BROADCAST AUDIO EQUIPMENT
Type BN-6B Transistor Portable Remote Amplifier
Model: MI-11221-B
Description
The Type BN-6B Amplifier, MI-11221-B, is a four-channel unit designed especially for broadcast programs remote from the studio. This unit amplifies low-level signals to a level suitable for transmission over a telephone line to the studio. Other features include complete cuing and monitoring facilities, etched circuit boards, use of transistors and germanium diodes, and operation from either batteries or a self-contained AC power supply.
The BN-6B amplifier is supplied in a rugged carrying case. The specially designed lid may be removed from the hinge pins, turned over, and replaced against the side with the mounting grommets. In this position, the lid forms a rest for the entire assembly at an angle which makes the control panel easily accessible for operation. The AC power cord is supplied clamped inside the lid of the carrying case. A weather-proof canvas carrying case, MI-11377, is also available as optional or accessory equipment.
Circuit
The BN-6B Amplifier has four separate input channels which can be operated either balanced or unbalanced. Balanced operation is accomplished by plug-in transformers MI-11776 (992074-1), which may be ordered separately. The 2N175 low-noise transistors are used in the input stages and in the following pre-amplifier stage. The driver stage is a conventional amplifier stage. No phase inversion is necessary since a complementary symmetry stage is used in the output. An isolation pad is connected to the transformer primary to protect the output transistors from damage in case of shorting the line.
The VU meter monitors both the output level and tests the battery current. A separate flashlight-type battery is necessary for VU meter illumination when the unit is operated from battery supply. Batteries are not supplied with the equipment. Lightweight, long-life mercury batteries should be ordered for the battery power supply. The battery kit MI-11778 may be ordered, which includes five PR. Mallory #TR135R (6.5 volt type) and one RCA #VS036 (1.5 volt type). The AC power supply consists of a full-wave rectifier circuit using germanium diodes and an r-c filter.
Technical Data
| Parameter | Specification |
|---|---|
| Power Required (AC) | 117 V, 50-60 cycles, 5 watt |
| Power Required (DC Supply) | Batteries MI-11778 (not supplied with MI-11221-B) |
| Fuse | 1/16 amp 3AG |
| Transistor and Diode Complement | 5 x 2N175 (RCA) Input stages and succeeding gain stage 3 x 2N109 (RCA) Gain stage, driver, and PNP part of complementary symmetry output stage 1 x 2N35 (Sylvania) NPN part of complementary symmetry output stage OR 2N214 2 x 1N91 Germanium rectifiers |
| Frequency Response | ±1-1/2 db at 30 to 15,000 cps |
| Source Impedance | 150 ohms |
| Load Impedance | 30 ohms |
| Input Impedance | 150/30 ohms |
| Output Impedance | 600/150 ohm + 18 dbm |
| Gain | 88 db ± 2 db (150-ohm source to 600-ohm load) |
| Harmonic Distortion | Less than 1.5% at 30 to 50 cps Less than 1% at 50 to 10,000 cps Less than 3% at 10,000 to 15,000 cps (+18 db output) Less than 1.25% at 30 to 50 cps Less than 1% at 50 to 10,000 cps Less than 1.25% at 10,000 to 15,000 cps (+8 db output) |
| Noise Level | -52 dbm (Single channel, gain 68 db, master gain control at 12, mixer gain at 16) Corresponds to signal-to-noise ratio 70 db at rated power output of +18 dbm. |
| Dimensions and Weight | Height: 5-5/8 inches Width: 16-1/2 inches Depth: 10-1/2 inches Weight: 25 pounds (approximately) Finish: Two-tone umber gray |
Diagrams and Illustrations
Figure 1 - Type BN-6B Transistor Portable Remote Amplifier: Photograph of the unit showing front panel controls and VU meter.
Figure 2 - Typical Frequency Response Curve: Graph illustrating the amplifier's frequency response from 30 Hz to 15,000 Hz.
Figure 3 - Block Diagram: Illustrates the internal signal flow, including input stages, preamplifiers, mixer, master control, isolation pad, and output transformer.
Figure 4 - Rear View of Amplifier: Shows input/output jacks (J1-J4), fuse holder (F1), line monitor output (J10), and power connectors (E1-E3).
