Lafayette Comstat 25 CB Transceiver

General Description

The Lafayette Comstat 25 is a combination transmitter and receiver designed for Class "D" operation in the 11-meter citizens radio service. It features a frequency-synthesized circuit for 25-channel crystal-controlled operation on both transmit and receive. The unit includes an S-meter, automatic noise limiter, adjustable squelch, and a "fine tuning" control for optimal reception. A public address (PA) feature is also included.

Receiver Section: Highly sensitive and selective dual-conversion superheterodyne type with one RF and two IF amplifiers. It uses 13 crystals for 25-channel operation. Features include an S-meter, full-time automatic noise limiter, adjustable squelch, and a 2.5 Kc "fine tuning" control.

Transmitter Section: Transmits amplitude modulated signals on the 11-meter citizens band. It utilizes the same frequency-synthesized circuit as the receiver. Features include plate modulation (up to 100% capability), up to 5 watts plate power input, and a "Range-Boost" circuit for enhanced modulation. A relative RF power meter indicates output power.

Power Supply: Operates from 105-120 volts AC, 50/60 cycles, or 11.5 to 14.5 volts DC. Includes separate power cables for AC and DC operation. The DC operation uses a built-in solid-state power supply circuit.

Specifications

Receiver

• Circuit Type: Dual conversion superheterodyne; crystal-controlled 25 channel with "fine tuning" capability of 2.5 Kc.
• Sensitivity: 0.8 μv for 10 db S/N to N ratio.
• Selectivity: 6 Kc bandwidth, -26 db.
• Intermediate Frequency: 1st IF 6.2 Mc, 2nd IF 455 Kc.
• Image Rejection: -75 db.
• Audio Output: 4 watts into 5 x 3" oval speaker.
• External Speaker Impedance: 4-8 ohms.
• Auxiliary Circuits: Full-time Automatic Noise Limiter, Adjustable Squelch, AVC.

Transmitter

• Plate Power Input to Final: 5 watts or 100 milliwatts, switchable.
• Modulation: AM, via plate modulation; up to 100% capability.
• Range-Boost: Yields high average modulation.
• Harmonic Suppression: Exceeds FCC requirements.
• Carrier Deviation: Exceeds FCC requirement of ±.005%.
• Antenna Matching: Matches resistive antenna loads of 50-75 ohms.
• Trans/Rec Switching: Relay switching, push-to-talk on microphone.
• Public Address: PA facilities using microphone and audio stage with an external speaker.

Tubes & Semi-Conductors

A list of tubes and their functions in the receiver and transmitter circuits is provided, along with semiconductor components like rectifiers and diodes.

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

Common Specifications:

• Power Supply: 105-120 volts AC, 50/60 cycles AC or 11.5-14.5 volts DC.
• Power Consumption: 117 volts AC: 80 watts max. 12 volts DC: 3-1/2 amps (rec), 4 amps (trans).
• Dimensions: 12" W x 5" H x 8-1/4" D (excluding knobs and plugs at rear).
• Net Weight: 17 lbs.
• Shipping Weight: 20 lbs.

General Instructions

IMPORTANT: Two extra channels (A and B) are provided for future operation on the proposed H. E. L. P. program. DO NOT TRANSMIT on these frequencies until the FCC has given approval for their use.

The Lafayette Comstat 25 is designed for Class "D" operation in the 11-meter citizens radio service and must comply with FCC regulations (Part 95). A station license is required for operation.

Base Station Installation

Place the transceiver in a convenient location near an AC power outlet, suitable ground, and antenna lead-in cable.

Mobile Installation

Mounting Bracket Installation

The microphone bracket can be installed on either the left or right-hand side of the transceiver in one of two positions, as shown in Figure 1 (illustrations of bracket placement). The position depends on the mounting bracket's placement. For a typical car installation where the bracket is attached to the top of the transceiver, position 2 is used.

Mounting Bracket

The mobile mounting bracket can be used for base installation by attaching it to the underside of the transceiver. Rubber feet can be attached to the bracket for stability. The bracket has additional holes to allow angling the transceiver for better front panel visibility.

