COLLINS KWM-380
RFI Analysis & Mitigation Guide
A Comprehensive Engineering Reference for Reducing Noise Floor and Improving HF Signal-to-Noise Ratios
Mike Peace VK6ADA • r-390a.net • vk6ada.com.au • April 2026 • Rev 1.0

1. Introduction

The Rockwell Collins KWM-380 (introduced ~1979–1982) is a fundamentally different machine from the tube-era S-Line and KWM-2 equipment. It is a fully solid-state, microprocessor-controlled, PLL-synthesized HF transceiver covering 0.5–30 MHz receive and 160–10 meters transmit in 10 Hz steps. It contains its own internal power supply, speaker, and space for accessories — a complete 50-pound station in one package.^[1]

Its RFI profile is completely unlike the 516F-2 power supply problems covered in our companion guide. The KWM-380’s noise issues stem from internal switching DC-DC converters, PLL synthesizer phase noise and spurious products, microprocessor clock radiation, and aging tantalum capacitor failures — plus the usual external shack RFI from LED lighting, switch-mode power supplies, and Ethernet/solar equipment that plagues every HF installation in 2026.

Rockwell Collins themselves identified and addressed many of these issues through a remarkable series of 18 Service Bulletins and multiple Service Information Letters. This guide consolidates that factory knowledge with modern ferrite, LC filter, and component substitution techniques to achieve the best possible signal-to-noise performance from the KWM-380.

Applicable Equipment: Collins KWM-380, Rockwell Collins HF-380, and military/commercial variants. All Service Bulletins through SB-18 and SILs through 2-88 are referenced.

2. KWM-380 Architecture Overview

2.1 Signal Path Summary

Understanding the RFI emission and susceptibility mechanisms requires mapping the KWM-380’s signal architecture. The receiver uses a triple-conversion design with an unusually high first IF:^[1][2]

Stage	Frequency	Key Components	RFI Significance
Front End	0.5–30 MHz	Passive: HP/LP filters, PIN diode AGC (CR104), SRA1H diode ring mixer	No RF amp — excellent IP3 (+15 dBm) but directly exposed to conducted noise
1st IF	39.145 MHz	4-pole crystal filter, IF amplifiers, optional noise blanker (AC-3801)	High IF eliminates image problems but synthesizer spurs can appear in passband
2nd IF	455 kHz	Mixer U102, crystal filters (2.1/1.7/8.0 kHz)	Standard 455 kHz — susceptible to power rail noise coupling
PBT	6.255 MHz	Up/down conversion for passband tuning	Additional frequency translation — each mixer is a potential spur source
Synthesizer	39.145–69.145 MHz	PLL with 39.6 MHz + 455 kHz reference oscillators, microprocessor control	PRIMARY internal noise source: phase noise, birdies, spurs
Power Supply	AC mains or 12–15V DC	Bridge rectifiers, linear regulators, internal DC-DC converter	DC-DC converter switching noise; tantalum cap failures degrade decoupling

2.2 Power Supply Architecture

The KWM-380 is strappable for 105/115/125/210/220/230/240/250V AC (50–60 Hz) or 12–15V DC negative ground. Power consumption is 120W receive, 600W transmit.^[1] The internal power supply uses:

• AC input: External power transformer → internal bridge rectifiers → linear voltage regulators for main rails

• DC-DC converter: Internal switching converter generates additional voltage rails from the main DC bus. This is the single most significant internal noise source.^[3]

• Decoupling: Extensive use of tantalum capacitors on all boards — known failure mode in aged units^[3]

• DC operation: 12–15V DC input bypasses the AC transformer but the internal DC-DC converter still runs

2.3 Why the KWM-380 Is Different from the S-Line

The 516F-2/S-Line RFI problem is primarily about rectifier switching transients and conducted noise. The KWM-380’s problems are more subtle: synthesizer phase noise that degrades close-in dynamic range, DC-DC converter hash that raises the broadband noise floor, microprocessor clock leakage that creates discrete birdies, and aging tantalum capacitors whose failure progressively degrades on-board decoupling. The passive front end (no RF amplifier) means the KWM-380 is less susceptible to strong-signal overload than most transceivers, but any noise present on the power rails or ground bus couples directly into the signal path through the diode ring mixer.

