Microwave oven HV cap. Working to shorted overnight?

[Doug Warner]

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.

 

[Sam Goldwasser]

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/
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| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

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[[email protected]]

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.

 

[Erich J. Schultheis, The Man with the 15 inch Cock]

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 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.

 

[Bob Shuman]

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


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.

 

[Sam Goldwasser]

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.

 

[CRaSH]

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/d...TALOG_SESSID=e26c29e1e99392d598299dc8126f219b

 

[Jim Yanik]

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.