AC Power Connection

Connect the 8-pin female connector of the AC power cord to the transceiver's 8-pin male power connector. Ensure the keyway is aligned before connecting. Turn the transceiver off (Volume control fully counter-clockwise). Plug the other end into a 105-120 volt, 50/60 cycle AC outlet. The AC input is fused internally.

Microphone Connection

Attach the 4-pin male connector of the coiled microphone cord to the 4-pin female connector on the 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 connector to the rear antenna connector.

Transceiver Installation (Mobile)

Choose a convenient location in a car, truck, or boat, ensuring it does not interfere with driving. The transceiver can be mounted to the underside of the instrument panel or dashboard using the supplied bracket. The bracket attaches to the panel with self-threading screws, and the transceiver secures to the bracket with large knurled screws.

DC Power Connection

CAUTION: This transceiver is designed for negative ground systems 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 fuse block). Connect the Black lead to the vehicle chassis (negative battery potential). Ensure the Red lead is connected to a point that provides power only when the ignition switch is turned on.

The DC power supply circuit operates within 11.5 to 14.5 volts DC. Ensure vehicle supply voltage does not exceed 14.5 volts. Adjust the voltage regulator if necessary.

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

Antennas

Antenna system efficiency significantly impacts transceiver performance. While this manual provides general information, specialized books are recommended for detailed antenna system guidance.

Antenna Cable

For mobile installations, RG-58/U 50 ohm coaxial cable lengths of 11 ft. 9 inches or multiples thereof are recommended for optimum results. Other lengths can be used if perfect antenna matching exists, though this is difficult to achieve due to impedance factors. Excess cable should be wound into a neat coil at least 8 inches in diameter.

For base station installations requiring long lead-in cables (over 50 feet), RG-8/U coaxial cable is recommended for lower loss.

Mobile Antennas

Vertically polarized whip antennas are best suited for mobile service. These can be loaded (top, center, or base) or a full quarter-wave. The vehicle's metal body acts as a ground plane. Common mounting locations are illustrated in Figure 3:

• Front Cowl Mount: Can replace the auto radio antenna, allowing dual use with a coupler. A short loaded whip can be used here with minimal efficiency loss. The radiation pattern is slightly irregular.
• Roof Mount: Offers an almost perfect omnidirectional radiation pattern. A shorter loaded whip is typically used here due to the impracticality of a full quarter-wave antenna.
• Rear Deck Mount: Allows use of a full quarter-wave or shorter loaded whip. The radiation pattern is somewhat irregular.
• Bumper Mount: Practical for full 108-inch quarter-wave whips. Allows easy removal without leaving holes in the car body. The radiation pattern is irregular.

Figure 3 Illustrations: Depicts four car antenna mounting positions: Front Cowl Mount, Roof Mount, Rear Deck Mount, and Bumper Mount.

Base Station Antennas

• Short Range: The Lafayette HE-19 (Stock No. 99-3015) is a small base loaded whip designed for short-range communication, mounting directly onto the transceiver. It measures 45" extended and 15" closed.
• Long Range: Three basic types are shown in Figure 4:
• (A) Vertical Ground Plane Antennas: Omnidirectional, providing optimum performance for contacting fixed stations and mobile stations.
• (B) Coaxial Antennas: High efficiency, omnidirectional radiators, ideal where ground plane installation is not feasible.
• (C) Directional Beam Antennas: Efficient and directional, intended for fixed-to-fixed long-range communications. A 3-element beam offers an 8 db increase in transmitter power.

Figure 4 Illustrations: Shows (A) Ground Plane antenna, (B) Coaxial Antenna, and (C) Directional Beam Antenna.

Ignition Interference

Standard vehicle radio suppression usually prevents serious ignition interference. However, the receiver's high sensitivity may pick up noise from your own vehicle. Noise suppressor kits are available, or a skilled technician can suppress ignition noise.

