FM92 for Packet

There are numerous ways of connecting a TNC to an Philips FM92 for packet use. This page aims to describe a method that is both simple and reliable:

  • No audio pre-amp is used for the input audio
  • Speaker audio out is utilised.
  • The TNC is connected to the head of the radio rather than the RX/TX unit

  1. Start with a radio that is correctly tuned for voice operations. In particular, the deviation should be correctly set.

  2. Make sure all the signals you require appear on the DIN connector on the back of the head. If you have an ex Victoria Police head they should all be there. If you have an ex CFA head you will need to connect Audio In and the PTT lines from the microphone connector on the front right of the head. WICEN’s own digipeaters use the CFA head pinout. Here are the pinouts for the various connections (Red connections are those you may need to make):


    Signal

    CFA Head
    5 or 8 PIN DIN

    Mic
    Connector

    VicPol Head
    8 Pin DIN

    Kantronics
    TNCs 9 Pin D

    Tiny-2 &
    MFJ1270B
    5 Pin DIN

    Tx Audio

    1

    4

    5

    1

    1

    Ground

    2

    3

    2

    6

    2

    PTT

    3

    1

    7

    3

    3

    Rx Audio

    4

     

    6

    5

    4

    Switched
    Rx Audio

    5

     

     

     

     

    Tx Audio
    Ground

     

    5

    3

     

     

    Cradle
    Switch

     

    2

    1

     

     

    NB Pin six on the microphone connector is closest to the right edge of the head.

  3. Make up a cable to go between the DIN connector on the rear of the head and the TNC

  4. If you are using an MFJ1270B TNC, see this article re equalising the levels of the high and low TX audio tones.

  5. Connect the TNC to your radio and your PC.

  6. You are now ready to adjust the deviation of your packet station. This should be done by adjusting the level of the audio sent by the TNC, NOT by adjusting the deviation of the radio. Your TNC manual will tell you how to alter the audio output level. (See the article at http://www.febo.com/packet/layer-one/transmit.html for why setting the deviation is important.)

  7. If you are in the Melbourne metro area you can use the automated deviation tester to set the deviation. See the newsletter article at: http://news.vic.wicen.org.au/apdnov01.htm

  8. If you are not within range of the deviation tester, you can use the following rule of thumb technique:
    1. Listen to the output of your packet station with another receiver.
    2. Monitor the level with a multimeter set to AC volts.
    3. Adjust the audio output level of the TNC to a low level.
    4. Increase the audio level in the TNC until the received audio level stops increasing
    5. Decrease the TNC audio level to 60% of the max level seen
    6. At the first oppoutunity, get your deviation checked on an RF test set.

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