Figure 5 - Battery Tubular Case on Bottom of Chassis: Internal view showing the battery holder assembly.
Figure 6 - Method for Replacement of Components on Printed Circuits: Diagrams detailing techniques for component removal and installation on circuit boards.
Figure 7 - BN-6B Remote Amplifier (Lid and Cable in Carrying Position): Image of the amplifier in its carrying case.
Figure 8 - Amplifier With Cover Removed Showing Printed Circuit Boards: Internal view revealing circuit boards and component layout.
Figure 9 - Schematic Diagram: Detailed electronic schematic of the BN-6B amplifier.
Figure 10 - Connection Diagram: Illustrates external wiring and component interconnections.
Installation
Transistors
All transistors are shipped in place and sockets are mounted on printed circuit boards. Replacement transistors are easily plugged in. CAUTION: Do not remove or insert transistors when the power is ON, as this may cause excessive noise and distortion. Allow approximately one minute after power is off before removing transistors.
Internal Connections
The BN-6B amplifier is shipped connected for either 150 or 30 ohm unbalanced input and a 600 ohm output. A plug is provided for each input socket. For an unbalanced line, either a 30 or 150 ohm microphone may be used. For a balanced line, plug-in transformer MI-11776 can be ordered. The sockets are connected for a 150-ohm input balanced line. For a 30-ohm balanced line, move connections on the socket from terminal 1 to terminal 2 and from terminal 5 to terminal 4.
Installation of Batteries
The preamplifier is shipped without batteries. These can be ordered separately as MI-11778, which includes five Mallory #TR135R (6.5 volt) and one RCA #VS036 (1.5 volt) batteries. To install the RCA battery, remove the cover and place it in the clips secured to the top shelf of the chassis in the right-hand corner. A cylindrical case holds the five batteries and is secured by two clips fastened to the bottom of the chassis.
VU Meter
The VU meter on the front panel measures output level and battery current. To measure battery current, turn the POWER switch to the BATT TEST position. When the meter indicates the center position or 100, the battery current is at its peak (32 volts). The amplifier should not be operated if the battery reading is below 10% less than the 100 reading. The VU meter attenuator is designed to show 0 on the VU scale with an output of +8 dbm. Mercury-type batteries offer approximately 40 hours of continuous operation. The RCA battery for meter illumination should be changed when dim.
Operation
After determining the power source (AC or battery), set the amplifier on a convenient surface and adjust the handle as desired. Proceed as follows:
- AC Power: Plug the cable connector into the receptacle on the back of the amplifier and connect to a convenient 117-volt AC outlet. Turn the POWER switch to the AC ON position.
- Battery Power: Turn the POWER switch to the BATT. TEST position and check the VU meter reading. If the reading is satisfactory (not below 10% less than the 100 reading), turn the POWER switch to the BATT ON position.
- Plug headphones into the PHONO jack for monitoring.
- Plug in microphones. All four receptacles can be used simultaneously. Low-impedance broadcast microphones like the RCA BK-5A or BK-6A are recommended. Ensure microphone cables have XLR-3-12C (MI-11089A) connectors matching the amplifier's receptacles.
CUE-MIC-I Switch
Plug headphones into the PHONO jack for output monitoring. If the studio signal is weak, turn the spring-return switch to the CUE position. This amplifies the signal through the BN-6B to the headphones, independent of line balance. This switch ensures operation on MIC-1 at all times.
Binding Posts
Three binding posts are mounted on the back of the chassis for line output connection and ground. Output connection is made via the telephone line for remote operation.
Maintenance
Fuse Replacement
When replacing a fuse, use one of the same type and rating (1/16 amp 3AG, slo-blo) as the original. Using a higher-rated fuse can damage the power transformer windings.
Servicing of the Etched Wiring Board Assemblies
The etched wiring boards are made of .062-inch thick phenolic laminate with a thin copper sheet bonded to one side, forming the conductor pattern. Component leads are threaded through punched holes and bent over against the copper conductors, then dip-soldered. Components can be replaced by following these instructions. Avoid breaking or cracking the board with excessive force or damaging the bonding adhesive with too much heat during soldering.
Tools Required:
- A small (35 watt or less) pencil-type soldering iron.
- A pair of small diagonal cutters.
- A pair of small long-nose pliers.
- A scribe or pick.
- A small knife.
