RCA BTS-101A Stereo Generator

Safety Information

WARNING: DANGEROUS VOLTAGES ARE PRESENT IN THIS ELECTRONIC EQUIPMENT. OPERATING PERSONNEL MUST OBSERVE ALL SAFETY REGULATIONS. DO NOT CHANGE TUBES OR MAKE ADJUSTMENTS INSIDE THE EQUIPMENT WITH VOLTAGES APPLIED. DANGEROUS CONDITIONS MAY EXIST IN CIRCUITS WITH POWER CONTROLS IN THE OFF POSITION DUE TO CHARGES RETAINED BY CAPACITORS. ALWAYS DISCHARGE AND GROUND CIRCUITS PRIOR TO TOUCHING THEM TO AVOID PERSONAL INJURY OR LOSS OF LIFE.

Emergency First Aid Instructions

Personnel engaged in the installation, operation, or maintenance of this equipment are urged to be prepared to give adequate Emergency First Aid.

Rescue Breathing

1. Find out if the person is breathing. Feel for air on your cheek, look for chest movement. If not breathing, place the person flat on their back.
2. If not breathing, open the airway by tilting the head backward. Lift the neck with one hand and push down on the forehead with the other. This may allow the person to breathe again.
3. If still not breathing, begin rescue breathing. Keep the head tilted backward, pinch the nose shut, and blow tightly into the mouth once every five seconds. Do not stop until help arrives.

General Care: Loosen clothing, keep the victim warm. Treat for shock if necessary.

Burns

• Skin Reddened: Apply ice-cold water to the burned area. Cover with a clean sheet or cloth. Consult a physician.
• Skin Blistered or Flesh Charred: Apply ice-cold water. Cover with a clean sheet or cloth. Treat for shock and take to a hospital.
• Extensive Burn - Skin Broken: Cover with a clean sheet or cloth. Treat for shock and take to a hospital.

Safety Precautions

This equipment is designed to safeguard personnel from operating hazards. Labels and caution notices point out potential hazards. Any module or Printed Wiring Board may have hazardous voltages exposed. Follow recommended procedures for care and maintenance. Always replace protective covers after servicing.

Warranty and Shipping Information

Warranty Items

Particular parts and/or equipment covered by warranty are specifically stated as such in the warranty or contract. To obtain a replacement, contact your local RCA sales office and supply Product Identification (Invoice Number, Ml Number, Type Number, Model Number, Serial Number) and Replacement Part Identification (Stock Number, Description). Failure to supply all information may cause delays.

Equipment Lost or Damaged in Transit

Inspect containers for visible damage and count containers against shipping papers before signing the carrier's receipt. If damage or shortage is noted, make a notation on the shipping papers. Unpack and inspect thoroughly for concealed damage immediately after receipt. Notify the carrier in writing and secure an inspection report within 15 days. Report all shortages and damages to RCA, Communication Systems Division, Camden, New Jersey 08102. RCA will file claims if an inspection report is obtained.

Field Engineering Service

RCA Field Engineering Service is available at current rates. Requests may be addressed to your RCA Broadcast Field Representative or the RCA Service Company, Incorporated, Broadcast Service Division, Camden, New Jersey 08102. Telephone: (609) 338-3434.

Tech Alert

Emergency 24-hour telephone consultation service for technical problems is available. Call TECH ALERT at (609) 338-3434. Western Union telex number is 83-4450.

Technical Summary

Performance

Electrical

Mechanical

Note 1: The BTS-101A will meet performance specifications with up to 10 feet of RG-58/U cable feeding into a load impedance of 5000 ohms or greater; and will meet FCC rules with up to 50 feet of RG-58/U cable.