Operating Controls and Features

The front panel features several controls and indicators:

• (1) Illuminated Meter: Indicates relative signal strength (S units) on receive or RF antenna power (watts) on transmit.
• (2) Range-Boost: Concentrates more audio power into sidebands for maximum communication distance in noisy conditions.
• (3) Channel Indicator: Illuminated dial showing the selected channel.
• (4) Channel Selector: Continuously rotating switch for 25 channels (23 CB + 2 extra).
• (5) Speaker: PM-type 5 x 3" oval speaker behind the grille.
• (6) Modulation Indicator: Functions as a modulation indicator on transmit.
• (7) PA-CB Switch: Selects "CB" for normal operation or "PA" for Public Address operation (using an external speaker). On receive, "PA" monitors through the external speaker.
• (8) Phone Jack: Standard phone jack for headphones or external speaker (4-8 ohms). Plugging in silences the internal speaker.
• (9) Mounting Bracket: Simplifies mobile installation and allows fast removal.
• (10) Microphone: Four-pin socket for push-to-talk ceramic microphone with coiled cord.
• (11) Fine Tuning: Allows "fine" tuning of the receiver for stations slightly off frequency.
• (12) Volume/On-Off: Varies speaker output and includes an on-off power switch.
• (13) Squelch/Standby: Quiets the receiver during "no-signal" conditions. Adjustable sensitivity. Fully counter-clockwise position switches to standby, reducing power consumption while maintaining warm-up.
• (14) Microphone Clip: Hang-up clip for the microphone.

Diagram of Controls: A front panel diagram with numbered callouts identifying the controls listed above.

Operating Instructions

Receiving

Set PA-CB to "CB". Rotate Squelch fully counter-clockwise. Select desired channel. Turn Volume knob until it clicks; the S-meter light should glow softly. Advance Volume to about 1/3. As the unit warms up, characteristic receiver noise should be heard. Adjust volume to a comfortable level.

Squelch Adjustment

The Squelch control eliminates background noise when no signals are present. Turn Volume up until noise is heard. Slowly rotate Squelch clockwise until noise disappears. Incoming signals will then overcome the squelch. Adjust sensitivity as needed. For weak signals or to disable squelch, turn Squelch fully counter-clockwise (without engaging standby).

Standby Position: In the "Standby" position, high voltages are switched off, but filament voltages remain. This reduces power consumption while keeping the unit ready for instant use.

Fine Tuning

The FINE TUNING control offers approximately 2.5 Kc of adjustment for reception of stations slightly off frequency. Tune for best reception and highest "S" meter reading. Be aware that tuning to an adjacent channel might yield better reception if the station is off-frequency.

"S"/RF Power Meter

The meter indicates incoming signal strength (S units) on receive and relative RF power output on transmit. On receive, the S-meter is factory-calibrated to indicate "S-9" with 100 microvolts. On transmit, it 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 only when the unit is used.

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 citizens band equipment license is posted.
2. Affix a signed mobile identification card (Form 452C).
3. Obtain, read, and understand Rules Part 95. Violators face severe penalties.

To transmit: Set PA-CB to "CB". Check RF POWER switch (rear) for 100 MW or 5 watts. Press the microphone button. Hold the microphone at the corner of the mouth and speak normally. The Modulation indicator light should flicker. The receiver is silenced during transmission.

Range-Boost

If reception is poor due to noise, switch "Range-Boost" on. This increases modulation density for better audibility. Do not shout; speak normally.

100 Milliwatt Operation

The 5W-100MW switch at the rear allows for reduced RF output for short-range operation.

Public Address Operation

For PA operation, use an external 4-8 ohm speaker connected to the PHONES jack. Set PA-CB to "PA" and press the push-to-talk button. Your voice will be heard from the external speaker.

Channels A and B

Channels A (27.235 Mc) and B (27.245 Mc) are for future use in the proposed H. E. L. P. program. DO NOT TRANSMIT on these frequencies until FCC approval is granted.

