Broadcast Equipment - BW-75A FM Monitor

Section 1: General Information

General Description

The RCA Type BW-75A Frequency and Modulation Monitor is a wideband, all-solid-state FM monitor designed to meet FCC requirements for measuring the center frequency and total modulation characteristics of monaural and multiplexed FM transmitters (88-108 MHz). It can also serve as a low-distortion, low-noise FM demodulator for driving audio monitor amplifiers and companion units like the RCA Type BW-85A Stereo Monitor and BW-95A SCA Monitor. The BW-75A features a deviation type modulation calibrator to ensure accurate modulation readings.

Physical Description

The BW-75A is constructed for standard 5½ x 19 inch rack mounting. Less frequently used controls and test points are located under a hinged front cover. Factory adjustments are housed within shielded compartments. The AC power input, RF input, and monitor outputs are situated on the rear panel via individual connectors or terminal blocks. The unit is entirely solid-state, utilizing silicon transistors for longevity, with circuits built on three military-grade, glass-epoxy, plated printed circuit boards. High-reliability industrial and military-grade components are used throughout.

Electrical Description

The BW-75A operates as a low-sensitivity, crystal-controlled, superheterodyne FM receiver. It incorporates a linear and stable pulse-counting discriminator to measure center frequency deviation and demodulate FM transmissions. The monitor includes various metering and testing provisions, such as a center frequency deviation meter, a peak reading total modulation meter (switchable for positive or negative polarity), and a peak modulation light that responds to both polarities. Metering circuits are present for setting RF input levels and standardizing the deviation meter with a calibrating oscillator. Outputs include a monaural output for monitoring, a distortion meter test output, a wideband test output, four wideband outputs for multiplex demodulators, an AM noise diode output, and an incidental AM noise diode output. FCC Type Approved remote metering is available for the deviation meter, modulation meter, and peak light.

Electrical Specifications

Mechanical Specifications

Dimensions: 5½ x 19 x 11 7/8 inches overall.

Net Weight: 15 pounds.

Shipping Weight: 19 pounds.

Instrument Identification

The instrument is identified by its type number and a six-digit serial number, both found on a plate on the rear panel. All correspondence should reference these identifiers.

Accessories

The BW-75A can be used with the Remote Meter Panel (MI-560736) or the BW-100B RF Amplifier (MI-560738). The Meter Panel includes a peak indicator lamp, deviation meter, and modulation meter. The RF amplifier provides pre-amplification and selectivity for off-air monitoring.

Section 2: Installation

Initial Inspection

Upon delivery, inspect the shipping carton for external damage. Unpack the unit carefully to avoid damage. Inspect all equipment for physical damage immediately after unpacking. Note any bent or broken parts, dents, or scratches. Refer to claim procedures if damage is found. Retain all packing materials for potential damage claims.

Claims

If concealed damage is found, notify the carrier in writing immediately and request an inspection report. File a claim for the damage, saving all shipping papers, letters, and invoices.

Repacking for Shipments

If the unit needs to be returned to RCA, contact the RCA representative for shipping instructions. Attach a tag with owner and address details, and a description of the required service. Use the original shipping carton if available, or repack using a double-walled carton (minimum 275 lb test strength), heavy paper/cardboard for surface protection, at least 4 inches of shock-absorbing material, heavy-duty shipping tape, and large "FRAGILE" labels. Return via air freight where possible, ensuring insurance for full value.

Preparation for Use

The BW-75A is designed for standard 19-inch rack mounting. Ensure adequate air space above and below the unit for heat dissipation. If mounted above high-heat equipment, consider cooling requirements to allow free air movement. Avoid ambient chassis temperatures above 50°C (122°F). Mount the BW-75A to the rack panel using four No. 10 screws and countersunk washers.

The unit requires a 105-125 VAC, single-phase, 50-60 Hz power source. Connect the three-wire grounded line cord to TB-1, with the ground wire to terminal 2 and AC line wires to terminals 1 and 3.

Connect a 50-ohm coaxial cable (RG-58) from the transmitter's monitor probe or RF amplifier to the RF input connector J7 on the rear chassis.

