This document is an engineering-grade analysis of RFI mechanisms affecting the HF-380, structured around three questions: what does the HF-380 radiate or conduct to the outside world; what couples into it through its cables, connectors, and chassis; and what can be done about both? The mitigation programme presented here is drawn from the Collins factory service bulletin record, the K9YC and ARRL RFI literature, and Fair-Rite, Würth, and Schaffner component data.

1. Architecture Recap — RFI-Relevant Circuits

The HF-380 is a fully solid-state, synthesiser-controlled, up-conversion transceiver. It was introduced in 1979 as the KWM-380 for the amateur market, with the HF-380 following as the militarised/commercial variant. Before analysing RFI it is worth enumerating the HF-380 subsystems that actually matter for emissions, susceptibility, and internal noise floor:

• Up-conversion to 39.145 MHz first IF. No bandpass preselector is needed — instead, a sequence of half-octave input low-pass/bandpass filters (diode-switched) precedes the first mixer. These switching diodes (A3CR800–CR807) are identified in factory service literature as vulnerable to T/R-relay-induced transients.
• Passband tuning. The 455 kHz signal is translated up to 6.255 MHz, filtered by the selected crystal filter (2.1 kHz SSB standard, optional 1.7 kHz, 8 kHz AM, 360 Hz CW, 140 Hz CW), and translated back. Two local oscillators are involved: 39.6 MHz reference and 455 kHz.
• Motorola 6802 microprocessor. A 2 MHz clocked 8-bit CPU (8-bit data bus internal, with external crystal) controls the synthesiser, display, filter selection, and tuning-knob input. The 6802 bus, address-decode logic, and Kiron memory (if installed) are the primary in-band digital emitters.
• Optical photo-chopper encoders. Two LEDs plus photo-transistors generate up/down pulses for tuning. These are inherently low-RFI compared with mechanical PTOs but contribute 50 µs-edged CMOS pulses onto the control board.
• Solid-state 100 W linear PA. Push-pull RF power stage with diode-switched low-pass harmonic filter per band, followed by the antenna relay and output connector. Harmonic rejection depends entirely on the band-pass output filter integrity.
• Linear power supply. The HF-380 uses a linear mains-derived supply, not a switching converter — a significant RFI advantage over later solid-state transceivers. However, the bridge rectifiers still exhibit reverse-recovery transients and the primary-to-secondary coupling path is worth addressing.
• Mobile DC operation. The HF-380 can be powered from 12–15 VDC (approximately 120 W on receive, 600 W transmit peak). Mobile installations introduce a new family of RFI paths: alternator whine, ignition noise, regulator switching, and accessory SMPS content.
• CU-380 / KC-380 interface. The HF-380 is normally used with the CU-380 antenna coupler (commercial version of the amateur KAT-380) via a multi-conductor control cable. This cable is a well-documented ingress path for external noise and can re-radiate HF from the PA.

This architecture is substantially cleaner than the later generation of fully microprocessor-controlled SMPS-powered transceivers — but it is also a product of 1979, with 6802 clock rates, optical encoder drivers, and bridge-rectifier DC supplies all creating specific, localised RFI signatures that can be addressed surgically.

2. Internal RFI Sources — What the HF-380 Generates

The HF-380 is neither silent nor especially noisy. Its internal emitters fall into five categories, each with its characteristic band and its characteristic mitigation approach:

Of these five, only the PA harmonic output carries HIGH severity in normal service — because it is the only emitter that, if neglected, can cause observable interference to other radio services and invite regulatory attention. Every other internal emitter is effectively a receiver self-noise issue: it affects the HF-380’s own noise floor but does not leak into the broader RF environment.

