Microwave advice
in [Repair]
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From: Mr. Land on 16 Jul 2010 17:47 OK, I see there is a long history of microwave over repair questions. But I don't seem to see a thread that has my question: So I have an older GE JVM1190 over-the-stove microwave unit. Symptom is no heat. When powered up and programmed to cook, the unit makes the expected humming noise (the same it's always made) but a cup of water in the chamber will not heat up. I've studied the microwave FAQ repeatedly and I believe I understand the dangers. I constructed a long wooden-stick-based discharger for the HV cap, discharged it (didn't get any spark at all), then ensured there was no residual voltage on either of its terminals. Then I shorted the terminals. First I "tested" the magnetron: infinite resistance from either cathode connection pin to ground, very low resistance between the two pins themselves. Seems OK. Then I tested the HV diode by placing it series with a 390 ohm resistor and applying 15 VDC, and meauring the voltage drop across the device. -15V negative biased, about 10 VDC forward biased...this seemed to be within range. Finally I tested the HV cap. It reads infinite resistance to the chassis from either terminal. Between the terminals my capacitance meters reads around 0.86 uF... which seems to be correct. I've heard mention of a possibly bad HV fuse. AFAICT, this unit doesn't have one of those. So...the HV cap seems good, the mag seems good, the diode seems good, the unit seems to draw appropriate current when in cook mode,yet the over won't heat food. What am I missing??? Could the magnetron still be "bad" despite the fact that it doesn't read shorted/open? Thanks.
From: Cydrome Leader on 16 Jul 2010 19:04 Mr. Land <graftonfot(a)yahoo.com> wrote: > OK, I see there is a long history of microwave over repair questions. > But I don't seem to see a thread that has my question: > > So I have an older GE JVM1190 over-the-stove microwave unit. > > Symptom is no heat. > > When powered up and programmed to cook, the unit makes the expected > humming noise (the same it's always made) but a cup of water in the > chamber will not heat up. > > I've studied the microwave FAQ repeatedly and I believe I understand > the dangers. > > I constructed a long wooden-stick-based discharger for the HV cap, > discharged it (didn't get any spark at all), then ensured there was no > residual voltage on either of its terminals. Then I shorted the > terminals. > > First I "tested" the magnetron: infinite resistance from either > cathode connection pin to ground, very low resistance between the two > pins themselves. Seems OK. > > Then I tested the HV diode by placing it series with a 390 ohm > resistor and applying 15 VDC, and meauring the voltage drop across the > device. -15V negative biased, about 10 VDC forward biased...this > seemed to be within range. > > Finally I tested the HV cap. It reads infinite resistance to the > chassis from either terminal. Between the terminals my capacitance > meters reads around 0.86 uF... which seems to be correct. > > I've heard mention of a possibly bad HV fuse. AFAICT, this unit > doesn't have one of those. > > So...the HV cap seems good, the mag seems good, the diode seems good, > the unit seems to draw appropriate current when in cook mode,yet the > over won't heat food. > > What am I missing??? > > Could the magnetron still be "bad" despite the fact that it doesn't > read shorted/open? > > Thanks. Is the transformer secondary good? check its resistance from the HV terminal to the chassis. Open = bad. Is that microwave old enough to use a reed relay for defrost mode? Do you have a way to measure the HV from a microwave oven safely?
From: Dani on 16 Jul 2010 22:25 The large transformer gives the 2000 volt approx ac high voltage as it gets about 120 vac input. A relay controls this. You could have a bad connection now burned at the relay output, or more likely a weak magnetron. If you have the 120 vac to the input of the big high voltage transformer, your magnetron is likely bad, considering your measurements. Please be carefull. Hope this helps.
From: Jeroni Paul on 17 Jul 2010 17:14 I was once in a similar trouble and it turned out that the press fit connectors to the magnetron were somewhat oxidized and did not make proper contact. I could not see oxide but I was measuring a few ohms of resistance in the contact. I cleaned both surfaces and it worked again. On 16 Jul, 23:47, "Mr. Land" <grafton...(a)yahoo.com> wrote: > OK, I see there is a long history of microwave over repair questions. > But I don't seem to see a thread that has my question: > > So I have an older GE JVM1190 over-the-stove microwave unit. > > Symptom is no heat. > > When powered up and programmed to cook, the unit makes the expected > humming noise (the same it's always made) but a cup of water in the > chamber will not heat up. > > I've studied the microwave FAQ repeatedly and I believe I understand > the dangers. > > I constructed a long wooden-stick-based discharger for the HV cap, > discharged it (didn't get any spark at all), then ensured there was no > residual voltage on either of its terminals. Then I shorted the > terminals. > > First I "tested" the magnetron: infinite resistance from either > cathode connection pin to ground, very low resistance between the two > pins themselves. Seems OK. > > Then I tested the HV diode by placing it series with a 390 ohm > resistor and applying 15 VDC, and meauring the voltage drop across the > device. -15V negative biased, about 10 VDC forward biased...this > seemed to be within range. > > Finally I tested the HV cap. It reads infinite resistance to the > chassis from either terminal. Between the terminals my capacitance > meters reads around 0.86 uF... which seems to be correct. > > I've heard mention of a possibly bad HV fuse. AFAICT, this unit > doesn't have one of those. > > So...the HV cap seems good, the mag seems good, the diode seems good, > the unit seems to draw appropriate current when in cook mode,yet the > over won't heat food. > > What am I missing??? > > Could the magnetron still be "bad" despite the fact that it doesn't > read shorted/open? > > Thanks.
From: Samuel M. Goldwasser on 18 Jul 2010 18:44 "Mr. Land" <graftonfot(a)yahoo.com> writes: > OK, I see there is a long history of microwave over repair questions. > But I don't seem to see a thread that has my question: > > So I have an older GE JVM1190 over-the-stove microwave unit. > > Symptom is no heat. > > When powered up and programmed to cook, the unit makes the expected > humming noise (the same it's always made) but a cup of water in the > chamber will not heat up. > > I've studied the microwave FAQ repeatedly and I believe I understand > the dangers. > > I constructed a long wooden-stick-based discharger for the HV cap, > discharged it (didn't get any spark at all), then ensured there was no > residual voltage on either of its terminals. Then I shorted the > terminals. > > First I "tested" the magnetron: infinite resistance from either > cathode connection pin to ground, very low resistance between the two > pins themselves. Seems OK. > > Then I tested the HV diode by placing it series with a 390 ohm > resistor and applying 15 VDC, and meauring the voltage drop across the > device. -15V negative biased, about 10 VDC forward biased...this > seemed to be within range. > > Finally I tested the HV cap. It reads infinite resistance to the > chassis from either terminal. Between the terminals my capacitance > meters reads around 0.86 uF... which seems to be correct. > > I've heard mention of a possibly bad HV fuse. AFAICT, this unit > doesn't have one of those. > > So...the HV cap seems good, the mag seems good, the diode seems good, > the unit seems to draw appropriate current when in cook mode,yet the > over won't heat food. > > What am I missing??? > > Could the magnetron still be "bad" despite the fact that it doesn't > read shorted/open? As someone else suggested, double check the continuity of the filament circuit - transformer and magnetron. Since that's high current bad connections are quite possible. However, the unit would draw less AC current if the magnetron was not getting filament power. But yes, everything could seem to check out but the magnetron could still be bad. -- 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.
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