Collins Solid-State Restoration Series · Commercial / Government Variant

Collins HF-380 Transceiver — Tantalum Capacitor Replacement Reference Guide

A risk-prioritised service reference for the ~200-unit commercial/military variant of the KWM-380, with specific attention to the high-duty-cycle service profile that distinguishes the HF-380 from its amateur sibling.

Companion document to the KWM-380 Tantalum Capacitor Replacement Reference · Mike Peace VK6ADA

At A Glance

The Collins HF-380 is the commercial/government-market variant of the amateur-band KWM-380 transceiver — approximately 200 units were produced by Rockwell-Collins versus roughly 1,753 KWM-380s. The two transceivers share the same Receiver/Exciter (A3), Synthesizer (A1/A2), and PA architectures, and therefore share an identical tantalum capacitor failure profile.

The HF-380 presents one important difference for a capacitor-replacement campaign: it was built for sustained duty-cycle operation — often supplied with the high-duty-cycle blower kit, speech processor, remote control interface, and spare-parts kit as standard — and therefore its tantalum population has typically accumulated more ripple-current hours, more thermal cycles, and more voltage-stress hours than a comparable ham-shack KWM-380.

Bottom line: every HF-380 arriving on the bench in 2026 should be treated as a candidate for proactive tantalum replacement on the Receiver/Exciter A3, the 24 V regulator, and the PA bias/control boards, prioritised by voltage-stress ratio and location on the DC bus.

Contents

Why Tantalum Capacitors Fail — Physics and History
HF-380 vs KWM-380 — What Matters for Capacitor Service
Failure-Mode Signatures in the HF-380
HF-380 Circuit Card Architecture
Risk Classification Matrix — Voltage Stress Ratio Method
Per-Board Tantalum Replacement Tables
Modern Replacement Technology Options
Recommended Replacement Strategy
Replacement Procedure — Bench Work
Testing, Verification, and Burn-In
Sourcing Guide — Current Distributor Part Numbers
References and Further Reading

1. Why Tantalum Capacitors Fail — Physics and History

The HF-380 and KWM-380 were designed in the late 1970s using the dipped solid-tantalum and axial-lead solid-tantalum capacitor technology that was the dominant bypass and decoupling component of the era. Tantalum was chosen over aluminium electrolytic for good reasons: higher capacitance per unit volume, better ESR at HF frequencies, wider temperature range, and longer shelf life with no dried-electrolyte failure mode. Those advantages are real and the original design choice was correct for the era.

However, solid tantalums from this era have a well-documented age-related failure mode that the amateur radio community, the military space-systems community, and the industrial electronics community have all characterised in detail: manganese dioxide (MnO₂) cathode self-healing becomes progressively less effective with age, and when a localised defect in the tantalum pentoxide (Ta₂O₅) dielectric finally exceeds the cathode’s ability to cauterise itself, the result is an explosive, low-impedance failure that draws large current from whatever DC bus is feeding it.

Historical Context — NASA Perspective

NASA’s Goddard Space Flight Center published formal derating guidance in the 1980s that led directly to modern tantalum-replacement practice: for mission-critical applications, solid tantalum capacitors must be operated at no more than 50% of their rated voltage. Rockwell-Collins designed the HF-380 to commercial, not NASA-grade, derating standards — typical working voltages on 35 V-rated parts run in the 20–28 V range, well above the 17.5 V NASA limit. This is the single most important predictor of which tantalums in your HF-380 will fail first.