3. External RFI Ingress Paths — What Gets In

This is where the modern RF environment fights the HF-380. In 1980 the HF noise floor was dominated by distant thunderstorms and household appliances with brush motors. In 2026, every nearby LED streetlamp, plasma TV, solar charger, VFD-driven pool pump, and broadband-over-powerline node is a candidate HF emitter. The HF-380’s six ingress paths, ranked by practical importance:

1. Antenna feedline common-mode current. By far the dominant coupling path. Noise sources radiate into the outer surface of the coax shield, which acts as an insulated vertical element with effective height typically exceeding the intended antenna. Without a common-mode choke at the HF-380 antenna connector, every watt of shield-borne noise reaches the first mixer.
2. AC mains cord. The HF-380 linear supply has limited primary-to-secondary isolation at HF. The mains cord presents a low-impedance RF path both outward (conducted emissions from bridge-rectifier transients) and inward (conducted immunity — mains-borne noise riding into the receiver via the AC inlet). Fair-Rite and Schaffner line filters with integral common-mode chokes are the correct treatment.
3. Microphone input. The HF-380 microphone jack is a known RFI susceptibility point — so much so that Collins issued Product Improvement Bulletin 1-84 specifically to address microphone input RFI and speaker talk-back during transmit. The fix changes input impedance and adds bypassing; a 1–2-turn ferrite choke on the mic cable at the HF-380 connector is a compatible external supplement.
4. CU-380 / accessory control cables. The control cable to the CU-380 coupler, the RS-232 line for Kiron memory programming, and any external tune/keying lines all penetrate the chassis shield. Each is a potential HF carrier for both emissions (PA energy radiating back into the shack) and immunity (shack noise entering the control/audio board).
5. DC power cable (mobile/base DC operation). When powered from a 12–15 V supply, the HF-380’s DC input is a major RFI ingress path. Vehicle alternator ripple (whine) and bench-supply SMPS switching transients both couple directly into the HF-380 rails. A common-mode choke plus bypass cap network on the DC cable at the HF-380 connector is mandatory for quiet mobile operation.
6. Chassis ground path. The HF-380’s rear-panel ground lug, when connected to a poorly bonded shack ground system, can become a resonant element that either radiates PA harmonics or couples in noise from adjacent equipment. Low-impedance bonding to a station ground bus is essential.

4. Service Bulletin Record — The Factory-Documented RFI Fixes

Collins issued 18 numbered Service Bulletins and 7 Service Information Letters over the production life of the KWM-380/HF-380. Of these, the following address RFI, spectral purity, or susceptibility directly and should be the first consideration in any HF-380 RFI programme — there is no point adding external ferrites to a unit that has not yet been brought up to current factory configuration.

A full SB/SIL compliance check is available from WA9Z / Exline Signal LLC (Jim Warner, +1-641-856-7772) or KX6K / Bob Struk (+1-408-725-8912). Both service shops will inspect a unit, report current configuration, and apply missing bulletins as part of routine service. The full SB/SIL set is archived at collinsradio.org/cca-collins-technical-archives.

5. Ferrite & Filter Component Catalogue

Ferrite material choice is the most commonly mis-specified element of amateur RFI work. For the HF-380’s operating range (0.5–30 MHz receive; 1.8–30 MHz transmit), only two Fair-Rite materials are appropriate for most applications: #31 (manganese-zinc, lossy from 1–300 MHz, ideal for HF common-mode chokes) and #43 (nickel-zinc, lossy from 25 MHz to several hundred MHz, excellent for upper-HF through VHF). Below 2 MHz, #75 material extends effective absorption down to mains ripple frequencies and is sometimes preferred for AC line filters.

6. The 30-Point Mitigation Programme

The programme is structured in three tiers. Tier 1 is mandatory for every HF-380 — service-bulletin compliance, the three external ferrite chokes that address 80% of typical installations, and bridge-rectifier snubber strapping. Tier 2 adds ingress-path ferrites for shack/accessory cables. Tier 3 addresses installation specifics — mobile, co-site, rack-mounted, and high-noise-floor environments.

