microwave oven for repair.It does not heat the material inside it.But, it takes 480 W as if it is working.Can the magnetron consume 480 W without radiating energy?I.e. if current flows into the magnetron,but it does not heat the material, does that mean it is broken?

author: TheMG
author_rank: Frequent Contributor
timestamp: August 29, 2020, 04:34:13 pm
content: A magnetron is a special type of vacuum tube and can suffer from the same failure modes as most vacuum tubes. It could be arcing internally, the vacuum seal could have failed letting some air into it, it can develop shorts to the filament, etc. All could cause the magnetron to draw lots of power without actually putting out much if any microwave energy. Additionally, if the magnets on the magnetron were damaged it could cause something like this as well as it wouldn't oscillate like it's supposed to.Of course, first start by measuring for correct high voltage to the magnetron (~2000V typical, high voltage probe a must!!!). If this seems ok, then magnetron is faulty and will need to be replaced.Also if it is a traditional transformer + voltage doubler capacitor setup (as opposed to modern high frequency inverter), check the cap.

author: wraper
author_rank: Unknown
timestamp: August 29, 2020, 04:49:00 pm
content: Pass through high voltage filter capacitor should be mentioned as well, it's an often cause of failure. It's a part of the magnetron but possible to replace.

author: Gyro
author_rank: Super Contributor
timestamp: August 29, 2020, 06:41:12 pm
content: Magnetrons can dissipate a lot of power internally when things go pear shaped. Here's an internal photo of a 900W Panasonic inverter one that I posted recently. It was still outputting some power, but it was dropping off very rapidly as it warmed up.You can see that alternate Anode segments (the ones on the Antenna output ring) were in the process of melting down to cylindrical blobs. Not sure of the exact sequence, but the magnets had broken up, affecting the uniformity of the magnetic field, and the Aluminium cooling fins had become a loose fit on the outside of the Copper Anode body. There was a little dust, but ventilation was still pretty good. It was well over 10 years old though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 30, 2020, 04:13:11 am
content: Also, is 480W inferred from an ammeter, or using a proper power meter?Tim

author: AndersJ
author_rank: Frequent Contributor
timestamp: August 30, 2020, 07:42:37 am
content: Simple consumer type power meter between line cord plug and wall outlet.About the same size as a hand.

author: profdc9
author_rank: Frequent Contributor
timestamp: August 30, 2020, 07:56:47 am
content: Do the magnetrons use beryllium oxide windows anymore? I have been always very concerned about accidentally damaging them and inhaling the dust because the windows on the tubes have been made of BeO.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 30, 2020, 08:06:26 am
content: Quote from: AndersJ on August 30, 2020, 07:42:37 amOK, usually reasonably accurate.Quote from: profdc9 on August 30, 2020, 07:56:47 amLook at the insulator color. White may be BeO. Though these days it might be AlN too, which would be cool. Pink is Al2O3 doped with Cr2O3 (in other words, polycrystalline ruby).Tim

author: SeanB
author_rank: Super Contributor
timestamp: August 30, 2020, 08:29:24 am
content: Pretty sure no consumer magnetron will use Beryllium as insulator, mostly because of the cost of it, and the heat it has to handle is not enough to need it. Just plain aluminia or some coloured oxide in it.

author: Gyro
author_rank: Super Contributor
timestamp: August 30, 2020, 10:06:32 am
content: They would have Beryllium Oxide warning labels if they did. They are all Alumina these days (in fact, they probably always have been). BeO was really the domain of RF power transistors.

author: chris_leyson
author_rank: Super Contributor
timestamp: August 30, 2020, 10:41:58 am
content: ^^^ SeanB and Gyro are correct. Magnetrons used in microwave ovens DO NOT use BeO anode coupler insulators, if they did it would be clearly stated in the magnetron data sheet and there would be a mandatory warning lable stuck on the anode box, but too many people are lazy and don't check data sheets. Besides there is no reason to cool the anode coupling electrode. The anode block has press fit cooling fins which remove a few hundred watts of heat by forced air cooling. Also you can't go by color. BeO used in RF power transistors or for anode cooling blocks that were at one time used for some of the larger 4CXxxx transmitter tunes, for example, is white. Anode couplers will sometimes arc to the cavity due to a miss match and then it's usually game over at that stage. Low heater emission or hot spots on cathodes caused by "back heating" of the cathode can cause a drop in RF output power. An incorrect operating current or magnetic field strength can cause a drop in output power as will a damaged coupling loop.Eimac 8873 was designed for conduction cooling and uses SK-1920 BeO "thermal link". BeO Cooling blocks were also available for 4CX250, but can't find data sheet. Eimac Y-799 or Y-808 may be the conduction cooled versions of the 4CX250s.

author: Gyro
author_rank: Super Contributor
timestamp: August 30, 2020, 11:30:53 am
content: Quote from: chris_leyson on August 30, 2020, 10:41:58 amApparently not... Replacements freely available. 5pcs sounds a bit optimistic though!https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313&_nkw=magnetron+cap&_sacat=0Quite what state the internals of the Magnetron look like by that stage of coupler damage is another matter, but the Anode coupler is a push-fit cap that doesn't interfere with the thick copper vacuum sealing pinch underneath it (just visible in the photo).P.S. The coupler on my melted Panasonic Magnetron was still in perfect condition.