KWM-380
Tantalum Capacitor Replacement Reference
Board-by-Board Location Guide, Risk Assessment & Replacement Recommendations
Mike Peace VK6ADA • r-390a.net • vk6ada.com.au • April 2026

This reference accompanies the Collins KWM-380 RFI Analysis & Mitigation Guide. A site visitor asked for a practical list of tantalum capacitors and their board locations to simplify the replacement process. This page consolidates what is documented publicly, provides a risk-based replacement strategy, and points you to the definitive factory source for exact component designators.

Why Replace Tantalum Capacitors?

Tantalum capacitors fail differently from aluminum electrolytics. When an aluminum electrolytic fails, it typically opens (loses capacitance) or leaks electrolyte. When a tantalum fails, it usually shorts — creating a dead short across the power rail it’s decoupling. This can destroy active components (ICs, transistors) fed by that rail and is one of the most common failure modes in KWM-380s that have sat unpowered for years.^[1]

Dave at W5RRR (Johnson Space Center Amateur Radio Club) documented a KWM-380 repair where a shorted 47 µF tantalum capacitor on the RF/Exciter board was found by monitoring resistance on the power bus while spraying individual caps with freeze spray — a resistance change identified the suspect cap. He counted 19 tantalum capacitors of 1 µF or larger on the RF/Exciter board alone, and reviewed the schematic to develop a risk listing based on voltage stress across each cap.^[1]

Key insight from W5RRR: Caps <1 µF are very low risk because there is less chemistry and energy density inside. Higher value (10 µF+) and higher voltage-rated tantalums fail first. The recommended compromise: replace all 47 µF tantalums as minimum, then all ≥10 µF, then remaining ≥1 µF as budget and time allow.^[1]

Definitive Source: KWM-380 Service Manual

The complete component-level parts list with exact designators (C101, C234, etc.) for every capacitor on every board is in the KWM-380 Transceiver Service Manual, Collins publication number 523-0769878. This manual contains full schematics, board layouts, and component locator diagrams.^[2]

It is available to Collins Collectors Association (CCA) members through the CCA Archives, and through Rockwell-Collins Amateur Products Distributor/Service Agencies. Jim Warner WA9Z at Exline Signal LLC stocks original parts and can provide detailed guidance specific to your unit’s serial number and revision level.^[3]

Recommendation: Before starting, obtain the Service Manual (523-0769878) and print the schematic sheets and board layout drawings for each board you plan to work on. Use the component locator diagrams to positively identify each tantalum by its circuit designator, value, and voltage rating. Do not rely on markings alone — the dipped tantalum “teardrop” packages used in this era often have faded or illegible markings after 40+ years.

Board-by-Board Tantalum Capacitor Guide

The KWM-380 uses through-hole dipped radial tantalum capacitors (the characteristic “teardrop” epoxy-dipped packages, typically yellow, orange, or blue). They are found on every major board. The following guide is organized by board location with estimated tantalum populations based on documented repair data and the known circuit architecture.^[1][2][4]

1. RF/Exciter Board (Receiver/Transmit Board)

This is the highest priority board — it contains the most tantalum capacitors and handles all receive and transmit signal processing. The W5RRR repair documented 19 tantalum capacitors ≥1 µF on this board.^[1]

Circuit Function	Typical Values	Est. Qty	Risk	Notes
Voltage regulator output decoupling	47 µF / 10–35V	2–4	HIGHEST	Near on-board regulators. The 47 µF is the documented failure.[1]
Power rail bulk decoupling	10–22 µF / 16–35V	4–6	HIGH	On +12V, +5V, and −12V rails distributed across the board
IF amplifier stage decoupling	4.7–10 µF / 10–16V	4–6	MODERATE	At 39.145 MHz IF, 455 kHz IF, and 6.255 MHz PBT stages
AGC loop / detector bypass	1–4.7 µF / 10–25V	2–4	MODERATE	AGC timing and hold circuits; PIN diode bias filter
Audio coupling / bypass	1–10 µF / 10–25V	2–3	MODERATE	Product detector output, mic preamp, AF chain

2. Synthesizer Board

Second priority. The PLL synthesizer generates the LO from 39.145–69.145 MHz. Degraded decoupling capacitors on this board directly increase phase noise and spurious products — the primary mechanism for receiver birdies. Tantalums here decouple the VCO, prescaler, PLL IC, and reference oscillator power rails.

