QEX

Resting atop the U-shaped frame is an assortment of parallel-connected, oil-filled capacitors that make up C4. Some of these capacitors have built-in rectifiers and/or bleeder resistors. Take care to connect those with a rectifier so that the rectifier is reverse biased—a forward-biased rectifier would short circuit the high voltage. (A simple multi meter check may not identify the diode polarity. Use a 9 to 12 V dc source with a 1 k£2 series resistor.)

Swing the top cover closed as shown in Fig 5, and everything is swallowed, as if inside a giant clam. Even the output connectors are inside. This allows me to use binding posts on the Plexiglas panel, instead of expensive high-volt

Fig 4—Double-backed tape and small cardboard strips between the oil-filled capacitors hold them in place, while fastening the strapping. All fuses are accessible with the cover in this position.
Fig 5—In addition to other features mentioned in the text, this power supply features a two-tone color scheme, four-wheel independent steering and chrome plated bumpers (handles). Mileage in QSOs per kilowatt hour is yet to be determined.

age connectors. Two screws, opposite the hinges, hold the lid closed.

Because of the mass of this power supply, I mounted four small swivel-ing casters on the bottom. The hinged top and casters prove most convenient when working on the amplifier and power supply.

Operation

A power supply like this one should be operated safely: with all high-voltage points inaccessible to the operator and others. I always make sure the ac plug is not in the outlet and within sight when I work on the amplifier or power supply. Use extreme caution when testing and troubleshooting. Your first contact with high voltage could very well be your last!

The efficiency of microwave-oven transformers seems lower than some others when lightly loaded. (The rectifier connections shown in Fig 3 result in half-wave operation of each transformer, with dc in the windings. This may lead to increased core loss and possibly some core saturation on voltage peaks. This may account for greater heating—along with the probability that the most economical core materials are used.) I've not experienced any heat-related failures, however, the enclosure becomes warm enough during extended operation that I may include some forced ventilation in the future.

Overall, I'm very satisfied with the performance of this power supply. It seems to loaf along with loads drawing

1000 to 1400 W dc input and could handle even heavier loads with a larger C4,

References

1 Many microwave ovens have the wiring diagram glued to the inside of their enclosure. I've had the privilege(?) of dismantling approximately 20 microwave ovens of assorted brands and models. Some were repaired. The rest were taking up the valuable space of a store owner who repairs, sells and rents appliances. Each of us took some of the salvaged parts. If you negotiate a deal like this, make sure you have (or get) the training necessary to safely repair and test these appliances. 2Steve Bender, "The Bender 2pp Tube Push-Pull Amplifier Rebuild Project, Part 3," Nuts and Volts Magazine, Apr 1997, Vol 18, No. 4, pp 65 to 70. 3lan White, G3SEK, "Power and Protection for

Modern Tetrodes," QEX, Oct 1997, p 15.

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