Unit 3 — Assembly and Setup

TM-GEAR-012 — Open Handout TM Chapter: Chapter 4 ELOs: Execute assembly steps in the correct sequence; verify build quality before operation Estimated time: 20 minutes

Step 1: Read the TM

Open TM-GEAR-012. Read Chapter 4 — Construction and Assembly completely.

Then come back here.

Chapter 4 Content

  1. Mount air variable capacitors in the enclosure with motor shafts coupled via flexible shaft couplers. Use Hall-effect limit switches at both ends of travel to prevent over-rotation.
  2. Mount the roller inductor vertically. Couple the DC motor to the roller shaft via a gear train (10:1 reduction for precise positioning).
  3. Connect the 10-turn pots to the capacitor and inductor shafts via 1:1 direct couplers. Wire pot center tap to ESP32 ADC via a 10 kΩ voltage divider.
  4. Mount the SWR bridge inline between the radio connector and the T-network input. Connect VFWD and VREF to ESP32 ADC.
  5. Wire L298N H-bridges: IN1/IN2 and IN3/IN4 to ESP32 GPIOs (PWM for speed control); OUT1/OUT2 and OUT3/OUT4 to motors. Enable pins to 3.3V for always-on.

Assembly Quality

Chapter 4 specifies 5 construction/assembly steps.

The assembly directly determines RF performance. Common errors: - RF leads too long — lead inductance raises SWR and limits high-frequency performance - Cold solder joints on RF nodes — high resistance causes signal loss and intermittent behavior - Ground loops — multiple ground paths at different potentials cause noise and calibration errors - Ferrite winding errors — wrong turn count or direction reverses transformer polarity or changes impedance ratio - Incorrect winding direction on toroidal transformers — affects phase and common-mode rejection

If Chapter 4 specifies a verification step after assembly (e.g., "verify DC resistance = X before proceeding"), do it. Those checks exist because they are the most common failure points.

Self-Check Questions

SC3-1. How many assembly steps does Chapter 4 specify?

SC3-2. What is the first assembly step? State it exactly from the TM.

SC3-3. Does Chapter 4 specify maximum lead length anywhere? If so, what is the limit and why?

SC3-4. Does Chapter 4 require a bench verification after assembly? What does it check?

SC3-5. What would you do if a winding resistance measurement came out wrong during assembly verification?

Answer Key

SC3-1. Count the numbered steps in Chapter 4.

SC3-2. See Chapter 4, step 1. Copy it exactly.

SC3-3. RF lead length limits are typically 10–15 mm for HF circuits. Longer leads add ~1–2 nH per mm, raising inductive reactance at high frequencies.

SC3-4. Scan Chapter 4 for verification steps. Common checks: DC resistance, winding balance, null depth on test signal, impedance ratio.

SC3-5. Stop assembly. Diagnose before proceeding — a winding error found before completion is much easier to fix than one discovered after the unit is boxed.

Checkpoint

Before proceeding: - [ ] You have read Chapter 4 completely - [ ] You can state the number of assembly steps and the first and last steps - [ ] You understand how assembly quality affects RF performance

→ Proceed to Unit 4