CAUTION: DO NOT APPLY MORE THAN 10 VOLTS RF TO THE MONITOR OR THE RF INPUT LEVEL CONTROL MAY BE DAMAGED.

External aural monitoring amplifiers can be connected to TB2 terminals 1 and 2 (unbalanced 600-ohm output). Remote center frequency deviation meters and modulation meters can be connected to TB2 terminals 7, 8 and 5, 6 respectively, observing polarity (terminals 6 and 7 are positive). The external loop resistance (excluding meters) must be 5000 ohms. These meters must be RCA-supplied to comply with FCC regulations. A remote peak modulation lamp can be connected to terminals 9 and 10. CAUTION: DO NOT SHORT TERMINAL 9 TO GROUND. Remote meters and lamps are available in the Remote Meter Panel (MI-560736).

Emergency First Aid Instructions

WARNING: DANGEROUS VOLTAGES ARE PRESENT. OPERATING PERSONNEL MUST OBSERVE ALL SAFETY REGULATIONS. NEVER CHANGE TUBES OR MAKE ADJUSTMENTS INSIDE THE EQUIPMENT WITH VOLTAGES APPLIED. DISCHARGE AND GROUND CIRCUITS BEFORE TOUCHING THEM.

Rescue Breathing Procedure:

• Start Immediately: Get air into the victim's lungs.
• Clear Mouth: Remove any mucus, food, or foreign matter.
• Loosen Clothing & Keep Warm: Keep the victim quiet and warm. Treat for shock if necessary.
• Don't Give Up: Continue until the victim breathes independently or medical advice dictates otherwise.
• Call a Physician: Summon medical aid for respiratory and other potential issues.

Burns:

• Reddened Skin: Apply ice-cold water, cover with a clean sheet/cloth, and consult a physician.
• Blistered or Flesh Charred: Apply ice-cold water, cover with a clean sheet/cloth, treat for shock, and transport to a hospital.
• Extensive Burn (Skin Broken): Cover with a clean sheet/cloth, treat for shock, and transport to a hospital.

Section 3: Operation

Initial Operation

Follow these steps for initial operation:

1. Set the FUNCTION switch to AMP BAL and the REMOTE METER switch to OFF [released].
2. Turn the POWER switch ON [depressed] and allow a 15-minute warm-up.
3. Depress the OSC switch; the modulation meter should read approximately 100% or more.
4. Turn the RF level control fully counterclockwise. Apply RF input to the RF input jack. Depress the RF switch and adjust the RF level control until the modulation meter reads 100%. The monitor operates with as little as 20% RF input, but 100% is used for AM noise calibration.
5. With the FUNCTION switch in AMP BAL, adjust the AMP BAL POTENTIOMETER for center zero on the carrier deviation meter and the MOD ZERO POTENTIOMETER for zero on the modulation meter.
6. With the FUNCTION switch in FREQ, adjust the FREQ CAL POTENTIOMETER for center zero on the carrier deviation meter.
7. Depress the MOD CAL switch; the modulation meter should read 100% to verify calibration accuracy.
8. Depress the OP switch to operate. The monitor is now ready.

Normal Operation

For normal operation, leave the BW-75A in the OP [operate] position. RF level changes do not affect accuracy. The FREQ CAL POTENTIOMETER may require occasional adjustment as per step 3-1-6. The PEAK MOD POTENTIOMETER is typically set to 100% so the PEAK LIGHT flashes at a 100% modulation peak. This light responds to both positive and negative peaks, independent of modulation polarity.

When the MOD CAL switch is depressed, the calibrating signal is applied to the metering circuit, peak light, and monitoring amplifier, producing an audible 60 Hz tone.

Local Oscillator Frequency Calibration

The BW-75A is factory-adjusted to the customer's frequency. For recalibration against an external standard, first set the transmitter to its assigned frequency. Measure the transmitter frequency as described in Operation, Paragraph 3-1. If the indicated center frequency deviation is not zero, adjust the local oscillator frequency using the crystal tuning capacitor C5 (accessible via the front panel) to achieve zero deviation.

