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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Micrwave oven - M8A fuse?
I am attempting to diagnose a Sharp combination microwave oven, it's a model R-7E53(W)M. This is a conventional/microwave oven and grill in one unit. The microwave output power is 850W. I am in the UK. After cooking some food last week using dual-cook (convection and microwave), the breaker for the circuit that the oven was plugged into tripped. On resetting the breaker, I found the oven worked normally - when asked to cook, the fans run, the oven light illuminates, the turntable turns, but no microwave energy is produced and the food does not heat. On opening the oven, I discovered the following: * the fuse (M8A 250V) in the feed to the HV transformer primary is blown. The main fuse (13A 250V) is okay. * the HV diode reads infinity both ways using a DMM * the magnetron reads a dead short across the filament terminals * the HV cap reads neither open nor short. If I set my DMM to the 20M ohm resistance scale, the resistance reading climbs up slowly. I assume this means the capacitor is charging and is thus likely OK. * the door interlock switches check out okay. I am following the Microwave oven guide in the S.E.R. FAQ. Question 1: the blown HV fuse is marked M8A 250V. Is the M significant? I'm familiar with T fuses (slow blow), F fuses (fast blow), but not M. A google has but turned up anything. Can I replace this fuse with a standard 8A fuse, and if so, should I select fast or slow blow? Question 2: Is this likely to be a magnetron failure? The magnetron is made by Sharp and is marked "RV-MZ A165 WREO" on one line, and "2M226 (16)" on another. Googling, this appears to be a type 2M226-16 magnetron. Thanks for any insight. If this were a standard cheapo microwave-only oven, I'd probably chuck it out. As it's a combination oven, it may be worth some effort to fix. -- A. Top posters. Q. What's the most annoying thing on Usenet? |
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Micrwave oven - M8A fuse?
Mike Tomlinson writes:
I am attempting to diagnose a Sharp combination microwave oven, it's a model R-7E53(W)M. This is a conventional/microwave oven and grill in one unit. The microwave output power is 850W. I am in the UK. After cooking some food last week using dual-cook (convection and microwave), the breaker for the circuit that the oven was plugged into tripped. On resetting the breaker, I found the oven worked normally - When exactly did it trip? when asked to cook, the fans run, the oven light illuminates, the turntable turns, but no microwave energy is produced and the food does not heat. On opening the oven, I discovered the following: * the fuse (M8A 250V) in the feed to the HV transformer primary is blown. The main fuse (13A 250V) is okay. * the HV diode reads infinity both ways using a DMM This would be correct for a DMM. You need to test it with a power supply since the forward drop is more than 6 V. * the magnetron reads a dead short across the filament terminals It will be very low ohms though not quite 0. * the HV cap reads neither open nor short. If I set my DMM to the 20M ohm resistance scale, the resistance reading climbs up slowly. I assume this means the capacitor is charging and is thus likely OK. Only thing would be if it fails at full voltage. * the door interlock switches check out okay. I am following the Microwave oven guide in the S.E.R. FAQ. Question 1: the blown HV fuse is marked M8A 250V. Is the M significant? I'm familiar with T fuses (slow blow), F fuses (fast blow), but not M. A google has but turned up anything. Can I replace this fuse with a standard 8A fuse, and if so, should I select fast or slow blow? Question 2: Is this likely to be a magnetron failure? The magnetron is made by Sharp and is marked "RV-MZ A165 WREO" on one line, and "2M226 (16)" on another. Googling, this appears to be a type 2M226-16 magnetron. A shorted magnetron or diode would probably not pop the breaker. A shorted cap might. When it popped the breaker would be significant. If it happened exactly when the heating was called on to stop, could be a triac failure also. Thanks for any insight. If this were a standard cheapo microwave-only oven, I'd probably chuck it out. As it's a combination oven, it may be worth some effort to fix. Yep. --- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/ Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/ +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm | Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html Important: The email address in this message header may no longer work. To contact me, please use the Feedback Form at repairfaq.org. Thanks. |
#3
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Micrwave oven - M8A fuse?
