Microwave Oven
You have already surmized (correctly, I think) that the capacitor is the problem, and I believe that follows the normal statistical pattern of component failure modes.
My main reason in writing is to caution you about trying to test the magnetron in any manner other than heating a glass of water in the food chamber with all doors, seals, and interlocks functioning and in good condition. Control of microwave leakage from an oven is very strongly dependent upon the condition and cleanliness of the seals around the door and around the food chamber, and trying to bring any antenna leads out of the oven could greatly increase leakage.
Additionally, I don't think it is possible to make any kind of accurate measurements inside such a small chamber because the pickup would be in such a complex, reverberant field. Measured levels would vary over a large range as you moved the pickup around the food chamber, and it would be very difficult to move the pickup without risking serious microwave leakage.
The standard method of evaluating the power output of a microwave oven is to place an accurately measured quantity of water in a thin vessel inside the food chamber and measure the temperature before and after operating the oven for a precisely known time period. (Don't leave a thermocouple, thermistor, or mercury thermometer inside the oven while it is on.) Sorry I don't have the formula for the calculation of power, but you could find it in the literature or in oven service manuals.
Or, you can figure it out from these relationships from Mark's Mechanical Engineer's Handbook, 5th edition:
One BTU is the heat that raises 1 pound of water 1 degree F and is equal to 251.996 IT calories. 1 IT calorie is equal to 1/860 of a watthour. For best accuracy, use a vessel with minimal thermal mass (i.e., a thin paper cup) and measure temperature quickly after the heating period.
There you go!
awright