Collins KWM-380 Transceiver
Failure Prevention Kit — Component & Modification Design

Section 1 — Architecture Overview, Variants, and Community Resources

Key Architectural Distinctions

The KWM-380 is fundamentally different from any other radio in this series. Understanding its architecture is essential before fault-diagnosing any problem:

• No tube, no band switch, no preselector: The front end is entirely passive before the first mixer — BC-band rolloff, three switchable high-pass filters (7, 14, or 20 MHz cutoff), a fixed 30 MHz low-pass filter, and a PIN diode AGC attenuator (CR104). No RF amplifier, by design: the 39.145 MHz first IF makes image rejection inherent without a tracking preselector.
• 39.145 MHz first IF: The first mixer (U100, a Mini-Circuits SRA-1H diode ring) produces a 39.145 MHz IF from any signal in the 0.5–30 MHz range. A broad 4-pole crystal filter at this IF provides some selectivity. The high first IF provides better than 60 dB image rejection across the entire frequency range.
• 455 kHz second IF with passband tuning: The second IF is 455 kHz. A unique passband tuning circuit translates the 455 kHz signal up to 6.255 MHz and back, placing the selected crystal filter (2.1 kHz SSB standard, optional 1.7 kHz, 8 kHz AM, 360 Hz, 140 Hz CW) within the passband. This provides true passband tuning without filter frequency change.
• 6802 microprocessor: A Motorola 6802 CPU controls the synthesiser, frequency display, filter switching, and tuning knob interface. The tuning knob uses optical photo-choppers (optical encoders) rather than a mechanical PTO. The CPU accepts up-count and down-count pulses from the encoders to step the synthesiser in 10 Hz increments.
• Two VFOs (A/B): The KWM-380 stores two independent operating frequencies. On power-up both VFOs always reset to 15.000 MHz; there is no non-volatile memory in the basic unit. The optional Kiron memory units add non-volatile frequency storage.
• Automatic PA turndown: Without the optional blower kit (KWM/HF-380 Blower Kit), the PA automatically reduces to 50 W after 10 seconds of CW key-down to prevent thermal shutdown. With the blower kit installed: 100 W average, 50% duty cycle.
• VSWR protection: Automatic PA output turndown at VSWR greater than 2:1. This is a solid-state protection circuit — not a fuse or tube protection resistor. If the antenna system has a fault, the KWM-380 will reduce power; check the antenna first if output is reduced.
• Synthesiser oscillators: Accuracy and stability depend on two oscillators: the 39.6 MHz reference and the 455 kHz oscillator. Both must be within ±3 Hz of nominal for the synthesiser accuracy specification of ±5 Hz after 10 minutes warm-up.

Production Variants and HF-380

Service Bulletins and Service Information Letters — Complete Index

Community Resources

Section 2 — Root Cause Failure Analysis

The KWM-380’s solid-state construction gives it failure modes entirely different from the tube-based Collins equipment in this series. Age-related semiconductor junction degradation, electrolytic capacitor aging, optical encoder mechanical wear, and ICs at the end of their MTBF curves are the dominant concerns.

