The KF/C6 is a small converter for VHF reception. It is intended to be placed in front of an ordinary short wave receiver to provide coverage of the 6 meter and 10 meter amateur bands.
It was originally advertised in the October 1947 issue of Australasian Radio World Magazine, where it was stated that it will be available in the immediate future as the KF/C610. It says that two converters would be released, one for the 6 meter band and one for the 10 meter amateur band. The engraved front panel has the KF/C610 name on it, but the scale has the name KF/C6 and only covers the 50 to 54 mHz. The 10 meter version would cover 28 to 29.7 mHz. The name comes from K for the Kingsley Radio Company, and F/C meaning Frequency Converter, and the 6 for the 6 meter amateur band. The photographs in the original advertisements, show two tuning scales, and a Kingsley oval nameplate, but this example has only the 6 meter scale and no nameplate. Like the K/S9 signal booster, this was an attempt to stimulate use of the new amateur VHF bands, and to provide some business with the manufacturing wind down after WW2. These bands were known as UHF in that period. It was advertised for 4 pounds 5 shillings, fully assembled and tested.
DESCRIPTION
The device is in a small steel black steel box, 6 inches x 8 inches x 5.5 inches deep, and is black wrinkle painted, with engraved legends. It contains two valves, a 6AK5 and an ECH35 valve and four tuned circuits. It has a small tuning scale and basic controls. There are two INPUT terminals for the aerial and two OUTPUT terminals for the receiver. The switch at the top is labelled ON/OFF. When in the OFF position, it connects the terminals together effectively bypassing the device. It also turns off the HT to the valves. When in the ON position, the HT is on, and the miniature 7 pin 6AK5 acts as an RF amplifier. The knob on the left hand side is labeled MATCH, and tunes the aerial with a variable series capacitor. This is different from the K/S9 which has a resistor across the tuned circuit to broaden the response. The KF/C6 does not have the resistor, so the tuning is sharper. The RF amplifier has a potentiometer in the screen circuit to adjust the RF gain. The knob located on the right hand side is labeld GAIN. This example of the KF/C6 actually has a 6AG5 fitted which is similar to the 6AK5. The central tuning knob is unlabeled. It inductively tunes three coils by moving slugs in and out of them. The tuned circuits are self supporting coils, each of 5 turns of heavy gauge wire. A slug rack on a lead screw, moves an iron slug into each of the three coils. The tuning knob has a 2:1 ratio which moves a dial pointer horizontally, using a string and pulley system. There is a "beehive" capacitor trimmer across each coil for alignment. Two coils are for the grid circuit and plate circuit of the RF amplifier. The third coil is for the oscillator in the converter valve, which is an octal ECH35. The signal from the RF amplifier is mixed with the oscillator signal, and changed to 10.7 mHz. There is a fixed 10.7mHz IF transformer in the plate circuit, and it is connected to the OUTPUT terminals. There are three wires coming out the back of the box, and these are for connection to the receiver, to supply 6.3 VAC for the valve heaters and 150 to 250 VDC for the valve plates.
OPERATION
Connect the aerial to the INPUT terminals.
Connect the receiver to the OUTPUT terminals.
Tune the receiver to 10.7 mHz.
Turn the KF/C6 switch to the ON position.
The MATCH control should be set for maximum volume.
The GAIN control should be set for the required gain.
Tune the KF/C6 across the 6 meter band.
RESTORATION
The restoration was minimal as there was little wrong with the unit. I did not replace any resistors, capacitors or valves, but the steel case needed repainting in black wrinkle. The dial stringing was tricky, and I had difficulty reassembling the string and pulley dial pointer. The scale is only paper and was lifting, so it was glued back on. I had a little trouble aligning the unit at first. There is an adjustment for each of the tuning slugs, and a varible capacitor across each coil. I found that moving the slug into the coil lowered the frequency, and reducing the capacitor increased the frequency. Finally I figured out, that the easiest way is to align the oscillator first. When I had this right, and tracking the dial scale correctly, I could peak the other coils. The RF coils are fairly broad and non critical, apart from the oscillator.
TOP VIEW
UNDERNEATH VIEW
BANDPASS
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REFERENCES
Australasian Radio World Magazine, October 1947Copyright:
Ray Robinson VK2NO