OMNI-500i
OMNI-1000i
OMNI-500 FA

500/1000 WATT

SOLID STATE
AM BROADCAST TRANSMITTER
TECHNICAL MANUAL

LPB COMMUNICATIONS, INC
960 Brook Road Unit #5
Conshohocken, PA 19428
TELEPHONE: (610) 825-4100
FAX: (610) 825-4047

Third Edition
Nov 2002
Copyright LPB Communications, Inc.

TABLE OF CONTENTS

1.1 GENERAL DESCRIPTION

1.1.1 Electrical Description
1.1.2 Mechanical Description
1.1.3 Limited Warranty
1.1.4 Technical Specifications
1.1.5 Electrical Specifications
1.1.6 Mechanical Specifications

2.1 THEORY OF OPERATION

2.1.1 Transmitter Overview
2.1.2 Motherboard

2.1.2.1 The RF Exciter

2.1.2.1.1 Crystal Oscillator
2.1.2.1.2 Frequency Synthesizer

2.1.2.2 PDM Exciter

2.1.3 700W Power Amplifier Module
2.1.4 Power Supply PCB

2.1.4.1 Overview
2.1.4.2 Transient Protection
2.1.4.3 Overcurrent Protection

2.1.5 Harmonic Filter

3.1 INSTALLATION

3.1.1 General
3.1.2 Unpacking and Inspection
3.1.3 Positioning the Transmitter
3.1.4 External Connections to the Transmitter

3.1.4.1 RF Ground
3.1.4.2 Power Mains
3.1.4.3 RF Output Connection
3.1.4.4 Audio Input Connections

3.1.5 Setting up the Transmitter

3.1.5.1 Step 1 Line Voltage
3.1.5.2 Step 2 Cooling
3.1.5.3 Step 3 Operating Frequency/Synthesizer Programming (Optional)
3.1.5.4 Step 4 Low Power Operation
3.1.5.5 Step 5 Modulation

4.1 MAINTENANCE

4.1.1 Preventive Maintenance

4.1.1.1 Mechanical Maintenance
4.1.1.2 Normal Operating Temperatures
4.1.1.3 RF Circuits Maintenance
4.1.1.4 Power Transformer Maintenance
4.1.1.5 Filter Maintenance
4.1.1.6 Fan Maintenance
4.1.1.7 Meter Maintenance
4.1.1.8 Components
4.1.1.9 Switches
4.1.1.10 Fuse Maintenance
4.1.1.11 Cabinet Interior and Exterior Maintenance

4.1.2 General Maintenance

4.1.2.1 Recommended Daily Schedule
4.1.2.2 Recommended Weekly Schedule
4.1.2.3 Recommended Monthly Schedule

5.1 TROUBLESHOOTING

5.1.1 Overview
5.1.2 Mother Board Problems
5.1.3 Power Amplifier Problems
5.1.4 Power Supply Problems
5.1.5 General Transmitter Problems

6.1 PARTS LIST

6.1.1 Ordering Parts

7.1 SCHEMATICS

8.1 ASSEMBLY DRAWINGS

SAFETY NOTE

Although the Omni transmitter incorporates interlocks and other design factors consistent with safe operation, it should be realized that the transmitter uses high voltages. The intended purpose of this manual is to serve as a general guide for trained and qualified personnel who are familiar with and aware of the dangers inherent to handling potentially hazardous electrical and/or electronic circuits. This manual is not intended to contain all safety precautions, which should be observed by personnel using this, or any other electronic equipment.

WARNING:

HIGH VOLTAGE EQUIPMENT CAN PRODUCE FATAL INJURY.

Operation of this electronic equipment involves the use of high voltages, which are dangerous to life. Operating personnel must at all times observe all safety regulations. Do not make any adjustments inside the equipment with the voltage supply on. Under certain conditions dangerous potential may exist in circuits with the power control in the off position due to charges retained by capacitors, etc. To avoid casualties, always discharge and ground circuits prior to touching them.

Persons engaged in the installation, operation and maintenance of this equipment or similar equipment are urged to become familiar with first aid procedures in theory and practical application thereof.

Local building codes and the fire protection standards must be observed during the installation and operation of this equipment.

1.1 GENERAL DESCRIPTION

The 'i' Series Omni transmitter competitively priced high efficiency and rugged AM Broadcast transmitters. These transmitters, like all Omni transmitters, are 100 percent solid state using extremely efficient pulse duration modulation (PDM) techniques. The nominal frequency range of the transmitter is 535 kHz to 1705 kHz.

The use of state-of-the-art Integrated Circuits (IC’s) and MOSFET's in the design provides reliability an order of magnitude better than that experienced with bipolar transistors or vacuum tubes. The use of a modular concept in the Omni design provides ease of maintenance.

Front panel status indicators facilitate diagnostics and operation. The high reliability, rugged construction and excellent AC to RF conversion efficiency translate into a very short payback period. The optional OMNI-SYN Frequency Synthesizer plugs into the transmitter motherboard.

All panels fit into standard 19-inch relay racks.

This Technical Service Manual contains the information necessary to install, operate, maintain, and service an Omni solid state transmitter. The sections of this book provide the following information about the transmitters:

Section 1. General Description: describes the transmitter and major components. A block diagram is included and the physical and electrical specifications are discussed.

Section 2. Theory of Operation: describes the design of the transmitter providing details of individual circuit operation. This section provides a basic tutorial for the interested student of solid state Pulse Duration Modulated (PDM) transmitter theory.

Section 3. Installation, provides the necessary instructions on how to interface with the input/output connections of the transmitter, the required inter-panel connections, initial adjustments, and component mounting instructions, where required.

Section 4. Operation: identifies and describes functions of the various transmitter controls and discusses transmitter-operating conditions.

