Usual failure modes of magnetrons?

[Phil Allison]

kevin.k...@gmail.com wrote:

----------------------------

Hey Lenny,

A repairman once told me that opening the door before the timer stopped the operation was bad for the magnetron. He said the unexpected interruption was a shock that over time led to more frequent magnetron failures. Do you have any knowledge or opinion about this?

-----------------------------------------------------

** There are hundreds of bullshit myths like this put around to blame users for failures that were nothing to do with them.

It a variation on the "blame the victim" logic.

Used when the real culprit desperately needs an excuse.

Or a supplier want to avoid a valid warranty claim.

Never fall for it.


..... Phil

 

[John Robertson]

On 2020/04/01 8:23 p.m., Phil Allison wrote:

kevin.k...@gmail.com wrote:

----------------------------


Hey Lenny,

A repairman once told me that opening the door before the timer stopped the operation was bad for the magnetron. He said the unexpected interruption was a shock that over time led to more frequent magnetron failures. Do you have any knowledge or opinion about this?

-----------------------------------------------------

** There are hundreds of bullshit myths like this put around to blame users for failures that were nothing to do with them.

It a variation on the "blame the victim" logic.

Used when the real culprit desperately needs an excuse.

Or a supplier want to avoid a valid warranty claim.

Never fall for it.


..... Phil

I agree with Phil.

The only thing I could see having a problem when you open the door prior
to the microwave timed out is the actual door safety switch, and only if
it carried a few amps of current - perhaps so on early microwaves. A
simple signal switch doesn't care much if the current is micro-amps it
will last as long as it would with no power. X number of door openings -
typically a million or so for a good quality switch.

The magnetron wouldn't care either, it has a certain number of ON
(powered up) hours in it, and considering the cycling the magnetron does
on reduced heating times, then that too is quite a long time - for
better quality units.

I've not seen the schematics for a microwave, but I assume that sensible
safety requirements would have at least two switches to disable the
magnetron if the door was opened when operating.

When I bought out our local TV/VCR/small appliance parts shop last year
there was also a bunch of microwave parts - magnetrons, big caps, etc. I
really should put them up on my site in case someone wants to fix older
units. Really, someone might want to!

John :-#)#

 

[John-Del]

On Thursday, April 2, 2020 at 1:36:22 AM UTC-4, John Robertson wrote:

I've not seen the schematics for a microwave, but I assume that sensible
safety requirements would have at least two switches to disable the
magnetron if the door was opened when operating.

Later generations have them wired in a make/break situation so in case a switch sticks closed or is intentionally jumped out, it shorts the supply and blows the main fuse.

 

[Phil Hobbs]

On 2020-04-02 10:38, John-Del wrote:

On Thursday, April 2, 2020 at 1:36:22 AM UTC-4, John Robertson wrote:

I've not seen the schematics for a microwave, but I assume that sensible
safety requirements would have at least two switches to disable the
magnetron if the door was opened when operating.


Later generations have them wired in a make/break situation so in case a switch sticks closed or is intentionally jumped out, it shorts the supply and blows the main fuse.

You can still stick a knife in the interlock if you need to fry
somebody's intruding drone.

Cheers

Phil Hobbs

 

[pfjw@aol.com]

On Thursday, April 2, 2020 at 10:47:20 AM UTC-4, Phil Hobbs wrote:

You can still stick a knife in the interlock if you need to fry
somebody's intruding drone.

Total immersion is simpler. Add a bit of salt or baking soda to the water, that makes the damage irreparable.

Peter Wieck
Melrose Park, PA

 

On Thursday, 2 April 2020 06:36:22 UTC+1, John Robertson wrote:

On 2020/04/01 8:23 p.m., Phil Allison wrote:
kevin.k...@gmail.com wrote:

----------------------------


Hey Lenny,

A repairman once told me that opening the door before the timer stopped the operation was bad for the magnetron. He said the unexpected interruption was a shock that over time led to more frequent magnetron failures. Do you have any knowledge or opinion about this?

