Yaesu FT-250R Technical Supplement

Also see for FT-250R: Manual Operating manual

Contents

Yaesu FT-250R Technical Supplement Manual pdf 7 page image

7FT-250R Technical SupplementCircuit DescriptionReceive Signal PathIncoming RF from the antenna jack is delivered to the RFUnit and passes through a low-pass filter and high-passfilter consisting of coils L1001, L1002, L1003, L1004, L1014& L1015, capacitors C1001, C1002, C1003, C1004, C1006,C1007, C1042, C1044, C1045, C1046 & C1047, and anten-na switching diode D1001 (RLS135).Signals within the frequency range of the transceiver arethen amplified by Q1011 (2SC5006) and enter a varac-tor-tuned band-pass filter consisting of coils L1016, L1017& L1018, capacitors C1049, C1050, C1051, C1052, C1053,C1054, C1056, C1057, C1058, C1059, C1060, C1061, C1062& C1063, and diodes D1004, D1005 & D1006 (allHVC350B) before delivery to the first mixer, Q1013(3SK296ZQ).Buffered output from the VCO is amplified by Q1018(2SC5374) to provide a pure first local signal between122.3 and 126.3 MHz according to the transceiver versionand the programmed receiving frequency for injection tothe first mixer Q1013. The 21.7 MHz first mixer productthen passes through monolithic crystal filter XF1001(21R12A4, 6 kHz BW) to strip away unwanted mixer prod-ucts, and the IF signal is then amplified by Q1014(2SC4400).The amplified first IF signal is applied to FM IF subsystemIC Q1017 (TA31136FN), which contains the second mix-er, limiter amplifier, noise amplifier, and S-meter ampli-fier.A second local signal is generated by the reference oscil-lator section of the PLL subsystem IC Q 1 0 3 0(MB15A01PFV1) using 21.25 MHz crystal X1001; a 450kHz second IF is produced when this signal is mixed withthe first IF signal within Q1017.The second IF then passes through the main selectivityelement, ceramic filter CF1001 (CFWM450E) to strip awayall but the desired signal; it is then applied to the limiteramplifier in Q1017, which removes amplitude variationsin the 450kHz IF, before detection of the speech by theceramic discriminator CD1001 (JTBM450CX24).Detected audio from Q1017 is applied to a low-pass filterconsisting of capacitors C1208, C1209 & C1210, resistorsR1320, R1321, R1322, R1323 & R1324, and Q1060(NJM2902V), then passes through the audio mute gateQ1044 and Q1063 (both 2SC4081) to the buffer amplifi-er Q1043 (2SC4617); it is then passed through the de-emphasis network consisting of capacitor C1057 and re-sistor R1208 to a high-pass filter consisting of capacitorsC1058, C1059 & C1060, resistors R1209 & R1210, andQ1064 (NJM2902V).The processed audio passes through the another audiomute gate Q1041 and Q1042 (both 2SC4081) to the vol-ume control potentiometer VR3001 on the VR Unit, thenis delivered to the audio amplifier Q1053 (DTA2822L),which provides up to 0.5 Watt to the headphone jack oran 8-W loudspeaker.Squelch ControlThe squelch circuitry consists of a noise amplifier, band-pass filter, noise detector & noise comparator withinQ1017, audio control gate Q1041, Q1042, Q1044, Q1063(all 2SC4081), microprocessor Q1035 (M3826AEFGP),and squelch controller S3001 on the VR Unit.When no carrier received, noise at the output of the de-tector stage in Q1017 is amplified and band-pass filteredby the noise amplifier section of Q1017 and the networkbetween pins 7 and 8, and then rectified by the noise de-tector section of Q1017. The resulting DC squelch controlvoltage outputs at pin 13 of Q1017, then it is passed to pin4 of the microprocessor Q1035.If no carrier is received, this signal causes pins 44 and 47of Q1035 to go “Low” and pin 54 to go “High.” Pin 47disables the supply voltage to the audio amplifier Q1053(TDA2822L), and pin 54 activates the audio control gatesQ1041, Q1042, Q1044, Q1063, Q1049 and Q1050. Thus, themicroprocessor Q1035 blocks output from the audio am-plifier, and silences the receiver, while no signal is beingreceived (and during transmission, as well). Meanwhile,pin 44 signals Q1056 (2SC4081) to hold the green (Busy)half of the LED D3001 (BRPY1211F) on the VR Unit off.When a carrier appears at the discriminator, noise is re-moved from the output, causing pin 4 of Q1035 to go “low”and the microprocessor to activate the audio amplifier,audio mute gate, and “Busy” LED.The microprocessor then checks for CTCSS or CDCSS codesquelch information, if enabled, or for DTMF data on theoptional DTMF Unit. If not transmitting and CTCSS orCDCSS is not activated, or if the received tone or codematches that programmed, the microprocessor stops scan-ning (if active) and allows audio to pass through the au-dio amplifier Q1053 to the loudspeaker by enabling thesupply voltage to it via Q1047 (2SB1132Q), Q1048(UMW1).