• 1
  Optical Encoder (Photo-Choppers) Failure — No Tuning Response [SB-4] The KWM-380’s main tuning knob uses optical photo-chopper encoders to generate up-count and down-count pulses for the 6802 CPU to step the synthesiser in 10 Hz increments. SB-4 “Replace Optical Encoder” is one of the most frequently required service bulletins in practice. The photo-chopper assembly consists of an LED light source and a phototransistor detector with a slotted disc interrupting the beam as the knob rotates. After 40+ years, the LED output degrades (reduced photon flux), the phototransistor sensitivity decreases, and the chopper disc may accumulate contamination on its slots. Symptoms: the main tuning knob appears to turn normally but the displayed frequency does not change, changes erratically, or changes in only one direction. In mild degradation, the tuning response becomes sluggish at slow rotation speeds while fast rotation still works. SB-4 provides the replacement encoder specification and installation procedure. The optical encoder assembly is a mechanical item with a finite service life; assume it requires inspection on every KWM-380 of unknown history.
• 2
  PA Oscillations — Uncontrolled Self-Excitation [SB-9] SB-9 “Stop PA Oscillations” is among the most safety-critical service bulletins issued for the KWM-380. An oscillating solid-state PA generates continuous RF at a frequency the ALC and power metering circuits may not correctly detect; this can drive the PA devices into excessive dissipation in a mode where normal protective feedback is ineffective. The oscillation may also produce broadband interference well outside the amateur bands. SB-9 modifies the PA driver and feedback network to eliminate the parasitic oscillation condition. Any KWM-380 whose PA has been producing distorted audio, excessive heat, or broadband noise should be checked for SB-9 implementation before any further transmit operation. Any unit with SB-9 not implemented should not be operated at full power until SB-9 is completed.
• 3
  Front-End Filter-Select Diodes Blown by T/R Relay Transients — Total Receive Loss [SIL 1-88] This is the most technically subtle failure mode in the KWM-380. Service Information Letter 1-88 documents that the front-end high-pass filter switching diodes A3CR800 through A3CR807 can be damaged or destroyed by voltage spikes produced when T/R relay A2AK1 switches during transmit/receive transitions. The relay switching transient exceeds the reverse breakdown voltage of the PIN diodes, permanently damaging their junction. The result is degraded or completely absent receive sensitivity on one or more of the high-pass filter selections, or erratic front-end behaviour that changes with filter position. Importantly, the T/R relay transient is a systemic design issue — it recurs on every transmit/receive switching cycle in unmodified units, meaning the diodes will be progressively stressed even after a repair if SIL 1-88 is not implemented. SIL 1-88 adds a transient suppression path (anti-static discharge and relay spike clamping) that protects the front-end diodes permanently.
• 4
  Frequency Synthesis Instability — Frequency Jumps, Lock Failures [SB-16 / SB-3] SB-16 “Improve Frequency Synthesis” supersedes the earlier SB-3. The KWM-380 synthesiser uses multiple PLL loops with the 6802 CPU providing frequency data. After 40+ years, ageing of components in the PLL loop filters, VCO varactor diodes, and reference oscillator circuits produces frequency synthesis anomalies: the frequency display shows one frequency while the transmitted/received frequency is different; the synthesiser loses lock on certain frequencies or frequency ranges; frequency jumps occur when tuning slowly through certain areas of the band. These failures are not immediately obvious because the frequency display (driven by the CPU) shows the commanded frequency, not the actual synthesised frequency. If the radio sounds or operates “off frequency” compared to other stations, check synthesiser lock status and reference oscillator frequency before assuming an operator error.
• 5
  Receiver Birdies from Internal Sources — Spurious Signals Throughout the Band [SB-7, SB-11] The KWM-380’s complex synthesiser with multiple PLL loops and a microprocessor generates many internal clock and mixing products. SB-7 “Reduce Receiver Birdies” and SB-11 “Add Receiver Low-Pass Filter” address specific birdie sources. In an unmodified KWM-380, birdies may be audible at various locations across the receiver passband — these are harmonics and mixing products of the synthesiser VCOs, the 6802 clock, the passband tuning oscillator, or the reference oscillators. Birdies are particularly intrusive in the CW receiver mode where the narrow audio filter would normally reveal weak signals below the noise floor. Any birdie audible in the receiver at S3 or stronger on a terminated 50 Ω input indicates an SB-7 or SB-11 implementation deficiency. Note that not all birdies are internal — AM broadcast band interference should be checked with the front-panel BC-band rolloff filter active.
