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Usual failure modes of magnetrons?

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Guest

Sun Jan 08, 2012 5:03 am   



Greetings All,
I have a Sharp brand "Half Pint" 400 watt microwave oven my wife and I
bought new in 1986. The thing still works like new. I use it now in
my shop because we have bought more powerful microwave ovens over the
years to use in our house. We have bought more than one because the
magnetrons in the others failed. At least I think the magnetrons
failed because there was still really high voltage going to the
magnetrons and the other oven functions worked (timer, digital
display, etc.). So I was wondering why my old Sharp Half Pint oven
still works and how magnetrons failed.
Thanks,
Eric

Paul Drahn
Guest

Sun Jan 08, 2012 5:03 am   



On 1/7/2012 7:03 PM, etpm_at_whidbey.com wrote:
Quote:
Greetings All,
I have a Sharp brand "Half Pint" 400 watt microwave oven my wife and I
bought new in 1986. The thing still works like new. I use it now in
my shop because we have bought more powerful microwave ovens over the
years to use in our house. We have bought more than one because the
magnetrons in the others failed. At least I think the magnetrons
failed because there was still really high voltage going to the
magnetrons and the other oven functions worked (timer, digital
display, etc.). So I was wondering why my old Sharp Half Pint oven
still works and how magnetrons failed.
Thanks,
Eric
Magnetrons are still vacuum tubes with heaters in them. They are

probably made in China and quality control is letting crap out the door.

Paul

mike
Guest

Sun Jan 08, 2012 2:00 pm   



etpm_at_whidbey.com wrote:
Quote:
Greetings All,
I have a Sharp brand "Half Pint" 400 watt microwave oven my wife and I
bought new in 1986. The thing still works like new. I use it now in
my shop because we have bought more powerful microwave ovens over the
years to use in our house. We have bought more than one because the
magnetrons in the others failed. At least I think the magnetrons
failed because there was still really high voltage going to the
magnetrons and the other oven functions worked (timer, digital
display, etc.). So I was wondering why my old Sharp Half Pint oven
still works and how magnetrons failed.
Thanks,
Eric
How did you measure the "really high voltage going to the magnetron"???


larry moe 'n curly
Guest

Sun Jan 08, 2012 2:32 pm   



e...@whidbey.com wrote:
Quote:

I have a Sharp brand "Half Pint" 400 watt microwave oven my wife and I
bought new in 1986. The thing still works like new. I use it now in
my shop because we have bought more powerful microwave ovens over the
years to use in our house. We have bought more than one because the
magnetrons in the others failed. At least I think the magnetrons
failed because there was still really high voltage going to the
magnetrons and the other oven functions worked (timer, digital
display, etc.). So I was wondering why my old Sharp Half Pint oven
still works and how magnetrons failed.

I think the www.repairFAQ.org has information about diagnosing
microwave ovens.

I had one magnetron fail when its permanent magnet (3" or so disk with
a large hole in the middle) cracked, and another one developed a short
in one of its feed-through capacitors where the high voltage went into
it.

Wolfgang Allinger
Guest

Sun Jan 08, 2012 2:37 pm   



On 07 Jan 12 at group /sci/electronics/repair in article
<pdrahn_at_webformixair.com> (Paul Drahn) wrote:

Quote:
Magnetrons are still vacuum tubes with heaters in them.

They are probably made in China and quality control is letting crap out
the door.

Sorry, you are wrong. CE aka China Engineering aka Crap Enforcement
doesn`t let quality in, so there is no quality production :]



Saludos Wolfgang

--
Wolfgang Allinger 15h00..21h00 MEZ: SKYPE:wolfgang.allinger
Paraguay mailer: CrossPoint XP 3.20 (XP2) in WinXPprof DOSbox
Meine 7 Sinne: reply Adresse gesetzt!
Unsinn, Schwachsinn, Bloedsinn, Wahnsinn, Stumpfsinn, Irrsinn, Loetzinn.

