HOW TO TAP A PHONE (Mar, 1957)


By Tony Karp

THERE are many ways to tap a phone; most of them against the law. Our little gadget, however, is quite legal and can be used to great advantage at home or in the office.

Basically, the unit consists of a pickup coil, an amplifier and a speaker. The pickup coil is placed under, or near, any transformer-type telephone without being in physical contact with it. As the electrical currents pass through the phone, part of the energy is induced into the pickup coil. This energy is fed into the amplifier where it is amplified to the point where it will operate the loudspeaker, enabling everyone within range to hear what is being said at the other end of the telephone line. This will come in handy when some relative is calling long-distance; your whole family can hear what he is saying. Or, in the office, the whole staff can hear a salesman’s report. There are other uses for the pickup, limited only by your own imagination.

The unit is a four-transistor audio amplifier with three transformer- coupled stages. The last stage is push-pull for greater clarity and higher output. When idle, it only draws about five milliamperes because the last stage does not draw any current until a signal is applied to it. Yet it has plenty of “sock” and on a good signal will put out over a quarter of a watt, enough to drive the four-inch speaker with plenty of volume. Since the amplifier draws 25 to 35 milliamperes only when there is a signal, the battery will give many hours of use before it needs replacing.

The first step in construction is to drill the holes to mount the chassis and the speaker in the meter case. Remove the decorative moulding on the front of the case. The two holes under the moulding must be drilled or reamed to a half-inch so that the nuts on the volume control and the jack will fit snugly and serve to support the front of the chassis. The case is made of light steel and no trouble in drilling the holes will be encountered if a sharp drill is used. Punch a slight indentation before the hole is started.

Since the amplifier is built in a fairly large case, there is plenty of space for a chassis that will hold all the parts without crowding. An unusual feature is that everything is attached to the chassis. This includes the input jack, the volume control and the battery.

By removing the two nuts holding the brackets in the back, the chassis can be removed intact for servicing.

Before construction of the chassis can be started, the mounting holes for the battery holder, transformers and brackets must be drilled. Using the photo and drawing of the chassis as a guide, lay out the transformers on the chassis and drill holes with a No. 30 drill. The same size holes should be made for the battery holder and brackets; the jack and volume control are mounted under the chassis. Make sure that the brackets for these parts are positioned carefully so that the nut on the volume control and the jack will match up with the two holes in the front of the case.

Next, mount the parts on the chassis. The first stage of the amplifier is mounted at the front of the chassis and the other two follow along the side with the push-pull stage being at the back. Flea clips are used to hold the transistors; they also act as binding posts. As a result, the usual rectangular holes for sockets do not have to be drilled. Three clips are used for each transistor. Mark the flea clip that holds the collector (C) of each transistor with a spot of red nail polish or paint so that you will be sure to connect the transistor with its proper polarity.

Insert the leads of the components into the holes in the chassis. Bend them slightly so they will stay in place. Now you can begin the soldering. Check the schematic carefully as you perform this operation. Be sure that the color coding on the leads of the transformer are correct to avoid oscillation. Another cause of oscillation is neglecting to ground the black lead on the output transformer. Solder this wire to a ground on the chassis and run a wire from there to the speaker. A solder joint on the frame of the speaker will not work. Check also for correct polarity on the electrolytic condensers.

Before testing the amplifier go over the schematic again to be sure that all connections are correct. A few minutes spent checking at this time may save you the trouble of later replacing a transistor.

Slide the chassis into the case so that the nuts pass through the holes in the front and support the chassis. Hold the chassis level and drill two holes for the bolts that will pass through the brackets in the back. Now remove the chassis and insert the transistors in the flea clips, making sure that the collector side is next to the red dot. When you clip the leads of the transistors, leave them a little long; this may save you trouble later. Put the battery into its holder and put on its snaps.