Section 5. Maintenance: describes procedures for preventive and corrective maintenance.

Section 6. Troubleshooting: provides fault location guidance and troubleshooting procedures.

Section 7. Parts List: provides information for ordering replacement components and assemblies. Industry standard part numbers are provided.

Section 8. Drawings and Illustrations: contain the transmitter schematics for transmitter maintenance. Additionally, assembly drawings and illustrations are provided for quick and easy identification and location of all components.

1.1.1 Electrical Description

Omni transmitters are completely solid state Amplitude Modulated (AM) transmitters for use in the Medium Wave (535 KHz to 1705 KHz) broadcast band. The Omni transmitters utilize Pulse Duration Modulation (PDM), a digital method for generating amplitude modulation by varying the width of a 0 to 300 volt 70 KHz waveform proportional to the audio information to be impressed on the carrier. This highly efficient technique utilizes 700 watt amplifier modules, each containing a modulator Metal Oxide Semiconductor Field Effect Transistor (MOSFET) and 4 RF MOSFET's in a bridge amplifier topology.

The Omni transmitters are modular in design with all components inside standard 19-inch aluminum panels. The 19-inch cabinets are utilized solely for mechanical support of these panels; no components are mounted to the cabinets. Each major circuit function (such as the PDM Exciter, the Crystal Oscillator, the Power Amplifier, etc.) is designed on a Printed Circuit Board (PCB) that plugs into the motherboard. This extensive use of modular construction affords ease of maintenance.

Referring to the block diagram of the transmitter, Figure 1, it can be seen that the transmitter consists of four major sub-assemblies. They are the Exciter, Power Amplifier, Harmonic Filter and Power Supply.

The Motherboard PCB contains interconnecting tracks, connectors for the exciter PCB’s, general housekeeping, and protection circuits. All system monitoring is accomplished with front panel status LED's.

The 700 Watt Power Amplifier Module plugs into the motherboard. Cooling is accomplished by drawing air in from the rear of the transmitter and exhausting it via the front panel grill.

The Harmonic Filter contains the elements required for signal purity. An adjustable spark gap is used to protect against antenna transient voltages. VSWR protection is accomplished by monitoring the high voltage current in each power amplifier. Additional, lightning protection is obtained by the bandpass topology of the combiner and harmonic filter.

The Power Supply utilizes a high voltage isolation transformer, low and high voltage rectifiers, a MOV for power line surges, capacitors, fuses, and circuit breakers.

1.1.2 Mechanical Description

The transmitter is enclosed in standard 19-inch chassis. The 500 watt transmitter is a single 7-inch high chassis. The 1000 watt transmitter is made up of two 7-inch high chassis. All cooling is accomplished with a low velocity air muffin fan. Low velocity cooling air enters at the rear of the chassis and exhausts via the front panel grill of the transmitter.

1.1.3 International Limited Warranty

LPB Communications, Incorporated, hereafter referred to as LPB, guarantees that equipment will be free from defects in material and workmanship for a period of ONE YEAR from the date of purchase. In addition, the equipment is warranted to conform to published specifications in effect on the date of the Purchase Agreement when the equipment is operated within the design parameters. LPB’s liability is limited, at the sole discretion of LPB, to repairing or replacing the equipment that is found, to the satisfaction of LPB, to have been defective.

The Warranty is subject to the following conditions:

• Warranty is void if the equipment has been not been installed, operated and maintained in accordance with LPB’s recommendations and instructions as detailed in the provided technical manual.
• Warranty does not apply to damage caused by misuse, neglect, unauthorized modification or accident.
• The Warranty is valid only to the original purchaser unless written agreement has been authorized by LPB.
• Semiconductors contained within the equipment carry the standard manufacturers warranty of 90 days.

What the Warranty Covers:

• LPB shall provide replacements for all parts at no cost to the Customer when they become defective during the warranty period, and upon the return of the defective part.
• In the event that a part fails during the warranty period and causes damage to a subassembly that cannot be readily repaired in the field, the entire subassembly so damaged may be returned to LPB for repair. The repairs will be made without charge to the Customer.
• Written authorization must be obtained before returning any equipment for any reason. Equipment returned under this warranty shall be delivered to LPB’s premises at the Purchasers Expense. Where no-charge warranty replacements or repair is provided under items 1 or 2, LPB will pay that part of the shipping costs incurred in returning the equipment to the Customer.

LPB makes no warranties other than those expressly stated, and none shall be implied. LPB’s sole obligation is to correct or replace defective or non-conforming equipment as set forth in the preceding paragraphs. Not-withstanding any other provision of this Agreement, in no event shall LPB be liable for indirect, incidental or consequential damages, whether caused by negligence or otherwise, and in no event shall LPB’s total liability exceed an amount equal to the price specified in the Purchase Agreement.

1.1.4 Technical Specifications

Power output: 500wt for 500i and 1000wt for 1000i.
Emission type: A3
Frequency Range: 535 kHz to 1710 kHz
Output impedance: 50 ohms, unbalanced
VSWR: <1.5
Exciter: Crystal Oscillator or Frequency Synthesizer (optional)
Frequency Stability +5 ppm (+ 8 Hz maximum)
Audio input impedance: 600 ohms balanced
Audio input level: -10 dBm to +10 dBm
Modulation: Up to 125% positive peak at 500 Watts (1000 watts for 1000i)
Frequency Response: +/-1.0 dB, 50 Hz to 8 kHz
Audio distortion: <3.0%, 50 Hz to 8 kHz
Carrier Shift: <2% at 100% power and 95% modulation

1.1.5 Electrical Specifications

Line voltage: 120/220 VAC + 15%, (500i); 220VAC+10%, (1000i)
Line frequency: 50/60 Hz
Power consumption with no modulation: 710va for 500i and 1420va for 1000i, at 100% sine wave mod: 1070va for 500i and 2140va for 1000i.
Overall Efficiency: >70%

