Can a lower voltage overheat a magnetron?

Question

Background

I am working on a microwave oven that starts to emit an odor after 2 minutes of full heating despite otherwise working correctly. The smell is the same as that "sweet", overheated coil varnish smell that anyone with much electronics experience knows well. It seems to be coming from the magnetron (inside the area where all the electronics are mounted, not the cooking cavity).

Question

When I measured what seemed to be low voltage going to the magnetron, it dawned on me: Since magnetrons are somewhat mystical in their operation to me, is it possible that a lower voltage could cause them to overheat? Would it matter if the electrons took a different path inside the tube?

Details

To assuage responders' questions about my methods, the following details are presented.

The magnetron's wave-guide looks clean via a bore-scope and the cooling fan spins.

With a multi-meter, I tested the magnetron, high voltage (HV) transformer, HV diode and HV capacitor while each was disconnected; all test fine that way.

I swapped, one at a time, the HV capacitor and the wave-guide cover, but the smell recurs after the 2 minutes. I replaced the magentron, too, and the smell changed slightly but was still present.

I can't see the HV diode causing such an issue, but I may try swapping it next.

I have an old, 30 kV, analog, DC voltmeter that I used to test the HV circuitry (I don't know of its accuracy). According to the oven's service manual for the oven, approximately 2400 volts should come from the HV transformer which is then "doubled" to about 4000 V and then fed to the magnetron. When disconnected, I measured 3000 V (maybe less) on the lead that is normally connected to the magnetron.

A new, replacement HV transformer is not available and a good used one may be hard to come by.

(If relevant, I remember replacing the HV capacitor years ago and noticed that that capacitor is rated for only 2100 V [what the appliance parts shop ordered for me back then]--I do not recall the original working voltage of that capacitor and it is not in the manual.)

---

Edit: Repair

For those who are curious about the end result of the repair, I fixed the magnetron overheating smell with an exact replacement, HV diode change. I didn't expect this. Again, the old diode behaves normally on a meter, forward-biased vs. reversed (though it measured 1/3 lower resistance in the conducting direction than did the new diode--I assume it has gone out of spec. and it has a hairline fracture in it). But I now measure 1 kV DC with the magnetron connected (versus 1.5 kV DC with the old diode), 2 kV DC when it's disconnected (versus 3 kV DC with the old diode) and there is no smell even after several minutes of full-power use. Thank you all for your helpful comments.

Answer 1

A cavity magnetron will be overheated by it's cathode heater if there is not sufficient bias voltage to operate correctly. Correct operation depends on thermal electrons being emitted in sufficient quantity and swept away to keep the cathode cool.

For this reason a 'conventional' microwave oven operates in On/Off mode to get reduced power (for defrosting).

An 'inverter' microwave oven has independent control of the cathode heater, and turns down the heater current to operate the magnetron at reduced voltage for reduced power.

Answer 2

Magnetrons don't have varnish in them, it is likely to be the transformer. Try measuring it's Temperature, it shouldn't be more than 70 centigrade.

Answer 3

Seems unlikely to me. The value of voltage you measure at the magnetron with an ordinary DMM will be a bit suspect because it's half-wave pulsating DC.

Check that the fan that cools the transformer and magnetron is working properly and air flow is not obstructed. They run the transformers in old-fashioned (non-switching power supply) microwave ovens very hard to save on cost and weight. If you compare the size and weight of a similar VA transformer that is rated conservatively for continuous operation without forced convection you'll see a large discrepancy. The smell is more likely to be coming from the fan motor or the transformer than the magnetron I would guess, because both of those have windings that are exposed to the air.

Answer 4

It is normal for a magnetron to heat up way more than the transformer.

Assuming here the usual domestic microwave oven with 1000..1200W input power and 700..800W microwave power.

~1 kW transformer is 95..98% efficient and has 20..50W of heat to dissipate, the magnetron is like 70-75% and has 250..350W heat. That's why it has fins and fan for cooling and the transformer doesn't.

Whatever coils there are in the magnetron, they are not insulated by varnish. Neither the working temperature nor the voltages present inside suggest the use of the same varnish that is used in transformers. They are also sealed. This means either no smell or different smell when they do overheat.

Measuring a distorted half-wave-voltage-multiplied-non-linearily-loaded voltage over the magnetron is hardly going to show anything. It is not the usual thermionic valve power supply that is ironed out by capacitors and inductors.

Measuring the AC output of the unloaded transformer may tell more.

The temperature of the transformer (or even the magnetron) can be measured by fingers (with some care taken) or by contact thermometer. Both of them are grounded outside. Then again the usual failure mode for a transformer is a short between windings that is not going to significantly increase the overall transformer temperature.

These people likely know what they are doing and they are not affraid neither of lowering the anode supply voltage nor of heating the magnetron with an independent power supply while doing something to the anode power. In fact, I am yet to see some tube schematics that modulates the heating according to the load. Some tubes even DO overheat when subjected to lower than the required heating.

Lowered anode power supply is also how microwave ovens are modified to be used in RVs/trucks with limited available peak power.

In short, if it smells like a burnt transformer, ...

p.s. I wish I was that brave (or qualified) to measure the anode voltage of a magnetron for a stake that low (a new microwave oven is like $50 and can be scavenged for all parts if the old one carries some sentiment). The last time I did something to a microwave oven, I connected mains AC to the output of the transformer and measured the lower voltage at the input and the heating voltage.

Answer 5

Magnetrons don't conduct until the voltage is within about 100V of their rated voltage, because the magnetic field that spirals the electrons around inside steers them back toward the cathode. The voltage across the magnetron is an almost perfect square wave going from about +10V (the diode has about 10 junctions in series) down to -4kV, so, with a meter, you will read about 2kV. If you have a CRO with x10 probes and 18 10MΩ resistors you can look at the waveform across the magnetron by connecting 9 of the resistors in series with the CRO x10 probe. The capacitor has a 2.1kV sine wave across it. that's why it is rated at 2.1kV. You can look at its voltage by using 2 strings of 10 meg resistors and both channels set in A+B mode with Ch B inverted. The reading will be 100x the difference between Ch A & Ch B. Click here for more details. What are the bare minimum parts needed to operate a magnetron?

Source of diagram: my own experiments.