Collins KWM-2A HF Transceiver RFI Analysis and Receiver Noise Mitigation

1. Executive Summary

The Collins KWM-2A is a double-conversion SSB/CW transceiver covering 3.4–30.0 MHz, produced by Collins Radio Company from 1961 to 1979.^[1] It represents the pinnacle of 1960s tube-era HF engineering, rated at a receiver sensitivity of 0.5 µV for 10 dB S+N/N and a mechanical-filter selectivity of 2.1 kHz at −6 dB.^[2] These characteristics remain impressive, yet the radio was designed for an electromagnetic environment vastly different from today’s shack.

Solar MPPT charge controllers, grid-tied string inverters, LED lighting drivers, and switch-mode power supplies generate dense broadband and narrowband HF noise that can raise the effective noise floor of the KWM-2A by 20–40 dB on the lower bands. This guide provides a systematic, technically grounded set of modifications and external add-on treatments that restore — and in many cases exceed — the KWM-2A’s original signal-to-noise performance in the modern noise environment.

Modifications are organised by cost-effectiveness. Start at Section 4 (antenna feedline chokes); those alone typically produce 10–20 dB of noise floor improvement at negligible cost. All modifications described are fully reversible and require no cutting or removal of original Collins wiring.

2. KWM-2A Receiver Circuit Topology & RFI Vulnerability Map

2.1 Signal Path Overview

The KWM-2A uses a double-conversion superheterodyne receiver architecture shared with the companion Collins S-Line receivers (75S-1 / 75S-3).^[3] The circuit uses 18 vacuum tubes, with the RF amplifier (V7, 6DC6), first and second IF amplifiers (V1B/V3B, 6AZ8; V15A/V15B, 6BN8), and audio chain (V16A/V16B, 6EB8) constituting the primary signal path.^[17] Full schematics across all production editions are archived by the Collins Collectors Association^[20] and the European Collins Collectors Association (CCAE).^[4]

2.2 Modern Noise Sources and Their HF Characteristics

3. Ferrite Material Selection Guide

Not all ferrite is equal. Fair-Rite Products Corporation — the primary North American manufacturer — designates materials by a two-digit ‘Mix’ number. Selecting the correct mix for the frequency range of interest is critical: the wrong mix yields negligible choking impedance and false confidence.^[5]

The FT-240-31 (2.4″ OD toroid, Mix 31, P/N 5943003801)^[14] is the workhorse component for the majority of modifications below.

4. Modification 1 — Antenna Feedline Common-Mode Choke

4.1 Why Feedline Common-Mode Current Is the Primary Noise Path

The KWM-2A’s permeability-tuned bandpass input filters are highly effective at rejecting out-of-band and image signals arriving as differential signals between the antenna conductor and shield. However, RFI carried as common-mode current — flowing on the outside of the coaxial shield — bypasses these filters entirely. Jim Brown K9YC’s comprehensive RFI reference^[8] documents this mechanism in detail and provides choking impedance measurements for Mix #31 toroids across the HF spectrum.

4.2 Choke Construction — FT-240-31 Coaxial Choke (Shack Entry)

Construct a coaxial common-mode choke using RG-8X wound on a single Fair-Rite FT-240-31 toroid (P/N 5943003801,^[14] Amidon equivalent T-240-31).

4.3 Second Choke at the Antenna Feedpoint

A second identical choke placed at the antenna feedpoint prevents the antenna from picking up noise radiated from nearby solar panel wiring and LED installations and conducting it down the feedline’s outer shield. K9YC’s RFI Cookbook^[8] provides detailed impedance data for this dual-choke configuration at each HF band.

4.4 Snap-On Clamp Ferrites Along the Feedline

Install Fair-Rite snap-on clamp ferrites (P/N 0431164281, Mix 31, 1.0″ cable diameter)^[15] at both ends of the feedline — 3–4 clamp-ons clustered at each location, no cable cutting required.

5. Modification 2 — Power Supply Noise Filtering

5.1 Original Collins Power Supplies (PM-2, 516F-2)

The primary noise ingress is the AC mains lead. Switching noise from other equipment on the same branch circuit conducts into the power supply and onto the B+ rail inside the KWM-2A (approximately 250 V at up to 300 mA).^[20]

5.1.1 AC Mains EMI Filter

Install a Schaffner FN2070-3-06 or equivalent 3 A dual-stage mains EMI filter^[13] on the AC input cord of the power supply.

5.1.2 Clamp Ferrites on AC Mains Cord

Clip two to three Fair-Rite 0431164281 Mix #31 snap-on ferrites onto the mains cord of the power supply, clustered at the supply end.

5.2 Modern Switching Supplies and Solar DC Systems

When operating from a modern switching supply, solar/battery DC system, or a third-party 12 V supply through an MP-1 voltage converter, the DC supply leads are the primary noise coupling path. MPPT controllers are particularly severe: their switching frequency (typically 40–100 kHz) and harmonics appear directly on the DC bus.