Tier 1 — Mandatory Baseline (Points 1–10)

1. Verify all SB/SIL compliance. Inspect serial number and date-code stickers against the CCA master SB list; apply any missing bulletins. Priority: SB-2, SB-7, SB-9, SB-15, SB-16, SB-18, SIL 1-84.
2. Antenna-feedline common-mode choke. 5 turns of coax through a Fair-Rite 2631803802 (#31 2.4” toroid) at the HF-380 antenna jack. Target: >500 Ω common-mode impedance from 2–30 MHz.
3. AC mains cord choke. 7–10 turns of the AC cord through a Fair-Rite 2675102002 (#75) at the rear of the transceiver. Choose #75 over #31 here to extend effective absorption down to the ripple-sideband range.
4. Microphone cable choke. Snap a Fair-Rite 2643625002 (#43) over the mic cable within 15 cm of the HF-380 front-panel connector.
5. CU-380 control cable choke. 3–5 turns of the control cable through a Fair-Rite 2631803802 at the HF-380 end. If the CU-380 is remote-mounted outdoors, add a second choke at the coupler end.
6. Chassis-to-ground-bus bond. Heavy copper strap (minimum 25 mm wide) from rear-panel ground lug to station ground bus. Keep under 30 cm; avoid coiled wire.
7. Ensure all diode-switched BPF diodes (A3CR800–CR807) are current-production. SIL 1-88 protection applies; HP-72 or 1N4148 diodes are common replacements.
8. Confirm PA low-pass filter integrity. Any band with an open LPF inductor or leaky bypass capacitor will dramatically raise harmonic output. Check with a spectrum analyser or tracking generator into a 50 Ω load.
9. Bridge-rectifier snubber strapping & electrolytic recap. Across each diode in the main PA-supply bridge, install an RC snubber: 100 Ω 1 W metal-film in series with a 10 nF 1 kV film capacitor (Vishay MKP1840-410-105). Keep leads under 25 mm; solder directly across diode pads. Reform or replace all aged main-filter electrolytics with modern 105 °C long-life types (Nichicon UHE, Panasonic FC). Together these eliminate the single largest class of internal RFI energy before any external mitigation is applied.
10. Baseline noise-floor measurement. Before further work, record the receiver noise floor with a 50 Ω termination on the antenna jack, on each band. This becomes the reference for all subsequent improvements.

Tier 2 — Shack & Accessory Ingress (Points 11–20)

11. External speaker cable choke. Small #43 bead at the HF-380 end. Speaker leads act as an extended antenna on HF.
12. Headphone-output cable choke. #43 bead at the HF-380 end. Ingress via headphone cables is surprisingly common during transmit.
13. CW key/keyer cable choke. #43 bead; add a 0.01 µF ceramic to chassis at the HF-380 end for RF bypass if keying instability is observed.
14. Alpha/amplifier keying line choke. If the HF-380 is used with an external amplifier, apply a #43 snap-on to the amp-key line. This path is notorious for causing PTT chatter under high-RF-field conditions.
15. RS-232 / Kiron programming line choke. #43 bead at the HF-380 end; second choke at the PC end.
16. PC/accessory USB cable choke. Any USB cable that enters the shack is a candidate HF emitter; #31 or #43 snap-on near the HF-380 end of any associated audio-interface cable.
17. LED shack-lighting replacement. If the HF-380 is operating in a room with commercial LED bulbs, replace them with verified-clean models (Cree PAR30 series and Philips Hue are typically acceptable; avoid dollar-store units).
18. Bench-supply SMPS review. Where the HF-380 shares a bench with SMPS-powered test gear, place all SMPS-equipped units on a filtered mains bar (Tripp-Lite Isobar series) separate from the HF-380 mains feed.
19. Ethernet cable choke (if PC interfaced). Cat-5/6 Ethernet cables entering the shack should pass through a #31 toroid (5–7 turns) before reaching networked equipment near the HF-380.
20. Coax double-shielding / SDR-tap isolation. For installations using a Web-888 or KiwiSDR tap on the HF-380 feedline, use double-shielded or quad-shielded coax from the tap to the SDR and ensure the SDR ground is bonded to the same bus as the HF-380 chassis.

