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Abstract:
This video details the teardown of a Collins 618M-1 avionics VHF transceiver. The presenter introduces the unit, its front and rear panel connectors, and associated components like a compatible ARINC-410 frequency selector and a large technical manual. The teardown reveals a complex hybrid design featuring both vacuum tubes and transistors. Key internal systems explored include a disconnected cooling blower, the RF signal path with a coaxial antenna relay, a triple-conversion superheterodyne receiver (IFs at 15MHz, 3MHz, 500kHz), and a sophisticated electromechanical frequency synthesizer using crystal oscillators, a DC motor, slip clutches, a differential gear system, and ganged variable capacitors. The transmitter output stage, employing 7558 driver and 8121 power amplifier tubes, along with its directional coupler and output filtering, is examined. The bottom side showcases the power supply components, including an autoresonant switched-mode power supply transformer, the modulation transformer for plate and screen modulation, and various power transistors.
Teardown of a Collins 618M-1 Avionics VHF Transceiver
0:00 Introduction: The Collins 618M-1 transceiver is introduced, showing front panel test points, jacks, and controls.
0:40 Rear Connectors & Accessories: The rear antenna and multi-pin connectors are shown. A compatible connector and an ARINC-410 standard frequency selector (used instead of the expensive original 313 model) are presented, along with the large technical manual.
1:56 Teardown Goal: The plan is to tear down the unit and potentially test its receiver functionality near an airport.
2:24 Initial Internal View: Upon opening, a blower fan is visible, with one wire noted as being desoldered, possibly to disable it due to noise.
3:31 Receiver Architecture: Referring to the block diagram, the unit is identified as a triple-conversion superheterodyne receiver with IF stages at 15 MHz, 3 MHz, and 500 kHz (with 40 kHz bandwidth for the 50kHz step version).
5:01 RF Path: The signal path from the antenna connector through a coaxial relay (RF switch for TX/RX) to the RF amplifier module is traced. The transmitter output stage location is also pointed out.
5:43 Frequency Synthesizer: The complex electromechanical frequency synthesizer is revealed, using banks of crystals (34 for VHF LO, 20 for HF LO) selected by motor-driven rotary switches.
6:47 Synthesizer Mechanics: A single DC motor drives the rotary switches for both VHF and HF oscillators via slip clutches and a differential gear system.
7:12 Differential & Tuning: The differential mechanism combines inputs derived from cams and gears to precisely position the shafts of multiple ganged tuning capacitors used across various RF circuits (e.g., input amplifier tuning).
8:52 Receiver Board: A glimpse of the receiver circuitry shows traditional "old school" point-to-point or turret board style wiring.
9:14 Transmitter Output Stage: The transmitter final stage is examined, identifying the 7558 driver tube and the 8121 power amplifier tube. A directional coupler for measuring forward/reflected power (SWR) is present.
10:16 PA Coupling & Filtering: The output coupling components, including high-voltage capacitors and tuning capacitors linked to the main tuning shaft, are shown, along with the output filter assembly leading to the antenna relay.
11:13 Bottom Side Components: The underside reveals spare fuses and the transformer for the high-voltage switched-mode power supply (identified as an autoresonant type).
11:40 Modulation System: The large modulation transformer is shown, used for modulating both the plate and screen grid voltages of the power amplifier tube. The audio amplifier/limiter path leading to it is mentioned.
12:16 Power Transistors: Several power transistors (TO-66 package) are identified, used for the modulator driver and the switched-mode power supply.