Microwave oven blows main fuse

[Dan_Musicant]

I found a MW oven in the street a few weeks ago and brought it home
thinking it either works or I'll tear it open and salvage the magnets.

It seemed dead, but yesterday I opened it up and the fuse tested dead
(250v, 15amp). I had a 125v 15 amp fuse and putting it in, the oven
appears to work.

Two questions:

1. Can I leave that fuse in there or should I get a 250v 15 amp instead?

2. A MW oven troubleshooting site says:

NOTE: Fuses do not usually blow on their own. A fuse may blow because of
problems with the interlock switches or with high voltage circuitry. It
is also possible, though rare, that a power surge will cause a fuse to
blow.

I have a MW oven I prefer, and figure to store this in my garage for
that day when my current one dies (they always seem to eventually). What
are the interlock switches and can I fix a bad one? Or is it possible I
could fix a high voltage circuitry problem? TIA for some help.

Dan

PS This oven was FILTHY! I cleaned tons of crud off the inside, but
more interestingly, the ventilation holes were 70+% clogged with grease.
It's all clean now. Could that bad ventilation have anything to do with
that fuse being blown?

 

[Ralph Mowery]

I found a MW oven in the street a few weeks ago and brought it home
thinking it either works or I'll tear it open and salvage the magnets.

It seemed dead, but yesterday I opened it up and the fuse tested dead
(250v, 15amp). I had a 125v 15 amp fuse and putting it in, the oven
appears to work.

Two questions:

1. Can I leave that fuse in there or should I get a 250v 15 amp instead?

2. A MW oven troubleshooting site says:

NOTE: Fuses do not usually blow on their own. A fuse may blow because of
problems with the interlock switches or with high voltage circuitry. It
is also possible, though rare, that a power surge will cause a fuse to
blow.

I have a MW oven I prefer, and figure to store this in my garage for
that day when my current one dies (they always seem to eventually). What
are the interlock switches and can I fix a bad one? Or is it possible I
could fix a high voltage circuitry problem? TIA for some help.

The 125 volt 15 amp fuse is fine as long as you use it on a 120 volt circuit
that is usually the case. Most will not be wired for a 240 volt circuit.
The voltage rating for a fuse is the maximum voltage and any voltage under
that is fine. The current rating should be the same.
When a fuse is operated near the maximum current rating and is turned off
and on a lot , they can blow for no apparent reason. If it is a glass fuse
you can see it, they usually just 'sag out'. That is the element seems to
sag down . If the glass is coated with the silver color then you may have
a major problem as the fuse blew all at once .

Interlock switches are prtoection switches. Usually if you open the door
there will be a switch to cut off the power. While your oven may not have
it, some electrical devices will have a switch that will short the high
voltage to the case or ground to discharge the capacitors when the case is
removed.

 

[Dan_Musicant]

On Mon, 04 Sep 2006 14:35:16 GMT, "Ralph Mowery"


:The 125 volt 15 amp fuse is fine as long as you use it on a 120 volt circuit
:that is usually the case. Most will not be wired for a 240 volt circuit.
:The voltage rating for a fuse is the maximum voltage and any voltage under
:that is fine. The current rating should be the same.
:When a fuse is operated near the maximum current rating and is turned off
:and on a lot , they can blow for no apparent reason. If it is a glass fuse
:you can see it, they usually just 'sag out'. That is the element seems to
:sag down . If the glass is coated with the silver color then you may have
:a major problem as the fuse blew all at once .
:
:Interlock switches are prtoection switches. Usually if you open the door
:there will be a switch to cut off the power. While your oven may not have
:it, some electrical devices will have a switch that will short the high
:voltage to the case or ground to discharge the capacitors when the case is
:removed.

This MW is an Emerson MT3070 manufactured in 1991 in Korea.

The fuse that was bad doesn't have transparent glass. I suppose it's
what they call a ceramic fuse. You'd never know it was blown, but it has
infinite resistance with my multimeter.

Can I assume that since the oven works with a replacement fuse that the
interlock switches are OK? Thanks for the help.

 

[Sam Goldwasser]

I found a MW oven in the street a few weeks ago and brought it home
thinking it either works or I'll tear it open and salvage the magnets.

It seemed dead, but yesterday I opened it up and the fuse tested dead
(250v, 15amp). I had a 125v 15 amp fuse and putting it in, the oven
appears to work.

Two questions:

1. Can I leave that fuse in there or should I get a 250v 15 amp instead?

2. A MW oven troubleshooting site says:

NOTE: Fuses do not usually blow on their own. A fuse may blow because of
problems with the interlock switches or with high voltage circuitry. It
is also possible, though rare, that a power surge will cause a fuse to
blow.

Actually, they do sometimes blow on their own after many years. So, it's
quite possible it will be fine. Since you're running it on 125 VAC, a
125 fuse should be acceptable electrically, though an exact replacement
type might be best from a liability point of view. All sorts of other
causes can result in a blown fuse but if you use the oven, it will
probably be obvious fairly quickly if there are still problems.