3. Factory-Identified Issues: KWM-380 Service Bulletins

Rockwell Collins issued 18 Service Bulletins and multiple Service Information Letters for the KWM-380. The following are directly relevant to RFI, noise floor, and signal quality. All of these should be verified as completed before pursuing additional modifications.^[4]

SB/SIL	Description	RFI Impact	Priority
SB-2	Improve transmit spectral purity	Reduces unwanted transmit mixer products and spurious outputs	CRITICAL
SB-6	Correct transmit hum	Eliminates 60/120 Hz modulation on transmitted signal from PSU coupling	CRITICAL
SB-7	Reduce receiver birdies	Suppresses internally-generated spurious signals audible in the receiver	CRITICAL
SB-9	Stop PA oscillations	Eliminates parasitic oscillation in the PA stage which generates broadband hash	HIGH
SB-11	Add receiver low-pass filter	Additional filtering to improve receiver front-end selectivity	HIGH
SB-12	CW waveshape improvement	Reduces key clicks (broadband transient splatter during CW)	HIGH
SB-16	Improve frequency synthesis (supersedes SB-3)	Reduces synthesizer-generated spurs and improves phase noise performance	CRITICAL
SB-18	Eliminate RF pulse	Removes spurious RF pulse generation — a discrete broadband transient	CRITICAL
SIL 1-84	Remove RFI susceptibility on mic input; speaker talkback during TX	Eliminates RF ingress on microphone line and audio feedback during transmit	HIGH
SB-8 / SB-13	Improve receiver AGC action	Corrects AGC overshoot and recovery — improves noise-burst handling	HIGH

IMPORTANT: All SBs are available from the CCA archives at collinsradio.org. Jim Warner WA9Z at Exline Signal LLC is the primary specialist for KWM-380/HF-380 service, carrying original Collins parts and providing factory-level repair and SB implementation.^[5]

4. Internal RFI Source Analysis

4.1 DC-DC Converter Switching Noise

SEVERITY: CRITICAL. The KWM-380 contains an internal transistor-based DC-DC converter that generates auxiliary voltage rails from the main DC bus. Unlike the purely linear supplies in the S-Line era, this switching converter produces broadband hash at its switching frequency and harmonics, coupling into every circuit it feeds. The W5RRR Amateur Radio Club documented a KWM-380 repair where the DC-DC converter’s switching transistor was failing, causing rail voltage drops and increased noise.^[3]

Mitigation approaches:

• Verify converter health: Check all DC-DC output voltages under load. A weak or oscillating switching transistor dramatically increases noise output.

• Add LC post-filtering: Install a 10–47 µH inductor (wound on a Fair-Rite #31 toroid) in series with each DC-DC converter output rail, followed by a 0.1 µF + 10 µF capacitor to ground. This forms a pi-network that attenuates switching hash before it reaches the synthesizer and IF boards.

• Modern DC-DC replacement: At least one aftermarket modern DC-DC converter module has been designed as a drop-in replacement for the original.^[3] Modern converters typically operate at higher switching frequencies with lower output ripple.

• Ferrite beads on converter output leads: Type 43 ferrite beads (Fair-Rite 2643802702) on each output wire from the DC-DC converter board suppress conducted harmonics before they enter the power distribution bus.

4.2 PLL Synthesizer Phase Noise and Spurious Products

SEVERITY: CRITICAL. The KWM-380’s synthesizer generates local oscillator injection from 39.145 to 69.145 MHz for the first mixer (U100, Mini-Circuits SRA1H diode ring). Any phase noise or discrete spurious products on this LO signal translate directly into the receiver’s noise floor via reciprocal mixing. SB-7 (reduce receiver birdies) and SB-16 (improve frequency synthesis, superseding SB-3) specifically address factory-identified synthesizer spur problems.^[4]

Mitigation approaches:

• Verify SB-7 and SB-16 are installed — these are the single most important factory modifications for receiver noise performance.

• AC-3807 High Stability Oscillator: If available, this optional crystal oscillator upgrade improves the 39.6 MHz reference stability and can reduce close-in phase noise.

• Synthesizer power rail decoupling: Replace aged tantalum capacitors on the synthesizer board with modern low-ESR electrolytics or solid polymer capacitors. Add 0.01 µF ceramic bypass caps at each IC power pin where not already present.

