Posted: Sun, Mar 18, 1990 6:23 PM GMT Msg: EGJA-4199-1763
From: UOSAT
To: AMSAT
Subj: FT736R/9600/FO-20/PACSAT Mods
Dumped on 16 Mar 90 23:57:08 Friday by ELE093
From: JAMES MILLER G3RUH via TMAIL Address: UOSAT
Date: 1990 Mar 16 =====
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FT736 & 9600 Baud Operation
---------------------------
by James Miller G3RUH
These notes tell you where to get FM RX audio direct from the
discriminator, and where to modulate the FM TX varactor directly. These
mods are non-destructive and take no more than a few minutes. The signal
bypass the "DATA SOCKET" for high grade FM operations.
The RX mod is suitable for:
* UOSAT-D 9600 baud downlink and terrestrial links
* 1200 baud AFSK/FM Standard Packet - BUT IT'S UNSQUELCHED.
The TX mod is suitable for:
* FO-20/PACSAT uplink (1200 bps Manchester FM)
* UOSAT-D 9600 baud uplink direct FSK and terrestrial links
* 1200 baud AFSK/FM Standard Packet.
---****---
FT736 - FM Direct from Discriminator
-------------------------------------
Detected FM direct from the receiver discriminator is available from the
RX UNIT at the junction of R91 and C83. These components are shown in the
top right-hand corner of the schematic.
Proceed thus:
1. Disconnect FT736 from the mains electricity. (Safety).
2. Remove top cover only.
3. RX Unit is the vertical module on the left.
4. Locate R91 which is about 25mm from the top, 50mm from the radio rear.
the resistor is "on-end", and near a couple of glass diodes.
5. Scrape any paint off R91's free end and wet with solder.
6. Your RXaudio lead should be a fine screened cable; connect the inner
to R91, and the outer braid to a ground point (e.g. can of TO09)
7. Route the cable out though any convenient aperture in the case.
8. The discriminator sensitivity (FM Normal) as about 6 kHz/volt.
Important note on 9600 Baud Use
-------------------------------
Some FT736 receivers are fitted with an LFH12-S IF filter for FM. (CF01 at
the top front of the RX Unit). This is a 12 kHz bandwidth filter which is
a little too narrow for 9600 bps FSK operation. It is recommended you
change this to 15 kHz or better still for UOSAT-D use, 20 kHz bandwidth
which will allow more tolerance for doppler shift, and give a far better
"eye". Suitable filters are: LFH-15S or CFW455E, and LFH-20S or CFW455D.
The first of these is a Yaesu spare part, and is often already fitted.
---****---
FT736 DIRECT VARACTOR FM MODULATION
------------------------------------
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.
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.
Proceed thus:
1. Disconnect FT736 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 any 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.
7a. 1200 BAUD PSK MODEM: TXAudio of 800 mV pk-pk can be obtained by
adjusting the components C9= 1uf, R3=47k, R5=infinity (i.e. remove).
C10 stays at 10nf (0.01uf).
7b. 9600 BAUD FSK MODEM: Adjust TXAudio level with VR1
Notes compiled by G3RUH @ GB7SPV 1990 Mar 16