18 OPERATION AND MAINTENANCE OF FM TRANSMITTERS Knowledge of operation and maintenance of any equipment is very important from working point of view as the life of equipment depends on it. We must understand the following very clearly : • Clear concept of the system. • Functional Diagram • Inter equipment Wiring • Knowledge of controls and indications available on various panels and their functions. • Cautions and checks. • Sequence of operation. • Switching ON/OFF. • Emergency operations. • Trouble shooting. System Concept These new generation Rohde & Schwarz transmitters are completely solid state. Three versions of power output have been opted by AIR depending upon the requirement envisaged in the plan. i) 3 kW ii) 2 x 3 kW Transmitter iii) 2 x 5 kW Transmitter The power amplifier VU 315 having an output power of 1.5 kW represents the basic module. 3 kW is made up by connecting two numbers of 1.5 kW units in parallel via a coupler unit. 2 x 3 kW units are thus obtained by combining two independent 3 kW transmitters. Similarly 5 kW transmitter comprises of 4 numbers of 1.5 kW units in parallel and 2 x 5 kW version is obtained by combining two independent 5 kW Transmitter in parallel. Each 3 kW/5 kW transmitter has its own AC power supply system. Each transmitter can be independently switched ON and OFF by use of Switch on Control Unit. Cautions • RF power transistors used in power amplifiers in the transmitter assembly contain Berylium Oxide material, under no circumstances these transistors be deliberately exposed to fire, broken or abraded. • RF voltage in the transmitter and combining racks are sufficient to cause RF burn in case of touching the part which is not properly terminated. • Before switching on, it is to be ensured that ‘No Port is Left Open” and ‘U’ links in the transmitter as well as in the combining rack are tightened properly to ensure that a firm contact has been established. • Before switching on the VHF amplifiers, ensure that blower set is normally working. In case of failure of blower set, it should not be tried to switch on the VHF amplifier. STI(T) Publication 181 /2003 007 FM Transmitter • Whenever RF power is dissipated on Dummy load, ensure that blower for Dummy laod is switched ON. • Level of the dielectric coolant in the reject load should remain constant. Check for possible leakage of coolant properly. • Pressure in the RF cable is to be checked regularly. • DC supply to Power amplifiers should never be connected in Reverse otherwise transistors are likely to be damaged. SWITCHING ON PROCEDURE • Switch on single phase mains to combining rack on the Mains distribution panel. • Switch on the circuit Breaker on the combining and Monitoring Unit. • Check Mains ON (Neon Lamp ON) and 24 V supply ON (LED ON) on the ACB & PSU unit and control panel respectively. • Connect U links on the U link panel and check lighting up of corresponding LEDs on the control panel as per table-1. The Mimic diagram on the front panel of control unit is shown in figure 1. During these checks system interlock fault LED remains in OFF condition. • Disconnect one of the U links and check whether system interlock fault LED comes ON. • Disconnect Dummy load contacts and check Dummy load fault LED turns ON restore the Dummy Load interlock connections. • Disconnect the reject load contacts. Check whether system interlock fault LED turns ON. Restore the connection of reject load contacts. • Switch on the 3 phase AC supplies for Tx A and Tx B on mains distribution panel. • Keep the Tx control switch on STBY position, check whether Tx interlock lamp lights up. • Select the Remote/local switch to local on the switch on unit of 3 kW transmitters. • Switch ON circuit breakers on the 3 kW Transmitters and press the Tx. ON control on the switch on unit. Check whether the ‘Tx ON’ lamp on Switch on Unit and mains ON lamp on Monitoring Unit turn green (carrier enable is not yet made available). • Change : Tx control switch position from standby to ON. Check whether Tx ON lamp turns Red. The RF present lamp on the selected exciter should light up. • Raise the power output of the transmitters to rated power gradually by operating the power control on the switch on unit. The RF present lamp on the switch on unit should light up. 2 x 3 kW Transmitter operation in case of failure of one In case of failure of one of the transmitters, the total power output in combined mode of operation drops to 1.5 kW from 5.5. By switching the working transmitter output to Antenna directly (by passing the combiner GD 008), the 3 kW power of the working transmitter can be restored to antenna. However it may be required to service /test the faulty transmitter without STI(T) Publication 182 007/2003 Operation and Maintenance of FM Transmitter affecting the service. It may also be required to remove certain sub systems such as exciter, trafowagon for detailed examination without affecting the service. Table I MIMIC Diagram Step No. Configuration Status Indication (On Control Panel) Mains on (dummy load) CR-1 CR-2 CR-3 CR-4 CR-5 CR-6 CR-7 CR-8 CR-9 1. TX (A+B) to Antenna ON ON ON ON 2. Tx (A+B) to D/L ON ON ON ON ON 3. Tx A to Antenna Tx B to D/L ON ON ON ON ON ON ON The checking of subsystem of the faulty transmitter can be done as explained below: • Switch OFF the transmitter completely. • Make the output co-axial U link connections appropriate to connecting working transmitter to antenna and faulty transmitter to Dummy load. • Switch ON the transmitter • Subsystem of the defective transmitter such as trafowagon power amplifier can be checked while in circuit using the schematic & explanation provided in the Technical manual. • The standby exciter can be energised by pressing ON the adapter plug in unit (KA-033A1) and checked. • In case sub-unit have to be removed from the transmitter for examination, the same can be done except the power amplifiers which are required to be in circuit for proper termination of the on-line Exciter. • The power output