Note: This file is updated from time to time by me. Discard old versions. James G3RUH. FT736 & 9600 Baud Operation --------------------------- by James Miller G3RUH 1994 Aug 10 These notes tell you where to get FM RX audio direct from the discriminator, and where to modulate the FM TX varactor directly. These modifications 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 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 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. Remove C82. It's by the end of CD01, the 455D block. 9. The discriminator sensitivity (FM Normal) as about 6 kHz/volt. Important notes on 9600 Baud Use -------------------------------- 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 barely OK for 9600 bps FSK operation. It is recommended you change this to 15 kHz. For UOSAT use, 20 kHz bandwidth will give more tolerance to doppler shift, and give a better "eye". Suitable filters are: LFH-15S / CFW455E, LFH-20S / CFW455D. You may prefer to replace the FM-N filter instead. Changing RX Unit crystal filter XF01 to type 13M20A (Yaesu part No. H1102126) will considerably improve reception. This is especially beneficial with UO-22. (Tnx: Yaesu-EA via EA1KT and muTek Tel: +44 115 9729467) For Full-Duplex satellite use ONLY, increase modem C25 to 1uF. 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. Use TXEPROM selection number 10 (ten) for UoSAT, i.e. JMP1,3 ON, JMP2,4 OFF Tidy Installation ----------------- I found it neat to make up a little aluminium bracket to affix under the rear left lid screw. This bracket carries a 5-pin DIN socket, and the TX RX and PTT wires are terminated here. This saves having leads dangling out of the interior. (Tnx EI6EH). Alternatively mount the connector in the hole where the fuse goes. Undo the fusefolder and push it safely inside the radio. If the fuse blows you have to open up the radio anyway! (Tnx EA1KT) Notes compiled by G3RUH @ GB7DDX.#22.GBR.EU 1990 Mar 16, rev 1994 Aug 10