List of Equipment

• BTS-101A Stereo Generator, less two Audio Lowpass Filters (MI-561061A)
• Plug-in 15 kHz Lowpass Filters (two) (MI-561064)
• Complete set of connector plugs for the BTS-101A Stereo Generator (required only when Interface Panel MI-561071 and/or Interconnecting Cable MI-561067 are not used)
• Set of chassis slides for servicing (optional) (MI-561069)
• Drilling Template for mounting units (optional) (MI-561073)

Recommended Test Equipment

• Precision AC Voltmeter (e.g., H.P. 400FL or equivalent)
• Audio Oscillator (e.g., H.P. 200CD or equivalent)
• Sweep Generator (e.g., Wavetec #180 or equivalent)
• Precision Differential Amplifier (See Figure 14)
• DC Voltmeter (e.g., Simpson 260 or equivalent)
• De-emphasis Network (See Figure 15)
• Oscilloscope (e.g., Tektronix Type 531 with Type H plug-in or equivalent; scope requirements detailed in Separation Adjustment procedure)
• Stereo FM Monitor (e.g., BW-85A (BW-185) fitted with external input attenuator)

General Description

The BTS-101A is a completely self-contained stereo generator that requires no parent equipment to generate the composite stereophonic signal. Although designed for the RCA BTE-115 FM Exciter, it can be used in any application requiring a high-quality stereo signal. It can be tested and adjusted independently. Program audio signals are applied to the input, processed through pre-emphasis and a 15 kHz low-pass filter (both switchable), and then fed to a 38 kHz electronic switch. The 38 kHz switching signal and 19 kHz pilot tones are derived from a stable 3.04 MHz crystal oscillator. The output is filtered, amplified to 3.5 volts peak-to-peak, and sent to the exciter. The unit also provides audio signal samples for metering systems. Remote mode switching is available via rear connector connections.

Installation

General

This procedure covers installation of the BTS-101A Stereo Generator for use with RCA transmitters (BTF-3ES1, BTF-5ES2, BTF-5ES1, BTF-10ES1, BTF-20ES1) and the BTE-115 Exciter. It can generally be applied to other FM exciters and transmitters. Ensure mounting holes are pre-drilled as per transmitter instructions.

Mounting the Interconnect Cable

An interconnecting cable assembly (MI-561067) may be supplied. Mount this cable along the inside edge of the front left-hand mounting rail in the transmitter. Position the cable so the 24-pin connector extends toward the bottom and breakout legs are near associated unit mounting areas. Connect the cable to existing transmitter wiring. For BTF-3/5 transmitters, follow specific instruction book figures. If optional chassis slides (MI-561073) are used, the same holes are used for mounting slides and cable ties.

Connections to the Main Cable Connector

The 24-pin plug on the interconnect cable provides audio input and remote control. Connections vary slightly between BTF-3/5 and BTF-5/10/20 transmitters. Specific pin assignments for Left Audio Input, Right Audio Input, Remote Stereo On, Remote Mono On, and Remote Common are detailed. Operational notes explain how momentary ground connections to specific pins switch between stereo and monaural modes. For BTF-5/10/20 transmitters, existing cable connectors may need to be removed and reconnected to the 24-pin plug.

Stereo Generator in Other Applications

When used independently or with other equipment, the BTS-101A requires an interconnecting cable (MI-561069) and appropriate connectors. Pin assignments for Left Audio Input High/Low, Right Audio Input High/Low, Remote Stereo On, Remote Mono On, and Common are provided. Switching can be accomplished by a switch, pushbutton, relay, or NPN transistor.

Final Mounting

Mount the Interface/Radiate Panel, Exciter Unit, and Stereo Generator according to specific instructions, ensuring cables do not interfere with servicing. Fill unused spaces with blank panels.

Interface/Radiate Panel

Mount the panel and connect the 4-pin plug and AC twist-lock connector. Route cables to avoid interference.

Exciter Unit

Connect the umbilical cord from the interface panel to the exciter's control connector. Connect the 10-pin plug from the interconnect cable to the exciter's input connector. Connect the RF connector from the interconnect cable to the composite input. Connect RF output and AC power cables. Mount the exciter.

Stereo Generator

Connect the 12-pin connector from the interconnect cable to the stereo generator's INPUT connector J101, and the RF connector to OUTPUT jack J102. Plug the accord into the interface panel and mount the stereo generator.