Service and Maintenance

General

The transceiver is designed for reliable service. Component failure may require replacement. Tubes and pilot lamps may age and require replacement, which can often be done by the user. More serious failures require a qualified technician.

WARNING

FCC regulations require that transmitter adjustments made while on the air must be supervised by or performed by a licensed radiotelephone operator.

The manual includes voltage charts, layout diagrams, schematics, and functional block diagrams to aid technicians.

Cabinet Removal

Disconnect power and antenna cables. Remove four large slotted-head screws on the top cover and six screws on the underside for the bottom cover.

Simple Trouble Shooting

Ensure the 5W-100MW switch is set to "5W" for transmission issues.

Tubes

Tubes can be tested using a tube tester or by a service shop. Replace weak or defective tubes with identical types. Refer to diagrams for correct locations.

Solid-State DC Power Supply

Uses a 2-transistor power supply for 12V DC operation. Transistors are on the rear panel and have a protective coating. Avoid contact with vehicle chassis or metal brackets to prevent short circuits.

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 power cable has an 8-amp in-line fuse. A 1-amp fuse is located within the transceiver for the 117-volt AC primary circuit (access via bottom cover).

Frequency Synthesizing System

The transceiver uses 13 crystals in combinations to produce 23 fundamental oscillator frequencies, enabling full 25-channel crystal-controlled operation. Failure of a single crystal can affect multiple channels.

* Note that the derived frequency is exactly 6.2 Mc higher than the channel frequency in each case. During transmit, the derived frequency is converted to the channel frequency by the 6.2 Mc crystal oscillator/converter (V11). During receive, the derived frequency is heterodyned with the incoming channel frequency at the 1st Mixer/IF to produce a 1st IF of 6.2 Mc.

Failure of any one of the eleven crystals used will cause a malfunction on a group of channels. For example, failure of crystal Q1 would cause the transceiver to be inoperative on channels 1, 2, 3 and 4; failure of crystal Q7 would cause the transceiver to be inoperative on channels 1, 5, 9, 13, 17 and 21.

Channel Selector Switch Diagram: Illustrates the internal mechanism of the channel selector switch, showing crystal connections.

Alignment Instructions

The transceiver is factory-aligned and normally does not require further adjustment. However, receiver and transmitter alignment may be necessary. Only qualified technicians with proper test equipment should perform these adjustments. FCC regulations require transmitter adjustments to be made by or under the supervision of a licensed radiotelephone operator.

Cabinet Removal

Disconnect power and antenna cables. Remove four large slotted-head screws (two each side) for the top cover, and six screws on the underside for the bottom cover.

Receiver Alignment

455 KC IF Adjustment

Connect transceiver to power, attach microphone. Set Volume to mid, Squelch to minimum, PA-CB to "CB". Set Fine Tuning to mid-position and Channel Selector to channel 13. Connect an AC voltmeter (VTVM) across the speaker terminals or to the "Phone" jack. Connect a 455 Kc signal generator (modulated 30% at 1 Kc) to pin 8 of V2(6AN8). Tune generator to 455 Kc. Increase output until VTVM reads approx. 0.5 volts. Adjust tuning cores of T3, T4, and T5 for maximum output, reducing generator output as needed.

6.2 MC IF Adjustment

Connect signal generator to pin 2 of V1 (6AN8), VTVM to speaker terminals. Ensure Fine Tuning is in the center position. Tune generator near 6.2 Mc for maximum VTVM reading (approx. 0.5 volts). Adjust top and bottom cores of T2 for maximum reading.

Second Oscillator

The second oscillator V2B (6AN8) operates at 5.745 Mc. The Fine Tuning control provides approx. 2.5 Kc of adjustment. A normally functioning oscillator develops -1.5 to -8 volts at pin 2 of V2B.

Local Oscillator

The master local oscillator V7A operates between 40.900 and 41.150 Mc, selected by the channel switch. It develops approx. -8 volts at pin 2 of V7. To tune, adjust the bottom core of L8 for maximum negative reading at pin 2 of V7A (channel 23), then back off 70% clockwise. Check all channels for activity.