Jim Yanik
Guest

Sun Jan 08, 2012 10:11 pm   



etpm_at_whidbey.com wrote in news:vb1ig7l9hnfpq1jattjg9nt0ca038qautv_at_4ax.com:

Quote:
Greetings All,
I have a Sharp brand "Half Pint" 400 watt microwave oven my wife and I
bought new in 1986. The thing still works like new. I use it now in
my shop because we have bought more powerful microwave ovens over the
years to use in our house. We have bought more than one because the
magnetrons in the others failed. At least I think the magnetrons
failed because there was still really high voltage going to the
magnetrons and the other oven functions worked (timer, digital
display, etc.). So I was wondering why my old Sharp Half Pint oven
still works and how magnetrons failed.
Thanks,
Eric


usually,the MW output drops,perhaps from reduced cathode emission.
or the filament opens,they don't last forever. Smile
or internal arcing causes the PS fuse to blow.

Not much else to go wrong.

--
Jim Yanik
jyanik
at
localnet
dot com

Paul Drahn
Guest

Sun Jan 08, 2012 10:42 pm   



On 1/8/2012 5:37 AM, Wolfgang Allinger wrote:
Quote:
On 07 Jan 12 at group /sci/electronics/repair in article
pdrahn_at_webformixair.com> (Paul Drahn) wrote:

Magnetrons are still vacuum tubes with heaters in them.

They are probably made in China and quality control is letting crap out
the door.

Sorry, you are wrong. CE aka China Engineering aka Crap Enforcement
doesn`t let quality in, so there is no quality production :]



Saludos Wolfgang

Yes, that is true, in some cases. On the other hand, I buy GPS receiver

assemblies, complete with antenna, by the thousands from China and have
never had a bad one.

Paul

Wolfgang Allinger
Guest

Sun Jan 08, 2012 11:45 pm   



On 08 Jan 12 at group /sci/electronics/repair in article
<pdrahn_at_webformixair.com> (Paul Drahn) wrote:

Quote:
On 1/8/2012 5:37 AM, Wolfgang Allinger wrote:
pdrahn_at_webformixair.com> (Paul Drahn) wrote:

They are probably made in China and quality control is letting crap
out the door.

Sorry, you are wrong. CE aka China Engineering aka Crap Enforcement
doesn`t let quality in, so there is no quality production :]

Yes, that is true, in some cases. On the other hand, I buy GPS
receiver assemblies, complete with antenna, by the thousands from
China and have never had a bad one.

Good to hear, that not every product is controlled by Crap Enforcement
:)





Saludos Wolfgang

--
Wolfgang Allinger 15h00..21h00 MEZ: SKYPE:wolfgang.allinger
Paraguay mailer: CrossPoint XP 3.20 (XP2) in WinXPprof DOSbox
Meine 7 Sinne: reply Adresse gesetzt!
Unsinn, Schwachsinn, Bloedsinn, Wahnsinn, Stumpfsinn, Irrsinn, Loetzinn.

Roger Jones
Guest

Mon Jan 09, 2012 12:58 am   



On Jan 7, 10:03 pm, e...@whidbey.com wrote:
Quote:
Greetings All,
I have a Sharp brand "Half Pint" 400 watt microwave oven my wife and I
bought new in 1986. The thing still works like new. I use it  now in
my shop because we have bought more powerful microwave ovens over the
years to use in our house. We have bought more than one because the
magnetrons in the others failed. At least I think the magnetrons
failed because there was still really high voltage going to the
magnetrons and the other oven functions worked (timer, digital
display, etc.). So I was wondering why my old Sharp Half Pint oven
still works and how magnetrons failed.
Thanks,
Eric

As someone said, there's not much to go wrong on the power side.
Magnetron can fail, either catastrophically or go low gain and not
oscillate. There's a HV transformer with a pile-wound secondary that
could short out (likely blowing a fuse) and a voltage doubler diode/
capacitor that could fail (either, but a cap failure would take out
the diode.)
See http://www.gallawa.com/microtech/dataselect.html
Safety! Microwave oven can give lethal electric shocks! Never try to
measure ANY voltages while oven is ON. Always discharge the HV
capacitor before touching ANY connection in the MW oven. See safety
note on above website.
If you don't know what you are doing, DON'T try to fix MW ovens.
Cheers,
Roger

Phil Allison
Guest

Mon Jan 09, 2012 3:42 am   



"Paul Drahn"
etpm_at_whidbey.com wrote:
Quote:

I have a Sharp brand "Half Pint" 400 watt microwave oven my wife and I
bought new in 1986.

