Lafayette Comstat 25A 23-Channel CB Transceiver

Introduction

The Lafayette Comstat 25A is a 23-channel Citizens Band (CB) transceiver designed for Class "D" operation in the 11-meter band. This manual provides comprehensive installation and operating instructions to ensure optimal performance and compliance with Federal Communications Commission (FCC) regulations.

Stock Number: 99-3130WX

The cover page features an image of the Lafayette Comstat 25A transceiver with a microphone, displaying the Lafayette logo and model name.

Specifications

General Instructions and Licensing

The Lafayette Comstat 25A is designed for use in the Class "D" citizens radio service. Users must comply with FCC Part 95 regulations. It is illegal to operate the transmitter without a valid station license and call sign. The licensee is responsible for any violations incurred during use.

Copies of FCC regulations for the Citizens Band Radio Service (Part 95) are available from the Superintendent of Documents, U.S. Government Printing Office. A station license application (Form 505) must be submitted.

An Identification Card Form 452-C must be attached to the transmitter.

Tubes and Semi-Conductors

The transceiver utilizes various vacuum tubes and semiconductor diodes for its operation:

General Description

The receiver is a highly sensitive and selective dual-conversion superheterodyne type operating in the 26.965 to 27.255 Mc (11-meter) citizens band. It features a frequency-synthesized circuit for 23-channel operation, an S-meter, full-time ANL, and an adjustable squelch control. A "fine tuning" control allows for precise reception of stations slightly off frequency.

The transmitter also operates in the 26.965 to 27.255 Mc band, utilizing the same frequency synthesizer for 23-channel crystal-controlled operation. It features AM modulation with up to 5 watts plate power input, and a "Range-Boost" circuit to enhance modulation density for improved communication in noisy conditions. A relative RF power meter indicates output power.

The unit includes Public Address (PA) facilities, allowing the microphone and audio stages to drive an external speaker.

Power Supply

The transceiver can be powered by either 105-120 volts AC (50/60 cycles) or 12.6 volts DC. It comes with separate power cables for AC and DC operation. For DC operation, it uses a built-in solid-state power supply circuit.

Installation

Proper installation is crucial for reliable operation.

Base Station Installation

Place the transceiver in a convenient location near an AC power outlet, with suitable ground and antenna connection. The microphone should be connected.

Mobile Installation

The transceiver may be mounted to the underside of a vehicle's instrument panel or dashboard using the supplied mounting bracket. Ensure the location does not interfere with vehicle operation.

Figure 2 on page 6 illustrates the mobile installation process, showing how the mounting bracket attaches to the transceiver and the vehicle's mounting surface. It also shows how securing screws allow for angling the unit.

Microphone Bracket Installation

The microphone bracket can be installed on either the left or right side of the transceiver, in one of two positions as shown in Figure 1 (page 5). For car installations where the mounting bracket is attached to the top of the transceiver, position 2 is typically used.

Figure 1 on page 5 shows two possible positions for mounting the microphone bracket.

AC Power Connection

Connect the 8-pin female connector of the AC power cord to the 8-pin male power connector at the rear of the transceiver. Ensure the keyway is aligned before connecting. Plug the other end into a 105-120 volt, 50/60 cycle AC outlet. The AC input is fused internally.

DC Power Connection (Mobile)

CAUTION: This transceiver is designed for vehicles with a negative ground system ONLY. DO NOT USE IN POSITIVE GROUND VEHICLES.

Connect the Red lead (fused for 8 amps) of the DC power cable to the main positive battery source (e.g., accessory post on ignition switch or accessory side of fuse block). Connect the Black lead to the metal firewall or chassis (negative battery potential). Ensure the Red lead is connected to a point that receives power only when the ignition switch is turned on.

Connect the 8-pin female connector of the DC power cable to the transceiver's 8-pin male power connector, aligning the keyway.

Microphone Connection

Attach the 4-pin male connector of the coiled microphone cord to the 4-pin female connector on the lower left front of the transceiver. Secure with the knurled ring.

Antenna Connection

Connect the antenna lead-in cable (RG-58/U or RG-8/U) terminated with a PL-259 type male coaxial connector to the matching antenna connector at the rear of the transceiver.

Antennas

Antenna system efficiency significantly impacts transceiver performance.

Mobile Antennas

Vertically polarized whip antennas are best suited for mobile service. These can be loaded (top, center, or base) or full quarter-wave. The vehicle's metal body serves as a ground plane.

