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Tom in Crozet  
#1 Posted : Thursday, April 28, 2011 7:07:55 PM(UTC)
Tom in Crozet

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Joined: 4/28/2011(UTC)
Posts: 4

Our spacesaver microwave is about 12 years old. The other day it suddenly stopped working just as I started it up on a normal cycle to warm up some food. The house circuit breaker was not thrown.
I opened up the unit and found that the 20amp fuse was blown. I investigated further and the high voltage diode is testing bad.
Before I invest in a new fuse and a new diode, how likely is it that other parts on this unit have failed? Can I test the magnetron without removing the unit? The HV capacitor doesn't show any obvious problems, but I'm not sure I'm testing it correctly (with my digital multimeter). (I did take steps to discharge it before handling it further. It did not seem to have a charge in it.)
Also, the troubleshooting diagram on the paper guide inside the unit says that if the 20 amp fuse is blown I should check the varistor. I cannot find this part, which is supposed to have green wires on it. Where should I look for the varistor?
Thanks for any help you can give.
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Joe / APP Team  
#2 Posted : Friday, April 29, 2011 8:41:06 AM(UTC)
Joe / APP Team

Rank: Advanced Member

Groups: Moderators
Joined: 7/14/2010(UTC)
Posts: 5,222

Originally Posted by: Tom in Crozet Go to Quoted Post
Our spacesaver microwave is about 12 years old. The other day it suddenly stopped working just as I started it up on a normal cycle to warm up some food. The house circuit breaker was not thrown.
I opened up the unit and found that the 20amp fuse was blown. I investigated further and the high voltage diode is testing bad.
Before I invest in a new fuse and a new diode, how likely is it that other parts on this unit have failed? Can I test the magnetron without removing the unit? The HV capacitor doesn't show any obvious problems, but I'm not sure I'm testing it correctly (with my digital multimeter). (I did take steps to discharge it before handling it further. It did not seem to have a charge in it.)
Also, the troubleshooting diagram on the paper guide inside the unit says that if the 20 amp fuse is blown I should check the varistor. I cannot find this part, which is supposed to have green wires on it. Where should I look for the varistor?
Thanks for any help you can give.


Tom,

If you have a varistor, it would be attached to the control board ot the L1 line to the control. But that's not the problem, an open varistor would cause the control or unit to stop working, it'd be dead.
Any of the components you mentioned, could cause the fuse to open, if there's a problem. Here are some checks to make at the high voltage components, with your meter,

Checking with a high ohm scale, if the high voltage capacitor
is normal, the meter will indicate continuity for a short
time and should indicate an open circuit once the capacitor
is charged. If the above is not the case, check the capacitor
with an ohmmeter to see if it is shorted between either of the
terminals and case. If it is shorted, replace the capacitor.

RECTIFIER/DIODE

The rectifier is located in the upper right hand portion of the
service area next to the capacitor. It can be checked for
continuity in one direction and an open in the opposite
direction (meter must be able to forward bias rectifier). If a
short is indicated in both directions, or if an infinite resistance
is read in both directions, the rectifier is probably

defective and should be replaced.
MAGNETRON ASSEMBLY TEST

High voltages are present during the cook cycle, so extreme
caution should be observed. Disconnect oven from power
supply and discharge the high voltage capacitor before
touching any oven components or wiring.
To test for an open filament, isolate the magnetron from the
high voltage circuit. A continuity check across the magnetron
filament leads should indicate less than I ohm.
To test for a shorted magnetron, connect the ohmmeter leads
between the magnetron filament leads and chassis ground,
This test should indicate an infinite resistance. If there is little
or low resistance the magnetron is grounded and must be

replaced.

It could be a bad door switch as well, check those circuits as well,

Good Luck,
:) :) :)
Tom in Crozet  
#3 Posted : Friday, April 29, 2011 12:46:28 PM(UTC)
Tom in Crozet

Rank: Member

Groups: Registered
Joined: 4/28/2011(UTC)
Posts: 4

Very helpful, Joe. Thanks for the response.
I didn't understand what it means to 'forward bias' the diode.
I'm also not sure where to find the magnetron on this spacesaver unit that's over the oven. Do I have to take the unit down in order to access the magnetron connections? Please advise.
Joe / APP Team  
#4 Posted : Friday, April 29, 2011 1:44:51 PM(UTC)
Joe / APP Team

Rank: Advanced Member

Groups: Moderators
Joined: 7/14/2010(UTC)
Posts: 5,222

Originally Posted by: Tom in Crozet Go to Quoted Post
Very helpful, Joe. Thanks for the response.
I didn't understand what it means to 'forward bias' the diode.
I'm also not sure where to find the magnetron on this spacesaver unit that's over the oven. Do I have to take the unit down in order to access the magnetron connections? Please advise.