• 6
  Transmit Hum and Degraded Audio [SB-6, SB-14, SIL 2-84] Three separate documents address transmit audio quality in the KWM-380: SB-6 “Correct Transmit Hum” addresses power supply ripple coupling into the transmit audio chain; SB-14 “Insure Transmit Audio Response” corrects frequency response rolloff in the transmit chain; SIL 2-84 substantially increases SSB talk power and improves VOX stability. Together, these represent a systematic issue with the original transmit audio design. An unmodified KWM-380 may produce a transmitted signal with audible hum sidebands (power-supply modulation), reduced talk power on SSB (compare with a good modern transceiver at equivalent microphone gain settings), and VOX that cuts in and out erratically. Implement all three documents as part of any complete restoration.
• 7
  Transmit Spectral Purity and RF Pulse [SB-2, SB-18, SIL 3-84] Three documents address transmit signal quality: SB-2 “Improve Transmit Spectral Purity” reduces unwanted sideband and harmonic products; SB-18 “Eliminate RF Pulse” addresses a brief RF burst produced during T/R switching that does not correspond to the intended transmitted signal; SIL 3-84 further reduces spurious emissions from the PA. An unmodified KWM-380 may not meet legal spectral purity requirements on all bands. SB-2 and SIL 3-84 are mandatory before on-air operation from a regulatory compliance perspective. SB-18 addresses a phenomenon that can also cause interference to nearby receivers (including the KWM-380’s own receiver during VOX operation): the RF pulse at each T/R transition.
• 8
  AGC Anomalies — Pumping, Overshoot, and Noise-Burst Responses [SB-8, SB-13] The KWM-380’s AGC system controls both the front-end PIN diode attenuator (CR104) and the final 455 kHz IF PIN diode attenuators, creating a dual-loop system of unusual complexity. SB-8 and SB-13 (a later revision) both address AGC action improvement — specifically AGC pumping on SSB signals with high peak-to-average ratios, AGC overshoot on strong signal appearances (producing a momentary audio loss after each SSB peak), and AGC noise-burst response (static crashes causing the AGC to suppress audio for longer than desirable). An AGC-anomalous KWM-380 produces what operators describe as “choppy” received audio on SSB: the signal level changes between syllables, pauses between words produce a brief gain surge, and static crashes silence the receiver for 1–2 seconds. Both SB-8 and SB-13 must be reviewed to determine which applies to the specific serial number range of the unit under restoration.
• 9
  WARC Band Absence on Transmit — No TX on 30, 17, 12 m [SB-10] Early production KWM-380 units were designed before the 1979 WARC bands were allocated. SB-10 “Add WARC Band Transmit” is the factory modification that adds transmit coverage on 10.1–10.15 MHz (30 m), 18.068–18.168 MHz (17 m), and 24.89–24.99 MHz (12 m). Without SB-10, the KWM-380 will tune to and receive on these frequencies but will refuse to transmit (the transmitter interlock prevents out-of-amateur-band transmission). For any operator wanting full amateur-band coverage, SB-10 is essential. Note: receive coverage is 0.5–30.0 MHz continuous on all units including pre-WARC production — only transmit is affected.
• 10
  Power Supply Electrolytic Aging and RF/AF Pot Taper Degradation [SB-5] Two aging-related issues share the tenth position. First, the internal power supply electrolytics: after 40+ years, increased ESR and reduced capacitance in the supply filter capacitors introduce ripple on the DC rails that manifests as transmit hum (compounding SB-6), audio instability, and in severe cases, synthesiser PLL instability from noisy supply rails. Replacing all electrolytic capacitors in the power supply section is the most effective single preventive step for an aged KWM-380. Second, SB-5 “Improve AF/RF Pot Tapers” addresses the audio and RF control potentiometers: the original tapers produce non-linear control response (most of the gain change occurring in one part of the knob rotation) and cleaning alone does not correct the taper issue. SB-5 adds fixed resistors to modify the effective taper characteristic to be more linear and usable across the full knob rotation.