Transmitter Measurements

The BW-75A facilitates normal transmitter proof-of-performance measurements. Frequency response, distortion, and noise can be measured via the front panel AUDIO TEST jack J8, which provides 5 volts RMS at 100% modulation. Measurements can be made with or without de-emphasis using switch S-3. The modulation meter has a flat frequency response; for pre-emphasized transmitters, the audio input level must be adjusted according to the standard 75µ sec pre-emphasis curve.

AM noise measurements can be made at J6 on the back panel. At 100% RF level, the normal output of J6 for 100% AM is 0 dB (0.78 volts) into a high impedance (500K or greater). A standard AC voltmeter can be used for these measurements.

Section 4: Principles of Operation

Block Diagram Description

Figure 4-1 provides a simplified block diagram of the BW-75A. The incoming RF sample is mixed with the tripled local oscillator frequency (650 kHz IF) and fed to the pulse-counter discriminator assembly (Card A2). The limiter stage removes AM components, and the Schmitt trigger squares the signal. A monostable multivibrator generates pulses whose spacing corresponds to the incoming frequency. This output is integrated by a low-pass filter to produce a signal proportional to the FM modulation. This signal is then amplified and processed by the modulation metering and peak flasher circuits (Card A3). The modulation metering includes a phase splitter, feedback amplifier, and peak diode detector. The flasher circuit uses an amplifier, phase splitter, Schmitt comparator, and monostable multivibrator to activate the peak lamp. The monitoring amplifier is a three-stage feedback amplifier providing low-distortion audio output. Power supply voltages are regulated.

Detailed Circuit Description

Oscillator, Tripler and Mixer Circuit (A1 Card): The RF input is attenuated and fed to the mixer. Diodes prevent overload. A modified Clapp-Gouriet crystal oscillator, stabilized by an oven, provides the base frequency. This is tripled and mixed to produce the 650 kHz IF signal.

Oscillator, Counter Discriminator and Amplifier Circuits (A2 Card): The 650 kHz IF signal is limited, squared by a Schmitt trigger, and fed to a monostable multivibrator. This multivibrator's pulse width is controlled by a constant current source, providing stability and linearity for frequency measurements. The output is integrated by a low-pass filter to yield a DC signal proportional to frequency, which is then compared with a reference voltage by a differential amplifier. Modulation calibration involves gating the 650 kHz signal. The unit includes 15V and 25V regulated power supplies.

Amplifier, Metering and Flasher Circuits (A3 Card): This section details the modulation metering circuit, which uses a phase splitter, feedback amplifier, and peak diode detector. The peak flasher circuit drives an indicator lamp. The monitoring amplifier is a three-stage feedback amplifier for aural monitoring and audio tests, with de-emphasis controlled by its frequency response. The main power supply uses a full-wave bridge rectifier and a voltage regulator.

Section 5: Maintenance

Introduction

This section provides information on performance checks, adjustments, calibration procedures, and troubleshooting techniques for the BW-75A.

Performance Checks

The BW-75A is partially self-checking. Key performance checks include:

• Power and Oven Check: Verify the oven light cycles correctly and the power light indicates operation.
• Oscillator Level Check: Depress the OSC switch and check the modulation meter reading.
• Frequency Meter Amplifier Check: Depress AMP BAL switch and verify the frequency meter pointer goes off scale.
• Freq Meter Discriminator Check: Depress FREQ CAL switch and verify the frequency meter pointer goes off scale.
• Mod Meter Amplifier Zero Check: Depress AMP BAL switch and adjust MOD ZERO POTENTIOMETER for zero reading.
• Modulation Calibrator Check: Depress MOD CAL switch and verify the modulation meter reads 100%.
• Remote Meter Switch Check: Verify remote meter readings remain unchanged when the REMOTE METER switch is operated.
• Percent Mod Potentiometer Check: Adjust PERCENT MODULATION potentiometer to just turn on the PEAK MODULATION light at 100% modulation.
• Modulation Polarity Check: Verify the modulation meter reads 100% for both positive and negative polarities.
• Carrier Level Check: Apply RF carrier and adjust RF level control to indicate 100% on the modulation meter.
• Operate Check: Depress the OPERATE switch and verify frequency deviation and modulation indication.
• De-Emphasis Switch Check: Verify a 17 dB output level change when the DE-EMPHASIS switch is operated.