• Front Panel Indicators - 300VDC, 18VDC Voltage, RF Power, MOV Status, Synthesizer Lock, Fault, Modulation Monitor, and Over-temperature LED indicators

Overheating: Automatic shutdown of individual amplifier module
Over current: Individual amplifier current limit foldback
Input Transient Protection: Isolation transformer and surge suppressors
Output Protection: Spark gap and Band-pass Harmonic Filter

1.1.6 Mechanical Specifications

Temperature: 0-50 ^oC, derate 2 ^oC per 300 meters (1000 ft)
Altitude: 0-3000 meters (10,000 ft)
Humidity <95% non-condensating
Cooling: Low velocity air

2.1 THEORY OF OPERATION

2.1.1 Transmitter Overview

The Omni transmitter contains four major blocks. They are the Control Circuitry Block, the Power Amplifier Block, the Antenna Interface Block, and the Power Supply. This section will present the theory of operation of each block providing detailed explanations of individual circuits. For best understanding of the following circuit descriptions it is recommended that the schematic diagrams be handy.

2.1.2 Transmitter Motherboard

The Transmitter Motherboard has connectors and interconnections for the Crystal Oscillator PCB (or the optional Frequency Synthesizer PCB set), the PDM Exciter PCB, and up to two Power Amplifier PCB’s. Other connections are via a wiring harness and made to the Power Supply PCB, the Combiner/Detector PCB, and the LED PCB.

Control circuits to detect power amplifier over current, loss of low voltages, and the front panel Reset Switch are located on the Motherboard, and control the transmitter via the TXINHIBIT Line.

The +12 volt and +5 volt regulators are also included to power the control and supervisory circuits such as the indicators on the LED PCB and it’s associated circuits.

2.1.2.1 The RF Exciter

The RF Exciter is either the Crystal Oscillator PCB or the Frequency Synthesizer PCB set.

2.1.2.1.1 Crystal Oscillator

The Crystal Oscillator contains a CMOS crystal oscillator set on the pre-determined frequency for operation in the monaural mode. The Oscillator operates at 6 to 8 times the carrier frequency depending on the crystal that is utilized. A chain of TTL flip-flops in the ripple counter topology that sets up either a 33% duty cycle or a 38% duty cycle divides the oscillator output down. The signal is buffered and sent to the power amplifier Metal Oxide Semiconductor Field Effect Transistors (MOSFET’s).

On this board is a crystal CMOS oscillator at 2.8 MHz and the dividers (74LS74) required for generating the 175 (or 350) KHz PDM sample frequency.

NOTE: JUMPERS ON THE MOTHERBOARD MUST BE INSTALLED WHEN USING THE CRYSTAL OSCILLATOR PCB.

2.1.2.1.2 Frequency Synthesizer

The Frequency Synthesizer consists of two printed circuit boards (PCB’s) and is capable of stepping in 9 or 10 KHz steps. Its output varies from 0.5 to 1.8 MHz and its internal operating frequency is at 8 times the carrier frequency.

The Frequency Synthesizer Analog PCB (B112250) contains all the analog circuitry. This includes the synthesizer integrated circuit (MC145159) incorporating a dual CMOS high stability crystal oscillator, the voltage controlled oscillator (MC1648), the phase lock loop filter and amplifiers (LF353). The synthesizer is locked to a 5.04 MHz high stability oscillator. The Synthesizer IC receives its programming 32-bit data word from the Frequency Synthesizer Digital PCB on the DATA line. The data is clocked into the Synthesizer with the CLOCK line and the SYNENAB lines. These are initiated only during the initial power up of the transmitter. The output of the Synthesizer is at 8 times the carrier frequency and is translated from ECL to TTL through a 2N5771 PNP transistor before going to the Digital PCB.

The Frequency Synthesizer IC outputs a signal that indicates whether the phase locked loop is locked or not. This line controls the LOCKENAB line that is used to enable the power amplifiers when the synthesizer is working properly. The lock line also is used to illuminate the front panel LOCK LED.

The Analog PCB contains a relay for automatically inserting one or two inductors for the voltage controlled oscillator's high-Q tank circuit. The inductor that is utilized depends on whether the carrier is above 960KHz (864 kHz for 9 kHz). For the low frequencies a 2.7 micro-Henry inductor is used and in the high frequency part of the band, a 1.5 micro-Henry is switched in parallel with the 2.7 micro-Henry. This switching is done automatically with a relay controlled by the programming lines on the Digital PCB. The two PCB’s are connected through a non-polarized jumper soldered to the Digital PCB (JB2201) and connected to the Analog PCB at JB2251.

The 5.04 MHz reference oscillator is buffered through a 2N2222 NPN transistor and sent to the Digital PCB.

The Synthesizer Digital PCB (B112200) contains the TTL logic required to program the Synthesizer IC. The Synthesizer IC requires a 32-bit serial data word to initialize its programmable divider circuits. This data word must be clocked into the Synthesizer IC in a very precise timing sequence set up by the CLOCK and SYNENAB lines. These signals are generated from the 5.04 MHz reference oscillator. The signal is divided and conditioned by the 74HC4040, 74LS14, and 74LS221 IC's.