• Shielding integrity: Verify all internal shield partitions between synthesizer, IF, and PA compartments are properly seated and making good contact. A displaced shield can allow synthesizer radiation to couple directly into the IF chain.

4.3 Microprocessor Clock Radiation

SEVERITY: MODERATE. The microprocessor that controls tuning, display, and band switching operates with a crystal-controlled clock. Harmonics of this clock frequency can couple into the IF chain and appear as discrete birdies at specific receive frequencies. SB-7 partially addresses this.^[4]

• Verify good RF grounding on the processor board mounting screws and inter-board ground straps.

• Type 43 ferrite beads on the clock oscillator output line and on data/address bus lines where they exit the processor board can reduce radiated harmonics.

• Add 100 pF ceramic bypass capacitors at the processor IC Vcc pins if not already present.

4.4 Tantalum Capacitor Degradation

SEVERITY: CRITICAL (age-dependent). The KWM-380 uses tantalum capacitors extensively on the RF/Exciter board and other PCBs for power rail decoupling and signal bypass. After 40+ years, these capacitors develop increased ESR, reduced capacitance, or outright short circuits. A shorted tantalum can destroy active components; an open or high-ESR tantalum silently degrades decoupling, allowing power rail noise to enter signal circuits.^[3]

The W5RRR repair documented finding a shorted 47 µF tantalum on the RF/Exciter board, and recommended categorizing all tantalum capacitors by failure risk (higher value/voltage units fail first) and replacing at least the high-risk population proactively.^[3]

Recommended capacitor replacement strategy:

• Priority 1: Replace all tantalum caps ≥10 µF on the RF/Exciter board with modern low-ESR aluminum electrolytics or solid polymer equivalents (same value, same or higher voltage rating).

• Priority 2: Replace tantalums on the synthesizer board and DC-DC converter board.

• Priority 3: Replace remaining tantalums on IF/AF boards.

• Add ceramic bypass: Parallel each replacement electrolytic with a 0.1 µF MLCC ceramic cap for high-frequency decoupling that the electrolytic alone cannot provide.

4.5 Transmit Path Issues

Several SBs address transmit-side noise that radiates back into the shack and can couple into the receive path during T/R transitions:

• SB-2 (Spectral purity): Addresses unwanted mixer products in the transmit chain. These spurious outputs radiate from the antenna and can re-enter the receiver via common-mode paths.^[4]

• SB-6 (TX hum): Corrects 60/120 Hz power supply coupling into the transmit audio chain. This is a power rail decoupling issue.^[4]

• SB-9 (PA oscillation): Parasitic oscillation in the PA generates broadband noise that couples back through the T/R switching path.^[4]

• SB-12 (CW waveshape): Reduces key-click transients that generate broadband splatter.^[4]

• SB-18 (RF pulse): Eliminates a discrete RF pulse — a transient burst that occurs during certain switching events.^[4]

• SIL 1-84 (Mic RFI): Removes RF susceptibility on the microphone input and eliminates speaker talkback during transmit — an RF feedback path through the audio chain.^[4]

5. Ferrite Core RFI Suppression

K9YC’s ferrite material selection guidance applies identically to the KWM-380 as to any HF station.^[6][7] Fair-Rite #31 is the primary broadband HF suppression material; #43 for higher frequencies.

5.1 External Cable Ferrite Application

Location	What To Do	Material	Priority
AC Mains Input	Snap-on #31 on power cord near rear panel, 4–6 turns. Or use Corcom IEC EMI filter.	Fair-Rite #31 (0431164281)	CRITICAL
Antenna Coax (SO-239)	Snap-on #31 cores on coax at rear panel. Stack #31 + #43 for broadband.	Fair-Rite #31 (2631102002) ×2	CRITICAL
Speaker / Audio Output	Both leads through one snap-on #31 core. Especially for long speaker cables.	Fair-Rite #31 (0431164281)	HIGH
Microphone Cable	Snap-on #31 near KWM-380 mic connector. Addresses SIL 1-84 RF susceptibility.	Small #31 snap-on	HIGH
DC Power Input (12V operation)	Both leads through #31 snap-on. Suppresses conducted noise from external DC supply.	Fair-Rite #31 (0431164281)	CRITICAL (DC op)
CU-380 / Accessory Cables	Snap-on #31 on any control interface cables (CU-380, keyboard AC-3805A).	Fair-Rite #31	MODERATE

5.2 Internal Ferrite Application

CAUTION: Internal modifications require careful work on densely-packed PCBs. The KWM-380 is a complex, expensive transceiver — do not improvise without schematics and test equipment.