Routine Operation

Frequent readjustment is not normally necessary. The primary operation is manual or remote mode switching between stereophonic and monaural modes. Switching is achieved by a momentary connection to ground, which can be activated by a switch, pushbutton, relay, or NPN transistor. The selected mode is indicated by front-panel LED lamps. The stereo pilot should be maintained at 9% modulation, adjusted via the front-panel OUTPUT LEVEL control. If this control is at an extreme, the internal PILOT LEVEL control (R46) may need adjustment, or audio limiter outputs may require resetting. The normal output level is 3.5 volts peak-to-peak for 100% modulation with a +10 dBm input level. Separation control settings should be checked during routine proof of performance tests, which may require removing program material from the channel under test.

Detailed Circuit Description

Left and right audio signals are applied to input connector J101. The left channel connects to pins 3 and 6, and the right channel to pins 1 and 4. Shields are connected to ground via pins 2, 5, 8, and 11. Audio signals pass through RF filters (L101-L104, C101-C104) to input pads and transformers T102 and T103. The left channel input is terminated by R5 and R6, with gain controlled by R7, restricted by R5 and R8. The output of R7 feeds R9 and C1, forming an on-board RF filter. Amplifier U1 amplifies this signal, with C4 providing a slight phase lag at 15 kHz. The output of U1 feeds a phase-linear network (R12, R13, C6) for square wave transmission, with C5 compensating for high-frequency loss. The signal then goes to amplifier U2, functioning as an active pre-emphasis network. The pre-emphasis time constant is determined by R15 and C12/C13, allowing selection of 0, 25, 50, or 75 µsec time constants via switches. R15 adjusts for correct frequency response. The output of U2 is filtered by FL1 (audio lowpass filter) and then fed to amplifier U7. Capacitor C14 and resistor R20 limit the slew rate to 60 kHz. The right audio channel operates similarly, with adjustments for precise phase and amplitude control. A 15 kHz linear crosstalk adjustment (R28) and a 1500 Hz linear crosstalk adjustment (R37) are present. A 15 kHz audio lowpass filter (FL2) is used. The outputs of U3 and U6 are applied to the electronic switch U9 via de-blocking capacitors. Diodes CR5 and CR6 protect against transients. The left signal is switched at pin 2 by a 38 kHz square wave from U15 (pin 1), and the right signal at pin 7 by an out-of-phase 38 kHz square wave (pin 8). The combined output feeds summing amplifier U10. Capacitor C22 limits the slew rate. Separation correction signals from R101 and R102 are also applied to U10. The pilot signal originates from a 3.04 MHz crystal oscillator (Q1, Y1), trimmed by C54. This signal is divided down to 304 kHz (U13), then 76 kHz (U14), and finally to 38 kHz pulses (U15) for the switch. A 19 kHz signal is also supplied by U15, buffered by U12A and Q3/Q4, and fed to the pilot filter (L1, C58). PILOT LEVEL control R46 provides the pilot signal amplitude, applied to U10 via R45. The gain of U10 is set by STEREO GAIN control R61. The output feeds the 53 kHz lowpass filter FL3. Signal input to the output buffer amplifier U11 is determined by relay K1, selecting either the mono input (pin M) or the stereo signal. The mono source is selectable between left and right channels. Relay K1 is controlled by transistor Q5, part of a latching circuit for mode switching. The power supply is conventional, with a split-primary transformer, full-wave rectification, and electronic regulation.

Figure 7: BTS-101A Stereo Generator Schematic Diagram

A detailed schematic diagram illustrating the electronic components and their interconnections for the BTS-101A Stereo Generator. It shows the signal paths for audio inputs, pre-emphasis, filtering, switching, pilot tone generation, summing, output amplification, and power supply regulation.

Field Modifications

Power-Up to Monaural

The BTS-101A normally returns to stereophonic mode on power-up. To revert to monaural operation, move capacitor C60 (10 µF 20 volts) to the pair of holes immediately to the left of its original location on the PW board. This change does not affect stereo-mono switching.

Mono Source Selection

The generator is shipped to use the left channel for monaural transmission. To select the right channel, unsolder the center conductor of the shielded (L) cable from the terminal in front of relay K1 and solder the center conductor of the companion (R) shielded cable in its place. Both L and R cables must remain connected for proper channel balance. Ensure the M pin end of the unused cable does not short.

