by Phil Atchley, KO6BB

Instructions to construct a variable audio attenuator cable

Purpose: to interface the microphone input jack of a tape recorder to either the line or speaker output of a shortwave receiver, for the purpose of being able to record directly from the receiver.

Notation: Being a variable attenuator, this will work with nearly any receiver/recorder combination. The following instructions assume the builder has little, if any, electronics knowledge. For this reason it is suggested the builder use pre-molded shielded cables to facilitate construction, rather than separate plugs and shielded wire.

Parts needed (from Radio Shack or other source):

1. Shielded patch cable with correct size male plugs to fit the mike input jack on the recorder on one end and the receiver line output or external speaker jack on the other.

2. 10K Audio Taper potentiometer (Radio Shack Catalog # 271-1721). 5K or 100K will work also.

3. Small plastic box to house the potentiometer in (optional). The smallest and cheapest box R/S has is good enough, OR you can just tape the "pot" after you adjust it.

Receiver Attenuator Patch Cord. Click here to enlarge image.

Assembly: See drawing.

1. Cut the shielded patch cable in the center creating two shorter cables.

2. Carefully strip the outer insulation from the cut end of each cable. Remove about 1.5 inches of the outer insulation. Be careful not to cut through the braided wire strands of the shield. Do NOT remove the insulation from the center conductor!

3. Comb out all the strands of the outer shield wire to straighten them. Roll them between your fingers to create a single conductor. Be sure not to miss any strands that can short to the center conductor.

4. Use a soldering iron and lightly tin, (put a layer of solder on), covering about the last inch of these shield wires. Leave the 1/2 inch of shield NEAREST the insulated part of the cable UNTINNED for flexibility. Be very careful not to melt the insulation on the center conductor!

5. Strip about 1/4 inch of insulation off the center conductor on both cable halves. Tin these wires.

6. Connect the braided SHIELD wires of BOTH cable halves to pin 1 of the potentiometer. Insert the wires through the hole and lightly bend them over. Carefully solder each wire.

7. Connect the bare center conductor wire from the RECORDER cable to pin 2, (center), of the potentiometer. Solder it.

8. Connect the bare center conductor wire of the RECEIVER cable to pin 3 of the potentiometer. Solder it.

9. There are several ways you can put this in a plastic box. The easiest is to cut two small notches for the cables in the box, generally between the lid and box. Insert the entire potentiometer INSIDE the box once it has been adjusted for proper operation.

The other method is to drill a hole large enough to permit the shaft to protrude out of the box with the pot sealed inside. Cut two small notches in the box for the cables. It's your call.

Because the control rarely requires adjustments once put into service, the first is as good as any.

Adjusting The Control For Proper Levels:

Connect one end of the cable from the line/speaker output of the receiver to the mic input on the recorder. Tune in a moderately strong station. Place the recorder in the record mode and adjust the record level control for the best possible audio recording without exceeding the distortion, or signal overload, red line. You will probably find that it is not necessary to turn the control up very high at all.

Contributed By: Phil Atchley, KO6BB

Merced, California, USA

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