The photograph on the left is of the left and right sides of the Hallicrafters HC-150 transceiver. The picture on the right is of the top and bottom of the transceiver. Notice the tabs on the bottom of the unit which is to allow the battery to be recharged without removal.
Receive theory of operation.
To keep things simple, the theory of operation that is described below is only for the receiver circuits that are used in the two channel models. The circuits that contain additional oscillators such as numbers 3, 4, 5 and 6, which were a user requested option, operate in the same manner as oscillators number 1 and 2 in the two channel models.
When an RF signal reaches the antenna it is first routed through tuned circuits L601 and L602. At this point the signal is applied to the base of the RF amplifier transistor Q601, which has a common emitter configuration. The now amplified signal is applied to a second pair of tuned circuits, L603 and T601. At this point the local oscillators come into play. The transistor for channel 1 is Q301 and the transistor for channel 2 is Q302 which are common to the double tuned oscillator output circuit consisting of coils L301 and L302. Each local oscillator utilizes crystals in the frequency range of approximately 15.7 to 17.8 MHz. The 4th harmonic signal of the oscillators is selected in the double tuned oscillator output circuit. The signal at this point has a frequency range of around 62.7 to 71.3 MHz for the 146 to 163 MHz. Keep in mind that this is for the two channel models only.
The output signal from the double tuned oscillator circuit L302 is now applied to a frequency doubler transistor Q303. The signal at the output of Q303 has a frequency range of approximately 125.5 to 142.5 MHz and is now applied to a double tuned circuit consisting of two RF coils, L303 and L304. The RF signal from T601 is now applied to the cathode of the first mixer diode CR603. The signal output from L304 is applied to the anode of CR603 and is utilized as an injection signal. The two signals are mixed in this stage producing the first intermediate frequency of 20.5 MHz. The output signal from CR603 is amplified by the 1st IF amplifier transistor Q401, the signal is tuned with coil L401 and applied to the 20.5 MHz crystal filter FL401.
The output signal from FL401 is amplified by the 2nd IF amplifier consisting of the two transistors, Z401A and Z401B. The signal is then applied to an amplifier stage at Z401C. The 2nd conversion oscillator, transistor Z401E, is a crystal controlled oscillator operating at 20.045 MHz. The output signals from Z401C and Z401E are applied to the 2nd mixer, transistor Z401D, where the signal frequencies are mixed to produce a 455 KHz signal. The 455 KHz output signal from transistor Z401D is then filtered by a ceramic filter network having two tuned transformers, T401 and T402, and two ceramic crystals, Y402 and Y403. The output signal from the crystal filter is amplified by a four stage cascade amplifier consisting of transistors Z402A, Z402B, Z402C and Z402D. The output signal from Z402D is applied to the 1st limiter stage, containing transistors Z403A and Z403B, which removes any amplitude variations from the signal.
The output signal from the 1st limiter is then applied to the 2nd limiter stage, containing transistors Z403C and Z403D, which further removes amplitude variations not completely removed by the 1st limiter. The output signal from the 2nd limiter is applied to the discriminator transformers T403 and T404. These transformers in conjunction with diodes CR402 and CR403 convert the signal to audio. The detected audio is coupled to the audio amplifier transistor Q304, through the On/Off and volume control variable 100k ohm resistor R501. The amplified audio is applied to the audio driver transistor Q305, which provides the audio signal necessary to drive the audio output transistors. The output signal from Q305 is applied to the Push-Pull audio output amplifier consisting of transistors Q306 and Q307. The output of this stage is capacitance coupled to the speaker.
The noise voltage used in the squelch amplifier circuitry is developed across resistor R443 and the ceramic capacitor C443, which is located in the collector circuit of transistor Z403C. This voltage is applied to the squelch amplifier transistor Q308 through the squelch control R502 which is a 5000 ohm variable resistor. The squelch detector circuit works when the output voltage from Q308 is rectified by the voltage doublers CR303 and CR304. A positive rectified voltage is developed which is proportional to the noise level(no signal). The rectified voltage is filtered by an RC network and applied to the squelch switch transistor Q309. When noise is present(no signal), this transistor is biased on. The emitter and collector voltages are DC coupled to the emitter and base respectively of the audio amplifier. When a signal is present, the squelch switch is biased off and Q304 operates normally and amplifies the audio signal.