Advice needed for microwave diagnosis

Hello all,

YES, I'm ->fully<- aware of the dangers associated with HV and RF.

The symptom is: about a couple of seconds after heating starts, one can hear
a half second low hum then either heating stops or resumes normally, until
another random occurence.

In case where it abnormally stops, the control panel returns to the stanby
state, just as if the STOP touch had been hit twice.

Of course, I first thought of a bad switch in the door's interlocks, or some
intermitent somewhere. Nope, all looks fine and, by the way it wouldn't
cause the controller to return to stanby.

I have checked as much as possible:
-) the mains part is OK, no loose connection, no dying relay, no burnt
thermal protection.
-) the filament seems in good shape, no worn insulation, connectors fine.
-) the diode works as expected (under a simple 15 V test @ 50 mA).
-) the capacitor looks good, well: under low voltage test.
-) the magnetron has no evidence at all of any arcing, loose internal
filament connection or shortening, as far as I can see or test.
-) the mains and HV fuses are in brand new condition.

The symptom disappear completely when the HV circuit is open by
disconnecting the HV transformer output.

What seems more likely is that when symptom occurs (possibly due to
intermittent shortening internal to the capacitor, hence the little hum) the
HV is brought to its knees and the low voltage transformer being in
undervoltage, the controller somehow "jumps" to iddle. However, I note that
it doesn't go in the RESET state.

Which test do you suggest to pinpoint the defective part more positively?

I've thought of monitoring either the filament's current or the HV. But then
how could I determine whether it is the filament that is arcing or
shortening somehow, or the capacitor, or the magnetron that's the culprit?

I don't think I have a suitable HV load to put in place of the magnetron for
testing the thing in actual operation. Operating the beast with the
magnetron's HV disconnected might create a deadly overvoltage situation if
no load is connected.

The best I can do is "borrow" the capacitor of my other oven (a 0.84
uF/1900V) and try it for a SHORT period. I'm a little reluctant at this, due
to the sligtly inferior voltage rating (but I suspect it can't be hurt
really if test is over a short period).

For anyone asking gory details, my microwave oven is a Samsung type RE576TC
produced in 1989. It is a 1050/600 W (absorbed/RF), is micro-controlled,
uses magnetron Samsung 2M204-M3, a 0.91 uF/2100V capacitor and HVR-1X 3
diode.

Thank you in advance for any hint.
--
Here's where you can reach me (fixed-width font required).
. ,-. .
.. . ,-. ,-. ,-. ,-. |- /,-.\ ,-. ,-. ,-| ,-. ,-. ,-.
| | `-. |-' | | |-' | |,-|| | | -- |-' -- | | | | | | |
`-^ `-' `-' ' ' `-' `' \`-^' `-| `-' `-^ :; `-' ' `-|
`-- | ,|
` `'

 

[Jerry G.]

Your address does not show properly to have a direct answer. Use
something like nameNOSPAM@isp.com for your address. This works well.

You have to properly test the transformer, rectifier, and tube. You
cannot do this by visual observation. One or a few of these devices are
at fault. The best test is substitution, unless you have the equipment
to actively test them.

Even if you know what you are doing, it may work out cheaper to give the
microwave out to a shop that fixes them all the time. They would have
the parts to try, or the test gear to determine the exact failure.
Sometimes servicing these yourself is more costly than giving it out.
As for the knowledge you seem to safely know where you are going.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
==============================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
Instruments http://www.zoom-one.com/glgtech.htm
==============================================
<see.address@post.sig> wrote in message
news:9h7rlvc4ctcppq0egmnqks785da4clcgrm@4ax.com...
Hello all,

YES, I'm ->fully<- aware of the dangers associated with HV and RF.

The symptom is: about a couple of seconds after heating starts, one can
hear
a half second low hum then either heating stops or resumes normally,
until
another random occurence.

In case where it abnormally stops, the control panel returns to the
stanby
state, just as if the STOP touch had been hit twice.