Diagram of Components: A diagram showing the internal layout of tubes, transformers, and other components with their interconnections.

RF Adjustments

After oscillators are functioning, connect a signal generator (modulated 30% at 1 Kc) to the antenna connector using RG58/U cable. Set generator output to approx. 10 µV. Tune receiver to channel 13. Tune generator around 27.115 Mc until a signal is heard. Adjust generator frequency for maximum VTVM reading. Adjust tuning cores of T1 for maximum output.

"S" Meter Adjustment

After receiver alignment, adjust VR1 for a "0" reading on the "S" meter with no signal input and channel 13 selected.

Transmitter Alignment

The local oscillator and synthesizer are used for transmit. B+ is removed from receiver tubes and applied to transmitter tubes during transmit. Bias on the RF power output tube is also adjusted.

NOTE: Connect a 50 ohm dummy load to the antenna connector before proceeding.

Connect VTVM to pin 1 of V9. With the mike button pressed, adjust L6 for maximum reading on channel 13 (approx. -2.0 volts). If no reading, check the 6.2 Mc converter stage. If the receiver is normal, check V8 or the 6.2 Mc crystal.

Connect VTVM to pin 2 of V10. Adjust L5 for maximum reading on channel 13 (approx. -15 volts). Check all channels with an RF wattmeter for equal power output. Adjust L5 slightly for equal readings on channels 1 and 23 if needed.

Maximum RF Output

Adjust VC3 (Load) and VC4 (Plate) for maximum power output on the RF wattmeter. Maximum RF input power is 5 watts. Check voltage across resistor R55 (1K, 2W); it should not exceed 21 volts. Adjust VC3 clockwise until 20-21 volts is measured across R55.

Modulation

Check modulation with a modulation monitor. With "Range-Boost" off, 60 mv of a 1.5 Kc tone should yield 100% modulation. With "Range-Boost" on, 9 mv should yield 100% modulation. Do not exceed 100% modulation.

TV Interference Trap

A built-in series-resonant trap helps suppress TV interference. If a "cross-hatch" or "wavy line" pattern appears on a TV screen during transmission, adjust the RF trap coil slug screw (L1) in the rear of the cabinet to minimize interference.

Returning the Unit for Repair

For necessary repairs, return the transceiver to the Lafayette store of purchase. For service at 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 prepaid express and mark "ELECTRONIC EQUIPMENT -- FRAGILE". Address to: SERVICE DIVISION, LAFAYETTE RADIO ELECTRONICS CORP., 111 JERICHO TURNPIKE, SYOSSET, L.I., N.Y.

Shipping Instructions

Pack the unit carefully to avoid transit damage. Use original carton or a sturdy carton with at least 3 inches of padding. Wrap the unit in paper first. Include a letter explaining the difficulties. Add 5¢ postage and mark "First Class Mail". Ship prepaid express and mark "ELECTRONIC EQUIPMENT -- FRAGILE".

Voltage Chart

All readings are taken with a VTVM from chassis (negative) to the indicated point. Input is 117 volts AC (or 12 volts DC). Transceiver set to channel 13, PA-CB in "CB", Volume and Squelch at minimum, Fine Tuning in center. A 50 ohm dummy load is connected to the antenna connector. Readings may vary by ±20%.

• NDV = No detectable voltage.
• NC = No connection.
• NM = Not measurable.

Voltage Chart Table: Details tube voltages at various pins for transmit (TR) and receive (REC) modes.

Power Supply Diagram: A simplified diagram showing power input points A, B, C and their corresponding voltage readings for TR and REC modes.

Functional Block Diagram

Functional Block Diagram: A diagram illustrating the signal flow and functional units of the transceiver, including receiver stages, transmitter stages, synthesizer, power supply, and controls.

Schematic Diagram

Schematic Diagram: A detailed electronic schematic diagram of the Lafayette Comstat 25 transceiver, showing all components, connections, and circuit paths.