KWM-2/2A Transceiver


Unmatched for versatility, dependability and mobility, the Collins KWM-2 maintains a reputation of outstanding performance in mobile and fixed station applications.

The KWM-2 power input is 175 watts PEP on SSB or 160 watts on CW. It transmits on voice or CW with a nominal output of 100 watts for complete coverage on 80 through 10 meters. Crystals are provided for all HF bands except 10 meters where one crystal is supplied with provision for two additional crystals.

The transceiver is finished in light gray enamel with a simulated leather front panel to match the S/Line.

The first available amateur mobile SSB transceiver was in the Collins KWM series. The KWM-2 continues to lead the field with the following features:

  • FILTER TYPE SSB GENERATION providing unsurpassed performance on both transmit and receive.
  • AUTOMATIC LOAD CONTROL which keeps the signal level adjusted to its rated PEP, resulting in an increase in average talk power.
  • INVERSE RF FEEDBACK which improves linearity and reduces distortion products, giving the cleanest signal on the air.
  • PERMEABILITY-TUNED VARIABLE OSCILLATOR with linearity and stability providing the best frequency calibration available. ONE KC DIVISION on all bands, eliminating frequency searching and allowing you to meet anyone on sked, on any band 80 through 10 meters.

Compactness and efficiency of the KWM-2 are achieved through Collins’ advanced design of having all tuned circuits and several tubes function in the dual role of transmitting and receiving. The same oscillators, Mechanical Filter and RF amplifier serve both the transmitter and receiver. CW break- in and monitoring sidetone circuits are built in.

Easily accessible controls on the front panel of the KWM-2 include the OFF-ON-NB-CAL SWITCH, EXCITER TUNING, ZERO SET, PA TUNING, LOADING, MIC GAIN, BAND SWITCH, AF GAIN, RF GAIN, EMISSION and METER SWITCH.

The KWM-2A is an extended frequency version of the KWM-2 for MARS (Military Affiliate Radio Service) and military applications. The KWM-2A has an additional crystal board permitting the operator to add 14 crystals to cover frequencies outside the amateur bands. The KWM-2A has a front panel switch and indicator, allowing instant switching between the two crystal boards.


FREQUENCY RANGE: 3.4-5.0 and 6.5-30.0 mc; with crystals furnished, bands are as follows:

80 meters – 3.4-3.6 mc, 3.6-3.8 mc and 3.8-4.0 mc.
40 meters – 7.0-7.2 mc and 7.2-7.4 mc.
20 meters – 14.0-14.2 mc and 14.2-14.4 mc.
WWV       – 14.8-15.0 mc.
15 meters – 21.0-21.2 mc, 21.2-21.4 mc and 21.4-21.6 mc.
10 meters – 28.5-28.7 mc.

MODE: SSB (either sideband selectable) or CW.

TYPE OF SERVICE: SSB continuous; CW 50% duty cycle.

POWER REQUIREMENTs: 115 v, 50-60 cps; power consumption approximately
235 watts in receive function and approximately 475 watts in transmit.

In mobile operation, 800 vdc required at approximately 175 ma; 275 vdc at 190 ma; a bias supply
adjustable between – 60 v and – 80 v; and 6, 12 or 24 v dc at 11.0, 5.5 or 2.75 amps respectively.

PLATE INPUT: 175 watts PEP on SSB; 160 watts on CW.

POWER OUTPUT: 100 watts PEP (nominal) into 50 ohms.

unwanted sideband -50 db; oscillator feed-through and/or mixer
products -50 db. Second harmonic -40 db. Third order distortion -30 db.

NOISE LEVEL: 40 db below single tone carrier.



ALTITUDE: 0-10,000 ft.

SIZE: With feet – 14-3/4″ W, 7-3/4″ H, 14″ D.

WEIGHT: 18 Lbs. 3 oz.

FREQUENCY STABILITY: Within 100 cps after warm-up.


BACKLASH: Not more than 50 cps.

VISUAL DIAL ACCURACY: 200 cps on all bands.

OUTPUT IMPEDANCE: Variable, 50 ohms nominal, capable of matching 2:1 SWR.

KEYING CHARACTERISTICS: Keying is free of chirps and clicks. Break-in CW and
sidetone provided.

AUDIO INPUT: High impedance microphone or phone patch.

AUDIO FREQUENCY RESPONSE: 300-2400 cps + 6 db.

stages and is capable of 10 db compression.

RF FEEDBACK: Approximately 10 db of RF feedback around PA and driver for improved linearity.

RECEIVER SENSITIVITY: 0.5 uv for 10 db signal-to-noise.

RECEIVER SELECTIVITY: 2.1 kc bandwidth at 6 db down; 4.2 kc bandwidth at 60 db down.

RECEIVER SPURIOUS RESPONSE: Image rejection better than 50 db. Internal
spurious below 1 uv equivalent antenna input.