Of course, I first thought of a bad switch in the door's interlocks, or
some
intermitent somewhere. Nope, all looks fine and, by the way it wouldn't
cause the controller to return to stanby.

I have checked as much as possible:
-) the mains part is OK, no loose connection, no dying relay, no burnt
thermal protection.
-) the filament seems in good shape, no worn insulation, connectors
fine.
-) the diode works as expected (under a simple 15 V test @ 50 mA).
-) the capacitor looks good, well: under low voltage test.
-) the magnetron has no evidence at all of any arcing, loose internal
filament connection or shortening, as far as I can see or test.
-) the mains and HV fuses are in brand new condition.

The symptom disappear completely when the HV circuit is open by
disconnecting the HV transformer output.

What seems more likely is that when symptom occurs (possibly due to
intermittent shortening internal to the capacitor, hence the little hum)
the
HV is brought to its knees and the low voltage transformer being in
undervoltage, the controller somehow "jumps" to iddle. However, I note
that
it doesn't go in the RESET state.

Which test do you suggest to pinpoint the defective part more
positively?

I've thought of monitoring either the filament's current or the HV. But
then
how could I determine whether it is the filament that is arcing or
shortening somehow, or the capacitor, or the magnetron that's the
culprit?

I don't think I have a suitable HV load to put in place of the magnetron
for
testing the thing in actual operation. Operating the beast with the
magnetron's HV disconnected might create a deadly overvoltage situation
if
no load is connected.

The best I can do is "borrow" the capacitor of my other oven (a 0.84
uF/1900V) and try it for a SHORT period. I'm a little reluctant at this,
due
to the sligtly inferior voltage rating (but I suspect it can't be hurt
really if test is over a short period).

For anyone asking gory details, my microwave oven is a Samsung type
RE576TC
produced in 1989. It is a 1050/600 W (absorbed/RF), is micro-controlled,
uses magnetron Samsung 2M204-M3, a 0.91 uF/2100V capacitor and HVR-1X 3
diode.

Thank you in advance for any hint.
--
Here's where you can reach me (fixed-width font required).
. ,-. .
.. . ,-. ,-. ,-. ,-. |- /,-.\ ,-. ,-. ,-| ,-. ,-. ,-.
| | `-. |-' | | |-' | |,-|| | | -- |-' -- | | | | | | |
`-^ `-' `-' ' ' `-' `' \`-^' `-| `-' `-^ :; `-' ' `-|
`-- | ,|
` `'

 

[Sam Goldwasser]

A low hum usually means a short on the outboard side of the HV cap - HV diode
or magnetron. May not blow main fuse. A shorted HV cap will probably blow
the fuse.

The HV diode and capacitor don't generally fail slightly. They are
either good or dead.

Since you can run the HV transformer with no load without problems, probably
not that.

Could be the magnetron. Difficult to test without swapping in a known good one
though.

--- sam | Sci.Electronics.Repair FAQ Home Page: 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 Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the Feedback Form at repairfaq.org. Thanks.



see.address@post.sig writes:

Hello all,

YES, I'm ->fully<- aware of the dangers associated with HV and RF.

The symptom is: about a couple of seconds after heating starts, one can hear
a half second low hum then either heating stops or resumes normally, until
another random occurence.

In case where it abnormally stops, the control panel returns to the stanby
state, just as if the STOP touch had been hit twice.

Of course, I first thought of a bad switch in the door's interlocks, or some
intermitent somewhere. Nope, all looks fine and, by the way it wouldn't
cause the controller to return to stanby.

I have checked as much as possible:
-) the mains part is OK, no loose connection, no dying relay, no burnt
thermal protection.
-) the filament seems in good shape, no worn insulation, connectors fine.
-) the diode works as expected (under a simple 15 V test @ 50 mA).
-) the capacitor looks good, well: under low voltage test.
-) the magnetron has no evidence at all of any arcing, loose internal
filament connection or shortening, as far as I can see or test.
-) the mains and HV fuses are in brand new condition.