Common mounting locations include:

• Front Cowl Mounting: Can replace the auto radio antenna; may require a two-way coupler for simultaneous use. Offers slightly irregular radiation, greater towards the rear fender opposite the mount.
• Roof Mounting: Provides an almost perfect omnidirectional pattern. A full 108-inch quarter-wave antenna is impractical; a shorter loaded whip is common, though less efficient.
• Rear Deck Mounting: Allows use of a full quarter-wave or loaded whip. Radiation pattern is somewhat irregular, favoring the front fender opposite the mount.
• Bumper Mounting: Practical for full 108-inch whips. Easy removal without body damage. Radiation pattern is irregular, with greatest radiation in two directions.

Figure 3 on pages 7/8 illustrates these mobile antenna mounting locations.

Antenna Cable

For mobile installations, RG-58/U 50 ohm coaxial cable length of 11 ft. 9 inches or multiples is ideal for optimum results. Excess cable should be coiled neatly (at least 8 inches in diameter).

For base station installations requiring over 50 feet of cable, RG-8/U coaxial cable is recommended for lower loss.

Base Station Antennas

Short Range: The Lafayette HE-19 is a small base loaded whip (45" extended) that mounts directly onto the transceiver.

Long Range: Three basic types are suitable:

• A. Vertical Ground Plane Antennas: Omnidirectional, providing optimum performance for contacting fixed stations and mobile stations.
• B. Coaxial Antennas: High efficiency radiators with omnidirectional characteristics, ideal when a vertical ground plane is not feasible.
• C. Directional Beam Antennas: Highly efficient and directional, intended for fixed-to-fixed long-range communications. A 3-element beam can provide an 8 dB increase in transmitter power.

Figure 4 on page 8 shows examples of base station antennas: A. Ground Plane, B. Coaxial, C. Directional Beam.

Ignition Interference

While standard broadcast radio suppression usually prevents ignition interference, the transceiver's high sensitivity may pick up noise from the vehicle. Noise suppressor kits are available, or a skilled auto radio technician can perform suppression.

Operating Controls and Features

The transceiver features several controls and indicators:

• (1) Illuminated Meter: Indicates signal strength (S units) during receive or RF antenna power (watts) during transmit.
• (2) Range-Boost: Concentrates audio power into sidebands for increased modulation density, improving communication in noisy conditions.
• (3) Channel Indicator: Illuminated dial showing the selected channel.
• (4) Channel Selector: Continuously rotating switch for selecting any of the 23 CB channels.
• (5) Speaker: 5" round PM-type speaker located behind the grille.
• (6) Modulation Indicator: Functions as a modulation indicator during transmit.
• (7) PA-CB Switch: Selects between normal CB operation and Public Address (PA) mode. In PA mode, the external speaker is used for transmitting audio. In receive, it allows channel monitoring over the external speaker.
• (8) Phone Jack: Standard phone jack for headphones or external speaker (4-8 ohms). Inserting a plug silences the internal speaker.
• (9) Mounting Bracket: Simplifies mobile installation and allows fast removal.
• (10) Microphone: 4-pin ceramic microphone with coiled cord and screw-on connector.
• (11) Fine Tuning: Allows fine tuning of the receiver for stations slightly off frequency, with a range of approximately 2.5 Kc.
• (12) Volume/On-Off: Controls audio output level and includes an on-off power switch (at extreme counter-clockwise position).
• (13) Squelch/Standby: Quiets the receiver during "no-signal" conditions. Adjustable sensitivity. Counter-clockwise position (without standby activation) disables squelch. In "Standby" position, high voltages (B+) are switched off while filaments remain powered, reducing consumption.
• (14) Microphone Clip: Hang-up clip for the microphone.

A diagram on page 9 labels these controls on the front panel.

Operating Instructions

Ensure the transceiver is properly installed and connected to power and antenna. Connect the microphone.

Receiving

Set the PA-CB switch to "CB". Rotate the Squelch control fully counter-clockwise (without engaging standby). Select the desired channel. Turn the Volume knob until it clicks. The S-meter dial light should illuminate. Advance Volume to about 1/3. A characteristic rushing sound indicates the receiver is active. Adjust Volume for a comfortable listening level.

Squelch Adjustment

The Squelch control eliminates background noise when no signals are present. Turn Volume up until background noise is heard. Slowly rotate Squelch clockwise until the noise disappears. This point allows incoming signals to break through the squelch. Increasing the Squelch setting requires stronger signals to be heard. Turning Squelch fully counter-clockwise (without standby) disables the circuit.

The "Standby" position of the Squelch control powers down high voltages (B+) while keeping filaments active, reducing power consumption.