• DC-DC converter output leads: Type 43 ferrite beads (2643802702) on each output wire — the highest-priority internal ferrite application.

• Synthesizer power input: Type 31 ferrite bead on the Vcc lead entering the synthesizer board.

• Microprocessor clock line: Type 43 bead in series with clock output (check SB-7 first — may already be addressed).

• Inter-board ribbon cables: Where practical, route through small #31 snap-on cores to suppress common-mode noise propagation between boards.

6. LC Filter Networks and Component Substitutions

6.1 AC Mains Filtering

Install a commercial IEC-inlet EMI filter at the AC power entry point. Schaffner FN 2060 series, Schurter 5500 series, or Corcom 6EQ1. Rated 6A minimum at 250V AC with >30 dB common-mode attenuation from 0.5–30 MHz. This single device often produces the most dramatic improvement in noise floor for any transceiver on AC mains, because it blocks both noise from entering and noise from leaving the radio.^[6]

6.2 DC-DC Converter Post-Filter

The most effective internal modification for noise floor reduction. Install a CLC pi-network on each DC-DC converter output rail:

Component	Value	Type / Rating	Purpose
C1 (input)	0.1 µF + 10 µF	Ceramic MLCC + polymer electrolytic	Wideband + bulk bypass to ground
L1 (choke)	10–47 µH	Wound on Fair-Rite #31 toroid	Series HF impedance
C2 (output)	0.1 µF + 10 µF	Ceramic MLCC + polymer electrolytic	Clean rail to downstream circuits

6.3 Tantalum Capacitor Replacement Program

This is both a reliability and an RFI improvement. Replace failing tantalums with modern equivalents:

Original	Replacement	Notes
Tantalum 10–100 µF (decoupling)	Panasonic OS-CON or Nichicon UPJ series polymer electrolytic, same value, ≥ original voltage	Much lower ESR than aged tantalum. Parallel with 0.1 µF ceramic.
Tantalum 1–4.7 µF (signal bypass)	High-quality MLCC ceramic (X7R dielectric) if PCB space allows, otherwise low-ESR aluminum	Ceramics provide superior HF bypass. Watch DC bias derating on ceramics.
Main PSU filter electrolytics	Nichicon KA or similar 105°C long-life electrolytic, same value and voltage	Reform slowly with variac before full voltage if originals still in circuit.

6.4 Synthesizer Decoupling Enhancement

After tantalum replacement, add 100 pF to 1000 pF ceramic capacitors (C0G/NP0 dielectric for frequency-critical locations) at each PLL IC power pin. These provide the high-frequency decoupling (>10 MHz) that electrolytic capacitors cannot. This directly reduces phase noise sidebands caused by power rail modulation of the PLL VCO.

7. Shielding, Grounding, and Shack Integration

7.1 Internal Shielding

The KWM-380 has multiple internal shield compartments separating the PA, synthesizer, IF chain, and processor sections. Verify all shield partitions are properly seated with clean metal-to-metal contact. Over time, shield retaining clips can loosen and screws corrode. A displaced shield allows direct radiation coupling between the synthesizer and IF sections — one of the most common causes of birdies that persist after SB-7/SB-16 installation.

7.2 Station Ground Bus

Standard K9YC grounding practice applies:^[6] single-point RF ground bus using ≥1″ copper strap or #4 AWG braid. The KWM-380’s chassis ground lug should connect to this bus with the shortest possible copper strap. If operating with an external amplifier, antenna tuner, or other accessories, bond all equipment to the same ground bus with ≤12″ straps.

7.3 Broader Shack RFI Reduction

The KWM-380’s passive front end makes it relatively resistant to strong-signal overload, but conducted noise from modern devices still raises the noise floor. Address these common sources:

• LED lighting: Replace with incandescent or high-quality filtered LEDs. #31 ferrite on LED driver power cords.

• Switching power supplies: Use linear supplies where possible. #31 ferrite on all SMPS AC cords in the shack.

• Solar inverters: Major noise source 2–30 MHz. Install AC line filter on inverter output. #31 ferrite on all DC cabling near panels.