Tier 3 — Installation-Specific (Points 21–30)

21. Mobile DC cable common-mode choke. 10–12 turns of the 12 V DC cable through a Fair-Rite 2631540002 at the HF-380 end. Add a 1000 µF / 25 V electrolytic plus 0.1 µF ceramic across the connector.
22. Mobile chassis bond. Short heavy strap from the HF-380 chassis to the vehicle body within 30 cm. Do not rely on the DC cable negative for RF return.
23. Alternator-whine suppression. If mobile, install an alternator spark-plug-style bypass capacitor (0.5 µF / 50 V) from the alternator output to the engine block.
24. Rack-mount shielding continuity. When the HF-380 is rack-mounted, verify that all rack screws make clean metal-to-metal contact with the chassis front panel; add tooth washers if paint is present.
25. Co-site PA isolation. If co-sited with a second HF transmitter (common in commercial installations) use a minimum of 40 dB antenna-system isolation and apply an external band-pass/band-stop filter to the receive path during the other transmitter’s active periods.
26. External 50 MHz LPF during transmit. Even with SB-2 and SIL 3-84 applied, an external Array Solutions or comparable harmonic filter in the transmit path is cheap insurance against 2nd-harmonic radiation onto 6 m.
27. Solar-installation bonding review. Where the station site has PV panels, verify inverter EMC compliance (FCC Part 15B at minimum) and ensure all PV DC runs are at least 3 m from the HF-380 antenna system.
28. Broadband-over-powerline (BPL) survey. Spot-check the HF-380’s noise floor at several times of day; BPL emissions can be intermittent. Report sustained interference to the local utility; the FCC still requires BPL operators to resolve confirmed interference.
29. Ferrite inventory documentation. Record every ferrite added and its location in a station log. Ferrite saturates if the conductor through it carries too much current; monitoring the station log makes fault-finding vastly easier.
30. Periodic re-verification. Repeat the Tier 1 noise-floor measurement every 6–12 months. RFI environments drift; new LED, SMPS, and solar installations may appear near the station between measurements.

7. Installation-Specific Considerations

Mobile & Field Installation

Because the HF-380 can operate from 12–15 VDC, many commercial and government installations ran it mobile. The receive current of approximately 8 A and transmit peak of up to 50 A at 12 V make heavy-gauge DC wiring mandatory — but heavy DC wiring is also the single biggest RFI ingress path in the mobile environment. The Tier 3 chokes (items 21–23) are non-negotiable for quiet mobile operation. A 10–12 turn common-mode choke on the DC cable, a short direct chassis bond to the vehicle body, and an alternator bypass capacitor are the baseline; any remaining alternator whine points to a ground-loop issue rather than a filtering issue.

Co-Site with Other HF Transmitters

Commercial HF-380 installations frequently shared antenna sites with other HF transmitters on adjacent frequencies. The HF-380’s front-end is robust (+15 dBm third-order intercept is specified) but not infinite. Where another transmitter is within a few hundred metres and within the same half-octave band-pass filter window, practical options are antenna-system isolation (physical separation, cross-polarisation, or directional nulls) and external band-pass/band-reject filtering in the HF-380 receive path during the adjacent transmitter’s active periods.

Rack-Mounted in Metal Enclosures

The HF-380 was supplied with rack-mount hardware for standard 19” racks. A rack-mounted HF-380 enjoys additional shielding — but only if the rack itself is properly bonded. Painted-panel rack rails often have insufficient contact to the chassis front panel; a single tooth washer per rack screw restores the bond. The rack frame should be tied to the station ground bus with a short, flat copper strap.