-----------------------------------------------------

** There are hundreds of bullshit myths like this put around to blame users for failures that were nothing to do with them.

It a variation on the "blame the victim" logic.

Used when the real culprit desperately needs an excuse.

Or a supplier want to avoid a valid warranty claim.

Never fall for it.


..... Phil




I agree with Phil.

The only thing I could see having a problem when you open the door prior
to the microwave timed out is the actual door safety switch, and only if
it carried a few amps of current - perhaps so on early microwaves. A
simple signal switch doesn't care much if the current is micro-amps it
will last as long as it would with no power. X number of door openings -
typically a million or so for a good quality switch.

The magnetron wouldn't care either, it has a certain number of ON
(powered up) hours in it, and considering the cycling the magnetron does
on reduced heating times, then that too is quite a long time - for
better quality units.

I've not seen the schematics for a microwave, but I assume that sensible
safety requirements would have at least two switches to disable the
magnetron if the door was opened when operating.

When I bought out our local TV/VCR/small appliance parts shop last year
there was also a bunch of microwave parts - magnetrons, big caps, etc. I
really should put them up on my site in case someone wants to fix older
units. Really, someone might want to!

John :-#)#

Door switches take more than a few amps, and that still applies to a lot of current prodution units.

Microwave interlocks use 4 switches in a self monitoring configuration. If a switch should open but doesn't, another monitoring switch shorts the power feed (via a resistor) which blows the main fuse. 2 series switches without monitoring were used in the 1970s. I've seen much worse in historic machines, I remember one that continued cooking with the door part way open.


NT

 

[Chris Jones]

On 03/04/2020 21:28, tabbypurr@gmail.com wrote:

On Thursday, 2 April 2020 06:36:22 UTC+1, John Robertson wrote:
On 2020/04/01 8:23 p.m., Phil Allison wrote:
kevin.k...@gmail.com wrote:

----------------------------


Hey Lenny,

A repairman once told me that opening the door before the timer stopped the operation was bad for the magnetron. He said the unexpected interruption was a shock that over time led to more frequent magnetron failures. Do you have any knowledge or opinion about this?

-----------------------------------------------------

** There are hundreds of bullshit myths like this put around to blame users for failures that were nothing to do with them.

It a variation on the "blame the victim" logic.

Used when the real culprit desperately needs an excuse.

Or a supplier want to avoid a valid warranty claim.

Never fall for it.


..... Phil




I agree with Phil.

The only thing I could see having a problem when you open the door prior
to the microwave timed out is the actual door safety switch, and only if
it carried a few amps of current - perhaps so on early microwaves. A
simple signal switch doesn't care much if the current is micro-amps it
will last as long as it would with no power. X number of door openings -
typically a million or so for a good quality switch.

The magnetron wouldn't care either, it has a certain number of ON
(powered up) hours in it, and considering the cycling the magnetron does
on reduced heating times, then that too is quite a long time - for
better quality units.

I've not seen the schematics for a microwave, but I assume that sensible
safety requirements would have at least two switches to disable the
magnetron if the door was opened when operating.

When I bought out our local TV/VCR/small appliance parts shop last year
there was also a bunch of microwave parts - magnetrons, big caps, etc. I
really should put them up on my site in case someone wants to fix older
units. Really, someone might want to!

John :-#)#

Door switches take more than a few amps, and that still applies to a lot of current prodution units.

Microwave interlocks use 4 switches in a self monitoring configuration. If a switch should open but doesn't, another monitoring switch shorts the power feed (via a resistor) which blows the main fuse. 2 series switches without monitoring were used in the 1970s. I've seen much worse in historic machines, I remember one that continued cooking with the door part way open.


NT

I saw an interesting very old Moulinex one which seemed to have a fairly
loose door, but when it started cooking, an electromagnet pulled the
door very tightly onto the oven. Before I tried it, I expected it to be
very leaky but it was one of the least leaky I tested.