The Three Stressors That Kill Vintage Tantalums

Voltage stress ratio: the ratio of applied DC voltage to rated voltage. A 10 µF 25 V tantalum operated at 24 V bus voltage is at 96% of rating — it is living on borrowed time. The same part operated at 12 V is at 48% of rating and may run for another four decades without complaint.
Inrush current: a low-impedance supply (bench power supply, large filter capacitor on the DC input, or a hard switch-on transient) can push a current spike through a tantalum that exceeds its self-healing capability even when the DC voltage is within rating. This is why many HF-380 tantalum failures occur at the moment of power-on after extended storage — the capacitor survived thousands of hours of operation but could not survive the inrush after being energised cold.
Ripple current heating and thermal cycling: the HF-380’s extended-duty commercial service profile means tantalums on the Receiver/Exciter, 24 V regulator, and PA bias boards have typically seen more hours at working temperature than those in a KWM-380 that was only fired up for weekend DX contests. Each thermal cycle drives a small mechanical strain into the Ta₂O₅ dielectric.

2. HF-380 vs KWM-380 — What Matters for Capacitor Service

The HF-380 and KWM-380 share approximately 95% of their circuit topology. The Receiver/Exciter (A3), Synthesizer output and variable loops (A1/A2), Crystal Oscillator reference (A7), PA board, Directional Coupler, Low-Pass Filter board, and 24 V regulator are electrically and mechanically identical between the two models. Service bulletins are issued jointly as “KWM/HF-380” documents — there is no separate HF-380 bulletin series.

Five differences matter for a tantalum campaign:

Attribute	KWM-380 (Amateur)	HF-380 (Commercial)
Units produced	Approximately 1,753	Approximately 200
Typical service profile	Amateur SSB, CW, RTTY — intermittent duty, typical 10–30% duty cycle	Commercial/government HF point-to-point, continuous receive with frequent extended transmit periods
Blower	Standard convection cooling; high-duty-cycle blower kit optional	High-duty-cycle blower kit supplied as standard — implies more thermal cycles on bias/PA boards
Speech processor	Optional accessory	Supplied as standard — additional tantalum population on processor daughter card
Control A5 board	Amateur-band firmware, 160–10 m TX limits	Control A5 638-6929-004: full-coverage RX 1.6–29.999 MHz; TX per government licence
Keypad / remote interface	AC-3805A optional keypad	Keypad plus remote control interface 639-6910-001 typically fitted

The practical implication is clear: on an HF-380, the 24 V regulator board and the Receiver/Exciter tantalums are a higher priority than on a KWM-380 for the same calendar age, because they have seen more cumulative ripple current and thermal cycling. Commercial-service HF-380s that operated on a point-to-point government circuit for a decade may have 50,000+ powered hours on them — substantially more than a typical amateur-owned KWM-380.

3. Failure-Mode Signatures in the HF-380

A failing tantalum in the HF-380 is rarely ambiguous. The five signatures below cover the vast majority of field failures documented by the CCA, by Jim Warner WA9Z / Exline Signal LLC, by Bob Struk KX6K, and by informal community accounts going back to the early 1990s.

Signature 1 — Fuse Blows at Power-On

The HF-380 power-on inrush now trips the mains fuse or, for 24 V DC operation, trips the DC input breaker. A shorted tantalum on the 24 V bus is the most common cause. The fault is easily localised by powering the chassis with boards removed one at a time — when fuse behaviour returns to normal, the offending board has been found. Within that board, a cold-spray-while-monitoring-bus-resistance technique (apply cold freeze spray to each tantalum in turn while watching a milliohm meter between bus and ground) reliably identifies the shorted device.

Signature 2 — Low-Rail Collapse, No Power Supply Output

A mildly-shorted tantalum draws enough current to prevent the 24 V regulator from coming up to rail voltage, but not enough to blow the fuse. The DC-to-DC converter transistor in the 24 V regulator is a common secondary casualty of this fault — a failed tantalum on the output of the regulator causes the switching transistor to run at maximum duty cycle trying to pull the rail up, which then fails from thermal stress. A non-responsive HF-380 that has no rail voltage but also does not blow fuses should be diagnosed with this sequence in mind: test tantalums first, then the regulator transistor.

