John, Heres another one to add to the collection. 73 Richard G3RWL

This is for the TH75E (dual band) hand-held. Some folks may wonder why use a
handy-talky for satellites ? - well, the PA transistor blew on my IC-471 and
I needed to stay QRV if possible while it went away for servicing; this
provides a crude way of doing so, but you need steady hands, confidence, a
low-power fine-tipped soldering iron, and knowledge that if you make a mistake
it could be expensive and I won't pay the repair bill.

Remove the battery. Remove the two screws thus exposed into the front panel.
Next remove the screws (2) above and below the PTT switch. Finally remove the
single screw beside the wrist-strap anchorage. Thats five screws in all,
front panel removal is also explained on page 31 of the TH75 manual (for
installation of optional CTCSS unit).

Since the TH75 is a dual band rig, there is a separate IF module for each
band; the VHF one is screened but the UHF one is not (I don't know the reason
for the difference). Discriminator output is taken from the UHF module as
below and is adequate (a couple of volts) to drive the G3RUH 9600bps modem.
A lower level output (but not enough for my RUH9600) is available from the
CTCSS option connector pins 9 (signal) and 10 (ground); when CTCSS is not
fitted, this connector has a resistor pushed into it between pins 5 and 10.

                     =     =     =
                    | |   | |   | |
                   |   | |   | |   |
                  -------------------
       screw ---> |                 | <--- screw
                  |       |--------||
fold front       P|       |        || <--- VHF IF unit (screened)
panel to         T|       |--------||
this side.       T|                 |
                  |       |--------||
       screw ---> |       |        || <--- UHF IF unit (not screened)
                  |       |--------||        see expanded view below
                  |                 |
                  |                 |
                   \     TH 75     /
                    |    below    |
                    |   control   |
                    |    panel    |
                    |             |
                    |             | <--- exit for THIN cable
                    ---------------
                     ^     2     ^
                     |---screws--|

On the IF module pcb you'll see a small potentiometer at the left-hand end,
some components, an IC, and a 455 filter module; all surface mounted with
tiny solder connections to the board, thats why you need a fine-tipped iron
and a steady hand. Raw discriminator output comes from the top left pin of
the IC to the top of the chip capacitor immediately beside it; thats where
I wired to, taking ground from the left-hand end of the array of connections
to the motherboard. I used two strands of thin ribbon cable for this (screened
cable would be too thick), fed the cable downwards and came out just below the
DC IN connector to a free phono connector. Then I used screened cable out to
the modem. All the screws go back easily except the one nearest to the output
wire; I GENTLY squeezed the two halves of the rig in a vise until the two
holes lined up then put the screw in.

                               Discriminator output,
          ------------ connect to this end of this tiny chip capacitor
         \|/
----------+-------------
|    r c  c ----- |---||     UHF IF module
|pot e a  a |d c| -----|     =============
|    s p  p |i h| | 4 ||
|           |s i| | 5 ||
|           |c p| | 5 ||
|           ----- -----|
|  o o o o o o o o o   |
------------------------
   ^
   |---------- ground


Operation: Set the rig to 5KHz steps (the smallest possible). Take coaxial
cable from the antenna connector to your normal antenna feedline. Optionally
set the squelch as desired. Use it.

I find that mostly I get a solid DCD indication and good data. What with 5KHz
steps and the rather crude S-meter its not as good as a REAL receiver but its
a fair substitute if nothing else is available.

I don't change anything until the DCD led starts to flicker; then, with no
centre-zero meter, you have to make your own guesses about whether to move a
5KHz step, swap polarization, swing the beam, etc.

A tip; the receiver is pretty sensitive and I found I got better results with
my mast-head pre-amplifier switched off except close to AOS/LOS - probably due
to cross-modulation from local cellular phones. Don't forget to re-charge the
battery sometimes if you're not using an external dc supply !!!