KWM-2 / KWM-2A
Failure Prevention Kit — Component & Modification Design

Section 1 — Root Cause Failure Analysis

The following ten failure modes account for the overwhelming majority of KWM-2/2A restoration casualties. They apply to both the KWM-2 and KWM-2A except where specifically noted. They are presented in priority order.

• 1
  R148 PA Screen Resistor Drift — Loss of Output Power on All Bands R148 is the 820 Ω / 2 W carbon composition resistor in the PA screen grid supply. It sits continuously in the heat stream of the two 6146 final tubes and drifts high over years of service. Documented real-world values range from 1,400 Ω to over 1,655 Ω in regularly-operated units. At the correct value the PA screen receives its rated voltage; as R148 drifts high, screen voltage drops, plate efficiency falls, and output power decreases across all bands. The symptom is insidious: the rig tunes up normally, grid current reads correctly, but RF output is 30–50% below specification and no amount of alignment will recover it. This is the most consistently documented single failure in KWM-2 restoration records. Measure R148 before any other transmitter diagnosis. Replace with a 5-watt metal film or non-inductive wirewound type to eliminate future thermal drift.
• 2
  516F-2 Selenium Bias Rectifier CR1 — Silent Degradation with Toxic Failure Mode The 516F-2 uses a stud-mount selenium rectifier CR1 to produce the −55 to −80 V variable bias voltage for the PA tubes. Selenium rectifiers degrade silently over decades: forward voltage drop increases, bias voltage drifts, and PA quiescent current shifts. The critical failure mode is acute: when a selenium rectifier fails it produces white smoke and an acrid odour that is potentially carcinogenic. This failure typically occurs under power with the operator present. The selenium must be replaced with a 1N5408 silicon diode before the supply is powered in any domestic environment. This is a ten-minute substitution that eliminates both the safety hazard and the long-term bias drift problem.
• 3
  K2 / K4 Relay Contacts — Intermittent T-R Switching and S-Meter Faults K2 is the VOX/PTT relay actuator. K4 is the combined T-R switching and meter-selection relay. Dust and oxide accumulate on the relay contacts during years of storage. Contaminated K2 contacts cause chattering (relay oscillates in and out when PTT is pressed), erratic keying, or complete keying failure. Contaminated K4 contacts at pins 3, 4, 5 and 6, 7, 8 cause loss of S-meter reading on receive — these contact groups switch the meter between TX and RX circuits. Both relays are accessible from below the chassis. Clean with contact cleaner on a strip of bond paper drawn between the closed contacts. In severely corroded cases, relay replacement is necessary.
• 4
  C225 Bypass Capacitor Short — Stuck in Transmit C225 is a 0.1 µF ceramic bypass capacitor in the PTT/VOX relay circuit. When it shorts, the PTT line is permanently grounded: the transceiver is stuck in transmit and cannot return to receive. The symptom is unmistakable but easily confused with a relay coil fault or VOX circuit problem. The diagnostic is simple: disconnect the PTT line and verify C225 is not shorted with an ohmmeter before condemning any relay or tube. Some production runs have this capacitor at the rear PTT RCA jack; others have additional instances near the MIC jack — verify which configuration your unit has and replace all instances found.
• 5
  R20 and R47 Drift — VOX Relay Chattering and PTT Instability R20 and R47 are the two 68 kΩ / 2 W resistors on top of the E50 terminal board that set the V4B (6AZ8 triode) grid operating point for the VOX relay actuator. When these resistors drift high — a predictable outcome of their operating temperature and age — the V4B grid voltage shifts, causing the VOX relay K2 to become unstable: it oscillates in and out when PTT is pressed, or drops out during transmission. Measure R20 and R47 in-circuit with power removed. Both should read 68 kΩ ±5%. Values above 75 kΩ indicate significant drift. Replace both together with 68 kΩ / 2 W metal film types — they work as a matched voltage divider pair and must both be replaced simultaneously.
• 6
  V4 (6AZ8) Grid Current — ALC Zero Unadjustable, VOX Misbehaviour V4 is a 6AZ8 dual triode/pentode used in the ALC and VOX circuits. The triode section V4B is the VOX relay actuator; the pentode section handles ALC functions. After decades of service, 6AZ8 tubes frequently develop grid current in one or both sections. Grid current in V4B upsets the voltage divider that controls K2, causing erratic VOX behaviour. Grid current in the pentode section makes it impossible to set the ALC zero correctly with the front-panel trimmer. If the ALC zero cannot be adjusted to null with V17 removed from its socket, suspect V4 grid current. Pull V4 and test specifically for grid current — do not rely on emission testing alone. Replace with a NOS 6AZ8 tested for zero grid current.
• 7
  516F-2 High-Voltage Filter Capacitors — Ripple, Sag, and Over-Voltage Risk The original 516F-2 HV supply uses three filter capacitors C2, C3, C4 with voltage-equalising resistors across each. After 60 years these electrolytics have dried dielectric, reduced capacitance, and elevated ESR. Consequences: increased HV ripple on the 800 V rail that modulates 6146 plate voltage and degrades transmitted audio IMD; and if one capacitor develops high leakage, the equalising network becomes unbalanced and over-voltages the remaining units. The 5R4 rectifier tube also produces a higher inrush surge than ideal for aged capacitors. A solid-state conversion kit (Harbach Electronics SS-516) replaces both rectifier tubes and all filter capacitors simultaneously and is the recommended solution.
• 8
  Paper Electrolytic Capacitors — Audio Chain Faults and Modulator Problems The KWM-2 contains paper-dielectric electrolytic capacitors in the audio, IF, and modulator sections. The most critical is C264 in the balanced modulator/cathode follower circuit. When C264 becomes leaky, it introduces a DC offset into the transmit audio chain, causing barely-detectable modulation (ALC meter barely moves even with mic gain at maximum), a VOX that drops out too early, and carrier balance problems that return after every adjustment. Collins incorporated a value change for C264 from 4 µF to 20 µF as Change #3 in the 9th edition manual — incorporate this if your unit has not had it done. The paper electrolytics throughout the audio chain should be replaced with modern types regardless of apparent condition.
• 9
  ALC Circuit Lockup — Zero Output on All Voice Modes ALC lockup produces a symptom easily misidentified as a transmitter alignment problem: full RF output in TUNE and LOCK modes, zero output in LSB, USB, and CW, with the ALC meter reading full scale. Causes include: leaky C200 in the ALC coupling path; drifted R119 (the 1.5 MΩ carbon composition type that drifts significantly with age); and a faulty V17 (6BN8) ALC rectifier tube. The diagnostic is fast: temporarily short the ALC jack (J-17 area) to ground. If the rig transmits normally on all modes with ALC shorted, the fault is within the ALC circuit. Trace through C200, R119, and V17 in that order.
• 10
  516F-2 Switched-Neutral Wiring Hazard — Live Chassis with Power Off The Collins 516F-2 was designed with the KWM-2 front panel switch interrupting the neutral side of the mains supply, not the hot. Pin 5 of the 11-pin connector is the switched neutral; pin 7 is the unswitched hot. If the mains cord is connected with hot and neutral reversed — done by technicians who believe they are “correcting” the fuse placement — the hot conductor is permanently connected to relay K4 inside the transceiver with the power switch in the off position. The chassis can be live when the rig appears off. Verify mains polarity with a mains tester before working inside either unit. Always ensure the hot conductor connects through the fuse.