Fine Tuning

Use the Fine Tuning control (approx. 2.5 Kc range) to optimize reception of stations slightly off frequency. Tune for best reception and highest S-meter reading. If a station is clearer when the control is moved left or right, it may be on an adjacent channel.

"S"/RF Power Meter

This meter indicates incoming signal strength (S units) during receive and relative RF power output (watts) during transmit. The S-meter is factory pre-adjusted to read "S-9" with 100 microvolts at the antenna input. It can be readjusted via a rear panel control. During transmit, the meter reads true antenna power output only when connected to a 50-ohm resistive load.

PRIVA-COM Socket

An 11-pin socket at the rear is for the Lafayette "PRIVA-COM" selective call unit. A jumper plug is normally inserted and should be removed when using the unit.

Transmitting

WARNING: NEVER PLACE THE TRANSCEIVER IN TRANSMIT MODE WITHOUT AN ANTENNA CONNECTED. THIS MAY DESTROY THE RF POWER OUTPUT TUBE.

Before transmitting:

1. Ensure a valid Class D license is posted at the main control location.
2. Affix a properly filled out and signed mobile identification card (Form 452C).
3. Read and understand FCC Rules Part 95. Violators face severe penalties.

To transmit: Ensure the PA-CB switch is in the "CB" position. Hold the microphone near your mouth, projecting your voice forward. Speak at a normal level. Press the microphone button to transmit. The Modulation indicator light should flicker. The receiver is silenced during transmit.

Range-Boost

If a station reports difficulty receiving due to noise or interference, switch Range-Boost "on" and speak normally. This circuitry increases modulation density in sidebands, improving audibility under adverse conditions. Do not shout when using Range-Boost.

Public Address (PA) Operation

For PA operation, use an external 4-8 ohm speaker connected to the PHONES jack. Set the PA-CB switch to "PA" and press the push-to-talk button on the microphone. Speak into the microphone; your voice will be heard from the external speaker. Releasing the push-to-talk button returns the transceiver to normal CB receive mode.

Operating Procedures

A Citizens Band station is not a replacement for a ham station. Transmitting a "CQ" (calling any station) to alert others is a violation of Citizens Band Regulations.

Service and Maintenance

The transceiver is designed for reliable service. Components may fail over time. Tubes and pilot lamps may age. Simple component replacements can be done by the user, but more serious failures require a competent technician.

WARNING: FCC regulations require that transmitter adjustments made while transmitting must be supervised by or performed by a person holding a valid radio telephone operator's license.

The manual includes a voltage chart, layout diagram, schematic, and functional block diagram to aid technicians. Instructions for aligning receiver and transmitter sections are also provided.

Cabinet Removal

Disconnect power and antenna cables. Remove four slotted-head screws on the sides to detach the top cover. Remove six screws on the underside to detach the bottom cover.

Simple Trouble Shooting

Tubes

Tubes can be tested at a neighborhood store or service shop. Replace weak or defective tubes with identical types. Refer to diagrams for correct locations.

Solid-State DC Power Supply

The 12-volt DC operation uses a 2-transistor power supply. Transistors are coated for protection and must not contact the vehicle chassis, as this will cause a short circuit.

Pilot Lamps

Three pilot lamps are used: two in the meter, one for the channel dial. They operate below maximum rating and should last indefinitely.

Fuses

The 12-volt DC cable has an 8-amp in-line fuse. A 2-amp fuse protects the 117-volt AC primary circuit and is located inside the transceiver. If a fuse fails, replace it with one of the same rating. Repeated fuse failure indicates a serious fault.

Receiver Alignment

455 KC IF Adjustment: Connect to power, attach microphone, set Volume mid-position, Squelch minimum, PA-CB to "CB". Set Fine Tuning to center, Channel Selector to 13. Connect AC voltmeter (VTVM) to speaker terminals or "Phone" jack. Connect a 455 Kc signal generator (30% modulated at 1 Kc) to pin 8 of V2 (6AN8). Adjust generator output until VTVM reads approx. 0.5 volts. Adjust tuning cores of T3, T4, T5 for maximum output, reducing generator output if necessary to keep VTVM reading below 0.5 volts.

11.275 MC IF Adjustment: Connect signal generator to pin 2 of V1 (6AN8), VTVM to speaker terminals. Ensure Fine Tuning is centered. Tune generator near 11.275 Mc for maximum VTVM reading. Reduce generator output to approx. 0.5 volts. Adjust top and bottom cores of T2 for maximum reading, reducing generator output if necessary.

