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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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#1
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Microwave oven HV cap. Working to shorted overnight?
Sharp oven, approx 8 years old. Turned it on tonight, loud hum for 1/2 sec, blue flash from rear, then nothing,. Since my background is electronics, I opened it, found the 20A fuse was open and installed a new 10A one ((biggest 250V fuse I had) Turned on, and observed that the flash came from the capacitor in the rear. (Can't tell excatly where it was arcing..) Checked cap for charge (dead), metered it,. 2 ohms!.. As a final test, I connected it in series with my bench supply and a #1157 automotive bulb, and it lights.. (it really is shorted.) Has anyone seen this failure mode before? In my past experience, caps generally fail slowly. BTW: The HV diode is good, I connected my supply across it with the curent limit set to a few mA, and ti conducts at about 7V. -- Email reply: please remove one letter from each side of "@" Spammers are Scammers. Exterminate them. |
#2
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Microwave oven HV cap. Working to shorted overnight?
Doug Warner writes:
Sharp oven, approx 8 years old. Turned it on tonight, loud hum for 1/2 sec, blue flash from rear, then nothing,. Since my background is electronics, I opened it, found the 20A fuse was open and installed a new 10A one ((biggest 250V fuse I had) Turned on, and observed that the flash came from the capacitor in the rear. (Can't tell excatly where it was arcing..) Checked cap for charge (dead), metered it,. 2 ohms!.. As a final test, I connected it in series with my bench supply and a #1157 automotive bulb, and it lights.. (it really is shorted.) Has anyone seen this failure mode before? In my past experience, caps generally fail slowly. The HV capacitor is NOT an electrolytic. It will likely fail spectacularly (at least from the cap's perspective). All it would take is a pinhole breakdown between the plates. However, a visible arc is strange. You sure it wasn't the fuse blowing again? BTW: The HV diode is good, I connected my supply across it with the curent limit set to a few mA, and ti conducts at about 7V. Probably, but it could still fail at full voltage. BTW, just because your background is in electronics, there is a big difference between a microwave oven with very lethal up to 5000 V and and an AMP or more of available current - and 3.3 V logic even if the power supply can deliver 100 amps. DO make sure you understand the safety issues involved in working on a microwave oven, by far the most dangerous piece of electronic equipemtn in the home to repair. --- sam | Sci.Electronics.Repair FAQ: 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 Sites: http://www.repairfaq.org/REPAIR/F_mirror.html Important: Anything sent to the email address in the message header above is ignored unless my full name is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. |
#3
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Microwave oven HV cap. Working to shorted overnight?
I am going to re-iterate that you FULLY understand the safety when
working on a microwave oven. The ONLY technicians that I know of that were killed while working on some equipment were working on a microwave oven. Even in the off state, the capacitor will hold a total charge at a voltage high enough to immediately stop your heart. Roughly 4.5 Joules of energy at 1.5KV is on that puppy when charged up. The lowest defibrillator setting used in Hospitals today is 2.5 Joules!!!!! As a result it is my own preference to never work on a microwave oven without a high voltage probe specially made for checking the capacitor voltage and discharging it. With the exception of the over the range hood microwaves, very few are expensive enough to be worth much in the way of a repair. |
#4
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Microwave oven HV cap. Working to shorted overnight?
Doug Warner wrote: Sharp oven, approx 8 years old. Turned it on tonight, loud hum for 1/2 sec, blue flash from rear, then nothing,. Since my background is electronics, I opened it, found the 20A fuse was open and installed a new 10A one ((biggest 250V fuse I had) Turned on, and observed that the flash came from the capacitor in the rear. (Can't tell excatly where it was arcing..) Checked cap for charge (dead), metered it,. 2 ohms!.. As a final test, I connected it in series with my bench supply and a #1157 automotive bulb, and it lights.. (it really is shorted.) Has anyone seen this failure mode before? In my past experience, caps generally fail slowly. BTW: The HV diode is good, I connected my supply across it with the curent limit set to a few mA, and ti conducts at about 7V. -- Email reply: please remove one letter from each side of "@" Spammers are Scammers. Exterminate them. my advice to you is to LEAVE THE HIGH VOLTAGE COMPOENTS ALONE. There is enough current in that cocksucker to burn your nutsack down to the floor. |
#5
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Microwave oven HV cap. Working to shorted overnight?