Signature 3 — Intermittent Hum, Instability, or Oscillation

A partially-degraded tantalum has elevated leakage current and elevated ESR without being fully shorted. On the Receiver/Exciter A3 board this manifests as transmit hum (directly addressed by service bulletin SB-6), receiver birdies (SB-7), or intermittent AGC instability (SB-8/SB-13). Owners often assume these symptoms require circuit modification when the underlying cause is a single degraded decoupling tantalum on the affected supply rail within the board. Service bulletin implementation plus tantalum replacement is almost always the correct approach — not one or the other.

Signature 4 — 4066 Analog Switch IC Cascade Failure

The HF-380 uses CMOS 4066 quad bilateral analog switches in several places on the Receiver/Exciter, most notably in the passband tuning, AGC time-constant, and audio routing sections. When a tantalum on the supply bus to a 4066 shorts, the resulting overvoltage or reverse-voltage transient often destroys the 4066 as a collateral casualty. Owners who experienced this cascade failure report that they recovered the radio only by replacing both the tantalum and one or more 4066 ICs — replacement 4066 parts (CD4066BE, MC14066B, or HEF4066B equivalent) should be on the shop shelf before starting this work.

Signature 5 — Characteristic Burnt-Manganese Smell

A tantalum that vented explosively leaves a distinctive acrid, sulphur-like smell inside the chassis. This is the combustion product of MnO₂ and the tantalum anode being driven through thermal runaway. Any HF-380 delivered to the bench with this smell has already suffered a catastrophic tantalum failure somewhere inside, and should be fully opened and visually inspected before power is reapplied. Look for PCB scorch marks, melted solder joints on the capacitor footprint, or a vent hole blown in the top of a dipped tantalum body.

4. HF-380 Circuit Card Architecture

The HF-380 is built around a motherboard that accepts pluggable card assemblies for the major subsystems. Each card contains its own local decoupling network of tantalum capacitors and ceramic bypass parts. The table below maps the card designators to Collins part numbers and identifies the cards with the heaviest tantalum population.

Card	Function	Collins P/N	Tantalum Risk
A1	Synthesizer Output Loop	638-6916-002 (541-7099-002 sub-assy)	Medium
A2	Synthesizer Variable Loop	638-6920-001	Medium
A3	Receiver/Exciter — the main signal-chain board	638-6908-002	HIGH — priority 1
A4	Passband Tuning (3-filter or 2-filter)	638-6912-002 (3F) / 638-6914-002 (2F)	Low-Medium
A5	Control — CPU/display/BCD interface	638-6929-004 (HF-380, gen-cov RX)	Medium
A7	Crystal Oscillator / Reference	638-6924-001 (oven 277-0494-020)	Low
PA	100 W Power Amplifier + bias regulator	634-8481-003 (638-6775-001 base)	Medium — bias tantalums at 28 V
DC	Directional Coupler	638-6788-001	Low
Reg	24 V Regulator / DC-DC Converter	638-6275-001 / 002	HIGH — priority 2
NB	Noise Blanker	641-7149-001 (638-6800-002 base)	Low-Medium
SP	Speech Processor (HF-380 standard)	Option assembly — see parts list	Low-Medium
CI	Remote Control Interface (HF-380 std)	639-6910-001	Low

Serial Number / Revision Variation

Collins issued board revisions throughout the HF-380/KWM-380 production run. The card part numbers above show the most commonly-encountered revision suffixes (-001, -002), but earlier production units may carry -000 suffix boards, and late-production or Rockwell-service-depot-updated units may carry -003 or higher. The tantalum population is broadly consistent across revisions, but specific reference designators can shift. Always cross-check against your unit’s service manual schematics and parts list before ordering replacements. The Collins Collectors Association archive holds the authoritative KWM/HF-380 service manual — citation in References.

5. Risk Classification Matrix — Voltage Stress Ratio Method

Rather than replacing every tantalum on every board (an expensive and invasive strategy that risks solder-damage to 45-year-old traces and through-holes), the defensible approach is to categorise each tantalum by its voltage stress ratio and replace the highest-risk parts first. The classification below is adapted from the methodology documented by Dave at W5RRR in his KWM-380 restoration narrative and aligns with the CCA repair-shop practice of Jim Warner WA9Z and Bob Struk KX6K.