5.2.1 DC Lead Common-Mode Choke

Wind both the positive and negative DC supply leads (bifilar — both wires side-by-side) through a Fair-Rite FT-240-31 toroid, 10–12 turns.^[14] This is the single most important filtering element for solar/battery DC installations.

5.2.2 Bypass Capacitor Bank at Supply Terminals

• 100 µF / 35 V electrolytic — suppresses low-frequency MPPT ripple
• 0.1 µF X7R ceramic, 50 V — suppresses mid-frequency switching harmonics
• 1 nF C0G/NP0 ceramic, 50 V — suppresses VHF resonance peaks

Combined, these three capacitors present an impedance below 0.5 Ω across 1 kHz to 100 MHz.

5.3 Solar MPPT Controller — Source Mitigation

• Relocate the MPPT controller as far from the antenna and feedline as physically possible.
• Run solar panel wiring in grounded metallic conduit (EMT) to prevent radiation from high-dV/dt switching waveforms.
• Wind the DC output cable of the MPPT controller through a Fair-Rite FT-240-31 toroid — 10 turns bifilar.
• Install a commercial DC-line EMI filter (TE Connectivity RSAL series, rated for actual DC current) inline between MPPT controller output and battery bus.

6. Modification 3 — Heater Supply Decoupling

The 6.3 V AC heater supply is distributed to all 18 tubes as a single-loop wire run. If conducted noise has entered the power supply, the heater winding distributes it to all tube heater pins — creating broadband noise injection throughout the RF chain.^[1]

6.1 Centre Tap Grounding Verification

Verify that the heater winding centre tap (or the artificial centre tap created by two 100-ohm resistors from each heater lead to chassis) is connected directly to the chassis. Each heater lead should read approximately 100–150 ohms to chassis through the centre tap resistors.

6.2 Heater Lead Common-Mode Choke

Wind both heater leads together (bifilar) through a Fair-Rite FT-114-43 toroid, 8–10 turns, between power supply connector J17 and the heater distribution.

6.3 Bypass Capacitors at Heater Pins

For tubes in the RF signal path — V7 (6DC6), V1B/V3B (6AZ8) — add 1 nF C0G/NP0 ceramic capacitors (50 V AC minimum) from each heater pin to the nearest chassis ground point.

7. Modification 4 — Audio Stage RF Bypass (LED Buzz Elimination)

A characteristic symptom of LED lighting interference in tube receivers is a high-pitched buzz in the audio — typically 30–100 kHz modulated onto the audio passband after rectification at the first audio tube’s grid. The V16A (6EB8 triode section) grid input connects to the AF GAIN control (R92) through a relatively long, unshielded wire inside the chassis, which acts as a pickup loop for HF noise fields.

7.1 V16A Grid Input RF Bypass

Install a 100 pF silver mica or C0G/NP0 ceramic capacitor directly across the V16A grid pin to the nearest chassis ground point. This forms a low-pass filter presenting ~454 Ω at 3.5 MHz and 114 Ω at 14 MHz — effectively shorting all HF energy to ground before it reaches the grid junction. The 3 kHz audio passband is entirely unaffected.

7.2 V16B Grid Input RF Bypass

Apply the same 100 pF bypass treatment to the V16B (6EB8 pentode section) grid pin.

7.3 Ferrite Bead on Grid Input Wiring

For installations with very severe LED interference, thread the V16A grid input wire through a single Fair-Rite 2743001111 Mix #43 ferrite bead (axial type, 0.25″ long).^[6] The bead adds ~180 Ω at 100 MHz in series with the grid wire — slipped over the existing wire, no soldering required.

8. Modification 5 — AVC/AGC Bus Decoupling

The AVC bus distributes AGC control voltage to V7 (RF amplifier), V1B (1st IF), and V3B (2nd IF). RF noise picked up by this bus wire is rectified at the AVC circuit and causes the receiver to attenuate gain in response to noise rather than signal — producing the characteristic pumping or breathing effect.

8.1 Collins Service Bulletin #8 (SB-8)

Collins issued Service Bulletin #8 to address AVC overshoot and add hang AGC action to the RF amplifier.^[16] If SB-8 has not been installed, its implementation is the most important electrical improvement to the receiver’s AGC performance in a noisy environment. It modifies V1B, V3B (6AZ8 IF amplifiers) and V7 (6DC6 RF amplifier). A detailed installation procedure is documented at jessystems.com.^[9]

8.2 AVC Bus Bypass Capacitor

Add a 0.01 µF (10 nF) ceramic capacitor from the AVC bus to chassis ground at V7’s AVC grid injection point. This reduces the effective noise bandwidth of the AVC rectifier, preventing switching transients from modulating the AVC voltage.