The Synthesizer IC reference divider is programmed through the R13 (MSB) to R0 (LSB) jumpers on the 74LS165 shift registers. This sets up the 80 kHz or 72 kHz synthesizer channel spacing required for 10 kHz or 9 kHz carrier channel spacing. The output divider circuits are programmed through the N9 (MSB) to N0 (LSB) jumpers on the 74LS165 shift registers. Changing the synthesizer output frequency is easily accomplished with the programming jumpers next to the shift registers on the edge of Digital PCB. If the carrier frequency is above 960 kHz (or 864 kHz for 9 kHz steps) the 1.5 micro-Henry inductor must be placed in parallel with the 2.7 micro-Henry voltage controlled oscillator tank circuit on the Analog PCB with the jumper or relay next to the voltage controlled oscillator. Refer to the Frequency Synthesizer Programming Chart in Table 1 for jumper locations for a given output frequency.

The Digital PCB contains a 74LS190 programmable divider and some 74LS74 flip-flops to divide the 5.04 MHz reference oscillator signal to 70 kHz. This 70 kHz square wave is used to generate the PDM triangle wave on the PDM Exciter PCB.

The Digital PCB divides and conditions the 8-times carrier Synthesizer output to generate an in-phase (QSIGNAL) and out-of-phase (QBARSIG) for driving the Power Amplifier MOSFET's at the desired carrier frequency. Due to a difference in turn-on and turn-off times, the MOSFET's require an approximately 30% duty cycle. This is generated in the digital delay generators (AD9501). QSIGNAL and QBARSIG are buffered through 74LS04's before being routed to the Power Amplifier Panels.

The Synthesizer can be programmed for 9 or 10 kHz steps by changing the Reference divider frequency section of the 32 bit programming word. This is R13 to R0 on the S2201 and S2201 digital switches on the Digital PCB. For 10 kHz spacing the programming word should be 00000000111111. To enable 9 kHz steps the R13 to R0 should be 00000001000110.

LOCKENAB and TXINHIBIT are monitored on the Digital PCB. If there is a problem with the synthesizer or a VSWR condition, the generation of QSIGNAL and QBARSIG is inhibited and the PAENABLE line is disabled.

2.1.2.2 PDM Exciter

The PDM (Pulse Duration Modulation) Exciter circuits convert the analog audio signal to a digital PDM waveform. This is accomplished by comparing the output of the sampling frequency triangle wave with the buffered and filtered audio. The output of the high-speed comparator is the PDM signal and is buffered and sent to the high level modulator MOSFET on the 500 watt Power Amplifiers.

The audio is conditioned through RF chokes, bypass capacitors, transient protector diodes and a variable gain instrumentation amplifier (LF353). The gain of the amplifier is controlled by the Audio Gain Potentiometer on this PCB. Following the instrumentation amplifier the audio signal passes through a selectable audio limiter and 2-section low pass active filter (LF353). The use of the limiter and filter is optional depending on user preferences in selecting audio performance.

Next, the audio signal is level shifted and passed through a piece-wise linear amplifier, (LF353) into the laser trimmed analog multiplier (AD534). The Piece-wise linear amplifier improves positive peak performance.

The analog multiplier is employed to multiply the audio signal with a DC voltage level proportional to the desired output power. This signal is then routed to the high speed PDM comparator (CMP05). The processed audio signal, whose range is 0-4 volts, is compared with the highly linear 0-4 volt triangle waveform to produce a TTL PDM signal. The PDM signal is gated with a 300 VDC presence detect circuit and the PAENABLE line before being sent through buffers (74LS04) to the Power Amplifier modulator MOSFET drivers. This gating prevents the operation of the power amplifiers if there is a problem (through PAENABLE) or if the 300 VDC filter capacitor bank is not fully charged.

In order to maintain constant modulation performance as power is cut back the two inputs to the PDM high speed comparator (CMP05) must have the zero voltage point of the 0-4 volt signals set very accurately (within 50 milli-volts). The adjustment point for the triangle wave is at the level shifter resistor divider just after the polystyrene capacitor. The adjustment point for the audio signal is at the offset resistor divider of the summing amp after the active filter.

The sampling frequency square wave drives a matched transistor (LM394) current mirror that charges and discharges a high quality capacitor. This produces a very linear triangle waveform. This waveform is level shifted and sent to the CMP05 high speed PDM comparator.

There is also the 300 VDC regulator circuitry that varies the PDM pulse width inversely with line voltage. A 10% line voltage variation will produce less than a 2% variation on the output power.

2.1.3 Power Amplifier

The 700 Watt Power Amplifier Modules plug into the Motherboard. Each power amplifier module includes a Modulator Metal Oxide Semiconductor Field Effect Transistor (MOSFET) and four RF Power Amplifier MOSFET's. The carrier frequency drive signals, QSIGNAL and QBARSIG, and the modulator drive signal, PDMSIG, are routed on the Power Amplifier motherboard to the power amplifier modules.

The individual power amplifier module's health is indicated by 4 LED's that are mounted on the edge of the Power Amplifier Module's printed circuit board and are visible through the Power Amplifier Panel's grill. One green LED indicates the presence of +18 volts DC. An amber LED indicates the presence of +300 volts DC. One green LED indicates the presence of RF output power (also connected to RF OUT LED’s on Front Panel). One red LED is used to indicate over-temperature or an inhibit condition.

The low (+18 volts) and high (300 volts) voltages on the Power Amplifier Modules are fused so that a failure on any module will not affect operation of the other Power Amplifier Modules. Conditioning of the +18 volt DC power supply is accomplished by an RF choke, a 24 volt high speed transient suppressing zener diode, bypass capacitors, a 15 volt regulator and two variable voltage regulators. In addition to the fuse, a 400 volt high speed transient suppressing zener diode and bypass capacitors protects the 300 volt power supply. The +18 volt power supply illuminates a green LED at the edge of the power amplifier PCB. The +300 volt power supply illuminates an amber LED on the edge of the power amplifier PCB. Both of these LED’s are visible through the front panel grill.

