4.1.2 FT736 MODS 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 signals 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 ------------------------------- Most 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 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. Try the standard first and see what happens; these filters have moderate part to part variations. 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 G3RUH 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 4.1.3 CRYSTAL CONTROLLED RADIO MODS If your crystal-controlled rig is true FM (varactor modulator in an OSCILLATOR stage), inject TXA through a 5 mF cap into the varactor cathode. However, a lot of crystal controlled rigs are phase modulated and need a varactor modulator added to the transmit crystal oscillator. Use an abrupt junction type, such as the MV2105 (available from Kantronics), and adjust the capacitance in the crystal circuit to compensate for the additional C of the varactor. If there are fixed caps, remove or pad them. If not, change the trimmer to one of a lower minimum value. You'll need to bias the varactor. I've used the transmit oscillator Vcc and a 20K trimpot to ground, feeding bias through 2 10 to 47k fixed resistors, feeding the modem TXA thru a 5 to 10 uF cap to the center junction of these, and feeding the free end to the cathode of the varactor. The bias needs to be adjusted for best received eye pattern on a service monitor or receiver. TRANSMIT MODS Transmit oscillator +Vcc Modem TXA o o | | 1 | 0 | 0 | K === 5 uF non-polarized (required for K9NG) | | \ 20K | MV2105 4-20 pF / <----- 47K ---o---- 47K ------o-------o----- to TXCO crystal \ | | | ^ / | | |/ \ === .001 === === / | /_\ /| | | | | o---------------o---------------o-------o----- to TXCO ground _|_ ////