Risk Class	Voltage Stress Ratio	Typical HF-380 Locations	Action
Class A — Replace now	> 80% of rated voltage	25 V-rated parts on the 24 V bus; 35 V-rated parts on the +28 V PA bias bus; the 47 µF bulk decoupler on A3	Unconditional replacement
Class B — Replace if accessible	50–80% of rated voltage	Larger-value decouplers on +12 V and +5 V CPU rails; IF-strip bypass tantalums on A3	Replace while the board is out
Class C — Monitor, defer	< 50% of rated voltage	Low-value signal-path coupling tantalums, audio-stage couplers operated at low bias	Test leakage; replace only if out of tolerance

The voltage stress ratio is calculated by dividing the measured DC bus voltage at the capacitor’s terminals by the capacitor’s rated voltage. Rated voltages on original-fit HF-380 tantalums are most commonly 10 V, 15 V, 25 V, and 35 V. For the 24 V regulated rail, any 25 V-rated part is by definition in Class A (stress ratio 96%); upgrading these to 35 V-rated modern replacements drops them to Class B (69%) and substantially extends service life.

6. Per-Board Tantalum Replacement Tables

The tables below summarise the typical tantalum populations by board, with recommended modern replacements. Capacitance values and voltage ratings are listed as Collins-specified; exact reference designators vary with board revision and must be cross-checked against the service manual schematic for your specific unit. Part numbers for modern replacements have been selected from current-production (2026) Mouser and DigiKey catalogue entries.

6.1 A3 Receiver/Exciter (638-6908-002) — Priority 1

The Receiver/Exciter carries the largest tantalum population in the HF-380 and is the single most important board for a capacitor-replacement campaign. A shorted tantalum here can take out the 4066 analog switch ICs (documented cascade failure), drop the 24 V rail, or produce any of the AGC/hum/birdie symptoms covered by the several service bulletins addressed to this board. Plan on a full afternoon for a comprehensive recap.

Value / Rating	Typical Function	Class	Recommended Modern Replacement
47 µF / 25 V	24 V bus bulk decoupler — documented single-point failure	A	Kemet T491D476K035AT (47 µF 35 V, 2917 SMD, or leaded T356E476K035AS)
22 µF / 25 V	+12 V, +15 V regulator output decouplers	B	Kemet T350E226K035AT (22 µF 35 V radial dipped)
10 µF / 25 V	IF-strip supply bypass, AGC time-constant, 4066 Vdd decoupling	B	Kemet T350D106K035AT (10 µF 35 V radial dipped) or T491C106K035AT (SMD)
4.7 µF / 15 V or 25 V	Audio chain coupling, product detector DC block	C	Kemet T350B475K035AT (4.7 µF 35 V) or Nichicon UWX1V4R7MCL1GB (aluminium polymer alternative)
2.2 µF / 15 V or 25 V	Op-amp decoupling, VOX time-constant	C	Kemet T350B225K035AT (2.2 µF 35 V)
1.0 µF / 35 V	Decoupling at various points in the mixer chain	C	Kemet T350A105K050AT (1 µF 50 V for headroom) or MLCC Kemet C1210C105K5RACTU (X7R)

6.2 24 V Regulator / DC-DC Converter (638-6275-001/002) — Priority 2

The 24 V regulator board handles the conversion of input DC (or rectified AC from the internal supply) to the regulated rails that feed the rest of the chassis. Tantalums here operate at high voltage stress and carry significant ripple current. On high-hours HF-380s operated on commercial circuits, this is the second most common recap priority after A3.

