Hi,

The above oven doesn't heat on microwave. Convection heating and all
control functions seem O.K.

Most notable symptom is that the primary transformer current is only 5
amps AC when the magnetron is to be energized. I would expect over
twice that much. It is modulated with a relay, whose contacts appear
fine and will click on and off properly at 50% power setting.

I discharged the High Voltage and did all the usual resistance
measurements on the magnetron, transformer and cap. I did remove the
diode and ran 10 volts in series with it and a 1.2K resistor. Got 3mA
forward and 0mA reverse current.

I have a high voltage probe on order to test the high voltage under
operating conditions.

Does anyone have a suggestions? The failure seems partial somehow and
I don't know how to proceede without replacing parts that may be
operating fine.

Thanks in Advance!

Regards,
Jim Shedden

I assume you checked the filament resistance of the magnetron, and the
integrity of the connections. An open filament or bad connections would
be the most likely cause, followed by a bad HV cap, though I don't know
that they fail open very often.

6 A is essentially the unloaded current into the HV transformer.

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(Jim shedden) writes:

Hi,

The above oven doesn't heat on microwave. Convection heating and all
control functions seem O.K.

Most notable symptom is that the primary transformer current is only 5
amps AC when the magnetron is to be energized. I would expect over
twice that much. It is modulated with a relay, whose contacts appear
fine and will click on and off properly at 50% power setting.

I discharged the High Voltage and did all the usual resistance
measurements on the magnetron, transformer and cap. I did remove the
diode and ran 10 volts in series with it and a 1.2K resistor. Got 3mA
forward and 0mA reverse current.

I have a high voltage probe on order to test the high voltage under
operating conditions.

Does anyone have a suggestions? The failure seems partial somehow and
I don't know how to proceede without replacing parts that may be
operating fine.

Thanks in Advance!

Regards,
Jim Shedden

(Jim shedden) wrote in message . com...
Hi,

The above oven doesn't heat on microwave. Convection heating and all
control functions seem O.K.

Most notable symptom is that the primary transformer current is only 5
amps AC when the magnetron is to be energized. I would expect over
twice that much. It is modulated with a relay, whose contacts appear
fine and will click on and off properly at 50% power setting.

I discharged the High Voltage and did all the usual resistance
measurements on the magnetron, transformer and cap. I did remove the
diode and ran 10 volts in series with it and a 1.2K resistor. Got 3mA
forward and 0mA reverse current.

That sounds a bit low- I would swap the diode and see what happens.
(compare readings with the replacement first)


I have a high voltage probe on order to test the high voltage under
operating conditions.

Does anyone have a suggestions? The failure seems partial somehow and
I don't know how to proceede without replacing parts that may be
operating fine.

Thanks in Advance!

Regards,
Jim Shedden

Andre wrote:

(Jim shedden) wrote in message . com...

I discharged the High Voltage and did all the usual resistance
measurements on the magnetron, transformer and cap. I did remove the
diode and ran 10 volts in series with it and a 1.2K resistor. Got 3mA
forward and 0mA reverse current.


That sounds a bit low- I would swap the diode and see what happens.
(compare readings with the replacement first)

Let's employ Ohm's law. 3mA * 1.2kOhms = 3.6V drop. Diode
drop is thus 10V - 3.6V = 6.4V. Sounds just about right to
me for a typical HVR-1X type diode, which is apparently a
string of ten Si diodes inside.

Dana

Hi,

Thanks to Sam and Andre for suggestions. I was inspired to repeat my
measurements, so I figured I would do a general thread update.

Magnetron:
FA to F terminal is 0.2 ohms (meter lead resistance) so assume
filament OK. Neither terminal shorted to case.

Capacitor:
Rated 0.91uF measures 0.9uF. Infinite resistance either terminal to
case. Between terminals measure exponential rise stopping at about 10
Meg. Cap has internal bleeder. Effect is symmetrical and brief
negative resistance implies charge storage when leads reversed. Dick
Smith ESR Meter reads 2.1

Diode:
Used 15 volts in series with 1.2K and measured voltage this time.
Forward drop across diode is 6.7, with diode reversed is 15.

Transformer:
Filament winding resistance basically zero. HV winding is 58.4 ohms.
Primary is about 0.2 ohms. No shorts between windings or chassis for
filament and primary. HV is tied to chassis one end. With secondaries
open and oven on primary is 120 volts at 4.3 amps. Secondary current
with magnetron filament only connected is 11 amps.

That's about the best I can do until my high voltage probe arrives
tomorrow. I suspect a bad magnetron, or I'm missing something. Are
there any doors that close off the microwave energy when the oven is
used on convection heat? There is a damper that is currently open, but
I thought that was to keep oven heat away from the electronics. I
imagine in combined heat mode, the damper would have to open when the
magnetron was energized to blow air across the magnetron fins and into
the oven. In any case, it seems to be more of an air control than RF
energy control, but I'm only learning...

Regards,
Jim

Hi,

Looks like we back in service with a new magnetron! My guess is that
something shorted in the old magnetron when the unit heated and dumped
filament current through the diode, burning it open.

Thanks to all for the help. (Bert, as a result of your suggestion I
did power up the high voltage circuit with a new diode before
replacing the magnetron. The diode held, so I figured the capacitor
OK. Good to check because the new magnetron not exact and needed some
work to fit in.)

Regards,
Jim

Jim shedden wrote:

Hi,

Looks like we back in service with a new magnetron! My guess is that
something shorted in the old magnetron when the unit heated and dumped
filament current through the diode, burning it open.

Thanks to all for the help. (Bert, as a result of your suggestion I
did power up the high voltage circuit with a new diode before
replacing the magnetron. The diode held, so I figured the capacitor
OK. Good to check because the new magnetron not exact and needed some
work to fit in.)

Interesting. I recently replaced the magnetron in a
GE JVM1642BB oven, only about 3.5 years old. In my case,
the HVR-1X diode measured 6V forward drop and maximum
reverse drop, the .86uF capacitor acted just like it
should (ohmeter reversal test) and the transformer
windings had appropriate DC resistance values.

Removing the magnetron, I found that the antenna
was partially burned/melted, though the filament
was .2 ohms or so.

Since the GE oven has a total of 10 year warranty
on the tube, I called GE and finally found the right
person to ask for a replacement tube. They sent one
out, along with a new stirrer (slightly changed
design) and a new HV diode. Kudos to GE for excellent
customer service.

I'd done a power measurement of the oven after
buying it, since it seemed to cook things more quickly
than expected. Sure enough, the original tube
was putting out around 1000W. The FCC test report
for this oven (really a branded Samsung oven with an
OM75P(10) tube) indicated around 770W output. I thought,
cool, I'm so lucky.

The new tube is good for around 765-770W. I suspect
now that the original tube was not quite right and
ended up killing itself. The power tests were done
using the calorimeter method, consistent with the IEC
standard. It just takes water and a thermometer.

My neighbor bought the same GE Profile oven the same
month from the same store that I did. His oven died two
weeks before mine did. My hunch is that Samsung made some
odd OM75P(10) tubes. Is this your tube? Is it under
warranty? Keep in the mind the GE ovens often have a
total of 10 years warranty on the tube.

Dana