• Ethernet / network equipment: Shielded STP Cat6 with #31 ferrite at both ends. Remove powerline network adapters entirely.

• Antenna feedline: 1:1 common-mode choke at feedpoint and at shack entry using stacked #31 and #43 toroids.^[6][7]

• Dedicated AC circuit: Run a separate electrical circuit to the shack. Whole-shack EMI filter at entry point.

8. Prioritized Modification Sequence

Work in this order, evaluating noise floor after each step. Stop when performance meets your requirements.

Step	Action	Expected Improvement	Invasiveness
1	Verify all Service Bulletins (SB-2, 6, 7, 9, 11, 12, 16, 18) and SIL 1-84 are installed	Varies; eliminates factory-known problems	Per SB instructions
2	External ferrite cores on AC mains, antenna coax, speaker, mic cable	5–15 dB noise floor reduction	None (snap-on)
3	AC mains EMI filter (Corcom/Schaffner)	5–15 dB (mains noise dependent)	External
4	Replace high-risk tantalum capacitors (RF/Exciter board, synthesizer board)	3–10 dB (depends on degradation)	PCB soldering
5	DC-DC converter health check; ferrite beads on output leads	3–8 dB	Internal wiring
6	DC-DC converter LC post-filter pi-network	3–6 dB additional	Internal; requires space
7	Synthesizer board ceramic bypass cap additions (PLL ICs)	1–3 dB phase noise improvement	Fine PCB work
8	Station ground bus improvement; antenna CM chokes	2–6 dB	Station infrastructure
9	Shack-wide RFI reduction (LEDs, SMPS, Ethernet, solar)	Highly variable; often dramatic	Station infrastructure

9. Known Issues and Age-Related Failure Modes

Beyond RFI-specific concerns, the following age-related issues affect KWM-380 performance and should be addressed during any comprehensive service:

• Tantalum capacitor shorts: The number one failure mode. Can destroy ICs and transistors. Test by monitoring resistance on each power bus while spraying individual caps with freeze spray — a resistance change indicates a suspect cap.^[3]

• DC-DC converter transistor failure: The switching transistor degrades over time, causing rail voltage drops, increased ripple, and elevated noise floor. Replace with modern equivalent or aftermarket module.^[3]

• Optical encoder (SB-4): The main tuning encoder degrades with age. SB-4 specifies a replacement.

• Electrolytic capacitor reformation: If the KWM-380 has been unpowered for years, slowly ramp AC voltage with a variac over several hours to reform electrolytic dielectrics before applying full operating voltage.^[3]

• Relay contact oxidation: The T/R and band-switching relays can develop contact resistance after decades. Clean with DeoxIT or replace.

• PIN diode degradation: The CR104 PIN diode AGC attenuator and the band-switching PIN diodes can degrade, affecting front-end linearity and noise figure.

10. Complete KWM-380 Service Bulletin & SIL Index

For reference, the complete list of factory Service Bulletins and Service Information Letters:^[4][5]

Bulletin	Description
SB-1	Change mic impedance
SB-2	Improve transmit spectral purity
SB-3	Superseded by SB-16
SB-4	Replace optical encoder
SB-5	Improve AF/RF pot tapers
SB-6	Correct transmit hum
SB-7	Reduce receiver birdies
SB-8	Improve receiver AGC action
SB-9	Stop PA oscillations
SB-10	Add WARC band transmit
SB-11	Add receiver low-pass filter
SB-12	CW waveshape improvement
SB-13	Improve receiver AGC action (additional)
SB-14	Insure transmit audio response
SB-15	Add anti-static discharge path
SB-16	Improve frequency synthesis (supersedes SB-3)
SB-17	Add external tune line for CU-380
SB-18	Eliminate RF pulse
SIL 1-81	Provide connections to Collins S-Line phone patches
SIL 2-81	(General service information)
SIL 1-84	Remove RFI susceptibility on mic input; speaker talkback during TX
SIL 2-84	(Service information)
SIL 3-84	(Service information)
SIL 1-88	(Service information)
SIL 2-88	(Service information)

11. References, Citations & Acknowledgments

73 de VK6ADA
Mike Peace VK6ADA / r-390a.net Administrator
vk6ada.com.au • © 2026 VK6ADA — Freely distributable for non-commercial use.