Second Oscillator

The second oscillator (V2B, 6AN8) is crystal-controlled. The Fine Tuning control adjusts its frequency. A normally functioning oscillator develops approx. -1.5 to -8 volts at pin 2 of V2B.

Local Oscillator

The master local oscillator (V7A) is crystal-controlled. Adjust the bottom core of L7 for maximum negative reading at pin 9 of V7A (with channel selector on 23), then back off clockwise to about 70% of maximum. Check all channels. A defective crystal yields zero voltage at pin 9 in four consecutive channels. After adjustment, check transmitter output frequency against FCC specifications and readjust L7 if needed.

Transmitter Alignment

The local oscillator and synthesizer are used for transmit. Bias is applied differently for receive and transmit modes. Connect a 50-ohm dummy load to the antenna connector.

RF Adjustments: Connect signal generator (30% modulated at 1 Kc) to antenna connector. Use RG58/U cable. Set generator output to approx. 10 µV, switch receiver to channel 13. Tune generator around 27.115 Mc for maximum VTVM reading. Adjust T1 for maximum output.

"S" Meter Adjustment: After receiver alignment, adjust VR1 for an "S-9" reading with 100µV at the antenna input and channel 13 selected.

Transmitter Alignment: Connect VTVM (with AC probe) to pin 1 of V9. With the mike button pressed, adjust T10 for maximum reading on channel 13 (approx. 1.4 volts is normal). If no reading, check for trouble beyond T9, possibly V8 or the 11.275 Mc crystal.

Maximum RF Output: Adjust VC3 (Load) and VC4 (Plate) for maximum power output on an RF wattmeter. Maximum RF input power is 5 watts (FCC limit). Check voltage across R55 (1K, 2W); it should not exceed 19 volts. Adjust VC3 clockwise and then peak VC4 for maximum. Then adjust VC3 clockwise until 19 volts is measured across R55.

Modulation Adjustment: Connect a modulation monitor and audio generator (1 Kc, 40 mv output). Place Range Boost switch ON, adjust VR4 for 80% modulation. Recheck by setting generator to 5mv for 50% modulation. With Range Boost OFF, modulation should be lower, requiring 15mv for 50% modulation. Speech should produce up to 100% modulation; never exceed 100%.

Page 14 shows the internal component layout diagram. Page 15 shows the rear panel layout. Page 16 details Maximum RF Output and Modulation adjustments.

TV Interference Trap

The transceiver has a built-in adjustable network to suppress television interference, particularly second harmonic radiation. If a "cross-hatch" or "wavy line" pattern appears on a TV screen during transmission, adjust the RF network coil slug screw (L1) in the rear of the cabinet to minimize interference. This is usually only necessary if the TV antenna is near the transmitter antenna.

Returning the Unit for Repair

For repairs, return the transceiver to the Lafayette store of purchase. If shipping to the main office, pack the unit carefully in its original carton or a sturdy carton with ample padding. Include a letter explaining the difficulties encountered. Ship via prepaid express, marking the carton "ELECTRONIC EQUIPMENT - FRAGILE".

Voltage Chart

The following chart provides voltage readings for various points in the transceiver under specific conditions (117 VAC input, channel 13, PA-CB in CB, Volume/Squelch minimum, Fine Tuning centered, 50 ohm dummy load connected).

* Measured with 1 megohm resistor in series with DC probe. Reading may vary at grid pins, depending on crystal activity.

Power Supply Points:

Diagram Descriptions

The manual includes several diagrams illustrating the transceiver's design and operation:

• Functional Block Diagram (Page 19): This diagram illustrates the signal flow through the various stages of the Comstat 25A, showing how RF signals are processed from the antenna through the receiver and transmitter sections, including the synthesizer and audio stages.
• Schematic Diagram (Page 20): This detailed electronic schematic provides the complete circuit layout, showing all components, their interconnections, and values. It is essential for advanced troubleshooting and repair.
• Internal Component Layout (Page 14): Shows the physical placement of tubes, transformers, coils, and other components within the chassis.
• Rear Panel Layout (Page 15): Identifies connectors and adjustment points on the rear of the transceiver.
• Front Panel Controls Diagram (Page 9): Labels the various knobs, switches, and indicators on the front panel.
• Mobile Installation Diagram (Page 6): Illustrates the mounting bracket and transceiver installation in a vehicle.
• Microphone Bracket Installation Diagram (Page 5): Shows options for mounting the microphone bracket.
• Mobile Antenna Mounting Diagram (Page 7/8): Depicts common locations for mounting mobile antennas on a car.
• Base Station Antenna Diagram (Page 8): Shows examples of different types of base station antennas.