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                         FO-20  &  the  Yaesu FT736R
                         ---------------------------
                           by James Miller G3RUH

If like me, one of the (many) features that attracted you to the FT736R
was the special DATA SOCKET, then you were probably very pleased to read
in the manual:
  "This 3-contact mini stereo jack allows direct connection to the FM
  receiver demodulator and FM transmitter modulator for digital equipment
  such as a packet radio TNC.  No pre-emphasis or de-emphasis is added to
  the signals at this jack" (page 14).

Don't believe a word of it!  A cursory inspection of the circuit diagram
shows that the TX side is simply merged with the microphone audio just
after the Mic Gain control, and is then murdered by several subsequent
filters.  On the RX side the data audio output is similarly indirect.

My initial day with FO-20 was disastrous.  (Yours too?)

So I looked at the FM signal on a calibrated monitor receiver when
transmitting FO-20/Microsat "Manchester" uplink signals.  The waveform was
appallingly distorted.

The problem is that the TX LF response cuts off at 800 Hz.  But the data
has substantial energy at 600 Hz and below.  Indeed, when transmitting a
600 Hz square wave it was clear from the droop distortion that FO-20 or a
Microsat would almost certainly not decode the uplink reliably.

The cure is simple; modulate the FM varactor directly.  Refer to the
circuit diagram; inject your TXaudio at the junction of R32/C29 on the TX
Unit.  The signal level at this point should be 800 mV peak-peak, and will
give +/- 3 kHz deviation.  DO NOT EXCEED THIS LEVEL.  Set Mic Gain to min.

The implementation is simple too.
 1. Disconnect FT736R from the mains electricity. (Safety).
 2. Remove top cover only.
 3. TX Unit is the module flat on the left (not the one tucked down the
    side vertically).
 4. R32 is just to the left of the rectangular shielded enclosure.  The
    resistor is "on end".  Scrape the paint off the free leg.
 5. Your TXaudio lead should be a fine screened cable;  connect the inner
    to R32, and the outer braid to the adjacent enclosure.
 6. Route the cable out though any convenient aperture in the case.

[ TXAudio of 800 mV pk-pk can be obtained from the G3RUH PSK modem by
adjusting the components C9= 1uf, R3=47k, R5=infinity (i.e. remove).  C10
stays at 10nf (0.01uf).]

Modulating the FM transmitter this way you get an LF response down to
18 Hz (at which point the associated synthesiser PLL begins to track the
modulation), and an HF response which is flat to some 10 kHz.

  *** FO-20/Microsat uplink modulation is now absolutely perfect. ***

 G3RUH @ GB7SPV 1990 Feb 23                 Amsat Tmail Address "UOSAT"