Magnetron replacement,, (substitution)

[[email protected]]

I have a large Kenmore over the stove type microwave oven that started growling last week. We pulled it apart and found that the magnetron had a shortbetween anode and filament. The diode and cap both check good. The cap is a 1.05uf 2100V unit. I have several new magnetrons in stock meant for comparable GE over the stove units. I don't know what value of cap is used with these though. In my travels I have seen caps as low as .90uf. If the GE magis the same physical size and mounts the same as the bad Kenmore does whatwould be the harm in using it if the present 1.05uf cap in the Kenmore were a different value from what the GE calls for? Thanks for any input. Lenny

 

[Baron]

dave Inscribed thus:

Electrolytics are often signaificantly over their rated capacity. The
difference between 0.9uF and 1.05uf is electrically insignificant.

I agree !

If the magnetron is physically the same and the heater voltage is the
same then just replace it. It should work ok.
You can always do the cup of water test...

HTH.

 

[[email protected]]

I have a large Kenmore over the stove type microwave oven that started growling last week. We pulled it apart and found that the magnetron had a short between anode and filament. The diode and cap both check good. The cap is a 1.05uf 2100V unit. I have several new magnetrons in stock meant for comparable GE over the stove units. I don't know what value of cap is used with these though. In my travels I have seen caps as low as .90uf. If the GE mag is the same physical size and mounts the same as the bad Kenmore does what would be the harm in using it if the present 1.05uf cap in the Kenmore were a different value from what the GE calls for? Thanks for any input. Lenny

I hadn't considered the heater voltage. I guess I figured that they would all be identical. Isn't it generally around two volts? Is there an easy way, or site where I might compare mag. Specs? This tube is real bear to replace. The oven was literally built around the magnetron. I would hate to openthe filament on the new tube the fist time I fired it up. Lenny

 

[[email protected]]

I have a large Kenmore over the stove type microwave oven that started growling last week. We pulled it apart and found that the magnetron had a short between anode and filament. The diode and cap both check good. The cap is a 1.05uf 2100V unit. I have several new magnetrons in stock meant for comparable GE over the stove units. I don't know what value of cap is used with these though. In my travels I have seen caps as low as .90uf. If the GE mag is the same physical size and mounts the same as the bad Kenmore does what would be the harm in using it if the present 1.05uf cap in the Kenmore were a different value from what the GE calls for? Thanks for any input. Lenny

 

[Phil Allison]

"dave"

Electrolytics are often signaificantly over their rated capacity.

** Completely irrelevant.

The difference between 0.9uF and 1.05uf is electrically insignificant.

** Not in a microwave oven it ain't.



.... Phil

 

[Jeff Layman]

"dave"


** Completely irrelevant.



** Not in a microwave oven it ain't.

Interesting. Took some time on Google to find out why. I had wondered
why the capacitances were so carefully stated over a very limited range,
and why the tolerance was so tight at +/- 3%.

For anyone interested, the answer appears to be here in section 9.5
(assuming it's correct):
http://www.repairfaq.org/REPAIR/F_micfaq7.html

By the way, if you want a laugh (and possible pair of future Darwin
award winners), have a look at the fourth paragraph of section 8.21!

 

[Jeff Layman]

I hadn't considered the heater voltage. I guess I figured that they
would all be identical. Isn't it generally around two volts? Is there
an easy way, or site where I might compare mag. Specs? This tube is
real bear to replace. The oven was literally built around the
magnetron. I would hate to open the filament on the new tube the fist
time I fired it up. Lenny

You might find some info here:
http://s3.amazonaws.com/szmanuals/8b1cbf510190d2b758c9b5cb5e139167

In the "typical" schematic on page 1, the heater voltage is shown as 3.15v.

 

[Phil Allison]

"dave"

Any capacitor can be significantly larger than its label says.

** Huh ??

Has this guy never heard of tolerance bands ?

Most film capacitors are rated at 10% tolerance but test much better.

Film caps with 1% tolerance are available and they are darn accurate.


..... Phil

 

[Phil Allison]

"Jeff Layman"

Interesting. Took some time on Google to find out why. I had wondered
why the capacitances were so carefully stated over a very limited range,
and why the tolerance was so tight at +/- 3%.

For anyone interested, the answer appears to be here in section 9.5
(assuming it's correct):
http://www.repairfaq.org/REPAIR/F_micfaq7.html

** That explanation is highly simplistic.

A crucial thing with a microwave oven is that the current drawn from the HV
transformer is the same on both half cycles of the AC supply - otherwise
the iron core saturates and that would blow the AC fuse.

On one half cycle, the transformer charges the film cap and on the next the
magnetron draws current from the cap and the transformer in series. There is
also series resonance between the cap and the transformer, due to its very
high leakage inductance.

Taken together, this explains why microwave ovens are made to suit the local
AC supply frequency.

BTW:

If you examine the AC current draw of a microwave oven, it is a pretty good
sine wave - remarkable considering what the load inside is like.


.... Phil

 

[David Lesher]

** That explanation is highly simplistic.

A crucial thing with a microwave oven is that the current drawn from the HV
transformer is the same on both half cycles of the AC supply - otherwise
the iron core saturates and that would blow the AC fuse.

Wouldn't it make more sense to use a switcher supply now-a-daze?
The ovens I've seen had large ergo $$$ transformers.

 

[Phil Allison]

"David Lesher"

Wouldn't it make more sense to use a switcher supply now-a-daze?

** Nope.

The ovens I've seen had large ergo $$$ transformers.

** Made in China for about $1 or $2 each and normally way outlast the rest
of the oven.

Any 1kW SMPS that does the same job is gonna cost 20 times more and be the
first thing to fail.



.... Phil

 

[[email protected]]

I have a large Kenmore over the stove type microwave oven that started growling last week. We pulled it apart and found that the magnetron had a short between anode and filament. The diode and cap both check good. The cap is a 1.05uf 2100V unit. I have several new magnetrons in stock meant for comparable GE over the stove units. I don't know what value of cap is used with these though. In my travels I have seen caps as low as .90uf. If the GE mag is the same physical size and mounts the same as the bad Kenmore does what would be the harm in using it if the present 1.05uf cap in the Kenmore were a different value from what the GE calls for? Thanks for any input. Lenny

Thanks for the great explanation of how the oven works Phil. I figured there was a resonance factor associated with this but wasn't completely sure. So then can I surmise that a different cap shifts resonance slightly and then does that affect maximum power transfer to the load, (Tube)? And then while we're at it, it occurred to me that the food must be the tube load, (output). I'm trying to understand this better so is at least part of that correct? Lenny