Icom ID-51A Service Manual - Page 15

TX AMP CIRCUITS (MAIN UNIT) The TX signal is passed through the buffer (Q48) and ATT (D20, D21), which controls the TX output power by the APC AMP (IC14). The TX signal is sequentially amplified by the YGR (Q39), drive AMP (Q41) and power AMP (Q45). • While operating on the VHF band The amplified TX signal is passed through the LPF (L501, C511), ANT SW (D503 and D504), TX output power detector (D512, D513) and two LPFs (L513, L515, L516, C547, C549, C553, C555 and C556), before being applied to the antenna. • While operating on the UHF band The amplified TX signal is passed through the HPF (L502, L503, C507, C508, C510 and C512), ANT SW (D501), TX output power detectors (D505 and D509) and two LPFs (L507, C524, C526, C529, and L511, C536, C539) and HPF (L512, L514, C545, C548, C550, C551 and C554), before being applied to the antenna. APC CIRCUITS (MAIN UNIT) The voltage produced at the LPFs (VHF band; L513, L515, L516, C547, C549, C553,C555 and C556, UHF band; L507, C524, C526 and C529) is rectified by the diodes (VHF band; D512 and D513, UHF band; D505 and D509), and it is used as the TX power sensing voltage. The voltage is applied to the APC AMP (IC14B, pin 6), and the output voltage controls the bias voltages of the ATT (D20 and D21) to keep the TX output power constant. • TX AMP AND APC CIRCUITS D14 LO SW Q48 BUFF D20,D21 ATT Q39 YGR AMP Q41 DRIVE AMP Q45 PWR AMP 1st mixer (Band A) -> IC14B APC CTRL PSET Q43 MUTE SW TXMUTE MAIN UNIT D501 D508 D510 HPF ANT SW D503,D504 D506,D903 LPF ANT SW LPF D505,D509 PWR DET PWR DET D512,D513 LPF ANTENNA TX:400.000M~479.000MHz RX:380.000M~479.000MHz LPF HPF TX:136.000M~174.000MHz RX:108.000M~174.000MHz LPF Receive circuit (UHF band) -> Receive circuit (VHF band) -> 4 - 6