The symptom disappear completely when the HV circuit is open by
disconnecting the HV transformer output.

What seems more likely is that when symptom occurs (possibly due to
intermittent shortening internal to the capacitor, hence the little hum) the
HV is brought to its knees and the low voltage transformer being in
undervoltage, the controller somehow "jumps" to iddle. However, I note that
it doesn't go in the RESET state.

Which test do you suggest to pinpoint the defective part more positively?

I've thought of monitoring either the filament's current or the HV. But then
how could I determine whether it is the filament that is arcing or
shortening somehow, or the capacitor, or the magnetron that's the culprit?

I don't think I have a suitable HV load to put in place of the magnetron for
testing the thing in actual operation. Operating the beast with the
magnetron's HV disconnected might create a deadly overvoltage situation if
no load is connected.

The best I can do is "borrow" the capacitor of my other oven (a 0.84
uF/1900V) and try it for a SHORT period. I'm a little reluctant at this, due
to the sligtly inferior voltage rating (but I suspect it can't be hurt
really if test is over a short period).

For anyone asking gory details, my microwave oven is a Samsung type RE576TC
produced in 1989. It is a 1050/600 W (absorbed/RF), is micro-controlled,
uses magnetron Samsung 2M204-M3, a 0.91 uF/2100V capacitor and HVR-1X 3
diode.

Thank you in advance for any hint.
--
Here's where you can reach me (fixed-width font required).
. ,-. .
. . ,-. ,-. ,-. ,-. |- /,-.\ ,-. ,-. ,-| ,-. ,-. ,-.
| | `-. |-' | | |-' | |,-|| | | -- |-' -- | | | | | | |
`-^ `-' `-' ' ' `-' `' \`-^' `-| `-' `-^ :; `-' ' `-|
`-- | ,|
` `'

 

[Curmudgeon]

Go here:
http://www.repairfaq.org/REPAIR/F_micfaq3.html#MICFAQ_015


<see.address@post.sig> wrote in message
news:9h7rlvc4ctcppq0egmnqks785da4clcgrm@4ax.com...

Hello all,

YES, I'm ->fully<- aware of the dangers associated with HV and RF.

The symptom is: about a couple of seconds after heating starts, one can
hear
a half second low hum then either heating stops or resumes normally, until
another random occurence.

In case where it abnormally stops, the control panel returns to the stanby
state, just as if the STOP touch had been hit twice.

Of course, I first thought of a bad switch in the door's interlocks, or
some
intermitent somewhere. Nope, all looks fine and, by the way it wouldn't
cause the controller to return to stanby.

I have checked as much as possible:
-) the mains part is OK, no loose connection, no dying relay, no burnt
thermal protection.
-) the filament seems in good shape, no worn insulation, connectors fine.
-) the diode works as expected (under a simple 15 V test @ 50 mA).
-) the capacitor looks good, well: under low voltage test.
-) the magnetron has no evidence at all of any arcing, loose internal
filament connection or shortening, as far as I can see or test.
-) the mains and HV fuses are in brand new condition.

The symptom disappear completely when the HV circuit is open by
disconnecting the HV transformer output.

What seems more likely is that when symptom occurs (possibly due to
intermittent shortening internal to the capacitor, hence the little hum)
the
HV is brought to its knees and the low voltage transformer being in
undervoltage, the controller somehow "jumps" to iddle. However, I note
that
it doesn't go in the RESET state.

Which test do you suggest to pinpoint the defective part more positively?

I've thought of monitoring either the filament's current or the HV. But
then
how could I determine whether it is the filament that is arcing or
shortening somehow, or the capacitor, or the magnetron that's the culprit?

I don't think I have a suitable HV load to put in place of the magnetron
for
testing the thing in actual operation. Operating the beast with the
magnetron's HV disconnected might create a deadly overvoltage situation if
no load is connected.