All drive signals are optically coupled into the power amplifier module. The RF amplifier is a full bridge topology. The floating modulator MOSFET and the two floating upper bridge MOSFET's receive their DC drive circuitry power from a DC-DC converter operating at approximately 12 kHz. It is normal to be able to hear the DC-DC converter oscillating when standing in front of the transmitter.

The two bottom bridge MOSFET's receive their drive circuitry power individually from three pin variable voltage regulators (LM317) whose voltage is preset at the factory through the resistor divider on the voltage adjustment pin.

All the MOSFET's are driven by drivers (TC4421) capable of delivering up to 9 amps of current to quickly charge and discharge the MOSFET gate capacity. Gate series resistors are employed to dampen oscillations in the drive circuit. Pull-up diodes and pull-down resistors protect all gates. The opto-isolators (HP2400) and the drivers are heavily bypassed due to the high frequency and high currents present in the drive circuits.

An on-board RF power meter illuminates a green LED on the center of the power amplifier PCB. This is visible through the front panel grill when the power amplifier is operating normally. A voltage divider connected to the RF output drives a comparator (LM311) which illuminates the LED at approximately half power. The Power Amplifier Modules are completely digital and generally either work or don't work.

A red LED, visible through the front panel grill, illuminates when the Power Amplifier Module is in the stand-by condition. This can be caused by an over-temperature condition. The Power Amplifier Module heat sink has a thermal switch attached, which inhibits the amplifier during an over-temperature fault. Once the heat sink cools down, the Power Amplifier Module will begin normal operation again and the red LED will extinguish.

A Power Amplifier Module stand-by condition (red LED illuminated) can be caused by the micro-switch on the edge of the printed circuit card not being sufficiently engaged by the Power Amplifier Module holding clamp.

All solid state PDM transmitter suffer from loss of modulation capability at low power levels due to the finite minimum pulse width that the high speed PDM comparator is capable of generating. By inhibiting some of the power amplifier modules the remaining operational power amplifier modules are run at a much higher power level where modulation characteristics are excellent.

2.1.4 Power Supply PCB

2.1.4.1 Overview

The linear power supply conditions all voltages required by the transmitter. The power supply contains isolation transformers, a Metal Oxide Varistor (MOV), fuses, circuit breakers, rectifiers, filter capacitors and control circuitry. Front panel LED's indicate proper operating voltages and MOV status.

2.1.4.2 Transient Protection

The power supply is protected from transient surges with a 40mm MOV across the secondary of the main isolation transformer. The MOV is bypassed with a high voltage 0.05 uF ceramic capacitor for additional transient suppression. The MOV can only absorb a certain amount of energy and then it fails shorted. When this occurs, a fuse takes the MOV out of the circuit, and the front panel LED illuminates. At this point the transmitter is not surge protected and the MOV must be replaced as soon as possible. Do not hesitate to correct this condition!

The isolation transformers are manufactured to rigorous standards to protect the transmitter from high line voltage transients. The isolation transformers appear as a low pass filter when a power main high voltage transient occurs, slowing down the rising edge and absorbing energy from the impulse. MOV, zener diode transient suppressors, filter capacitors and voltage regulators efficiently absorb any transient energy that is passed through the isolation transformer. These protection circuits are in the power supply and on each printed circuit board.

The main isolation transformer provides 240VAC across its secondary. Under this condition the DC high voltage should be between 280 and 340 volts DC at full power operating conditions with the +18 volt DC positive supply between 18 and 19 volts DC. Negative DC voltage is not critical and can range from -18 to -23 volts DC.

2.1.4.3 Over Current Protection

Two power MOSFETS, along with inrush current limit resistors provide the soft start for the 300VDC supply. Initially the MOSFETS are off allowing time for the 6200uF capacitors to charge through the ten 100 ohm resistors. After 0.6 seconds, the MOSFETS turn on providing a low resistance shunt to the resistors.

Fuses are mounted on the PCB to protect the primary of the low voltage transformers, fans, and MOV.

2.1.5. Harmonic Filter / Detector

In the 1000i, the outputs of the Power Amplifiers are fed to a broadband, in-phase combiner, then to a 25 to 50 ohm low pass filter. In the 500i, the power amplifier output goes directly to a low pass filter.

A detector circuit provides the signal for the front panel modulation/power level indicator. The RF sample is fed for test purposes to the back panel.

3.1 INSTALLATION

3.1.1 General

The transmitter is installed in four steps: 1) Unpacking and inspecting the transmitter; 2) Positioning the transmitter; 3) Connecting audio, antenna, power mains, and remote control lines (if applicable); and 4) Setting up the transmitter. The installation of the transmitter should be planned ahead, taking into account the most important electrical and mechanical requirements of the equipment.

3.1.2 Unpacking and Inspection

The transmitter is shipped completely assembled and ready for installation except for the isolation transformer on some larger transmitters.

WARNING:

THE TRANSMITTER COMPONENTS MAY BE DAMAGED IF THE TRANSMITTER IS DROPPED OR SEVERELY JARRED. USE CARE IN MOVING THE TRANSMITTER. USE APPROPRIATE LIFTING AND MOVING EQUIPMENT.

Unpack and inspect the transmitter as follows:

1. Carefully remove the transmitter from the transportation vehicle and place near the final installation location.

2. Remove the shipping material from the transmitter taking care not to scratch or dent the transmitter.

3. Open the transmitter front panel and verify the exciter PCB’s and power amplifier PCB’s are firmly seated. Care should be taken during the insertion of the Control Panel PCB's into the motherboard sockets. The card sockets are keyed to only accept the correct board. Each PCB is labeled and its respective slot on the Control Motherboard is labeled.

4. Open the transmitter cover and remove all packing material within the transmitter. Remove all the packaging tape and retaining material used to protect the circuit boards and other sensitive parts within the equipment during shipment. Carefully, clean the inside of the transmitter.