Rewiring Power Transformer for 240 V AC Input

The BTS-101A is normally wired for 120 V AC input. The split-primary power transformer T101 can be rewired for 240 V AC input by connecting its windings in series, as shown on the schematic. The transformer supports both 50 and 60 Hz. When rewired for 240 V AC, the fuse should be changed to a 1/8-ampere rating.

Alignment Procedure

Internal adjustments are factory sealed and should not normally require field adjustment. This procedure is for component replacement or other conditions necessitating realignment. The test equipment listed in the "Recommended Test Equipment" section is required. Ensure test equipment is calibrated.

Power Supply Check

1. Apply power and check DC voltages at TP6 (gray) and TP7 (white). They should be +24V and -24V, ±10%.
2. Check voltage regulators: U1 pin 7 should be +12V ±5%, U1 pin 4 should be -12V ±5%, and U15 pin 14 should be +5V ±5%.

Audio Stage Alignment

1. Connect left and right audio inputs in parallel and in phase. Apply a 2.45V RMS audio signal at 1 kHz (equivalent to +10 dBm L=R).
2. Ensure STEREO mode is selected (S101 to STEREO) and PILOT LEVEL control (R46) is fully counterclockwise.
3. Set audio amplifiers flat: switch out pre-emphasis (S1-S4 to left) and audio filters (S5, S6 up).
4. Set LEFT GAIN control (R7) for 0 dBV (0.775 V RMS) output at TP3 (green).
5. Set right channel 150 Hz linear crosstalk control (R28) for 0 dBV at TP4 (blue).
6. Reduce oscillator level until output at TP3 is -3 dBV (0.548 V RMS). Switch in left channel pre-emphasis (S1, S2 to right) and adjust LEFT PRE-EMPHASIS control (R15) until output at TP3 is 0 dBV. Switch out pre-emphasis and return oscillator to original level.
7. Switch in left channel 15 kHz audio lowpass filter FL1 (S5 down). Set oscillator to 19 kHz. Monitor TP3 with an oscilloscope: Adjust L4 for a null at 19 kHz, L6 for a null at 22 kHz, L2 for a null at 30 kHz. Recheck 19 and 22 kHz. Switch filter out (S5 up).
8. Move scope to TP4. Switch in right 15 kHz lowpass filter FL2 (S6 down). Set oscillator to 19 kHz and adjust L4 for a null at 19 kHz, L6 for a null at 22 kHz, L2 for a null at 30 kHz. Recheck 19 and 22 kHz. Switch filter out (S6 up).
9. Disconnect audio oscillator; connect sweep generator. Set scope for external horizontal input, driven by sweep generator control voltage. Sweep range: 200 Hz to 16 kHz, rate approx. 5 sweeps/sec.
10. Use a precision differential amplifier (Figure 14). Connect its inputs to TP3 and TP4, and its output to the oscilloscope's vertical input. Set scope to 1V/cm. Feed a left-only reference signal to the sweep generator, yielding 2V peak-to-peak on the scope. Feed an L=R signal to the stereo generator. Set scope to 5mV/cm. The scope displays amplitude and phase errors. Adjust to reduce errors to 5 mV peak-to-peak (1 cm on 5mV/cm setting). Confirm pre-emphasis networks are switched out (S1-S4 to left).
11. Adjust right channel 150 Hz linear crosstalk control (R28) for minimum amplitude on scope (mid/low frequencies). Adjust 15 kHz linear crosstalk (FLAT) capacitor C24 for minimum amplitude at higher frequencies.
12. Insert a de-emphasis network (Figure 15) between sweep generator and stereo generator. Switch in pre-emphasis networks (S1-S4 to right). Adjust right channel 1500 Hz linear crosstalk control (R37) for minimum amplitude at mid-frequencies. Adjust right channel 15 kHz linear crosstalk control (R35) for minimum amplitude at high frequencies.
13. Repeat steps 10 and 11 (with de-emphasis out for step 10).