High-Ambient-Noise Urban QTH

The modern urban HF environment frequently runs S7 or higher on the lower bands simply from ambient electrical noise. In such environments the full 30-point programme plus careful antenna siting (loops, receive-only Beverages, K9AY or pennant loops) offers the best improvement. The HF-380’s noise blanker (SB-2 service-bulletin version) is effective against impulse noise but is not a substitute for source suppression.

8. Component-Level SNR Improvement Programme

External ferrites and mains filters treat the environment around the HF-380. Internal component substitution treats the radio itself — addressing the incremental noise floor degradation that four decades of component ageing have contributed. The programme below is organised in three tiers parallel to the RFI programme; Tier 1 items are universally worthwhile, Tier 2 targeted at weak-signal operators, and Tier 3 at full restoration.

Tier 1 — Universal Improvements

Electrolytic recap programme. The single biggest improvement available. Every HF-380 now 40+ years old has aged aluminium electrolytics throughout the supply and audio chain. ESR climbs with age, filtering degrades, and 120 Hz ripple that was originally 50 mV can now be several hundred mV — directly raising the noise floor on the DC rails feeding the synthesiser and IF chain. Recommended parts:

• Nichicon UHE or Panasonic FC — general-purpose filter positions, 105 °C, 2000+ hour endurance.
• Nichicon FG (“Fine Gold”) — audio chain coupling and decoupling.
• Panasonic OS-CON / Al-polymer hybrid — local decoupling at each board power entry.
• Cornell Dubilier DCMC or Vishay 36DA — main PA-rail bulk capacitors (screw-terminal, 10,000–22,000 µF). Avoid NOS parts of uncertain shelf age.

Bridge-rectifier snubber strapping. Already included in the Tier 1 RFI programme (item 9); noted here because it also directly reduces the 120 Hz spectral content injected into the DC rails.

Board-entry decoupling supplement. At each board’s power entry point, add a 10 µF aluminium polymer (Panasonic SEPC or Nichicon PCF) in parallel with a 0.1 µF X7R ceramic, directly across the supply-to-ground pins where the harness enters. This costs under a dollar per board and significantly reduces board-to-board noise coupling that shows up as synthesiser-spur modulation on receive.

Tier 2 — Targeted Phase-Noise & Signal-Path Work

Low-noise LDO regulators on oscillator supplies. The 39.6 MHz reference oscillator and 455 kHz secondary oscillator are the biggest beneficiaries of supply-noise reduction. The original 7812/7805 regulators feeding these circuits have noise figures around 40–100 µV/√Hz — fine for 1979, poor for 2026. Substitute:

• Analog Devices LT3042 — 0.8 µV RMS noise (10 Hz–100 kHz), ultra-low-noise positive LDO.
• Texas Instruments TPS7A4700 — 4 µV RMS, selectable output voltage.

Neither is pin-compatible with a 7812 so this is a small daughter-board job, but the phase-noise reduction is substantial — typically 5–10 dB reduction in close-in reference spurs, which translates to cleaner weak-signal reception on crowded bands. Apply to the synthesiser board’s oscillator supplies specifically; there is no audible benefit to upgrading the logic-rail regulators.

Reference oscillator upgrade — AC-3807 or modern OCXO. The factory already offered this: the AC-3807 High Stability Oscillator option replaces the stock 39.6 MHz reference with an oven-controlled unit, delivering tighter long-term stability and lower phase noise. If an AC-3807 is not findable, a modern substitute using a Connor Winfield OH200 or Morion MV-89A OCXO with a simple buffer amp into the original injection point works well. Typical improvement: 10–15 dB reduction in synthesiser-spur levels and essentially zero temperature drift. This is the single cleanest “no regrets” improvement if nothing else is done.

Op-amp substitutions in audio & AGC. The audio output amp (LM380 or similar) and the AGC op-amps are candidates for quieter modern parts. For the audio output, TDA2003 or LM1875 run cleaner than the original LM380 with no pin changes needed. For AGC op-amps typically LM324 or LM301 sockets, substitute NE5532, OPA2134, or LME49720 (discontinued but still obtainable). Higher gain-bandwidth product in modern parts may require a small compensation cap across the feedback path, but settling behaviour improves and the noise floor drops by 3–6 dB. Keep TL071/TL082 sockets populated with TL-series parts — those were already the low-noise choice of 1979.