5. Inspect the transmitter for loose or missing hardware.

6. Verify the 700 watt Power Amplifier Module is firmly seated. The clamps holding the modules do not need to be tightened at this point.

7. Verify all cables are installed in the rear of the transmitter and dressed neatly. Verify the grounding straps are tight and attached to each panel.

8. File any damage claims promptly with the transportation company. Retain all packing material if a claim is filed.

3.1.3 Positioning the Transmitter

The installation of the transmitter should meet the following requirements:

1. Allow at least 3 feet (1 meter) clearance at the front and rear of the transmitter for ease of servicing.

2. Ensure that the environmental conditions are within the temperature, humidity, and altitude limits listed in the mechanical specifications.

3. Verify that the transmitter installation site is clean and that the cooling air is not dusty or dirty. The transmitter can be installed in most places without air conditioning; it is recommended that an exhaust fan be used to provide air circulation. If insufficient cooling air is not provided the power amplifier modules will cycle on and off causing RF output power fluctuations.

WARNING:

HIGH VOLTAGE EQUIPMENT CAN PRODUCE FATAL INJURY.

 

Operation of this electronic equipment involves the use of high voltages, which are dangerous to life. Operating personnel must at all times observe all safety regulations. Do not make any adjustments inside the equipment with the voltage supply on. Under certain conditions dangerous potential may exist in circuits with power control in the off position due to charges retained by capacitors, etc. To avoid casualties, always discharge and ground circuits prior to touching them.

 

3.1.4 External Connections to the Transmitter

The Omni transmitters require 50 or 60 Hz, single or three phase, and the voltages specified in the electrical specification. A fused main power disconnect switch or a main power circuit breaker should be provided. Consult with local electrical codes on the proper gauge wire to be utilized.

3.1.4.1 RF Ground

For proper operation a good RF ground connection is required. Using a low inductance ground strap (4-6 inch wide copper strap) connect the transmitter ground to an external earth ground. This external ground should be connected to the antenna ground system independent of the power company ground.

3.1.4.2 Power Mains

With the power mains master disconnect circuit breaker OFF connect the power lines to the transmitter. If the transmitter is wired for single phase, connect the single phase balanced 240 volt AC power line to the cable exiting the rear of the transmitter.

3.1.4.3 RF Output Connection

High voltage and currents are present at the RF output connection of the transmitter. Please ensure that a proper connection is made at this point.

3.1.4.4 Audio Input Connections

The transmitter accepts the audio input signal at 0 dBm +10 dB from a source with a 600 ohm source impedance. The potentiometer closest to the front panel on the PDM Exciter PCB adjusts the input gain of the instrument amplifier on the PDM Exciter PCB over this range.

Use shielded twisted pair wire to connect audio source to the terminal board TB1 on the rear of the Control Panel. Programming audio is on terminals 1 and 2. Ground is terminal 3.

3.1.4.6 Output Monitoring Points

The transmitter has the following monitoring points available on the back of the Control Panel:

RF Monitor - located on the rear of the transmitter; this is an RF sample for driving a modulation monitor. This is accomplished by a tap on the harmonic filter output tank inductor and puts out approximately 5 volts RMS at full power. As power is cut back, the voltage scales down proportionally.

3.1.5 Setting up the Transmitter

The transmitter has been tuned and tested on the customer's frequency before shipment, some of the following steps may not be required; however the complete procedure is given for informational purposes.

3.1.5.1 Step 1 Line Voltage

Be sure that the circuit breaker on the front panel of the Power Supply Unit is turned off. Measure the Mains voltage. Select the proper taps on the secondary of the Power Supply Transformer according to the following chart.

Note: Select tap setting that corresponds to the max line voltage that is greater than or equal to your measured line voltage.

Max Line Voltage	Tap Setting TB2
139 / 278	1 & 2
118 / 236	1 & 3
103 / 206	1 & 4

3.1.5.2 Step 2 Cooling

Turn on the main circuit breaker. Verify that the fans are functioning properly. Verify that the filters are clean. If necessary, remove and clean the filters in a warm solution of mild soapy water. The panel fan filters must be vacuumed on a regular basis to remove dust and debris.

3.1.5.3 Step 3 Operating Frequency/Synthesizer Programming (optional)

Verify that the correct operating frequency is being generated by the Crystal Oscillator by monitoring the output Frequency. The variable capacitor, CV1, on the Crystal Oscillator board is used to make fine adjustments to the operating frequency.

If the optional Frequency Synthesizer is installed, the synthesizer frequency and carrier signal duty cycle are set by the thumbwheel switches located on the Controller front panel. Set these switches according to the programming chart (10kHz spacing or 9 kHz spacing).Verify that the oscillator stage is now operating properly, before applying the high voltage, by monitoring the frequency at the Control Panel rear panel Frequency Monitor BNC.

WARNING: Serious damage will occur if the synthesizer is set to a frequency outside of the Harmonic Filter Pass-Band or the duty cycle of the carrier drive signal is set incorrectly.

3.1.5.4 Step 4 Low Power Operation

With the transmitter operating in stand-by, tighten the Power Amplifier Module clamp against the Power Amplifier Module inhibit microswitch. The Power Amplifier Module red LED should extinguish when the clamp is properly seated against the micro-switch. Engage the High Voltage Circuit Breaker on the Control Panel. Verify the 300 volt DC is present using the front panel LED. Verify RF Power is present using the front panel LED. Verify that the High Voltage is 315 volts + 10% using a voltmeter. If not, adjust the tap on the Isolation Power Transformer.

3.1.5.5 Step 5 Modulation

Connect an audio generator to the Audio Input Terminal Board on the rear of the transmitter. Select whether operation is to include the active filter on the Motherboard PCB. This is easily selected with the jumper switches on the Motherboard PCB. Connect an oscilloscope to the RF Sample BNC connector. Set the audio generator output frequency for 1000 Hz and slowly increase the output control until some modulation is indicated on the oscilloscope.