Insert the speaker and grille cloth. Now, install the chassis and bolt it in place. Solder the two leads from the speaker to the two flea clip binding posts on the rear of the chassis. Leave a little slack in these wires so that the chassis can be removed without unsoldering.

Solder the shielded cable from the phone pickup to the miniature plug. The inside wire goes to the short side of the plug, the braided part to the long part. Now the unit is ready to be tested.

Turn the amplifier on. If the unit is functioning correctly, the only sound from the speaker will be the normal hiss generated by the transistors. If the unit oscillates, check the ground on the input, the volume control and the output. If it still oscillates, check the leads on the transformers and be sure that R4 is the right value.

If no sound is heard, check the battery polarity and make sure that the transistors are inserted properly. If this fails to produce results, a check of the schematic against your unit is in order.

Now plug in the phone pickup jack for a final test. Turn on the amplifier and place the pickup coil underneath the base of your telephone. Remember, it must be the kind of telephone which has its transformer in the base and not in a separate box on the wall. In the latter case, the pickup coil of course must be placed near the wall box. At any rate, you should now be in business and all your family or friends will be able to listen in comfort to long distance calls. •

  1. Al Bear says: December 30, 20084:47 pm

    Awesome! showing folks how to commit a Federal crime!

    *wonders if it WAS a Federal crime* in 1957?

  2. LightningRose says: December 30, 20085:38 pm

    Dick Cheney was how old in 1957?

  3. Warren says: December 30, 20085:42 pm

    There are other uses for the pickup, limited only by your own imagination.

    Now we know what W was reading as a lad…

  4. Jeff says: December 31, 200812:57 am

    Not all readers of Modern Mechanics are from the USA, at least I’m not.
    I use something similar to get that classic “telephone voice” effect in audio samples.
    It isn’t illegal to record telephone conversations in Australia, as long as both parties are informed and agree.
    A better circuit replaces the pick-up coil with a 600 Ohm to 600 Ohm audio transformer wired in parallel across the voice wires (some PABX use the other 2 wires to ring the phone buzzer).

  5. Jack says: December 31, 20087:44 pm

    I think the illegality the article mentions is not the wiretapping laws but the old Bell-monopoly era prohibition on connecting non-Bell issued equipment to your phone line. This devices neatly sidesteps that using induction.

    The article is hardle about encouraging illegal behavior, though. First off, anyone can buy an equivalent device at any RadioShack. Further, given that the coil has to pick up induced currents from the phone’s transformer, I don’ t think this could even conceivably be used to “wiretap” someone surrepticiously.

    Regardless, there’s no harm in having people know how to interface with the telephone system; it’s not even illegal anymore unless you’re listening in on someone else’s conversation. It’s also no big secret: As Jeff says, just connect a 1:1 audio “telephone” transformer across the voice pair and you’ve got your tap. Or, if you really want a low budget wiretap that’s far more suitable for illegal use, just connect the voice wires of your phone to the voice wires of your target with alligator clips. Wow. Federal Criminal knowledge there…

  6. Jim Jones says: December 31, 20089:52 pm

    Oh yeah, I remember the good ole days!


  7. NFM says: December 31, 20089:53 pm

    Has anyone of you tried this?

  8. Dragon says: January 1, 20091:04 am

    For 1957, I’m amazed this circuit didn’t use a 12AT7 tube! That was pretty leading edge, using transistors that far back.

  9. lunarmagpie says: January 1, 20092:32 pm
  10. VijayK says: January 1, 20094:12 pm

    Now we can finally beat them Reds!!

  11. mercedes says: January 31, 200911:05 am

    this is just what i needed brb going to radioshack

  12. milkman says: January 28, 20103:12 am

    has anyone read this article. http://cellphonespyrevi… It is about how to actually perform real cell phone spy taps. I have an issue with my wife and would like to know if this is really possible

  13. Dave says: October 1, 201010:33 pm

    @milkman – Rather than spy on your wife, talk to her. It will either result in you understanding, or divorce. Best to get it out of the way.

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