The best I can do is "borrow" the capacitor of my other oven (a 0.84
uF/1900V) and try it for a SHORT period. I'm a little reluctant at this,
due
to the sligtly inferior voltage rating (but I suspect it can't be hurt
really if test is over a short period).

For anyone asking gory details, my microwave oven is a Samsung type
RE576TC
produced in 1989. It is a 1050/600 W (absorbed/RF), is micro-controlled,
uses magnetron Samsung 2M204-M3, a 0.91 uF/2100V capacitor and HVR-1X 3
diode.

Thank you in advance for any hint.
--
Here's where you can reach me (fixed-width font required).
. ,-. .
. . ,-. ,-. ,-. ,-. |- /,-.\ ,-. ,-. ,-| ,-. ,-. ,-.
| | `-. |-' | | |-' | |,-|| | | -- |-' -- | | | | | | |
`-^ `-' `-' ' ' `-' `' \`-^' `-| `-' `-^ :; `-' ' `-|
`-- | ,|
` `'

 

On Tue, 9 Sep 2003 07:24:53 -0400, "Jerry G." <jerryg@total.net> wrote:

|> Your address does not show properly to have a direct answer.
Doesn't it mention the *fixed-width font* requirement, does it?
Anyhow, direct answer is not an issue, when using Usenet.

|> Use something like nameNOSPAM@isp.com for your address. This works well.
It also works well for spambots!

|> You have to properly test the transformer, rectifier, and tube. You
|> cannot do this by visual observation.
Sorry if I didn't made crystal clear I do possess and have used the
equipment and knowledge to test most of it, except the HV part ;-(((

|> One or a few of these devices are at fault.
Right, at least marginally for the moment.


Thank you for your answer.

(Umm, I guess this other .sig will make it much better in Outlook).
--
You usenet
can at
reach q-e-d
me dot
here org

 

On 09 Sep 2003 08:07:33 -0400, Sam Goldwasser <sam@saul.cis.upenn.edu>
wrote:

|> The HV diode and capacitor don't generally fail slightly. They are
|> either good or dead.
Do you know which type (technology) those caps are?

|> Could be the magnetron. Difficult to test without swapping in a known good one
|> though.
In this situation, would you go trying the known good 0.84 uF / 1900 V in
place of the suspect 0.91 uF / 2100 V for, say, as long as one minute or two
(e.g. 10 tests of 10 seconds each)? I believe it would be OK since there
must be a large margin in the rating of these parts.

I now think I can easily insert a small resistor in series with the HV
transformer's primary, reducing the HV output. This will minimize the
possibility of damage to the known good cap, yet produce enough HV to make
it a real test.

I'll try that and let the group know.

--
Here's where you can reach me (fixed-width font required).
. ,-. .
.. . ,-. ,-. ,-. ,-. |- /,-.\ ,-. ,-. ,-| ,-. ,-. ,-.
| | `-. |-' | | |-' | |,-|| | | -- |-' -- | | | | | | |
`-^ `-' `-' ' ' `-' `' \`-^' `-| `-' `-^ :; `-' ' `-|
`-- | ,|
` `'

 

On Tue, 9 Sep 2003 18:16:06 -0400, "Curmudgeon" <curmudgeon@noway.com>
wrote:

|> Go here:
|> http://www.repairfaq.org/REPAIR/F_micfaq3.html#MICFAQ_015

Thanks, but that's where I'm coming from ;-)
--
Here's where you can reach me (fixed-width font required).
. ,-. .
.. . ,-. ,-. ,-. ,-. |- /,-.\ ,-. ,-. ,-| ,-. ,-. ,-.
| | `-. |-' | | |-' | |,-|| | | -- |-' -- | | | | | | |
`-^ `-' `-' ' ' `-' `' \`-^' `-| `-' `-^ :; `-' ' `-|
`-- | ,|
` `'