Emergency Repairs
If a defective component is known, it may be replaced without removing the board. For small components like resistors, cut the component in half with diagonal pliers, then crush the body with long-nose pliers to gain extra lead length. For larger components, clip leads close to the body.
Permanent Repairs
- Remove hardware fastening the board to the chassis and tilt the board up.
- Isolate the defective component. To disconnect a component for testing, heat the junction of the component lead and the etched wiring with the soldering iron, concentrating heat on the lead. Pry up and straighten the bent-over portion of the lead with a knife blade, then pull the lead through the hole with pliers.
- To remove the defective component, snip its leads off at the component side of the board.
- Using a small soldering iron (35 watts or less), heat the leads and remove them from the printed wiring side of the board. Be careful not to apply too much heat or force to avoid damaging the thin copper conductors.
- Clean and preform the leads of the new component, insert them through the holes until the component body is tight against the board.
- On the circuit side, grasp the component lead and bend it over in the direction of the circuit pattern.
- Crimp the wire tightly against the board and cut off the excess component lead, leaving about 1/16 inch of wire protruding from the hole.
- Heat the lead and apply rosin-core solder. Do not use paste or acid flux. Remove excess rosin with alcohol.
- Replace the circuit board using the original hardware.
Replacement of Transistor Sockets
Follow the same procedures as for components. Transistor sockets are mounted on top of the circuit board.
Voltage Readings
The table below shows typical voltage-to-ground readings at transistor socket terminals, obtained with an RCA Voltohmist. Values are approximate and may vary ±20% due to transistor production tolerances. Measurements were made with all gain controls at zero, plugs P5-P8 in the unit, and line voltage at 115v AC.
| Transistors | Pin 4 | Capacitors | (-) Terminal |
|---|---|---|---|
| XQ1 | -4.2V | C5 | -15.1V |
| XQ2 | -4.2V | C12 | -34.3V |
| XQ3 | -4.2V | C13 | -38.7V |
| XQ4 | -4.2V | C14 | -42.9V |
| XQ5 | -11.1V | C15 | -26.9V |
| XQ6 | -9.0V | C16 | -15.9V |
| XQ7 | -15.0V | C18 | -34.3V |
| XQ8 | -34.3V | ||
| XQ9 | 0 V |
Parts List
| Symbol No. | Stock No. | Drawing No. | Description |
|---|---|---|---|
| RESISTORS: | |||
| AT1 to AT4 | 213300 | 758505-2 | Resistor: variable, comp., 25,000 ohm ±10%, 2 w |
| AT5 | 213299 | 758505-1 | Resistor: variable, comp., 10,000 ohm ±10%, 2 w |
| CAPACITORS: | |||
| C1 | 106114 | 990130-1 | electrolytic, 10 μf, 10 v |
| C2 | 213301 | 984655-26 | electrolytic, 1.0 μf, 50 v |
| C3 | 79181 | 984655-16 | electrolytic, 2.0 μf, 50 v |
| C4 | 213301 | 984655-26 | electrolytic, 1.0 μf, 50 v |
| C5 | 213302 | 984655-27 | electrolytic, 50 μf, 50 v |
| C6 | 106114 | 990130-1 | electrolytic, 10 μf, 10 v |
