Yaesu FT-7800R Technical Supplement

Also see for FT-7800R: Brochure & specs Operating manual

Contents

Circuit DescriptionVHF ReceptionThe incoming VHF signal is passed through a low-passfilter network, antenna switching diodes D1074 (RLS135),D1075 (RLS135) and D1015 (HVC131), and another low-pass filter network to the RF amplifier Q1008 (3SK296ZQ).The amplified RF signal is passed through a varactor con-trolled bandpass filter consisting of L1017, L1018, andD1010, D1011, and D1012 (all HVC365), then applied tothe first mixer Q1009 (3SK296ZQ) along with the firstlocal signal from the PLL circuit.The first local signal is generated between 189.05 MHzand 193.05 MHz, depending on the receiving frequency,by the VHF VCO, which consists of Q1076 (2SC5374) andvaractor diodes D1049 (HVC365), D1050 (HVC131),D1051 (HVC365), and D1052 (HVC362).UHF ReceptionThe incoming UHF signal is passed through a low-passfilter network, antenna switching diodes D1077(UM9401F) and D1018 (RLS135), D1019 (RLS135), andD1084 (HSC277), and another low-pass filter network tothe RF amplifier Q1001 (3SK296ZQ). The amplified RFsignal is passed through a varactor-controlled band-passfilter consisting of L1006 and L1007, and D1002, D1003,and D1004 (all HVC350B), then applied to the first mixerQ1002 (3SK296ZQ) along with the first local signal fromthe PLL circuit.The first local signal is generated between 384.95 MHzand 404.95 MHz, depending on the receiving frequency,by the UHF VCO, which consists of Q1072 (2SC5006)and varactor diodes D1046 (HVC375B) and D1047(HVC350B).IF and Audio CircuitsThe 45.05 MHz first IF signal is applied to the monolithiccrystal filter XF1001 which strips away unwanted mixerproducts, and the IF signal is applied to the first IF ampli-fier Q1016 (2SC4400). The amplified first IF signal is thendelivered to the FM IF subsystem IC Q1023 (TA31136FN),which contains the second mixer, limiter amplifier, noiseamplifier, and FM detector.The 44.6 MHz second local signal is derived from 11.15MHz crystal X1002, the frequency of which is multipliedby four at Q1022 (2SC4400), producing the 450 kHz sec-ond IF signal when mixed with the first IF signal withinQ1023 (TA31136FN).The 450 kHz second IF signal is applied to the ceramicfilter CF1001 which strips away all but the desired shig-nal, and then passes through the limiter amplifier withinQ1023 (TA31136FN) to the ceramic discriminator CD1001which removes any amplitude variations in the 450 kHzIF signal before detection of speech.The detected audio passes through the de-emphasis net-work, a low-pass filter consisting of Q1028 (NJM2902V)and associated circuitry, and a high-pass filter consistingof Q1028 (NJM2902V) and associated circuitry. The fil-tered audio signal is passed through the audio volumecontrol IC Q1034 (M51132FP) which adjusts the audiosensitivity to compensate for audio level variations, thendelivered to the audio switch Q1036 (DTC144EE).Squelch ControlWhen no carrier received, noise at the output of the de-tector stage in Q1023 (TA31136FN) is amplified and band-pass filtered by the noise amp section of Q1023(TA31136FN). The resulting DC voltage is applied to pin2 of main CPU Q1065 (M3826AEFGP), which comparesthe squelch threshold level to that which set by the frontpanel SQL knob.While no carrier is received, pin 53 of Q1065(M3826AEFGP) remains “low,” to disable audio outputfrom the speaker.Transmit Signal PathThe speech signal from the microphone passes throughthe MIC jack J2001 to AF amplifier Q2011 (NJM2904V)on the PANEL unit. The amplified speech signal is sub-jected to amplitude limiting by Q1040 (NJM2902V) onthe MAIN unit. The speech signal then passes throughlow-pass filter network Q1040 (NJM2902V) and bandswitch Q1039 (BU4066BCFV) to the VHF VCO or UHFVCO.VHF Transmit Signal PathThe adjusted speech signal from Q1040 (NJM2902V) isdelivered to VHF VCO Q1076 (2SC5374) which frequen-cy modulates the transmitting VCO made up of D1049(HVC365). The modulated transmit signal passes throughbuffer amplifiers Q1077 and Q1078 (both 2SC5374). Theamplified transmit signal is then applied to the Pre-Driveamplifier Q1084 (2SK2596) and Driver amplifier Q1086(RD07MVS1), then finally amplified by Power amplifierQ1087 (RD70HVF1) up to 50 Watts. This three-stage pow-er amplifier’s gain is controlled by the APC circuit. The50 Watt RF signal passes through high-pass filter and low-pass filter network, antenna switch D1072 and D1073(both XB15A709A), and another low-pass filter network,and then is delivered to the ANT jack.D-1