4.1 MAINTENANCE

4.1.1 PREVENTIVE MAINTENANCE

The long-term reliability of the transmitter is almost directly proportional to the degree of preventive maintenance performed. Preventive maintenance is a systematic series of operations performed periodically on equipment to prevent early failure of equipment. Definite schedules should be established and records maintained of work actually done.

Prior to performing preventive maintenance, review the transmitter operation logbook, which should include a record of all operating parameters. Check particularly for any slow variation in any meter reading. This can be an indication of a potential problem, which may go unnoticed because the day-to-day deterioration is so slight as to be of no concern.

4.1.1.1 Mechanical Maintenance

Although a broadcast transmitter is mainly thought of in terms of its electrical and electronic characteristics, of equal importance is its mechanical integrity. Although the majority of the mechanical connections within a transmitter relate to its physical structure, of greater importance are the mechanical junctions, which are a part of the electrical or electronic circuitry.

Routinely check that all bolts, nuts and screws are tight. They only need to be secure. Over-tightening can strip threads and ultimately damage or break the fastening device.

4.1.1.2 Normal Operating Temperatures

Maintain circulation around the transmitter, obstructed filters will cause overheating and possible damage to the electronic circuits.

4.1.1.3 RF Circuits Maintenance

Particular attention should be given to the mechanical connections made to coils and capacitors in the high level RF output circuits of the Harmonic Filter. The RF currents carried by these components are relatively high. Heat generated by a poor mechanical condition will generally raise the connection temperature to a point where discoloration and/or oxidation will occur. Check RF capacitors for any sign of bulging caused by abnormally high operating temperature or possible lightning damage.

4.1.1.4 Power Transformer Maintenance

Check operating temperature immediately after shutdown. Normal losses in transformer core and windings will generate an overall temperature rise in the power transformer; however, excessive temperature indicates either an internal defect of the transformer itself or operation at an abnormal load or input voltage.

Check mechanical connections to the windings for localized heating. These carry relatively high currents. A loose connection acts as a resistor. Resistors generate heat. Check that all connections are secure.

Periodically check core securing hardware for tightness.

4.1.1.5 Filter Maintenance

The transmitter is equipped with a maintainable air intake filter located on the rear panel. The filter should be regularly checked for dust accumulation. A filter clogged with dust or dirt restricts the airflow and excessive heat can result. If the filter element is made of fiberglass, replace filter element with a new filter of the same type and size. If the filter element is made of steel mesh or plastic, remove the filter from the frame, soak filter in warm water using a mild soap or detergent. Let dry and reinstall in frame. Using compressed air is an excellent alternative to cleaning the filters and transmitter cabinet.

It is advisable that each time the air filters are cleaned or replaced the inside of the cabinet be thoroughly vacuumed to remove dust accumulation.

4.1.1.6 Fan Maintenance

Regularly inspect fan blades for dust accumulation. Remove any dust accumulation with a brush or vacuum cleaner. The fans are made with sealed ball bearings that have lifetime lubrication.

4.1.1.7 Meter Maintenance (non frequency agile transmitters)

The quality meters used are extremely delicate devices. Repair of these devices should be done by a qualified technician.

Periodically check mechanical mounting.

Clean meter faces with a soft, damp cloth.

4.1.1.8 Components

Examine for any sign of overheating of components on the PCB's. Color coding discoloration, cracking, or chipping indicates a potential problem area. A defective component or shorted solid state device on the "load" side of the resistor may cause overheating.

If discoloration has made the resistor value illegible, consult schematic diagram and replace, after solving the cause of failure, with same value and wattage shown (do not make substitutes).

Keep resistor surfaces clean, using a soft brush or dry cloth.

4.1.1.9 Switches

Check that Front Panel switches are securely fastened to prevent rotation of the overall assembly during adjustment.

Ensure that knobs are securely fastened and the knob pointer is properly oriented.

4.1.1.10 Fuse Maintenance

When a fuse blows, determine the cause before installing a replacement. Inspect fuse holder caps and fuse holder mounts for charring and corrosion.

Examine fuse holders for dirt, improper tensions and loose connections. The tension of the fuse clips may be increased by carefully pressing the clip sides together.

Replace a blown fuse with one of the same rating only. Warning: Replacing a blown fuse with one of higher ampere rating will cause damage to circuits and components.

4.1.1.11 Cabinet Interior and Exterior Maintenance

Remove any dust accumulation from the cabinet interior with a soft dry cloth.

Remove dust and dirt accumulations from the cabinet base with dry brush or cloth or vacuum cleaner brush.

Wipe down exterior cabinet surfaces.

Check for any rust or corrosion on painted surfaces. Sand surfaces to raw material and repaint. Remove dust and dirt accumulations from the internal PCB's with a dry brush.

4.1.2 GENERAL MAINTENANCE

4.1.2.1 Recommended Daily Schedule

Inspect meter readings for any abnormalities.

4.1.2.2 Recommended Weekly Schedule

Vacuum base of cabinet. Record all meter readings of the transmitter so they can be referred to when replacing parts and re-tuning of the transmitter.

4.1.2.3 Recommended Monthly Schedule

Clean intake filters. If permanent type filters (aluminum), wash and spray with special dust attractant. If fiberglass filters, remove any dust or replace if required.

Visual inspection is the most important preventive maintenance operation because it determines the necessity for others. Become thoroughly acquainted with normal operation conditions in order to recognize abnormal conditions readily. The remedy for most visible defects is obvious.