| C7 | 213301 | 984655-26 | electrolytic, 1.0 μf, 50 v |
| C8 | 106114 | 990130-1 | electrolytic, 10 μf, 10 v |
| C9 | 213301 | 984655-26 | electrolytic, 1.0 μf, 50 v |
| C10 | 106114 | 990130-1 | electrolytic, 10 μf, 10 v |
| C11 | 213301 | 984655-26 | electrolytic, 1.0 μf, 50 v |
| C12 | 213304 | 984655-29 | electrolytic, 250 μf, 50 v |
| C13 to C16 | 213303 | 984655-28 | electrolytic, 100 μf, 50 v |
| C17 | 210001 | 984655-30 | electrolytic, 250 μf, 6 v |
| C18 | 213304 | 984655-29 | electrolytic, 250 μf, 50 v |
| C19 | 106055 | 990130-10 | electrolytic, 100 μf, -10 +250%, 15 v |
| C20 | 79181 | 984655-16 | electrolytic, 2.0 μf, 50 v |
| CR1, CR2 | 210963 | Rectifier: germanium (IN91) | |
| E1 to E3 | 46907 | 181193-5 | Post: binding |
| F1 | 211428 | 990157-102 | Fuse: 1/16 amp., 3 AG |
| J1 to J4 | 213288 | 8926959-1 | Connector: female, 3 cont. |
| J5 to J8 | 105783 | 882886-2 | Connector: female, 8 cont. |
| J9 | 98593 | 180882-1 | Jack: female, single cont. |
| J10 | 54472 | 889482-3 | Connector: AC |
| M1 | 213286 | 486639-1 | Meter: VU |
| P1 to P4 | Not used | ||
| P5 to P8 | 213298 | 8926985-501 | Connector: male, 9 cont. |
| RESISTORS: (Fixed, composition unless otherwise specified) | |||
| R1 | 735730-89 | 180,000 ohm ±10%, ½ w | |
| R2 | 735730-77 | 18,000 ohm ±10%, ½ w | |
| R3 | 735730-129 | 100 ohm ±5%, ½ w | |
| R4 | 735730-132 | 75 ohm ±5%, ½ w | |
| R5 | 735730-74 | 10,000 ohm ±10%, ½ w | |
| R6 | 735730-210 | 130,000 ohm ±5%, ½ w | |
| R7 | 735730-50 | 100 ohm ±10%, ½ w | |
| R8 | 735730-72 | 6800 ohm ±10%, ½ w | |
| R9 | 735730-68 | 3300 ohm ±10%, ½ w | |
| R10 | 735730-212 | 160,000 ohm ±5%, ½ w | |
| R11 | 735730-133 | 82 ohm ±5%, ½ w | |
| R12 | 735730-70 | 4700 ohm ±10%, ½ w | |
| R13 | 735730-73 | 8200 ohm ±10%, ½ w | |
| R14 | 735730-68 | 3300 ohm ±10%, ½ w | |
| R15 | 735730-130 | 62 ohm ±5%, ½ w | |
| R16 | 735730-87 | 120,000 ohm ±10%, ½ w | |
| R17 | 735730-130 | 62 ohm ±5%, ½ w | |
| R18 | 735730-70 | 4700 ohm ±10%, ½ w | |
| R19, R20 | 735730-125 | 39 ohm ±5%, ½ w | |
| R21 | 735730-139 | 150 ohm ±5%, ½ w | |
| R22 | 735730-70 | 4700 ohm ±10%, ½ w | |
| R23 | 735730-89 | 180,000 ohm ±10%, ½ w | |
| R24 | 735730-77 | 18,000 ohm ±10%, ½ w | |
| R25 | 735730-129 | 100 ohm ±5%, ½ w | |
| R26 | 735730-132 | 75 ohm ±5%, ½ w | |
| R27 | 735730-74 | 10,000 ohm ±10%, ½ w | |
| R28 | 735730-89 | 180,000 ohm ±10%, ½ w | |
| R29 | 735730-77 | 18,000 ohm ±10%, ½ w | |
| R30 | 735730-129 | 100 ohm ±5%, ½ w | |
| R31 | 735730-132 | 75 ohm ±5%, ½ w | |
| R32 | 735730-74 | 10,000 ohm ±10%, ½ w | |
| R33 | 735730-89 | 180,000 ohm ±10%, ½ w | |
| R34 | 735730-77 | 18,000 ohm ±10%, ½ w | |
| R35 | 735730-129 | 100 ohm ±5%, ½ w | |
| R36 | 735730-132 | 75 ohm ±5%, ½ w | |
| R37 | 735730-74 | 10,000 ohm ±10%, ½ w | |
| R38 | 735730-65 | 1800 ohm ±10%, ½ w | |
| R39 | 735730-175 | 4300 ohm ±5%, ½ w | |
| R40 | 735732-164 | 1600 ohm ±5%, 2 w | |
| R41, R42 | 735731-137 | 120 ohm ±5%, 1 w | |
| R43 | 735730-146 | 300 ohm ±5%, ½ w | |
