Acom 2000A Operating Manual

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Acom 2000A Operating Manual Manual pdf 38 page image

36BOARD which are not to be replaced by the user. Should one of these fuses be blown, it may beindicative of other more serious failures. This is a complex and possibly dangerous operation.For this reason we recommend this work be carried out by a trained service technician. Seethe TECHNICAL SUPPLEMENT for details.8-4. Tube ReplacementNew tubes must be supplied as a Matched Pair with close electrical characteristics. Otherwisethere may be an imbalance and one of the tubes will bear an excessive share of the operatingcurrent, which can lead to overheating and reduced tube life.Two 4CX800A (GU74B / ГУ74Б) high performance ceramic-metal tetrodes manufactured bySvetlana (Светлана) are employed in the amplifier. Tube replacement is a complex and possiblydangerous operation. For this reason, we recommend that this work be carried out by a trainedservice technician. See the TECHNICAL SUPPLEMENT for details.8-5. The ACOM2000A Simplified Schematic DiagramSee Fig.6-1 ACOM2000A Simplified Schematic Diagram.The two 4CX800A (GU74B / ГУ74Б) Svetlana (Светлана) high performance ceramic-metaltetrodes (V1 and V2) with plate dissipation of 800W each are grid-driven. The input signal fromthe RF INPUT jack is passed through a broadband input matching circuit which is comprisedof components in the PHASE DETECTOR PCB and also L1, L2, C2, C3 and R1. This circuittunes out the input capacitance of the tubes. The resistor R1 is a termination load of this circuitand can dissipate up to 100W of drive power.Cathode resistors R2 and R5 create DC and RF negative feedback and equalize the gain ofthe tubes. The varistor RV1 in the screen grid circuit protects the tube screen grid, and voltageregulator in the events of a flashover.The combinations of L3-R3 and L4-R6 in the plate circuits are the VHF/UHF parasitic suppressors.DC plate voltage is fed through chokes RFC2-RFC3 and the capacitor C10 blocks it from theoutput. The output circuit is comprised of L5-L6 and C12-C16, which form a classic Pi-L networkand suppresses harmonic frequency emissions. It is switched and tuned over the bands byS1A-S1D and the stepping motors M3-M5 via the MOTOR DRIVER PCB. The output signal isfed through the vacuum antenna relay K1, wattmeter current transformer TA1 and high-passfilter RFC5-C20 and finally to the antenna output. The chokes RFC4 and RFC5 keep track of theantenna relay contact condition and together with C20 prevent the plate supply from reachingthe antenna and RFC5 shunts it to ground if the DC blocking capacitor C10 fails. The resistorR9 and FV1 protect the amplifier from static discharge via the RF OUT connector.The PHASE DETECTOR together with the PLATE CAPACITIVE DIVIDER, RF WATTMETERand three OPTOSENSOR circuits are the main sources of information for the control circuitof the amplifier during the autotune process. The control circuit consists of the CENTRALPROCESSOR UNIT, based on the 80C31 microprocessor as the main system and the Z86E04Zilog single chip microprocessor which is used as the antenna relay QSK controller. They areinterfaced through the INTERFACE PCB to all other circuits.All voltages delivered from the LOW VOLTAGE and HIGH VOLTAGE SUPPLY PCBs, tubecurrents, temperatures and airflow cooling capability are permanently monitored. Numeroussoftware-derived protection limits are based on this information.The SIGNAL FILTER PCB and RFC1 eliminate RF feedback from incoming cables, including the