 

[Phil Hobbs]

On 2020-04-02 11:05, pfjw@aol.com wrote:

On Thursday, April 2, 2020 at 10:47:20 AM UTC-4, Phil Hobbs wrote:

You can still stick a knife in the interlock if you need to fry
somebody's intruding drone.


Total immersion is simpler. Add a bit of salt or baking soda to the water, that makes the damage irreparable.

Peter Wieck
Melrose Park, PA

You have to catch it first though.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Paul Hovnanian P.E.]

tabbypurr@gmail.com wrote:

> another monitoring switch shorts the power feed

I'm looking ata schematic from an old Sharp microwave. There is such a
'monitor switch' which shorts the power supply. It is located immediately
across the magnetron primary. So, should al other interlocks and control
relays fail, this will indeed blow the fuse.

Now, back to the original rumor. Should someone open the door on the oven
rather then wait for the control to shut off power, that short may be
applied to the transformer before it and/or the magnetron has bled off
energy. The resulting transient might be enough to shorten the life on
something.

--
Paul Hovnanian mailtoaul@Hovnanian.com
------------------------------------------------------------------
Someone figured out my password. Now I have to rename my dog.

 

On Monday, 13 April 2020 02:13:34 UTC+1, Paul Hovnanian P.E. wrote:

tabbypurr wrote:

another monitoring switch shorts the power feed

I'm looking ata schematic from an old Sharp microwave. There is such a
'monitor switch' which shorts the power supply. It is located immediately
across the magnetron primary. So, should al other interlocks and control
relays fail, this will indeed blow the fuse.

that's the plan

Now, back to the original rumor. Should someone open the door on the oven
rather then wait for the control to shut off power, that short may be
applied to the transformer before it and/or the magnetron has bled off
energy. The resulting transient might be enough to shorten the life on
something.

The short only occurs if the primary interlock switches fail, no other situation. At which point no-one cares what it does to equipment life expectancy.


NT

 

[Paul Hovnanian P.E.]

tabbypurr@gmail.com wrote:

The short only occurs if the primary interlock switches fail, no other
situation. At which point no-one cares what it does to equipment life
expectancy.

The shorting monitor switch closes every time the door opens. If the timer
shuts off the magnetron and you open the door a few seconds later, there is
no effect. If you open the door on a running oven, even if the series
interlock switch removes AC power from the HV transformer, the shorting
switch may close within milliseconds of that. So what is the time constant
of the decaying field in the transformer? And what hapens if you give that
field a shorted winding?


--
Paul Hovnanian mailtoaul@Hovnanian.com
------------------------------------------------------------------
My password is my dog's name. My dog is named %8Nk=14hD

 

[Cydrome Leader]

Paul Hovnanian P.E. <paul@hovnanian.com> wrote:

tabbypurr@gmail.com wrote:


The short only occurs if the primary interlock switches fail, no other
situation. At which point no-one cares what it does to equipment life
expectancy.

The shorting monitor switch closes every time the door opens. If the timer
shuts off the magnetron and you open the door a few seconds later, there is
no effect. If you open the door on a running oven, even if the series
interlock switch removes AC power from the HV transformer, the shorting
switch may close within milliseconds of that. So what is the time constant
of the decaying field in the transformer? And what hapens if you give that
field a shorted winding?

The delay in the mechanical timing of those switches is clearly always enough. If there
was overlap, you'd blow the fuse every time.

fun fact, the monitor swtich as it's being called here is these days (at least the past
decade if not longer) is a non-snap action switch in a standard microswitch form factor.
The only rating is 1 use and enough current to blow the mains fuse. They never switch any
load during the life of the microwave unless there is a fault condition. Microwave ovens
are sort of like cars and have every possible extra cost engineered out. Polaroid instant
cameras were like that too. It was amazing how many parts they tweaked out of the design.

 

[three_jeeps]

Link points to a website that is under construction???