Audio Filter Alignment

1. With filters and pre-emphasis switched out, adjust inductors L2, L4, L6 for a flat response from 10 Hz to 15 kHz, with a maximum deviation of ±0.5 dB.
2. Switch in pre-emphasis networks (S1-S4 to right) and remove sweep de-emphasis. Confirm filters meet specification.
3. Connect a 19 kHz sine wave signal (10 dBm) to both inputs. Monitor TP3 and TP4. Adjust inductors L2, L4, L6 for a null at 19 kHz. The null depth should be at least 40 dB.
4. Repeat step 3 for the right channel at TP4.

Separation Adjustment

1. Insert de-emphasis network and switch in stereo generator's left and right pre-emphasis networks (S1-S4 to right).
2. Set front-panel SEPARATION controls to mid-position. Set 53 kHz lowpass filter (FL3) variable capacitor (C6) to half mesh.
3. Set audio oscillator to 500 Hz at +10 dBm, feeding left channel only.
4. Set stereo generator to MONO mode and monitor composite output (J102) with a scope. Synchronize scope sweep with audio oscillator. Adjust OUTPUT LEVEL control for 3.5V peak-to-peak output.
5. Set stereo generator to STEREO mode and adjust STEREO GAIN control (R61) for 3.5V peak-to-peak output at J102.
6. Verify scope performance: monitor signal at U9 pin 2. Set scope vertical sensitivity to 5 mV/cm DC. A flat baseline (≤ 2.5 mV peak-to-peak variation) is acceptable.
7. Keep scope at 5 mV/cm DC and monitor J102. Adjust LEFT SEPARATION control for a flat baseline at 500 Hz.
8. Set oscillator to 14 kHz. Adjust variable capacitor C6 on 53 kHz lowpass filter (FL3) for a flat baseline. (Note: If FL3 fails, the entire assembly must be replaced).
9. Repeat steps 7 and 8 until baseline is flat at both 500 Hz and 14 kHz.
10. With oscillator at 500 Hz, feed a right-only signal and adjust RIGHT SEPARATION control for a flat baseline.
11. Manually sweep oscillator from 500 Hz to 15 kHz for both left-only and right-only inputs. Verify worst-case baseline amplitude is no greater than 15 mV peak-to-peak (3 cm on 5mV/cm setting), corresponding to 47 dB separation.

Pilot Adjustment

1. Feed signal to both inputs in phase (L+R signal). Set oscillator to 500 Hz at 2.45V RMS and switch stereo generator to MONO mode.
2. Verify output at J102 is 3.5V peak-to-peak. Adjust OUTPUT LEVEL control if necessary.
3. Connect output (J102) through a 5000 ohm pot to the baseband input of a BW-85A (BW-185) stereo monitor. Set monitor to TOTAL and MOD range. Adjust pot until monitor reads 100% modulation.
4. Switch stereo generator to STEREO mode and adjust STEREO GAIN control (R61) until monitor indicates 91% modulation.
5. Switch stereo monitor function to PILOT, and center PILOT LEVEL control (R46).
6. Using a plastic tuning tool, adjust PILOT PHASE control (L1) for maximum pilot level.
7. Adjust PILOT LEVEL control (R46) for a pilot amplitude of 9% on the stereo monitor.
8. While watching the Hz deviation meter, adjust PILOT FREQUENCY control (C54). Ensure frequency can be adjusted on either side of 19 kHz, then set it 0.5 Hz high.
9. Reverse signal leads to the right input (L-R signal) and monitor J102 with a scope. Adjust PILOT PHASE control (L1) for correct pilot phase (scope "bow tie" pattern).
10. Check that pilot amplitude is still 9%. Adjust PILOT LEVEL control (R46) if necessary.

Remote Functions

1. Verify metering samples (J101 pins 7 and 9) are approximately 0.8V RMS. Pin 7 is left sample, pin 9 is right sample.
2. Check external mode lines: Short pin 12 of J101 to ground for mono mode; short pin 10 of J101 for stereo mode.

Final Considerations

The stereo generator is now optimally aligned. Separation and crosstalk will exceed 45 dB for frequencies between 50 Hz and 15 kHz. No further adjustments should be performed. During final verification, use the BW-85A (BW-185) FM monitor for readings. If unfavorable readings occur, recalibrate the monitor, do not adjust the stereo generator.