Doug,
In answer to your question: Yes, I've seen the capacitor go bad instantaneously and will clearly show as shorted in both directions when measured out of circuit using a DVM. (I am assuming your 2 ohms is out of circuit.) Also check the magnetron resistance and the zener diode bias voltage as well. The only other thing that regularly fails in most microwaves are the door safety microswitches. Good luck and be very careful with the lethal voltages and current generated by the unit. Bob "Doug Warner" wrote in message ... Checked cap for charge (dead), metered it,. 2 ohms!.. As a final test, I connected it in series with my bench supply and a #1157 automotive bulb, and it lights.. (it really is shorted.) Has anyone seen this failure mode before? In my past experience, caps generally fail slowly. BTW: The HV diode is good, I connected my supply across it with the curent limit set to a few mA, and ti conducts at about 7V. -- Email reply: please remove one letter from each side of "@" Spammers are Scammers. Exterminate them. |
#6
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Microwave oven HV cap. Working to shorted overnight?
"Bob Shuman" writes:
Doug, In answer to your question: Yes, I've seen the capacitor go bad instantaneously and will clearly show as shorted in both directions when measured out of circuit using a DVM. (I am assuming your 2 ohms is out of circuit.) Also check the magnetron resistance and the zener diode bias voltage as well. The only other thing that regularly fails in most microwaves are the door safety microswitches. Good luck and be very careful with the lethal voltages and current generated by the unit. "zener bias voltage"? Huh? --- sam | Sci.Electronics.Repair FAQ: 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 Sites: http://www.repairfaq.org/REPAIR/F_mirror.html Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. |
#7
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Microwave oven HV cap. Working to shorted overnight?
Doug Warner wrote:
Sharp oven, approx 8 years old. Turned it on tonight, loud hum for 1/2 sec, blue flash from rear, then nothing,. Since my background is electronics, I opened it, found the 20A fuse was open and installed a new 10A one ((biggest 250V fuse I had) Turned on, and observed that the flash came from the capacitor in the rear. (Can't tell excatly where it was arcing..) Checked cap for charge (dead), metered it,. 2 ohms!.. As a final test, I connected it in series with my bench supply and a #1157 automotive bulb, and it lights.. (it really is shorted.) Has anyone seen this failure mode before? In my past experience, caps generally fail slowly. BTW: The HV diode is good, I connected my supply across it with the curent limit set to a few mA, and ti conducts at about 7V. My 15yr old Sears did exactly the same thing a few months ago. Fuse blows like a flash bulb and shorted cap. Here's a good source - reasonable prices and quick service (30% cheaper than Sears)! http://www.electronix.com/catalog/de... 9dc8126f219b |
#8
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Microwave oven HV cap. Working to shorted overnight?
"CRaSH" wrote in
newsMQuf.37813$Mi5.12378@dukeread07: Doug Warner wrote: Sharp oven, approx 8 years old. Turned it on tonight, loud hum for 1/2 sec, blue flash from rear, then nothing,. Since my background is electronics, I opened it, found the 20A fuse was open and installed a new 10A one ((biggest 250V fuse I had) Turned on, and observed that the flash came from the capacitor in the rear. (Can't tell excatly where it was arcing..) Checked cap for charge (dead), metered it,. 2 ohms!.. As a final test, I connected it in series with my bench supply and a #1157 automotive bulb, and it lights.. (it really is shorted.) Has anyone seen this failure mode before? In my past experience, caps generally fail slowly. My 1979 Sharp MW failed the same way(mine was a loud bang),it only cost me $25 for a new cap (and diode) from a local appliance repair store. I replaced the HV diode at the same time,because I could not test it. This oven is STILL working,a good product. -- Jim Yanik jyanik at kua.net |
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