Value / Rating	Typical Function	Class	Recommended Modern Replacement
100 µF / 35 V	Input filter, output filter of DC-DC converter	A	Retire tantalum. Use Nichicon UPW1V101MPD (100 µF 35 V aluminium, 105°C) or Kemet A700V107M006ATE015 (polymer 100 µF 6.3 V not suitable here — confirm voltage). Aluminium is preferred on this high-ripple rail.
47 µF / 35 V	Regulator output decoupling	A	Kemet T491D476K050AT (47 µF 50 V) or Nichicon UWX1V470MCL1GB (aluminium polymer)
10 µF / 35 V	Error amplifier compensation, reference bypass	B	Kemet T350D106K050AT (10 µF 50 V)

DC-DC Converter Transistor — Common Collateral Failure

When the output tantalum on the 24 V regulator shorts, the switching transistor in the DC-DC converter frequently fails as a secondary casualty. Several field reports document needing to replace both the tantalum and the switcher transistor before the board returns to service. A modern alternative approach is to lift out the entire 638-6275 regulator and substitute a contemporary drop-in DC-DC module engineered for this specific application — third-party replacement modules are available from specialist Collins service providers. This is worth considering for high-hours HF-380 units where the original regulator has suffered multiple previous repairs.

6.3 PA Board (634-8481-003) — Priority 3

The PA board carries five RF power transistors and the final amplifier bias regulator. Tantalums on this board are typically in the bias-regulator section and on the DC feed to the RF stages — they carry significant ripple current during transmit and benefit from voltage-rating upgrades on replacement.

Value / Rating	Typical Function	Class	Recommended Modern Replacement
22 µF / 35 V	Bias regulator output decoupling	B	Kemet T350E226K050AT (22 µF 50 V)
10 µF / 35 V	RF-stage supply bypass	B	Kemet T350D106K050AT (10 µF 50 V)

6.4 A1/A2 Synthesizer (541-7099-002) — Medium Priority

The synthesizer is the most alignment-sensitive section of the HF-380 and should not be recapped unless there is a specific reason to do so (frequency instability, failed lock, birdies traced to the synthesizer, or an individual tantalum identified as failed). Tantalums on the synthesizer generally operate at modest voltage stress and are in Class C territory. If a full recap is performed, the unit will require complete synthesizer re-alignment against the procedure in the service manual.

6.5 Control A5 (638-6929-004) — Low-Medium Priority

The Control A5 board carries the 6802 CPU, EPROM, display drivers, and BCD-to-decimal decoders. Most tantalums here are decoupling the +5 V logic rail and operate at about 42% voltage stress (Class C). The exception is the 12 V rail bypass tantalum that feeds the display multiplexer, which is typically a 10 µF 15 V part operating at 80% stress (Class A/B boundary) and worth proactive replacement while the board is out. If the display flickers or loses segments intermittently, this is the first part to check.

7. Modern Replacement Technology Options

Four technologies are routinely used as modern replacements for the original solid tantalums in the HF-380. Each has trade-offs that matter in specific circuit positions.

7.1 Modern Solid Tantalum (Kemet T491, T350, T356; AVX TAJ series)

Functionally identical to the original 1979 parts but with substantially improved MnO₂ cathode manufacturing, tighter leakage tolerance, and — most importantly — the ability to get 35 V or 50 V rated parts in the same footprint as the original 25 V parts, dropping voltage stress ratio by 30–50%.

Use for: signal-path couplers, low-ripple decoupling, locations where the original ESR/ESL signature matters. Do not use for: high-ripple positions on the 24 V regulator — use aluminium or polymer there.

7.2 Polymer Tantalum (Kemet T520, T521; AVX TCJ series)

Conductive polymer cathode instead of MnO₂. Very low ESR, benign failure mode (opens rather than shorts when it fails), and no ignition risk. Cost roughly 2–4× solid tantalum equivalents.

Use for: any position where a shorted-failure mode would cause collateral damage — in particular the Vdd rail of 4066 ICs, the +5 V CPU rail on A5, and the 24 V bus bulk decoupler on A3. A polymer tantalum in the 47 µF position on A3 all but eliminates the documented Signature 1 failure.