Magnetrons are still vacuum tubes with heaters in them. They are probably
made in China and quality control is letting crap out the door.


** Magnetrons for Sharp microwave ovens made in China back in 1986 ????

That was the era when Japan and Taiwan were dominant.



..... Phil

Phil Allison
Guest

Mon Jan 09, 2012 11:54 am   



"Jim Yanik"
Quote:


usually,the MW output drops,perhaps from reduced cathode emission.
or the filament opens,they don't last forever. Smile
or internal arcing causes the PS fuse to blow.

Not much else to go wrong.


** Must be some cases of loss of vacuum.

Affects lots of valves, big and small.



..... Phil

Winston
Guest

Mon Jan 09, 2012 3:39 pm   



Phil Allison wrote:
Quote:
"Jim Yanik"


usually,the MW output drops,perhaps from reduced cathode emission.
or the filament opens,they don't last forever. Smile
or internal arcing causes the PS fuse to blow.

Not much else to go wrong.


** Must be some cases of loss of vacuum.

Affects lots of valves, big and small.

I've dissected a few microwave ovens to salvage
transformers and for fun.

I've found cracked magnets on a few magnetrons.

I don't know if a cracked magnet would cause the
failure of a magnetron, if that was the root cause
of the failures or if some other failure caused
the magnets to crack.

--Winston

hrhofmann@att.net
Guest

Mon Jan 09, 2012 5:28 pm   



On Jan 9, 8:39 am, Winston <Wins...@BigBrother.net> wrote:
Quote:
Phil Allison wrote:
"Jim Yanik"

usually,the MW output drops,perhaps from reduced cathode emission.
or the filament opens,they don't last forever. Smile
or internal arcing causes the PS fuse to blow.

Not much else to go wrong.

** Must be some cases of loss of vacuum.

     Affects lots of valves, big and small.

I've dissected a few microwave ovens to salvage
transformers and for fun.

I've found cracked magnets on a few magnetrons.

I don't know if a cracked magnet would cause the
failure of a magnetron, if that was the root cause
of the failures or if some other failure caused
the magnets to crack.

--Winston

Every one I have ever salvaged had low magnetron output or a bad HV
transformer. Never seen a cracked magnet. But those old magnets are
great for fooling around with as they ar so strong.

Winston
Guest

Tue Jan 10, 2012 5:16 am   



hrhofmann_at_att.net wrote:

(...)

Quote:
Every one I have ever salvaged had low magnetron output or a bad HV
transformer. Never seen a cracked magnet. But those old magnets are
great for fooling around with as they ar so strong.

I wonder if a cracked magnet could cause low microwave
output?

Sounds like a chicken and egg problem.
Excessive power dissipation in the magnetron might
cause a magnet to crack. (Line transient perhaps?)
The cracked magnet would push the magnetron
away from cutoff, forcing it to dissipate more
power which heats the magnets, etc.

--Winston

klem kedidelhopper
Guest

Tue Jan 10, 2012 4:29 pm   



On Jan 9, 11:16 pm, Winston <Wins...@BigBrother.net> wrote:
Quote:
hrhofm...@att.net wrote:

(...)

Every one I have ever salvaged had low magnetron output or a bad HV
transformer.  Never seen a cracked magnet.  But those old magnets are
great for fooling around with as they ar so strong.

I wonder if a cracked magnet could cause low microwave
output?

Sounds like a chicken and egg problem.
Excessive power dissipation in the magnetron might
cause a magnet to crack. (Line transient perhaps?)
The cracked magnet would push the magnetron
away from cutoff, forcing it to dissipate more
power which heats the magnets, etc.

--Winston

Like you we have a small Goldstar oven that we got as an incentive for
visiting a time share back around 1985. The thing has a simple wind up
clock timer that rings a bell when it runs down. It too has worked
flawlessly all these years. I've been repairing consumer electronics
all my life and although I've never heard the term before I firmly
believe that "crap engineering" is in everything today and is
definitely alive and well in China. Lenny

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