of the 2 x 3 kW FM transmitter reduces to: 500 W under the following conditions : i) VSWR > 1.5 at the output of any one/all the power amplifiers. ii) Temperature exceeds 70oC at the resistor in absorber unit. iii) Rack blower fault. Under these circumstances, the transmitter should be operated in single transmitter mode only. For operation on Tx B, the following procedure is to be followed: • Remove the switch on control unit from its original positions and insert it in the place of Adaptor plug-in unit. • Switch ON the transmitter by turning the transmitter control switch on combining and monitoring unit. STI(T) Publication 183 007/2003 FM Transmitter • Turn the level adjuster to get an output of 6 to 8 W as monitored on the front panel of online Exciter. • Adjust the power control on switch-on control unit (GS033 A1) to obtain 3 kW output. MAINTENANCE Routine check-panel meter readings No particular periodic maintenance is required. Various parameters of the exciter and the VHF amplifier can be checked using the selector switches and meters on the front panels. Following approximate nominal values are valid for the amplifiers in normal operation (2 x 3 kW). Exciter a) RF Output = 10-12 watt b) Deviation (AF) = 30-75 kHz c) Pilot tone = 9.5 dBm d) Audio left/Right For 75 kHz deviation = +6 dBm Power Amplifier a) RF voltage (forward) = 25-28 Div (300 V-FSD) b) RF voltage (Reverse) = 0-10 Div (60V – FSD) c) Current IV1 and IV2 = 3.5 – 6 A IE1 to IE8 = 7.5 to 12 A d) Differential voltage Ur = 0-10 Div (30 V FSD) Switch on control unit Forward power (6 kW FSD) = 1-1.2 Div Reverse power (300 W FSD) = 0-0.3 Div Combining and Monitoring Rack Check the parameters by pressing appropriate switches Tx A forward power = 3 kW Tx A Reverse power = < 50 W Tx B Forward power = 3 kW Tx B Reverse power = < 50 W Tx. (A+B) Forward power = 5.5 to 6 kW Tx. (A+B) Reverse power = < 100 W Reject load power = < 100 W STI(T) Publication 184 007/2003 Operation and Maintenance of FM Transmitter General Maintenance M/s Rohde and Schwarz, the manufacturer of these transmitters normally do not suggest any special maintenance. However dust, humidity, high temperature and frequent power supply fluctuations are our known enemies. It is therefore, absolutely necessary to keep these factors in check. Though lot of redundancy is provided in these transmitters, yet following preventive measures must be followed: - Ensure clear knowledge of the system. - Do not feedle the equipments wrongly. - Various controls inside the units are set in the factory and should not be disturbed. - Ensure dust free atmosphere to the extent possible. - Clean the filters of exhaust /inlet ducts regularly. - Clean the tag block and printed ckt boards with soft brush. Use suction type of light duty blower (vaccum cleaner) for removing the dust from racks, use Isoproz/Alcohol for cleaning printed ckt boards. Do not use CCL4. - See no bristles of brush or foreign materials is left on the boards or tag blocks. - Do not allow cobwebs to be formed in the equipments. - Keep the entry holes of trenches and conduits always plugged with glasswool so that no rats could enter and damage the equipments. - Ensure that blower are working satisfactory. No attempt should be made to switch on PA when both blowers are not working. - Keep the window airconditioners working and avoid overheating of components. - Check symptoms of overheating (if any). - Ensure that exhaust fan at the output of exhaust duct is always kept ON during transmission. - Ensure tightness of RF connections and clamps. - While removing or plugging in the units ensure that connector pins are not damaged. - Check pressure of RF cable and ensure that there is no leakage. - Check the oil level in reject load. - Check water/oil level (GCEL) (BEL) dummy loads. - Check the working of interlocks regularly. - Ensure that main AVR is kept on Auto mode and is working satisfactorily. - Ensure that fan of dummy load is always ON when transmitter is switched ON to dummy load. - During each transmission temperature of components be felt for excessive heating. The temperature of ‘U’ links should be observed to ensure that it does not exceed 60oC. - In case of fault in any one of the PAs, good transmitter should be selected manually at a suitable pause by change over of ‘U’ links. STI(T) Publication 185 007/2003 FM Transmitter STI(T) Publication 186 007/2003 - In the event of the removal of failed PA unit from the rack, the distribution of air to other PAs get affected which can result in unintended overheating. So the opening in the duct formed by removal of faulty unit has to be closed suitably. - While switching ON transmitter, power shall be raised gradually by potentiometer while observing the power on panel meters. Trouble Shooting These transmitters are supposed to be very rugged and reliable. These are even meant for unattended and remote operation. In spite of all redundancy, faults are bound to come. Before attempting to solve the problem, need to clear understanding of its working is to be emphasised fully. Health of various units is indicated by large number of LEDs provided for the purpose. The function of each control and indication must be understood clearly. Always keep simplified drawings handy. In case of any fault of problem. - Notice the abnormality. - Check all visible indications. - Note all panel meter readings and check the abnormality - Identify the faulty unit. - Isolate the faulty unit by following emergency arrangements to maintain continuity of service. - Repair the faulty unit. - Perform functional checks to ensure proper working. Make adjustments, if necessary. Ensure the correctness of point being touched. - Perform performance measurements for checking the specifications. System Concept Cautions SWITCHING ON PROCEDURE Table I MIMIC Diagram MAINTENANCE Routine check-panel meter readings Exciter Power Amplifier Switch on control unit Combining and Monitoring Rack General Maintenance Trouble Shooting