Tom,
Thats a weird way of saying that the diode will only allow a circuit to flow in one direction, and that the meter leads have to be in the proper position to test the part.
(you should also have a meter that operates on 12 VDC, or has a Diode test symbol, looks like a triangle with a line at the point)

"A diode is an electrical device allowing current to move through it in one direction with far greater ease than in the other. The most common kind of diode in modern circuit design is the semiconductor diode, although other diode technologies exist. The term “diode” is customarily reserved for small signal devices, I ≤ 1 A. The term rectifier is used for power devices, I > 1 A."

Your magnetron tube is mounted to the top of the wave guide on the oven cavity, it usually has 2 heavy guage wires attached to the 2 encased terminals on the tube.
You don't have to "drop" the unit, it does make it easier, though. It sounds like you would be able to get to everything without dropping it.

:) :) :)
Tom in Crozet  
#5 Posted : Friday, April 29, 2011 8:14:58 PM(UTC)
Tom in Crozet

Rank: Member

Groups: Registered
Joined: 4/28/2011(UTC)
Posts: 4

Hi Joe,
Thanks for the further info.
I was able to reach the magnetron by removing the fan assembly that sits behind the control panel. Removed two screws, pulled a couple of connectors, unthreaded some wires that come through the fan panel, and the white plastic panel containing the fan was easily removed. (A flatblade screwdriver is needed to pry up four plastic catches that hold the panel in place, two on top and two on the bottom.)
The magnetron sits behind the panel. Two red wires go into it.
It chacked out as good: just 3/10 of an ohm across the terminals, and infinite resistance between each terminal and the magnetron housing.
I was also able to get a favorable reading on the HV capacitor: some resistance showing for a few seconds across the two terminals, then changing to infinite resistance.
So: it seem s I have a blown 20 amp fuse and a bad diode. I am going to go ahead and order these parts and see if that brings the unit back to life.
BTW, I did check the door switches and monitor switches and they are all opening and closing as designed.
Thanks again for your help.
Tom
Joe / APP Team  
#6 Posted : Monday, May 2, 2011 6:03:35 AM(UTC)
Joe / APP Team

Rank: Advanced Member

Groups: Moderators
Joined: 7/14/2010(UTC)
Posts: 5,222

Originally Posted by: Tom in Crozet Go to Quoted Post
Hi Joe,
Thanks for the further info.
I was able to reach the magnetron by removing the fan assembly that sits behind the control panel. Removed two screws, pulled a couple of connectors, unthreaded some wires that come through the fan panel, and the white plastic panel containing the fan was easily removed. (A flatblade screwdriver is needed to pry up four plastic catches that hold the panel in place, two on top and two on the bottom.)
The magnetron sits behind the panel. Two red wires go into it.
It chacked out as good: just 3/10 of an ohm across the terminals, and infinite resistance between each terminal and the magnetron housing.
I was also able to get a favorable reading on the HV capacitor: some resistance showing for a few seconds across the two terminals, then changing to infinite resistance.
So: it seem s I have a blown 20 amp fuse and a bad diode. I am going to go ahead and order these parts and see if that brings the unit back to life.
BTW, I did check the door switches and monitor switches and they are all opening and closing as designed.
Thanks again for your help.
Tom


Tom,

Great job, just like a PRO, diagnostician.

Everything sounds right,

Great Job, Good Luck,
:D :D :D
Tom in Crozet  
#7 Posted : Monday, May 2, 2011 6:25:44 AM(UTC)
Tom in Crozet

Rank: Member

Groups: Registered
Joined: 4/28/2011(UTC)
Posts: 4

Thanks for all the help and the good wishes. Once I get the parts and install them, I'll let you know if the unit is working again.
Tom

Originally Posted by: Joe / APP Team Go to Quoted Post
Tom,

Great job, just like a PRO, diagnostician.

Everything sounds right,

Great Job, Good Luck,
:D :D :D
Joe / APP Team  
#8 Posted : Monday, May 2, 2011 7:39:53 AM(UTC)
Joe / APP Team

Rank: Advanced Member

Groups: Moderators
Joined: 7/14/2010(UTC)
Posts: 5,222

Originally Posted by: Tom in Crozet Go to Quoted Post
Thanks for all the help and the good wishes. Once I get the parts and install them, I'll let you know if the unit is working again.
Tom


Good deal, we'll appreciate it.

:) :) :)
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