Mike Tomlinson writes:
In article , Sam Goldwasser writes When exactly did it trip? I'm not completely sure. It was either at the very end of cooking one lot of food or at the very beginning of cooking the next. There was a ~2 minute interval between. I only noticed power had gone when I opened the fridge to get something. This would be correct for a DMM. You need to test it with a power supply since the forward drop is more than 6 V. Thanks. I have found the test cct in the FAQ. When it popped the breaker would be significant. If it happened exactly when the heating was called on to stop, could be a triac failure also. The supply to the HV transformer is controlled by a relay, not a triac in this model. Update: I replaced the 8A fuse with a 5A fast-blow, and the oven appears to work. It boiled half a pint of water in about 3 minutes. The mica splash protector for the waveguide was very dirty (even on the waveguide side.) Have cleaned it up and will see how it goes. It's not totally out of the question for the fuse to have just died of old age and there actually being no problem. A. Top posters. Q. What's the most annoying thing on Usenet? Sorry. --- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/ Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/ +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm | Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html Important: The email address in this message header may no longer work. To contact me, please use the Feedback Form at repairfaq.org. Thanks. |
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Micrwave oven - M8A fuse?
Mike,
Keep your eye on that capacitor as well ... it may just be a matter of time and this was a warning signal of what is to come. My above the range GE microwave popped the fuse about 4-6 months before the capacitor went out completely. Bob "Sam Goldwasser" wrote in message ... Mike Tomlinson writes: Update: I replaced the 8A fuse with a 5A fast-blow, and the oven appears to work. It boiled half a pint of water in about 3 minutes. The mica splash protector for the waveguide was very dirty (even on the waveguide side.) Have cleaned it up and will see how it goes. It's not totally out of the question for the fuse to have just died of old age and there actually being no problem. |
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Micrwave oven - M8A fuse?
In article , Sam Goldwasser
writes It's not totally out of the question for the fuse to have just died of old age and there actually being no problem. Understood, thanks, though I was puzzled that it popped the breaker as well. That was what made me think there had to be another fault. By the way, the oven is still working well. A. Top posters. Q. What's the most annoying thing on Usenet? Sorry. Wasn't aimed at you personally. -- A. Top posters. Q. What's the most annoying thing on Usenet? |
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Micrwave oven - M8A fuse?
On Fri, 29 Aug 2003 13:30:13 +0100, Mike Tomlinson
wrote: In article , Hotspur writes I've repaired plenty of these,where in the UK are you?I'm at the Borders near Holy Island.If it's tripped whilst cooking and you are fully up to date with microwaves I'll walk you through the repair.You shouldn't have opened it and tested if you did not discharge th hv. I did, following the instructions in the FAQ, and kept a wire croc clipped across the cap terminals while I was poking around. Did remember to disconnect it before I switched on though it's possible the thermal cutouts at that control the oven/mag have failed.They are small round discs with the operating temp.on them. According to the diagram pasted to the side of the cover, these are in series with the HV circuitry and would not cause a blown HV fuse. Replace them they are cheap.You need the standard 8A microwave fuse to replace,buy them in packs of ten for a quid. Thanks. Where can I get them? -- A. Top posters. Q. What's the most annoying thing on Usenet? CPC Preston tel 01772-654455 they sell to none trade customers any problems I'll get them on my account and send you them next order but you should be ok,All orders are minus VAT and postage |
#8
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Micrwave oven - M8A fuse?
A. Top posters. Q. What's the most annoying thing on Usenet? CPC Preston tel 01772-654455 they sell to none trade customers any problems I'll get them on my account and send you them next order but you should be ok,All orders are minus VAT and postage Also no matter what the diagram states,and i know the cutouts would not have popped the fuse,replace the door switches and the cutouts you will prevent trouble later for the sake of a small outlay unles of course you want to sell the unit,i still replace them all. |
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