5.1 TROUBLESHOOTING

5.1.1 Overview

This section contains brief suggestions on troubleshooting to isolate faults. When the transmitter is operating properly, all green and amber LED's should be illuminated and all red LED's should be extinguished.

5.1.2 Controller Panel

The green +18 or -18 vdc LEDs are not illuminated:

1. Check the fuse F2 on the Power Supply PCB.
2. Verify the plugs are seated firmly in the Power Supply PCB and Motherboard PCB sockets.

The green Lock LED is not illuminated:

1. Verify that the Oscillator or Synthesizer is operating properly.
2. Verify that the carrier frequency is present on TP5 & 6 on the motherboard.
3. Verify that the Reference Oscillator is operating. On the Frequency Synthesizer Analog PCB it will oscillate at 5.04 MHz at TP19. On the Crystal Oscillator PCB it will oscillate at 4 or 8 (TP17) times the carrier frequency depending on the reference crystal installed.

The red Fault Trip LED is illuminated:

1. Verify that the antenna is connected to the transmitter RF output, and is a 50 ohm resistive impedance.
2. Verify the cables to the Filter/Power chassis on the 1000i are OK.

5.1.3 Power Amplifier Problems

The amber Power Amplifier Module +300 volt LED is not illuminated:

1. Check the 300 volt DC fuse F1 on the Power Amplifier Module PCB.
2. If the fuse on the module fails repeatedly it is likely that some of the MOSFET's are damaged. They can be tested with an ohmmeter. Damaged MOSFET's generally fail in the shorted mode. They will exhibit very few ohms across the Drain-Source junction. Remove and replace only the damaged MOSFET's. Generally a modulator MOSFET and two RF MOSFET's will fail.

Good MOSFET's can be detected without removing them from the circuit. Using an ohmmeter across the Drain-Source junction, the interested student, should be able to detect the MOSFET's inherent Drain-Source diode junction. The ohmmeter leads may have to be reversed to see the diode junction conduct.

The green Power Amplifier Module +18 volt LED is not illuminated:

1. Check the +18 VDC fuse F2 on the Power Amplifier Module PCB.

The middle green Power Amplifier Module LED is not illuminated:

1. This LED is an indicator of RF power. It will illuminate at approximately half power or more. Verify that the LED illuminates when the transmitter is at full rated power. When the LED will not illuminate check the fuses and the MOSFET’s as in the above paragraphs. Note: Power amps in frequency agile transmitters are operated at lower power levels to protect the amps from damage. This may keep the RF LED from lighting. Adjusting the power level up from the factory setting will cause damage to power amps.

The middle green Power Amplifier LED blinks intermittently:

1. It is common for this LED to blink on and off with modulation due to its relatively short time constant detection circuitry. This allows it to detect the RF power surge in positive peaks (and conversely the negative peaks).

The red Power Amplifier Module LED is illuminated:

1. Verify the clamping bracket is engaging the inhibit micro-switch on the lower front corner of the Power Amplifier Module PCB and that it has fully depressed the switch.
2. Verify that the over-temperature sensor on the heat sink is not open due to an over-heating condition. This sensor should be a short circuit when the heat sink is cool.

5.1.4 Power Supply Problems

The green +18 and -18 vdc LEDs are not illuminated:

1. The 18 VDC fuse F2 on the power supply board is open.

The green 300 VDC LED on the Power Supply is not illuminated:

1. Check the high voltage circuit breaker CB1.

The red MOV LED is illuminated:

1. Check the MOV fuse F3 on the Power Supply PCB. If the fuse is blown the MOV could be bad. With the fuse removed, using an ohmmeter, check across the MOV. You should measure the conducting diode junction of D3 in one direction and by reversing the meter leads, the reverse biased diode junction (open-circuit). If a short circuit appears in both directions, the MOV needs to be replaced. MOV's fail shorted and the fuse removes the MOV "off-line" to prevent damage to the transmitter. Replace these components immediately as with out them the surge suppression capacity of the transmitter is seriously compromised.

5.1.5 General Transmitter Problems

CW operation is fine but Fault trips with modulation

This condition usually results from the antenna not being properly tuned to an impedance of 50 +J0 ohms.

1. Verify the transmitter operates into a Dummy load with full modulation.
2. Bridge the antenna and related components to verify impedance. Tune if necessary.

Transmitter does not generate rated power

1. Verify that the 300 volt DC is not dropping below 280 volts DC. Check taps on the High Voltage transformer and if necessary increase voltage to be between 300 and 330 volts DC.

Output power fluctuates

1. Most often the air filters not being properly cleaned cause this. The red LED on the Power Amplifier Module will illuminate and then extinguish when the temperature returns to normal. To correct this situation, remove and clean the cabinet air filter and vacuum the air filters on the back power amplifier panel.
2. Verify that the Power Amplifier Module micro-switch is properly engaged. If necessary adjust contact pressure with the Power Amplifier Module holding clamps.
3. Power Main feed fluctuating more than +10%, can cause RF power fluctuations. Monitor incoming voltage to the transmitter and change isolation transformer taps as necessary.

No RF output power

1. The Power Amplifier Module PDM may not be enabled. Check that the FAULT LED on the front of the transmitter is not illuminated.

6.1 PARTS LIST

Replacement parts are available from LPB. Telephone to contact the Service and Parts Department, or address correspondence to:

LPB Communications, Inc. Telephone: (610) 825-4100
960 Brook Road Unit #5 FAX: (610) 825-4047
Conshohocken, PA 19428

6.1.1 Ordering Parts

If replacement parts are not available locally contact LPB Communications, Inc. When ordering parts be sure to specify the circuit reference designator and part description in full. It is advisable to obtain minimum order information from the Omni Component Sales Department before ordering parts.

7.1 DRAWINGS AND ILLUSTRATIONS

8.1 ASSEMBLY DRAWINGS