Audio-chain coupling capacitor substitution. Bipolar electrolytics in audio coupling positions should be replaced with Wima MKS or Vishay MKT polyester film. Film caps eliminate the small distortion that electrolytics introduce and are immune to the leakage-current drift that plagues aged electrolytics in coupling service.

Tier 3 — Full Restoration & High-Performance Installation

Carbon composition resistor substitution in the signal path. Collins used carbon composition resistors through much of the HF-380’s RF and audio sections. These drift with age and humidity, and more importantly exhibit measurable current noise — up to 3 µV per V of DC across them, compared with under 0.1 µV for metal film. In the IF gain stages, AGC loop, and audio preamp, replacement with 1% metal-film (Vishay/Dale CMF55 or Yageo MF series) reduces floor noise by a few dB across the receive path. Tedious work and only worth doing during a full service cycle, but the improvement is real and permanent. Keep carbon comp in place only where intentionally used as a parasitic suppressor across RF chokes.

Front-end switching diode upgrade beyond SIL 1-88. The A3CR800–CR807 substitution to HP-72 or 1N4148 is SIL 1-88 baseline. Going further, PIN diodes — BAR63/BAR64 or the HSMP-3860 series — offer lower IMD and better linearity under strong-signal conditions. This is a significant change (bias network modifications required) and only worth considering in high-RF environments with adjacent strong signals.

Shielding supplements. Copper foil tape along any chassis seam that has developed paint or corrosion, a copper-tape strip across the top of the synthesiser board cage, and ferrite beads on any ribbon cables crossing from digital to analog sections. Individually small, collectively worth another 2–3 dB floor reduction.

9. Verification Procedure — Before/After Measurement

RFI work without measurement is guesswork. The following procedure produces a repeatable record suitable for comparing pre- and post-modification noise floors on the HF-380 itself; no external spectrum analyser is required for the basic check, though a Web-888, KiwiSDR, or similar wideband SDR tap greatly improves diagnostic resolution.

1. Establish a warm-up baseline. Power the HF-380 on and allow 15 minutes of warm-up before any measurement. The synthesiser’s ±5 Hz stability spec is valid only after 10 minutes.
2. Terminate the antenna port. Fit a clean 50 Ω dummy load directly at the antenna connector. Verify the load is within 1.1:1 VSWR.
3. Configure the receiver. SSB mode, AGC slow, RF gain full clockwise, no additional preamp. Passband tuning centred.
4. Record S-meter reading at 8 frequencies. 1.850, 3.750, 7.150, 10.125, 14.200, 18.125, 21.300, 28.500 MHz. The S-meter on the HF-380 is a useful relative-noise indicator when used with a fixed-gain configuration.
5. Swap the dummy load for the station antenna. Record the same 8 frequencies. The delta between dummy-load and antenna readings is the shack+site noise floor above the HF-380’s intrinsic noise.
6. Apply modifications one at a time. Repeat the two-step measurement after each modification. Retain only modifications that measurably improve either the dummy-load reading (intrinsic noise) or the delta (ingress).
7. Wideband verification with Web-888/KiwiSDR. If available, tap the HF-380 feedline through a directional coupler to a wideband SDR and record the 0.5–30 MHz noise floor as a single waterfall. Before/after waterfalls make 3–5 dB improvements immediately visible; they are almost invisible on the HF-380’s own S-meter.
8. Transmit-side verification. With the transmitter into a 50 Ω load, measure harmonic output with a spectrum analyser (or via a wideband SDR with appropriate attenuation). Harmonics should be at least 50 dB below carrier on all bands for a factory-spec-compliant unit.

10. References & Further Reading