Reducing Magnetron Voltage

[dacflyer]

any possibility of morphing another control board to theone you have now..there is not a whole lot to them now days.. i have even used some as timers on certain things,
there was even a N&V article on them this year, a few months back..
sure its not just easier to replace the oven ? i see many here dirt cheap on craigs list
i seen a huge JC penny convection oven / microwave for sale. $15.00
worked and looked like new.
i still have here a Amanna microwave, stainless steel inside, and the door pulls down like a oven

[Joseph]

A lot of the issue involves my dislike digital control for cooking. I make my food from scratch and don't know how much power or how long it will take to cook.

It wouldn't be expensive to replace the whole thing, but then that takes me back through to the less than optimal functionality again. Also, I can't discard my broken microwaves because they have nice parts in them, yet I don't want another used one to have to store right now.

[dacflyer]

gotchya...
maybe later on you might find another junked microwave with a manual timer.
my amanna here has mechanical timer and a selector switch for slo-cook and defrost.
good luck

[Joseph]

Thanks!

[MrAl]

Al, may I ask why the power to your home fluctuates so much. That is definitely very inconvenient.

Hi Joe,


Ask the power company
I dont know.

[Joseph]

Hi Al

NPR just aired a news story about plans to run an under water power cable from an off-shore Virginia wind farm to New Jersey.

[MrAl]

Hi again Joe,


Nice to hear back from you again.
When is that supposed to take place, anytime soon?

[Joseph]

Al, thank you kindly.

I had heard the radio spot a little wrong. The power cable seems like it will be more of a trunk line along the coast. The proposed cable would be finished by 2016. Google is a main participant in the project.

http://www.npr.org/templates/story/stor ... =130508508

An update on the microwave project is that I have settled upon a simple remote on/off switch for the power transformer for the time being. But I have placed a 25 watt incandescent light bulb across the switch contacts. When the switch is closed, the bulb filament is cold, having a resistance of 50 ohms. When I open the switch contacts, the voltage spike should dissipate in the light bulb filament. That should make the switch contacts last longer.

I am contemplating what will happen if I reduce the resonance capacitor from .7uF to .35uF by adding another one in series with the original. This might detune the circuit and have more disadvantages than the advantage of lowering the power output, however.

[radionut8888]

This may help explain what is happening in your microwave oven. The magnetron is a crossed field microwave tube. The magnetic field and electric field are at right angles to each other. The electric field travels in a circle about the axis, and because of the physical size and configuration of the cavity needs to see a really constant high voltage so that the "cloud" of electrons traveling around in the cavity are in synchronous with the vanes in the cavity. As soon as the voltage changes very much, like 2-3%, the speed of the cloud reduces and couples much less energy out of the tube.

That's why the high voltage likes to be held in tight tolerance. Most microwave ovens attempt to hold the high voltage relatively constant with a constant voltage transformer - The kind with a capacitor across one winding. To achieve power control, most modern microwave ovens vary the ratio of high voltage on/off time, proportioning on and off times and thus holding the high voltage fairly constant for the pulsed on times.

Placing a variable transformer in series with the high voltage does reduce the power, but since the couples energy out of the magnetron drops precipitously with the lower voltage, the power has to go somewhere. It is absorbed in the tube, resulting in excessive heating and possible premature failure of the magnetron.

The best solution is to use time proportioning power control.

[Edd]

" Now that the contacts on the timer board have eroded away "

Any chance that the more exacting semantics would be that the contacts on the "power cycling relay" have eroded away.
Any magnetron contol method that I am familiar with be it radar transmitters on downwards through microwave ovens are dependent upon on off duty ratio cycling to effectively reduce average power output proportionatively.
To wit . . . my circa '76 AMANA had its relay contacts " erode away" and I cleaned/burnished its contacts their final time and then eventually replaced the whole re4lay proper. Its next failure was in '85 , at which time I just set in a 240 vac rated SOLID STATE relay in its place, with it still working to date . . . . with no further "contact" erosion what-so-ever!
My secondary . . . Panasonic unit failed in the same manner, with its power relay being but the meager size of a domino.
i didn't like that power to physical size ratio of the OEM relay, so I also put in a SS relay in its place and has being running from 2001 until date.
If your unit is NOT a mechanical springwound on/off timer and actually pulses power on off for acquiring its reduced power incrementing, you should also be able to do the same thing .
(If vari power is even being offered . . .in which case . . . I am thinking of those $69-99 cheapie units, which only run at full power.)
In my case , my units were deee-luxe $1K and 500 dollah units.

If the former technology. . . .let me round up another SS relaY and me be your Sandy Paws this year.


73's de Edd

[Joseph]

Radionut8888, thanks for that explanation of the operating theory. I had come to the deduction that the efficiency of the tube was dropping as I was lowering the voltage. These ovens tend to creak a little with the power off and door open while the magnetron cools back down. I noticed it doing that more than I felt comfortable with. I didn't want to take a chance with overheating the tube, so I set aside the option of lowering the magnetron voltage. I use power cycling now, corresponding to your suggestion.

[Joseph]

Hello Edd. Thank you kindly for that awesome offer to help out. I like how you solved those problems in those ovens.

Indeed, my oven uses a mechanical timer with a rotating cam that opens and closes a set of physical contacts according to an on/off time determined by the setting on a power knob.

The way that I seem to be happy using the oven now involves my having placed a switch in series with the power transformer primary winding and controlling the on/off time manually. It requires me to tie myself up with the cooking process, but I probably should for better cooking results and reduction of steam generation anyway, which tends to damage my tight and energy efficient living space. I try to ventilate and dehumidify my living space as best as I can.