7.3 Aluminium Polymer (Nichicon UWX, Panasonic OS-CON)

Aluminium anode with conductive polymer cathode. Very low ESR, high ripple-current capability, no ignition risk. Much larger physical size than equivalent tantalum.

Use for: high-ripple positions on the 24 V regulator and PA bias. Verify physical fit — some positions have insufficient clearance for an aluminium polymer in the same capacitance/voltage class as the original tantalum.

7.4 MLCC (X7R, Class II Ceramic)

Multi-layer ceramic. Available in values up to 100 µF in large case sizes (1210, 1812, 2220). Zero failure risk from cathode thermal runaway. DC voltage coefficient reduces effective capacitance — a 10 µF X7R MLCC at half-rated DC bias typically delivers only 5–7 µF of actual capacitance.

Use for: bypass and decoupling in low-capacitance positions where the original tantalum was an impedance element rather than a bulk storage element. Do not use for: AGC time-constant, audio coupling, or any position where the loss of capacitance under DC bias would shift a time constant or corner frequency. Mike recommends X7R only with case-by-case engineering judgement.

8. Recommended Replacement Strategy

A practical sequencing strategy for a HF-380 restoration, ordered from lowest risk to highest risk and from greatest payback to most discretionary:

Phase 1 — Power supply and regulator. Replace all Class A tantalums on the 24 V regulator board (638-6275). Use aluminium polymer for the high-ripple input and output positions. Verify the DC-DC converter transistor’s health and replace if marginal. Test the complete regulator on the bench with a dummy load before reconnecting to the chassis.
Phase 2 — Receiver/Exciter A3. Replace all Class A tantalums on A3, with specific attention to the 47 µF 24 V-bus bulk decoupler (use polymer tantalum here for failure-mode safety — this is the single most important capacitor change in the entire restoration). Replace Class B tantalums in the IF strip and AGC chain while the board is out. Test the board in isolation on a bench fixture if possible before reinsertion.
Phase 3 — PA and Directional Coupler. Replace Class A and B tantalums on the PA bias regulator. Inspect the directional coupler for the known-failure reed relay K1 (Wabash 1017-20-1, Collins P/N 410-0585-020) — if the coupler is shown to be failing on receive, relay replacement is often required in parallel with capacitor service.
Phase 4 — Control A5, Synthesizer, and Speech Processor. Replace Class A tantalums only (the 10 µF 15 V on A5 display multiplexer being the most common candidate). Leave the synthesizer alone unless a specific failure is in evidence; re-alignment is non-trivial and should not be triggered casually.
Phase 5 — Service bulletin implementation. With the boards out for recap, this is the optimal moment to implement the relevant service bulletins and SILs. SB-2 (transmit spectral purity) and SIL 3-84 (PA spurious emissions reduction) are essentially mandatory for regulatory compliance on-air. SB-8 or SB-13 (AGC improvement) resolves the receiver-choppy-on-SSB complaint. SIL 1-88 (front-end filter select diode protection) addresses a common failure mode on A3 and should be implemented on any HF-380 that will be operated near other transmitters.

9. Replacement Procedure — Bench Work

The HF-380 boards are plated-through-hole double-sided PCBs with through-plating on nearly all signal and power vias. This construction is unforgiving of careless desoldering; lifting a pad on the Receiver/Exciter or Synthesizer means either a delicate jumper repair or a full board replacement. The procedure below is derived from several decades of Collins community experience.

9.1 Tools and Workstation Setup

Temperature-controlled soldering station, set to 330°C / 625°F for leaded tantalum removal and 340°C / 645°F for plated-through-hole cleaning.
Solder sucker (mechanical) and solder wick — both are needed. Mechanical sucker for the bulk, wick for cleanup.
ESD-safe wrist strap and mat. The HF-380’s CMOS ICs (4066, 6802 CPU, EPROM) are vulnerable to ESD and cost real money to replace.
Magnification: 3× at minimum; 5–10× with ring light for via inspection.
Digital capacitance/ESR meter capable of measuring small tantalum values (Bob Parker’s Blue ESR, Peak Atlas ESR70, or equivalent).
Variable bench supply and milliohm meter for cold-spray short-hunting if a failed tantalum is already suspected.
Spare CD4066BE, MC14066B, or HEF4066B in the drawer before starting work on A3.