I have a MW oven I prefer, and figure to store this in my garage for
that day when my current one dies (they always seem to eventually). What
are the interlock switches and can I fix a bad one? Or is it possible I
could fix a high voltage circuitry problem? TIA for some help.

Read the safety warnings at:

http://www.repairfaq.org/sam/micfaq.htm

Then decide if you're experienced enough to try. But, yes, it's possible
to fix almost any problem in a microwave oven that hasn't resulted in
physical damage.

WARNING: Microwave ovens are probably the MOST dangerous consumer appliance or
electronic equipment to service, and not due to the microwaves.

PS This oven was FILTHY! I cleaned tons of crud off the inside, but
more interestingly, the ventilation holes were 70+% clogged with grease.
It's all clean now. Could that bad ventilation have anything to do with
that fuse being blown?

Quite possible..

--- 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.

 

[Sam Goldwasser]

On Mon, 04 Sep 2006 14:35:16 GMT, "Ralph Mowery"


:The 125 volt 15 amp fuse is fine as long as you use it on a 120 volt circuit
:that is usually the case. Most will not be wired for a 240 volt circuit.
:The voltage rating for a fuse is the maximum voltage and any voltage under
:that is fine. The current rating should be the same.
:When a fuse is operated near the maximum current rating and is turned off
:and on a lot , they can blow for no apparent reason. If it is a glass fuse
:you can see it, they usually just 'sag out'. That is the element seems to
:sag down . If the glass is coated with the silver color then you may have
:a major problem as the fuse blew all at once .
:
:Interlock switches are prtoection switches. Usually if you open the door
:there will be a switch to cut off the power. While your oven may not have
:it, some electrical devices will have a switch that will short the high
:voltage to the case or ground to discharge the capacitors when the case is
:removed.

This MW is an Emerson MT3070 manufactured in 1991 in Korea.

The fuse that was bad doesn't have transparent glass. I suppose it's
what they call a ceramic fuse. You'd never know it was blown, but it has
infinite resistance with my multimeter.

Can I assume that since the oven works with a replacement fuse that the
interlock switches are OK? Thanks for the help.

Probably. With the oven plugged in, open and close the door a bunch of
times. There will usually be an interlock switch that actually is wired
across the line and blows the fuse if the interlocks don't take place in
the correct sequence. If the fuse survives opening and closing the door,
the interlocks are probably OK, at lesat with respect to fuse blowing.

See the Microwave Oven Repair Guide at:

http://www.repairfaq.org/sam/micfaq.htm

or the info at: http://www.gallawa.com/microtech/

--- 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.

 

[Dan_Musicant]

On 04 Sep 2006 11:01:52 -0400, Sam Goldwasser <[email protected]>
wrote:

:WARNING: Microwave ovens are probably the MOST dangerous consumer appliance or
:electronic equipment to service, and not due to the microwaves.

Yes, I think I'm aware of the danger, and that it's principally that of
being badly shocked, and generally by virtue of the big capacitor. When
I have the cover off I'm always really careful to stay away from that
cap and the wires that go to it. Thanks!

 

[Sam Goldwasser]

On 04 Sep 2006 11:01:52 -0400, Sam Goldwasser <[email protected]>
wrote:

:WARNING: Microwave ovens are probably the MOST dangerous consumer appliance or
:electronic equipment to service, and not due to the microwaves.

Yes, I think I'm aware of the danger, and that it's principally that of
being badly shocked, and generally by virtue of the big capacitor. When
I have the cover off I'm always really careful to stay away from that
cap and the wires that go to it. Thanks!

It's worse than the big capacitor, though that's what people also bring up.

That has a chance of killing you but it's not 100 percent since the
worst case stored charge is *only* about 12 Joules or so.

And, yes, it should be discharged before touching anything.

The HV transformer, on the other hand, probably is closer to 100 percent
lethal if you touch it with the power on since there's 2 or 3 thousand
volts at 1/2 AMP or more sustained current available.

--- 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.

 

[Dan_Musicant]

On 04 Sep 2006 11:05:26 -0400, Sam Goldwasser <[email protected]>
wrote:

robably. With the oven plugged in, open and close the door a bunch of
:times. There will usually be an interlock switch that actually is wired
:across the line and blows the fuse if the interlocks don't take place in
:the correct sequence. If the fuse survives opening and closing the door,
:the interlocks are probably OK, at lesat with respect to fuse blowing.

I opened and closed the door several times, restarting the oven between
each. No blown fuse so far. I suppose it might be OK. Thanks.

 

[Fred McKenzie]

I opened and closed the door several times, restarting the oven between
each. No blown fuse so far. I suppose it might be OK.

Dan-

I missed the beginning of this. I have an old Sharp microwave. When I
first bought it about ten years ago, I plugged it in, put something inside
to cook, and pressed the start button. There was a loud band and a flash
of light, and the circuit breaker blew.

I took it back and exchanged it for another unit of the same model. That
one worked OK for a while, but soon blew the breaker again. It was
blowing the breaker in one out of about every ten times.

It turned out to be a bad circuit breaker! The breaker was replaced in
2000 when the house was renovated, and that same microwave hasn't blown
the new breaker yet.

Fred