9. Modification 6 — IF Stage Screen Grid Bypass Renewal

Original Collins bypass capacitors in the IF chain are typically 0.01–0.1 µF disc ceramics in service for 60+ years. These age in two ways relevant to RFI: increased ESR (raising bypass impedance, allowing screen voltage ripple to modulate IF gain) and reduced capacitance (raising reactance at the noise frequency).

9.1 Screen Grid Bypass Renewal

Replace all screen grid bypass capacitors on V1B, V3B, and V15A/V15B with 0.1 µF / 100 V X7R ceramic capacitors. The X7R dielectric is stable over temperature and voltage — superior to the original disc ceramic composition.

9.2 IF Shield Can Contact Verification

Verify each IF coil shield can lid makes solid electrical contact with its can body, and that the can body contacts the chassis through its mounting bracket. Clean with DeoxIT and ensure lid clips are firm.

10. Modification 7 — Station Ground Architecture

Grounding is the foundation of all other RFI mitigation. Ground loops inject hum, buzz, and noise directly into the signal path regardless of how well the antenna and power leads are filtered. K9YC’s RFI Cookbook^[8] provides the definitive treatment of HF station grounding architecture.

10.1 Star Ground Architecture

All equipment — KWM-2A chassis, PM-2/516F-2 chassis, 30L-1 linear amplifier, 312B-5 network, and external accessories — must connect with a short, low-impedance ground strap to a central bus bar.

• Use 2″ wide copper strap (not wire) for all chassis bonding. Wire has inductance of ~250 nH/metre at HF; 2″ copper strap reduces this by approximately 10:1.
• Keep all ground strap lengths under 30 cm.
• Bond the KWM-2A chassis directly to the PM-2/516F-2 chassis with a dedicated strap — not through the control cable shield.
• Bond the AC mains safety ground and RF ground together at a single point — the service entrance panel.

10.2 Coax Shield Grounding at Shack Entry

Ground the feedline shield to the station star ground at the point of entry into the shack using an SO-239 bulkhead chassis connector with the body bonded to the building ground conductor and station star ground bus.

10.3 Interconnect Cable Ferrites

Install Fair-Rite 0431164281 Mix #31 snap-on ferrites on all cables between the KWM-2A and accessories: control cable to PM-2/516F-2, NB ANT / RELAY PATCH ANT relay cable, and audio patch cables. Cluster 3–4 clamps at each cable end.^[15]

11. Modification 8 — External Noise Blanker and DSP Noise Canceller

11.1 KWM-2A Built-In Noise Blanker Provision

The KWM-2A includes a noise blanker switch position (NB) on the function switch and a dedicated NB ANT relay connection.^[1] This provision accepts modern external noise blanker hardware, as detailed in the CCAE tools archive.^[4]

11.2 MFJ-1026 Noise Cancelling Signal Enhancer

The MFJ-1026^[11] is a phase-cancellation device that uses a separate sense antenna to sample the noise environment, then adjusts phase and amplitude of the noise sample and subtracts it from the main antenna signal. Under ideal conditions it provides 40–60 dB of cancellation for single-source noise. It connects in-line between the feedline and the KWM-2A antenna terminal.

11.3 Timewave DSP-599zx Audio DSP Filter

A Timewave DSP-599zx^[12] or equivalent DSP audio filter placed in the audio output chain provides 20–40 dB attenuation of impulse noise and narrowband tones while preserving SSB and CW intelligibility. Connects between the KWM-2A external speaker jack and the speaker — no RF-path modifications required.

11.4 Modern DSP Noise Blanker — F6EXG (CCAE) Technique

F6EXG of the CCAE documented a technique for installing a modern DSP noise blanker into the KWM-2A by tapping the 455 kHz second IF signal.^[10] The DSP blanker uses a digital detector to generate blanking pulses precisely aligned with noise impulses. This is the most technically ambitious modification, but delivers the most effective impulse noise rejection. Procedure documented at ccae.tm6cca.com/tools.html.

12. Modification 9 — Band-Specific Notch Filters

If a noise source operates at a specific fixed frequency — common for MPPT controllers and PLT/smart meter systems — a high-Q notch filter placed in series with the antenna provides targeted attenuation of 40–60 dB at that precise frequency while leaving adjacent frequencies unaffected.

12.1 Parallel LC Notch Filter

A parallel LC tank circuit resonant at the noise frequency presents very high impedance (notch) when placed in a series T configuration in the feedline. Silver-plated copper wound on a Micrometals T-68-6 powdered-iron core achieves Q values of 200–400, giving notch depths of 40–60 dB with a 3 dB bandwidth of 5–25 kHz.

12.2 MFJ-704 Commercial HF Low-Pass Filter

For simplified implementation, the MFJ-704 or equivalent commercial HF low-pass filter at the antenna input rejects all energy above 30 MHz. In-line, no-modification device.

13. Prioritized Implementation Plan

14. Parts Reference

15. Noise Floor Diagnostic Procedure