9.2 Desoldering Sequence

Mark capacitor polarity on the silkscreen with a fine permanent marker before removal. Original Collins silkscreen is readable on most boards but can fade near high-heat components.
Apply fresh solder to both leads of the capacitor to be removed — this rehydrates the old joint and makes desoldering much cleaner.
Remove solder with a mechanical sucker, one lead at a time, with the capacitor body lightly flexed away from the board so the lead clears the hole as solder is drawn.
Once both leads are free, lift the capacitor from above — never pull sideways or twist, which can tear the pad.
Clean the through-hole with wick and a tiny amount of flux until the hole is clear and the pad is bright.
Inspect the pad and the opposite-side via for cracking or lifting with magnification before installing the replacement.

9.3 Installation

Verify polarity of the new capacitor against the silkscreen mark. Modern Kemet tantalums mark the positive lead with a dark stripe (unlike aluminium electrolytic practice where the stripe indicates negative) — confusion here is a common failure source.
Lead-form the new part so it sits flat against the PCB with minimal lead length above the board — long leads increase ESL and can act as inadvertent RF antennas on A3.
Solder both leads with a single controlled heat application. Avoid extended dwell; the pad was just thermally cycled during removal and does not need a second full thermal excursion.
Trim leads close to the solder fillet on the reverse side.
Clean flux residue with isopropyl alcohol and a dedicated flux brush. Do not soak the board — the potentiometers and 4066 switches on A3 are not wash-tight.

10. Testing, Verification, and Burn-In

The golden rule after any recap: never apply full power directly to a just-recapped HF-380. Use a current-limited bench supply and a variac-fed Isolation transformer (for the AC supply path) to bring the rails up slowly while watching for anomalous current draw, smoke, or abnormal heating. The recommended sequence:

Cold resistance check. With power off, confirm the 24 V bus to chassis ground reads greater than 100 Ω (it should; a failed tantalum here would read < 1 Ω). Also check for reversed polarity on all recently-installed tantalums using an ohmmeter — a reversed tantalum reads very low resistance in one polarity.
Low-voltage ramp. Apply +12 V DC to the 24 V input terminal (half rated voltage) from a current-limited bench supply set to 500 mA compliance. Confirm no fault and confirm the 24 V regulator produces an output (it will be lower than nominal at this input voltage). Let the chassis sit at this voltage for fifteen minutes.
Full-voltage ramp. Raise to 24 V input with the current limit still at 500 mA. Verify rail voltages at 14 V, 12 V, and 5 V test points. If everything is clean, increase current compliance to 5 A for operational testing.
Receiver soak. Operate the receiver on an antenna for two hours. Check for the characteristic tantalum-failure smell, listen for AGC anomalies, and monitor 24 V rail voltage under varying signal conditions.
Low-power transmit test. With the PA drive turned well down, key into a dummy load for short periods. Watch the 24 V rail for sag and the PA bias voltage for stability. If the unit remains clean, progressively raise drive to full output over several cycles.
Extended burn-in. Run the HF-380 on receive for at least 24 hours before returning it to service. Most tantalum infant-mortality failures occur in the first few hours of operation.

11. Sourcing Guide — Current Distributor Part Numbers

The table below lists current (2026) Mouser and DigiKey part numbers for the complete set of replacement tantalums suitable for a full HF-380 recap. Part numbers are subject to distributor catalogue changes; verify availability before ordering a batch.

