Telephone to Transceiver Interface

The circuit is very simple. The Hybrid circuit is two Magnetek transformers, part # TY-302P,

available from Mouser for $8.33. Part # 553-TY-302P. The other parts you may have in your

goodie box. This circuit requires 600-ohm termination. Thanks to Norman,

K5EBL, for his part in putting together this project. Norman came up with the hybrid ckt.

If you would rather buy one, JK Audio has the TAP-1 for $245.

It appears to basically be the same as our home brew box.

I'm now using the JK Audio Tap-1, and it works the same

as my homemade one. I did have to do some rf shielding

by adding a ferrite choke to the telephone line.

The 900 Mhz portable telephones are ideal to remotely control Amateur Radio Transceivers
these days. They are inexpensive and many models to choose from. I paid $30 for mine with
a $10 rebate about two years ago. The one I have is a Vtech 910 ADL. .
If you have or can find a Vtech 910 ADL, this photo shows where to make the connections.
Your Transceiver must have the capability to run VOX . (Voice Operate Xmitt) .

This is a project with lasting enjoyment. I never get on the air without my cordless phone
in my shirt pocket with a cell phone type mic/earphone. Check me out, operating from my
easy chair in the living room while watching SSTV on the big screen TV. .

To modify one of these, all you need to find, is the place to feed audio in from your speaker .
and audio out to the mic input of your radio. This would be easier if one could get a schematic.

I have done this on several cordless phones, without a schematic, using the following procedure:

Open up the Base unit and apply power. With an Oscilloscope, probe around on the pc board,
while talking into the telephone and looking for the audio wave form of your voice. When you
find it, place one end of a .01 uF capacitor to that point and the center conductor of a well
shielded cable to the other end. Find the ground plane on the pc board and connect the shield.
This audio goes to your mic circuit. I recommend that you feed it through an audio equalizer
to lessen the distinctive telephone sound. .
To find the point to feed audio in from your speaker circuitry, use an audio generator or any
low voltage audio signal, such as, speaker audio with the volume turned down low. Use a .01
uF BLOCKING CAPACITOR so you won't damage the telephone and probe around on the
pc board until you hear the audio in the telephone. Using the same procedure as above, a
.01 cap and shielded cable will go to your speaker circuitry. You may need a pot attenuator,
out boarded, so you don't overdrive the telephone. .
Do not plug the telephone into the telephone line and take the usual antistatic precautions.

I used two Radio Shack (273-105) ferrite chokes, one on the power lead and the other on
the two audio cables. I used the small ( 1/10" ) 50 Ohm coax for the audio cables. This was
necessary to prevent RFI. .
Good Luck
Wayne, W5FJS

Below, is the methed we used before Norman came up with the Hybrid ckt.