Adjustments, Calibration and Troubleshooting

This guide provides troubleshooting steps for common issues:

• Power and Oven: Checks for fuse issues, oven heater continuity, thermostat function, and power light circuit.
• Oscillator Level: Troubleshooting for transistor malfunction or defective crystal.
• Frequency Meter Amplifier: Addresses issues with meter scale readings, saturation, and remote meter line faults.
• Freq Meter Discriminator: Guides on adjusting potentiometers, checking waveforms, and diagnosing issues with the calibrating oscillator or diode switch.
• Mod Meter Amplifier Zero: Troubleshooting for meter zeroing issues and bias adjustments.
• Modulation Calibrator: Checks for errors in modulation meter readings and potential issues with the 15V regulator.
• Remote Meter Switch: Verifies proper operation of remote meters.
• Percent Mod Potentiometer: Instructions for adjusting the knob and checking the span.
• Modulation Polarity: Guidance on adjusting for equal readings and troubleshooting noise issues.
• Carrier Level: Checks for issues with diode A1CR3 if no RF level is observed.
• Operate: Troubleshooting for failures in the OPERATE position, checking diode switch A2CR1, oscillator peaking, oven cycling, and transistors A1Q2/A1Q3.
• De-Emphasis Switch: Checks for capacitor issues related to the de-emphasis function.

Section 6: Replaceable Parts

Introduction

This section lists parts for ordering, including their reference designations, descriptions, and RCA stock numbers.

Ordering Information

To order parts from RCA, provide the unit's type number and serial number, a description of the part (including reference designation and location), and the RCA Stock or Master Item (MI) Number.

Reference Designators and Abbreviations

Reference Designators: A=assembly, C=capacitor, CR=diode, DS=device signaling (lamp), F=fuse, FL=filter, J=jack, L=inductor, M=meter, P=plug, Q=transistor, R=resistor, S=switch, T=transformer, TB=terminal board, W=cable, X=oven, Y=crystal.

Abbreviations: CER=ceramic, COMP=composition, CONN=connector, ELECT=electrolytic, FXD=fixed, GE=germanium, K=kilo (1000), MEG=1,000,000, METFLM=metal film, MY=mylar, PC=printed circuits, PIV=peak inverse voltage, POLY=polystyrene, PORC=porcelain, POT=potentiometer, SEMICON=semiconductor, SI=silicon, U=micro, VDCW=dc working volts, W=watts, WW=wirewound.

Parts Lists

The manual includes detailed parts lists for the Main Chassis and various circuit cards (A1, A2, A3), specifying component type, stock number, and description. Key components listed include capacitors, diodes, lamps, fuses, meters, transistors, resistors, switches, transformers, and crystals.

Diagram Descriptions

Figure 1-1: Shows a front view of the RCA BW-75A FM Monitor.

Figure 1-2: Illustrates the BW-75A front panel with controls and indicators.

Figure 1-3: Depicts the rear panel connectors and terminal blocks of the BW-75A.

Figure 1-4: Provides side and front views of the BW-75A, showing its dimensions.

Figure 2-1: Shows the connector layout for the FMM-1 (BW-75A) rear panel.

Figure 3-1: Identifies the front panel controls and indicators of the BW-75A.

Figure 4-1: A block diagram illustrating the signal flow and main functional blocks within the BW-75A.

Figure 4-2: Displays various waveforms generated or processed by the BW-75A circuits.

Figure 5-1: Shows the component layout for the A1 Card (Oscillator, Tripler, Mixer).

Figure 5-2: Shows the component layout for the A2 Card (Oscillator, Counter Discriminator, Amplifier).

Figure 5-3: Shows the component layout for the A3 Card (Amplifier, Metering, Flasher).

Figure 5-4: An internal view of the unit, showing the top side of the chassis with components.

Figure 5-5: An internal view of the unit, showing the bottom side of the chassis with components.

Figure 7-1: A schematic diagram of the Oscillator, Tripler, and Mixer Card A1.

Figure 7-2: A schematic diagram of the Oscillator, Counter, and Amplifier Card A2.

Figure 7-3: A schematic diagram of the Amplifier, Metering, and Flasher Card A3.

For detailed component listings and troubleshooting, refer to Sections 5 and 6.

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