Section 3 — Kit Component Reference

Section 4 — Pre-Operational Safety Protocol

Pre-Power Visual Inspection Checklist

• Mains voltage strapping: verify before any mains connection (see above).
• SB-9 status: determine whether SB-9 (PA oscillation fix) has been implemented before attempting transmit. If unknown: contact Exline Signal LLC (WA9Z) or KX6K who can assess service history.
• SB-2 / SIL 3-84 status: determine spectral purity compliance before on-air operation.
• SIL 1-88 status: inspect front-end diodes CR800–CR807 for damage if receive sensitivity is suspect; protect against recurrence with SIL 1-88 before regular transmit operation.
• Power supply capacitors: inspect for any visible swelling or electrolyte residue in the power supply section.
• Blower kit: determine whether the optional KWM/HF-380 Blower Kit is installed. Without it, CW key-down is automatically limited to 50 W after 10 seconds. With the blower: 100 W average, 50% duty cycle. Plan operation accordingly.
• Optional accessories: determine which optional accessories are installed (noise blanker AC-3801, speech processor AC-3802, keyboard entry AC-3805A, high stability oscillator AC-3807, general coverage AC-3808, CW filters AC-3810/3811) to assess alignment requirements.

Section 5 — Circuit Modifications

  KWM-380 RECEIVE SIGNAL PATH (simplified)

  Antenna → BC rolloff filter (L800-804, C800-801)
          → High-pass filter selection (3 options: 7/14/20MHz cutoff)
             [PIN diode switched by CPU via K7, L7, M7, N7 lines]
          → Zener overload clamp (VR100-101, 7V limit)
          → PIN AGC attenuator (CR104, driven by AGC chain)
          → 30MHz fixed low-pass filter
          → 1st mixer U100 (SRA-1H diode ring)
            + Synthesiser LO (39.145 + f_rx to 69.145 MHz range)
          → 39.145 MHz first IF (broad 4-pole crystal filter)
          → Optional noise blanker (AC-3801)
          → 2nd mixer U102
          → 455 kHz second IF
          → Passband tuning (455kHz → 6.255MHz → 455kHz)
            with crystal filter selection (2.1/1.7/8.0/0.36/0.14 kHz)
          → Product detector (SSB/CW) or AM detector
          → CW audio filter
          → AF amplifier → speaker

  KEY DIAGNOSTICS:
  ┌────────────────────────────────────────────────────────────────────┐
  │ No RX on one HP filter only → CR800-CR807 diode damaged (SIL 1-88)│
  │ Display shows freq, no TX   → SB-9 oscillation, or pre-SB-10     │
  │ Tuning unresponsive         → Optical encoder degraded (SB-4)     │
  │ RX birdies throughout band  → SB-7 / SB-11 not implemented        │
  │ Synthesiser frequency error → SB-16 / oscillator calibration      │
  │ Choppy SSB audio            → AGC issue (SB-8 or SB-13)           │
  └────────────────────────────────────────────────────────────────────┘

Figure 1. Collins KWM-380 receive signal path summary and key diagnostic table.

Section 6 — Installation Sequence

• 1
  Documentation, serial number, and service bulletin status Obtain all 18 SBs and 7 SILs from collinsradio.org. Download the service manual and self-study guide. Identify serial number and determine production era (pre/post WARC). Contact WA9Z or KX6K for service history if available.
• 2
  Mains strapping verification and power supply inspection (K-006, K-005) Verify primary voltage strapping. Inspect all power supply electrolytics for distress. Replace all electrolytics with 105°C high-ripple types before applying mains.
• 3
  Implement SB-9 before any transmit (K-001) PA oscillation prevention is mandatory before transmit testing. Implement SB-9 and confirm implementation before any RF drive is applied.
• 4
  Implement SB-2, SIL 3-84 before on-air operation (K-002) Transmit spectral purity compliance is mandatory. Implement and verify with a spectrum analyser before connecting an antenna.
• 5
  Implement SIL 1-88 and SB-15 (K-003, MOD-2) Front-end diode protection from T/R transients and ESD. Inspect CR800–CR807 for pre-existing damage. Implement both modifications.
• 6
  Implement SB-16 for frequency synthesis (K-004) Frequency synthesis improvement (supersedes SB-3). Verify both reference oscillators within ±3 Hz after implementation.
• 7
  First power-up, tuning test, and optical encoder evaluation (K-007) Power on into dummy load. Test tuning at very slow rotation speed. Any sluggishness = optical encoder degraded; implement SB-4.
• 8
  Receiver birdie survey and AGC test (K-008, K-009) 50Ω terminator on antenna. Scan receive for birdies with audio and spectrum monitoring. Implement SB-7/SB-11. Test AGC range; implement SB-8 or SB-13 as appropriate.
• 9
  Transmit audio, hum, and talk power (K-010, MOD-4) Implement SB-6, SB-14, SIL 2-84. Verify transmitted audio quality on an adjacent receiver. Calibrate reference oscillators per MOD-4 for ±5 Hz accuracy.
• 10
  WARC bands, CW, pots, and remaining bulletins (MOD-1, MOD-3, M-003, K-010) Implement SB-10 (WARC TX). SB-12 (CW waveshape). SB-5 (pot tapers). SB-18 (RF pulse). SIL 1-84 (mic RFI). SB-1 (mic impedance). SB-17 if CU-380 is used. Document all implementations.

Section 7 — Verification Tests

Optical Encoder Tuning Function

Receiver Sensitivity — Front-End Diode Check

Synthesiser Frequency Accuracy

PA Power and Spectral Purity (post-SB-2 and SIL 3-84)