| R44 | 735730-203 | 68,000 ohm ±5%, ½ w | |
| R45 | 735730-154 | 620 ohm ±5%, ½ w | |
| R46 | 735730-180 | 7500 ohm ±5%, ½ w | |
| R47 | 735730-70 | 4700 ohm ±10%, ½ w | |
| R48 | 735730-61 | 820 ohm ±10%, ½ w | |
| R49 | 735730-154 | 620 ohm ±5%, ½ w | |
| R50 | 735730-158 | 910 ohm ±5%, ½ w | |
| R51 | 735730-120 | 24 ohm ±5%, ½ w | |
| R52 | 214949 | 722384-230 | wire wound, 1.5 ohm ±5%, 1 w |
| R53 | 735730-72 | 6800 ohm ±10%, ½ w | |
| R54 | 735730-74 | 10,000 ohm ±10%, ½ w | |
| S1 | 213296 | 486640-1 | Switch: rotary, wafer type, 3 sec., 4 pos. |
| S2 | 213297 | 486641-1 | Switch: rotary, wafer type, 1 sec., 2 pos. |
| T1 | 213305 | 992043-1 | Transformer: output |
| T2 | 213306 | 992086-1 | Transformer: power |
| T3 | 213627 | 992074-1 | Transformer: input |
| XF1 | 205914 | 8811104-1 | Holder: fuse |
| MISCELLANEOUS: | |||
| XQ1 to XQ9 | 215818 | 8888533-7 | Socket: transistor |
| 32098 | 8926984-501 | Cable: power, with male and female connectors | |
| 32661 | 180707-3 | Cable: power, with 2 contact male connectors | |
| 213290 | 878243-1 | Connector: female, 2 contact | |
| 213291 | 8926966-1 | Cap: battery tube | |
| 203645 | 47764-1 | Escutcheon: panel overlay | |
| 30925 | 181654-1 | Foot: rubber | |
| 214647 | 875443-9 | Handle: carrying case | |
| 17268 | 8941269-1 | Holder: battery | |
| 30075 | 737820-505 | Knob: microphone controls | |
| 213293 | 712336-507 | Knob: "off-on" and mic switch | |
| 213292 | 8926999-501 | Printed Circuit: amplifier component assembly | |
| 213295 | 8926994-501 | Printed Circuit: power supply filter | |
| 213289 | 8924067-1 | Spring: battery contact pressure Tubing: battery holder | |
Audio Facilities Measurements
The following tables detail measurements taken for the BN-6B amplifier, including distortion, noise level, and frequency response, conducted by CBS Radio.
Distortion Measurements in % RMS
| CPS | 50 | 100 | 200 | 400 | 1000 | 5000 | 7500 |
|---|---|---|---|---|---|---|---|
| 50 | 0.26 | 0.23 | 0.18 | 0.18 | 0.27 | 0.21 | 0.68 |
| 100 | 0.2 | 0.14 | |||||
| 400 | 0.2 | 1.4 | |||||
| 1000 | 0.18 | 1.5 | 0.14 | 0.15 | 0.12 | 0.16 | 1.1 |
| 5000 | 0.25 | 1.54 | |||||
| 7500 | 0.31 | 1.1 | 0.3 | 0.8 | 0.74 | 0.33 | 0.75 |
Noise Level Measurements in DB Below Output Level at 400 CPS
| CPS | Unweighted | ||||||
|---|---|---|---|---|---|---|---|
| 400 | -6.4 | -7.3 | -6.2 | -7.2 | -7.0 | -5.7 | -6.7 |
Response-Freq. Measurements in DB Deviation from 1000 CPS Level
| CPS | 50 | 100 | 200 | 500 | 1000 | 2000 | 5000 | 10000 | 15000 |
|---|---|---|---|---|---|---|---|---|---|
| 50 | -11.2 | -11.1 | |||||||
| 100 | -6.4 | -1.5 | -6 | -8 | -6 | -7 | |||
| 200 | -1.2 | -2 | |||||||
| 500 | 0 | -2 | |||||||
| 1000 | 0 | 0 | 0 | -2 | 0 | -2 | 2 | 0 | |
| 2000 | -1 | ||||||||
| 5000 | 0 | 0 | |||||||
| 10000 | -1.7 | -1.8 | -1.0 | -8 | -1.8 | -1.0 | -2 | -1.8 | |
| 15000 | -1.6 |
AN-6B VU Reads 74 db Low
MEASURED BY: [Not specified in document]
Note: The website www.SteamPoweredRadio.Com is referenced throughout the document.