Section 2 — Kit Component Reference

The table below lists all components in the KWM-2/2A failure prevention kit. Tier 1 items are mandatory pre-power replacements. Tier 2 items address secondary aging. Modification components are listed separately.

Section 3 — 516F-2 Power Supply Restoration

The 516F-2 Is the Foundation

The 516F-2 supplies filament voltage (6.3 V AC), low B+ (+275 V DC), high voltage (800 V DC), and variable bias (−55 to −80 V DC). Drifted bias from the degraded selenium rectifier shifts the 6146 quiescent current away from the 40 mA specification. Low or ripple-laden B+ directly degrades transmit audio IMD. No KWM-2 restoration is complete without restoring the 516F-2 first.

Solid-State Rectifier Conversion (MOD-1)

Collins engineers recommended solid-state conversion of the 516F-2 rectifiers in Service Information Letter SIL 1-76, citing concern with arc-over in the tube rectifiers under high-voltage transients. The Harbach Electronics SS-516 kit addresses this in a single assembly session.

Mains Polarity Verification

Before connecting any S-Line or KWM-2 equipment, verify mains polarity at the 516F-2 power cord with a plug-in mains tester. The hot conductor must connect to the fused side of the line. With the 516F-2’s original design, incorrect polarity places mains potential on the KWM-2 chassis with the power switch off.

  516F-2 CONNECTOR PIN ASSIGNMENTS (11-pin female P1)
  ─────────────────────────────────────────────────────────────────
  Pin 1   GND (chassis)
  Pin 2   6.3 V AC filament supply
  Pin 3   6.3 V AC filament supply return
  Pin 4   +275 V DC (low B+)
  Pin 5   AC mains — switched by KWM-2 ON/OFF switch  (SWITCHED NEUTRAL)
  Pin 6   +275 V DC return (GND)
  Pin 7   AC mains hot (unswitched) — HOT AT ALL TIMES — connects to K4
  Pin 8   Bias voltage (–55 to –80 V DC)
  Pin 9   Bias return (GND)
  Pin 10  HV (+800 V for linear amplifiers)
  Pin 11  HV return (GND)

  HAZARD: If 516F-2 cord is connected reversed (or outlet wired reversed):
    → Pin 7 (K4 internal connection) becomes energised with switch OFF
    → The KWM-2 chassis area near K4 is at mains potential with rig "off"
  ALWAYS verify with a mains tester before working inside either unit.

Figure 1. 516F-2 connector pin assignments and switched-neutral hazard.