RECEIVER OUTPUT LEVEL: 1.0 watt maximum.

AUTOMATIC GAIN CONTROL: The audio output level does not change more than 20 db as the input
signal is changed from 5 uv to 1 v. Fast attack and slow release provide excellent AVC action on
voice and CW.

VIBRATIONS: 2 g at 10-33 cps.

Under the hood…


The KWM-2/2A is an SSB or CW transceiver operating in the range between 3.4 and 30.0 mc. It consists of a double- conversion receiver and a double-conversion exciter- transmitter. The transmitter and receiver circuits use common oscillators, and a common mechanical filter, as well as a common r-f amplifier. The transmitter low-frequency i-f and the receiver low-frequency i-f is 455 kc. The high- frequency i-f for both is 2.955 to 3.155 mc. This is a band- pass i-f which accommodates the full 200-kc bandwidth.

Transmitter Circuit


Microphone or phone-patch input is connected to the grid of the first audio amplifier, V1A, amplified, and coupled to the grid of the second audio amplifier V11B. Output from V11B is coupled to the grid of cathode follower V3A through the MIC GAIN control, R8. Output from the cathode follower is fed to the resistive balance point of the balanced modulator. In TUNE, LOCK, and CW positions of the EMISSION switch, output from the tone oscillator, V2B, is fed to the grid of the second audio amplifier. The amplified tone oscillator signal is taken from the plate of V11B and coupled to the grid of the vox amplifier V14B to activate the vox circuits in CW operation. This signal is also fed to the grid of the first receiver a-f amplifier, V16A, for CW monitoring.


Audio output from the cathode of V3A and the bfo voltage are fed to a diode quad balanced modulator (CR1, CR2, CR3, and CR4). Both upper and lower sideband outputs from the balanced modulator are coupled through i-f transformer T1 to the grid of the i-f amplifier, V4A. Output from the i-f amplifier is fed to the mechanical filter, FL1. The passband of FL1 is centered at 455 kc. This passes either upper or lower sideband, depending upon the sideband selected when the EMISSION switch connects bfo crystal Y16 or Y17. The single-sideband output of FL1 is connected to the grids of the first transmitter mixer in push-pull.


The 455-kc single-sideband signal is fed to the first balanced mixer grids in push-pull. The plates of the mixer are connected in push-pull, and vfo signal is fed to the two grids in parallel. The mixer cancels the vfo signal energy and translates the 455-kc single-sideband signal from the balanced modulator to a 2.955- to 3.155-mc single-sideband signal. The T2-L4 combination between the first and second mixer provides broadband response to the 200-kc variable i-f output (2.955 to 3.155 mc) from the first transmit mixer V5. The band-pass i-f signal is fed to one of the grids of the second balanced mixer, and the high-frequency injection signal energy from crystal oscillator V13A is fed to the cathode and the other grid. This arrangement cancels the high-frequency injection signal energy within the mixer and translates the band-pass i-f signal to desired operating band.


The slug-tuned circuits coupling V6 to V7, V7 to V8, and V8 to the power amplifier are ganged to the EXCITER TUNING control. The signal is amplified by the r-f amplifier, V7, and the driver, V8, to drive the power amplifier, V9 and V10. Output from the parallel power amplifiers is tuned by a pi- network and fed to the antenna through contacts of transmit- receive relay K3. Negative r-f feedback from the PA plate circuit to the driver cathode circuit reduces distortion in the output signal. Both the driver and PA stages are neutralized to ensure stability. When r-f driving voltage to the PA becomes great enough that positive peaks drive the PA grids positive, the grids begin to draw current and the signal is detected. This produces an audio envelope. The audio is rectified by the alc rectifier, V17A, which is connected to produce a negative d-c voltage. The voltage is filtered by C159, C160, R118, and R119 (which also determine the alc time constants), and used to control the gain of V4A and V7. This system allows a high average level of modulation without driving the PA tubes well into the grid current region, which would result in increased distortion.

Receiver Circuits


Signal input from the antenna is connected through relay contacts to the tuned input circuit, T3. The signal is applied from T3 to the grid of the receiver-transmitter r-f amplifier, V7. Amplified signal from V7 is applied from the tuned circuit, consisting of L10 and band switch selected capacitors, to the grid of the receiver first mixer, V13B.


The input r-f signal is fed to the grid of V13B, and the high-frequency oscillator injection signal is fed to the cathode of V13B. The difference product of the first mixer is applied from the plate of the tube to variable i-f transformer T2. Output of T2 in the range of 2.955 to 3.155 megacycles is applied to the grid of the second receiver mixer, V17B, across parallel-tuned trap circuit Z5. This trap circuit minimizes a spurious response which would otherwise result from harmonics of the high-frequency crystal oscillator. When signal input is applied to the grid of V17B and vfo injection signal is applied to the cathode of V17B, the 455-kc difference product is fed from V17B plate to mechanical filter FL1.