Parts Ordering Information

Replacement Parts

Order parts with a Stock Number from RCA, Distributor and Special Products Division, Deptford, New Jersey 08096. Order items with a Master Item (Ml) Number from RCA, Commercial Communications Systems Division, Camden, NJ 08102. Due to product modifications or part unavailability, substitute parts may be supplied and may require mounting modifications. Substitute parts will not impair operation or performance. For information on using substitute parts, contact RCA Tech Alert, Bldg. 2-8, Camden, NJ 08102, or call (609) 338-3434.

Emergency Part Service

For emergency part service during working hours, contact RCA Distributor and Special Products Division at (609) 848-5900 or (609) 541-3636 ext. 2234 or 2235. After working hours (Eastern time), call (609) 853-0560.

Ordering Instructions

• Continental United States, Alaska, Hawaii: Order parts with STOCK NUMBER from RCA Distributor and Special Products Division, 2000 Clements Bridge Road, Deptford, NJ 08096. Order parts with MASTER ITEM (Ml) NUMBER from RCA, Commercial Communications Systems Division, Camden, NJ 08102, or nearest RCA Regional Office. Standard components with no stock or Ml number should be obtained from local distributors.
• Dominion of Canada: Order from your local RCA Sales Representative or RCA Victor Limited, 1001 Lenoir Street, Montreal, Quebec.
• Outside Continental US, Alaska, Hawaii, Canada: Order from your local RCA Sales Representative or RCA International Division, Clark, New Jersey - U.S.A. (Wire: RADIOINTER). Emergency: Cable RADIOPARTS, DEPTFORD, NJ.

Replacement Parts

BTS-101A STEREO GENERATOR Ml-561061A

Suggested Station Spares

Diagrams and Illustrations

Figure 1: BTS-101A External Views

Shows the front panel with controls for STEREO MODE, MONO, OUTPUT LEVEL, SEPARATION LEFT, SEPARATION RIGHT. It also indicates input connector J101 and composite output connector J102.

Figure 2: Stereo Generator Block Diagram

Depicts the signal flow from Left and Right Program Inputs, through pre-emphasis, low-pass filters (LPF), an electronic switch (38 kHz), summing amplifier, and output amplifier. It highlights the 3.04 MHz crystal oscillator, 19 kHz pilot tone generation, metering samples for BTE-115, and remote mode control connections.

Figure 3: Oscilloscope Waveforms

Illustrates various signal patterns observed on an oscilloscope, including composite stereo waveform, (L-R) subcarrier with pilot, (L+R) main channel with pilot, and examples of insufficient (L-R) signal, excess (L-R) signal, phase error, and correct/incorrect pilot phase.

Figure 4: BTS-101A Top View

A diagram showing the physical layout of components and adjustment points on the Printed Wiring (PW) board.

Figure 5: PW Board Switches and Adjustments

Details the location of switches (S1-S6) and variable resistors (R7, R15, R28, R35, R37, R46, R61, C6, C54) on the Printed Wiring board for controlling pre-emphasis, low-pass filters, separation, pilot level, and gain.

Figure 6: Dual Input Pad Components

Shows the configuration of components for the input signal pads.

Figure 7: BTS-101A Stereo Generator Schematic Diagram

The comprehensive circuit schematic detailing all components, interconnections, and signal paths for the stereo generator's operation.

Figure 8: Audio Lowpass Filter Schematic

A schematic diagram of the audio low-pass filter circuit.

Figure 9: Audio Lowpass Filter Components Location

Diagram indicating the physical placement of components for the audio low-pass filter.

Figure 10: 53 kHz Filter Schematic

A schematic diagram of the 53 kHz low-pass filter circuit.

Figure 11: 53 kHz Filter Components Location

Diagram indicating the physical placement of components for the 53 kHz filter.

Figure 12: BTS-101A Stereo Generator Components Location

A diagram showing the physical layout of components on the main Printed Wiring board.

Figure 13: Interconnecting Cable Schematic

Illustrates the wiring and connectors for the interconnecting cable assembly used with the BTE-115 Exciter.

Figure 14: Precision Differential Test Generator Schematic

A schematic for a test generator used in alignment procedures.

Figure 15: De-emphasis Network Schematic

A schematic diagram of a de-emphasis network used for testing and alignment.