Section 4 — Tier 1 Mandatory Replacements

Selenium Rectifier CR1

Locate the stud-mount selenium rectifier near the bias filter network. Note the polarity. Unsolder the original. Mount the 1N5408 silicon diode with correct polarity orientation. Resolder. The silicon diode will produce a slightly lower bias voltage — re-adjust BIAS ADJUST potentiometer R9 after the first power-up to restore 40 mA quiescent current.

All 516F-2 Electrolytics

Replace all electrolytic capacitors in the 516F-2, prioritising C2, C3, C4 (HV filter) and the bias supply filter caps C6 and C7. The Harbach SS-516 kit includes the complete electrolytic set. Test the 516F-2 independently with a dummy load before connecting to the KWM-2.

R148 Measurement and Replacement

With the KWM-2 unpowered and 516F-2 disconnected, locate R148 in the PA screen supply circuit. Measure with a DMM. Values above 850 Ω confirm drift and require replacement. Values between 800 and 840 Ω indicate the component has been previously serviced. Replace with 5-watt K-001 regardless if value cannot be confirmed from service records.

C225, R20, R47, C264, and V4/V17 Tube Tests

Replace C225 (all instances, K-004). Measure and replace R20/R47 (K-003). Replace C264 with 20 µF type (K-005). Pull V4 and test for grid current — replace with K-006 if grid current is present. Pull V17 and confirm it tests correctly — keep K-007 as a verified spare.

Clean K2 and K4 relay contacts with DeoxIT D5 on bond paper strips. Verify contact resistance below 100 mΩ per contact set if milliohm measurement is available. Perform the cleaning before any transmit testing, not as a corrective step after faults appear.

Section 5 — Circuit Modifications

Section 6 — Installation Sequence

Service the 516F-2 first and verify it independently before connecting to the KWM-2. This protects the transceiver from supply faults and simplifies diagnosis at every subsequent step.

• 1
  Documentation and mains polarity check Photograph the 516F-2 and KWM-2 interiors before touching anything. Verify mains polarity of the 516F-2 supply cord with a mains tester. Confirm correct polarity before any further work.
• 2
  516F-2 — Replace selenium CR1 (K-002) and all electrolytics (K-011, K-012) Install the Harbach SS-516 kit (or equivalent). Replace the mains cord with a 3-wire type (K-008). Verify mains polarity with new cord fitted. Install MOD-1.
• 3
  516F-2 — First power-up via Variac, voltage verification With 516F-2 disconnected from the KWM-2, power via Variac and raise from 0 to full over 30 minutes. Verify all DC outputs at the 11-pin connector pins. Adjust BIAS ADJUST R9 for −70 V bias. Confirm all voltages within specification before connecting the transceiver.
• 4
  KWM-2 — Replace R148, R20/R47, C225, C264 With KWM-2 unpowered and 516F-2 disconnected: measure and record R148 value; install K-001. Replace R20 and R47 (K-003). Replace all C225 instances (K-004). Replace C264 with 20 µF type (K-005). Install MODs 2, 3, 4.
• 5
  KWM-2 — Relay contact cleaning (K-013) Clean K2 and K4 contact sets with DeoxIT D5 on bond paper strips before applying power. This step is prevention, not repair.
• 6
  KWM-2 — Connect 516F-2 and first power-up Connect 516F-2 to KWM-2 via Variac. Power up. Verify receiver operation on all bands. Check S-meter reads correctly. Press PTT: verify clean T-R switching without chatter. Monitor PA plate current: should read 40 mA at idle.
• 7
  V4 grid current test and ALC zero verification Remove V17 from its socket. With V17 out, ALC meter should read zero. If it does not, V4 has grid current in the ALC section — replace with K-006. Reinstall V17 and verify ALC zero can be nulled with the ALC ZERO trimmer.
• 8
  PA quiescent current bias setting Remove antenna connection. Set MODE to TUNE. Read no-signal PA plate current. Adjust BIAS ADJUST on 516F-2 for 40 mA total quiescent current. If meter does not read correctly, check K4 contacts before adjusting bias.
• 9
  Transmit ALC lockup test Key transceiver in CW mode into 50 Ω dummy load. If zero output in CW/LSB/USB but output in TUNE/LOCK: temporarily short the ALC jack to confirm lockup. Trace through C200 (K-010), R119 (K-009), and V17 (K-007) in sequence.
• 10
  PA tune-up and output power verification Tune up on each band. Verify approximately 100 W PEP output on all bands. Low output that alignment cannot fix: re-check R148 value and PA screen voltage. Also verify 516F-2 B+ under TX load is not sagging below 255 V.
• 11
  Full alignment and performance baseline Perform full alignment per the KWM-2/2A manual. Verify 70K-2 PTO end-point, carrier balance, ALC zero, and S-meter zero. Verify VOX operation. Record all settings as a post-restoration baseline.

Section 7 — Verification Tests

R148 / PA Screen Voltage Verification

ALC Circuit Verification

Relay Switching Verification

516F-2 Under TX Load