Circuit Function	Typical Values	Est. Qty	Risk	Notes
VCO power rail decoupling	10–47 µF / 10–16V	1–2	HIGH	Noise on VCO supply = phase noise on every frequency
PLL IC / prescaler decoupling	4.7–10 µF / 10V	2–4	HIGH	Digital switching noise couples into PLL loop bandwidth
Reference oscillator (39.6 MHz, 455 kHz)	1–10 µF / 10V	2–3	MODERATE	Clean reference = clean LO output. Consider AC-3807 upgrade.
PLL loop filter	1–4.7 µF / 10V	1–2	MODERATE	Value-critical: must match original exactly or PLL loop dynamics change

3. DC-DC Converter & Power Supply Board

Third priority. The internal DC-DC converter and power supply regulation circuitry. Tantalums here serve as output filter capacitors for the converter and as decoupling for the linear regulators. The DC-DC converter switching transistor itself is also a known failure point.^[1]

Circuit Function	Typical Values	Est. Qty	Risk	Notes
DC-DC converter output filter	22–47 µF / 16–35V	2–4	HIGHEST	High voltage stress + ripple current. Replace with low-ESR polymer.
Linear regulator input/output	10–47 µF / 25–35V	2–4	HIGH	Regulator stability depends on output cap ESR. Match carefully.

4. Microprocessor / Control Board

Lower priority — the processor board operates at +5V with relatively low current. Tantalums here decouple the processor, RAM, EPROM, and display driver ICs.

Circuit Function	Typical Values	Est. Qty	Risk	Notes
+5V rail bulk decoupling	10–22 µF / 10V	2–3	MODERATE	Lower voltage stress = lower failure risk
IC-level bypass	0.1–1 µF / 10V	4–8	LOW	Small values rarely fail. Replace only if testing shows issues.

Estimated Tantalum Population Summary

Board	Tantalums ≥1µF	High-Risk (≥10µF)	Replace Priority
RF/Exciter (Rx/Tx)	19 (documented)[1]	6–10	1st
Synthesizer	6–11 (estimated)	2–4	2nd
DC-DC / Power Supply	4–8 (estimated)	4–8	3rd
Processor / Control	6–11 (estimated)	2–3	4th
TOTAL (est.)	35–49	14–25

Replacement Recommendations

Replacement Component Selection

Original Type	Recommended Replacement	Notes
Tantalum 10–100 µF (decoupling)	Panasonic OS-CON SVP/SEP series (polymer aluminum) or Nichicon UPJ series	Much lower ESR. Same or higher voltage rating. Parallel each with 0.1 µF MLCC ceramic.
Tantalum 1–4.7 µF (signal bypass)	MLCC ceramic (X7R, 2× voltage rating) or Nichicon KL series (low-leakage)	Ceramics lose capacitance under DC bias — use 2× voltage rating to compensate. Film caps also work.
Tantalum <1 µF	Leave in place unless testing indicates failure	Very low risk. Less chemistry / energy density inside.[1]
PLL loop filter tantalum	Replace with exact same value — polymer or new tantalum	VALUE-CRITICAL: Do not change capacitance. PLL loop dynamics depend on this.
DC-DC converter output filter	Low-ESR polymer electrolytic (Panasonic EEHZA or Nichicon UCJ)	Must handle ripple current. Check ESR spec vs. original. Add 0.1 µF ceramic in parallel.
Linear regulator output	Polymer electrolytic or low-ESR aluminum electrolytic	CAUTION: Some older linear regulators (LM78xx/79xx) require minimum ESR on output cap for stability. Very low-ESR replacements can cause oscillation. Verify with datasheet.

How to Test Tantalum Capacitors In-Circuit

The freeze-spray technique documented by the W5RRR repair team is practical and effective:^[1]

1. Power off, discharge all capacitors.

2. Connect an ohmmeter across the power bus feeding the board under test.

3. Note the baseline resistance reading.

4. Spray each tantalum cap individually with freeze spray (component cooler).

5. A resistance change (especially a drop toward zero) identifies a suspect cap.

6. Desolder one lead of the suspect cap to confirm. Test out-of-circuit with ESR meter or capacitance meter.

For a more thorough approach: use an ESR meter (such as the Peak Atlas ESR70 or equivalent) to test each tantalum in-circuit. High ESR (>5× nominal for the value) indicates a degraded cap even if it hasn’t fully shorted. Any tantalum that measures high ESR should be replaced regardless of its value or risk category.

Parts Sources

• Jim Warner WA9Z, Exline Signal LLC — Original Collins KWM-380 parts. [email protected], (641) 856-7772^[3]

• Surplus Sales of Nebraska — KWM-380/HF-380 boards and parts. surplussales.com

• Mouser / Digi-Key — Modern polymer and low-ESR electrolytic replacements. Search by capacitance, voltage, and radial lead spacing.

• KE9PQ / Nationwide Radio — Collins KWM-380/HF-380 parts inventory. ke9pq.com

References

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