Value	Voltage	Type	Part Number	Distributor SKU
47 µF	35 V	Polymer Ta (recommended for A3 bulk)	Kemet T521D476M035ATE070	Mouser 80-T521D476M035E70
47 µF	35 V	Solid Ta (radial dipped, budget)	Kemet T356E476K035AS	Mouser 80-T356E476K035AS
22 µF	35 V	Solid Ta radial dipped	Kemet T350E226K035AT	Mouser 80-T350E226K035AT
10 µF	35 V	Solid Ta radial dipped	Kemet T350D106K035AT	Mouser 80-T350D106K035AT
10 µF	35 V	Polymer Ta (for 4066 Vdd)	Kemet T521D106M035ATE070	Mouser 80-T521D106M035E70
4.7 µF	35 V	Solid Ta radial dipped	Kemet T350B475K035AT	Mouser 80-T350B475K035AT
2.2 µF	35 V	Solid Ta radial dipped	Kemet T350B225K035AT	Mouser 80-T350B225K035AT
1.0 µF	50 V	Solid Ta radial dipped	Kemet T350A105K050AT	Mouser 80-T350A105K050AT
100 µF	35 V	Aluminium 105°C (for 24 V reg)	Nichicon UPW1V101MPD	DigiKey 493-13332-1-ND

11.1 Specialist Service Suppliers

For owners who prefer to have this work done professionally, two specialist service providers in North America are known to the CCA community for competent KWM/HF-380 restoration work:

Jim Warner WA9Z / Exline Signal LLC — collinsradio.org/category/kwm-380-service-and-parts/. Full commercial production service shop. Kiron memory units (OEM replacement), remote interface kits, programming, troubleshooting, and repair for all KWM/HF-380 series. CCA member discount available. Phone: (641) 856-7772.
Bob Struk KX6K — Expert repair and alignment of KWM-380 and HF-380. Full stock of OEM Collins parts. Repairs, rebuilds, and upgrades. Phone: (408) 725-8912.
Surplus Sales of Nebraska — surplussales.com/collins/CollKWM380-1.html. Current stock of replacement circuit boards including crystal oscillator A7, 24 V regulator A8/Reg, directional coupler, and oven sub-assemblies. Useful when a board is beyond economical repair.
KE9PQ Nationwide Radio Sales — ke9pq.com/collins-items/parts/new-category/. NOS KWM-380/HF-380 parts including the hard-to-find Wabash K1 reed relays, knob sets, service manuals, and front panel reproductions.

12. References and Further Reading

Collins Collectors Association archive: collinsradio.org/cca-collins-technical-archives/collins-radio-equipment-manuals/. Complete HF-380 Service Manual (de Katz), HF-380 Users Manual (de Katz), all 18 joint KWM/HF-380 service bulletins, and all 7 SILs. Also holds the Collins factory Self-Study Guide (KWM380_Self_Study_.pdf) which is the best introduction to the signal chain available.
Dave’s KWM-380 Repair Narrative (Johnson Space Center ARC, W5RRR): w5rrr.org/2025/06/06/the-anatomy-of-a-radio-repair/. First-hand narrative of a KWM-380 recap including the shorted 47 µF tantalum identification methodology and the risk-categorised tantalum list approach adopted by this reference.
WA3KEY KWM-380 Reference (John J. Schultz W4FA): wa3key.com/kwm380.html. The 1982 CQ Magazine review reprinted in full, with detailed circuit description of the front-end, IF chain, passband tuning, and synthesizer. Best single reference for the signal-chain theory of operation.
RigPix Database: rigpix.com/collins/hf380.htm for HF-380 specifications and internal photographs.
K9ZW — “With Varying Frequency” HF-380 articles: k9zw.wordpress.com/tag/collins-hf-380/. Detailed owner’s experience with an original (non-converted) HF-380 including rack setup, accessories, and interface to modern amplifiers.
VK6ADA KWM-380 Tantalum Capacitor Replacement Reference (companion document): vk6ada.com.au/kwm-380tantalum-capacitor-replacement-reference/.
NASA Goddard EEE Parts Derating Guidelines: the 50%-of-rated-voltage tantalum derating standard originates from NASA space-flight hardware practice and is the definitive technical basis for the voltage-stress-ratio classification used throughout this reference.