The output from FL1 is applied to the grid of the first i-f amplifier, V1B. The i-f signal is amplified by V1B and V3B and applied through T5 to avc rectifier V15A and to the grid of product detector V15B. Beat-frequency oscillator signal is applied to the cathode of V15B, and the product of mixing is the detected audio signal. Output of the avc rectifier circuit is applied to the two receiver i-f amplifiers and through contacts of relay K4 to the receiver-transmitter r-f amplifier. This avc voltage controls the gain of the receiver and prevents overloading.


Output from the product detector is applied through the A.F. GAIN control, R92, to the grid of the first a-f amplifier, V16A. Amplified audio output of V16A is coupled to the grid of the a-f output amplifier, V16B, which produces the power to operate a speaker, headphones, or phone patch.


The transceiver contains the tone oscillator, the beat- frequency oscillator, the variable-frequency oscillator, the high-frequency crystal oscillator, and the crystal calibrator.


The tone oscillator operates when the EMISSION switch is in LOCK, TUNE, or CW position. It is a phase-shift oscillator operating at approximately 1500 cps. Its output is fed to the transmitter audio circuits for CW operation. Some of the output from the tone oscillator is applied to the receiver audio circuits for sidetone monitoring in CW operation. Due to the 1500-cps tone applied to the balanced modulator during CW operation, the actual transmitted CW signal will be 1500 cps above the KWM-2/2A dial reading.


The bfo is crystal controlled at either 453.650 or 456.350 kilocycles, depending upon whether Y16 or Y17 is selected by EMISSION switch section S9H. The unused crystal is shorted out by this switch section. These crystal frequencies are matched to the passband of the mechanical filter, L1, so that the carrier frequency is placed approximately 20 db down on the skirts of the filter response. This 20-db carrier attenuation is in addition to the 30-db suppression provided by the balanced modulator.


The vfo uses fixed capacitance and variable inductance to tune the range of 2 .5 to 2 .7 mc . The series combination of capacitor C308 and diode CR301 is connected in parallel with capacitor C303. The diode switches C308 into or out of the circuit, depending upon the polarity of a bias voltage impressed across the diode junction. When USB emission is selected, the bias is positive and C308 is switched into the circuit. The capacitor then is adjusted to shift the vfo frequency by an amount equal to the frequency separation of bfo crystals Y16 and Y17. This allows the selection of either sideband without upsetting tuning or dial calibration.


The high-frequency crystal oscillator, V13A, is crystal controlled by 1 of 14 crystals selected by BAND switch S2. Output from the high-frequency crystal oscillator is fed to the transmitter second mixer and to the crystal oscillator cathode follower. The cathode follower provides isolation and impedance match between the crystal oscillator and the receiver first mixer cathode. The output frequency of this oscillator is always 3.155 mc higher than the lower edge of the desired band. This high-frequency injection signal is the crystal fundamental frequency for all desired signals below 12 megacycles. For operating frequencies higher than 12 mc, the crystal frequency is doubled in the plate circuit of the oscillator. Instructions for calculating crystal frequencies for the desired bands are given in section 2.


The 100-kc crystal calibrator, V12A, is the pentode section of a type 6U8A tube. Its output is coupled to the antenna coil, T3 . The calibrator may be trimmed to zero beat with WWV by adjustment of capacitor C76.

Vox and Anti-vox Circuits

Audio output voltage from the second microphone amplifier, V11B, is coupled to the VOX-GAIN control R39. A portion of this voltage is amplified by vox amplifier V14B and fed to the vox rectifier, which is one of the diodes of V14. The positive d-c output of the vox rectifier is applied to the grid of vox relay amplifier V4B. causing it to conduct current and actuate the vox relay, K2. Contacts of K2 switch the receiver antenna lead, the other relay coils, and bias voltage . Relays K3 and K4 switch the metering circuits from receive to transmit, the low plate voltages from receive to transmit tubes, and the avc and alc leads.

The anti-vox circuit provides a threshold voltage to prevent loudspeaker output (picked up by the microphone circuits) from tripping the KWM-2/2A into transmit function. Some of the receiver output audio voltage is connected through C235 to the ANTI-VOX GAIN control, R45. Signal from the slider of this potentiometer is rectified by the anti-vox rectifier, which is the other diode of V4. Negative d-c output voltage from the anti-vox rectifier, connected to the grid of V4B, provides the necessary anti-vox threshold. ANTI-VOX GAIN control R45 adjusts the value of the anti-vox voltage threshold so that loudspeaker output will not produce enough positive d-c output from the vox rectifier to exceed the negative d-c output from the anti-vox rectifier and cause V4B to actuate K2. However, speech energy into the microphone will cause the positive vox voltage to overcome the negative anti-vox voltage and produce the desired action of K2.