Collins KWM-1 Transceiver
Failure Prevention Kit — Component & Modification Design

Section 1 — Circuit Overview and Community Resources

Key Circuit Architecture

The KWM-1 is a double-conversion SSB/CW transceiver sharing common circuits for transmit and receive. The major signal processing blocks are:

• Balanced modulator: Ring-type using four matched 1N67 germanium diodes (CR1–CR4). These diodes are the critical SSB generation elements — their matching determines carrier suppression quality.
• Mechanical filter FL1: 455 kc (nominal) Collins mechanical filter, lower sideband. This is the roofing filter for both transmit (sideband selection) and receive (selectivity). No other component determines transmitted and received SSB quality as decisively.
• VFO/PTO: Permeability-tuned oscillator, 3.445–3.545 Mc, with the characteristic Collins lead-screw-and-slug mechanism. This provides the continuous frequency coverage within each 100 kc crystal segment.
• Crystal HF oscillator V11: Crystal-controlled, 10 positions on the plug-in crystal unit. Each crystal positions the transceiver in a 100 kc segment from 14.0 to 30.0 Mc. The crystal plug-in unit contacts and the crystal selector switch S1 are critical mechanical contact elements.
• PA section: V23 and V24, two 6146 triodes in parallel grounded-grid configuration (cathode drive). PA screen voltage divider R154/R155 for TUNE position. Screen voltage resistor R148 documented to drift from heat.
• Receive-transmit amplifier V4: Shared with both transmit and receive paths. Gang-tuned with the EXCITER TUNE control. Critical for transmit mixer injection and receive signal path.
• ALC: Dual time-constant self-adjusting circuit. Service bulletins SB-1D and SB-1E address specific ALC and power supply hum/tweet artefacts.
• VOX and anti-trip: Integral to the design. VOX relay is one of the contacts subject to oxidation-induced failure.
• Pi-L output network: Matches 52-ohm antennas with up to 2.5:1 SWR. C146 (10 m load trimmer) is a documented short-circuit risk affecting all bands.

Tube Complement

Collins Service Bulletins for the KWM-1

Community Resources

Section 2 — Root Cause Failure Analysis

The KWM-1 is a pure all-tube design from 1957 with no solid-state content except the balanced modulator diodes and rectifier diodes. At 65+ years, its failure modes are dominated by germanium device aging, mechanical contact oxidation, tube emission depletion, and electrolytic capacitor degradation.

• 1
  Balanced Modulator Diodes CR1–CR4 (1N67) — Carrier Leakthrough and Sideband Imbalance The balanced modulator in the KWM-1 uses four matched 1N67 germanium point-contact diodes (CR1–CR4) arranged in a ring topology. The carrier suppression quality of the entire SSB signal depends entirely on the matching of these four diodes. After 65 years, germanium point-contact diodes degrade non-symmetrically: their forward voltage drops drift, their reverse leakage increases, and the inter-diode matching that Collins established at the factory is lost. The consequences: carrier leaking into the transmitted sideband (producing a buzz or heterodyne detectable by receiving stations), unequal sideband levels, and reduced audio fidelity on both transmit and receive. The 1N67 is an obsolete germanium type. Its closest modern equivalent for this application is a matched set of Schottky diodes (such as 1N5711 or HP-5082 series) selected from a batch for matched forward voltage at the carrier frequency. Alternatively, NOS 1N67 devices are occasionally available. Test the existing diodes with an in-circuit carrier null measurement before replacing — if the null is deep and symmetrical, the originals may still be serviceable.
• 2
  455 kc Mechanical Filter FL1 — The Non-Replaceable Heart of the KWM-1 Collins mechanical filter FL1 is the 455 kc roofing filter that simultaneously selects the lower sideband on transmit and provides the primary adjacent-channel selectivity on receive. Without a functional FL1, the KWM-1 cannot produce a legal SSB signal and its receive selectivity collapses to that of a wideband IF amplifier. The Collins mechanical filter is a precision electromechanical device; it is not a commodity component. Failure modes include: metal disc resonator fatigue producing a frequency shift or degraded passband; end-cap seal failure that allows humidity to reach the resonators, producing irregular passband response; physical shock damage causing transducer detachment. A cracked or failed mechanical filter may produce distorted audio, partial sideband suppression, or dramatically reduced sensitivity. Before attempting any alignment: verify the 455 kc filter passband with a signal generator and oscilloscope or spectrum analyser. Do not perform any mechanical work near the filter housing; the internal elements are shock-sensitive. Store carefully and never expose to magnetic fields from external sources.
• 3
  PTO Lubrication Failure — Stiff Tuning, Backlash, and Frequency Run-Out The Collins permeability-tuned oscillator (PTO) in the KWM-1 uses a precision lead screw and ferrite slug mechanism to continuously tune the VFO over a 100 kc range (3.445–3.545 Mc). This mechanism requires a specific high-quality grease on the lead screw and bearings. After 65 years, the original lubrication has almost certainly dried, hardened, or migrated. Symptoms of lubrication failure: stiff or notchy tuning feel, increased tuning backlash, audible squeaking when tuning slowly, and frequency run-out (the kHz scale is not linear across the tuning range). The PTO bearings should also be inspected for wear. The CCA PTO service document (available at collinsradio.org/rx/) is the definitive guide for KWM-series PTO service: it covers lead screw removal, grease type specification, bearing inspection, and frequency run-out measurement. A PTO service is one of the first tasks for any KWM-1 restoration.
• 4
  PA Screen Resistor R148 Thermal Drift — Reduced and Band-Selective Output Power Resistor R148 sets the screen grid voltage for the 6146 PA tubes (V23/V24) during normal operation. This resistor is specified at 820 Ω / 2 W in the KWM-1, but is located in the heat path from the PA tubes and runs continuously warm. The Collins mailing list community documents this failure explicitly: “Check PA screens resistor R148. S/b 820 ohm 2 watt — drifts from heat to 1400 ohms. Replace with 820 ohm 5–7 watt.” A drifted R148 reduces the screen voltage below the design operating point, reducing the PA transconductance and output power, particularly on the higher bands. The fix is straightforward: replace R148 with an 820 Ω resistor rated at 5–7 watts to reduce its operating temperature and prevent future drift. This is a preventive measure worth implementing on every KWM-1 regardless of current performance.
• 5
  Transmit/Receive and VOX Relay Contact Oxidation — No Transmit, Intermittent Switching The KWM-1 uses non-sealed relays for transmit/receive antenna switching and VOX operation. The WorldwideDX community confirms this as the root cause of a common KWM-1 transmit failure: “It is a little bear to get to. These relays were very high quality back in the 60s. They are even rebuildable by changing out any of the parts, coil, a single contact, etc. Most relays of this time were not sealed.” After 65 years, the non-sealed relay contacts develop a layer of silver oxide/sulphide that increases contact resistance or causes intermittent connection. A completely oxidised relay contact produces no transmit output despite all other circuits functioning. The relay contacts can be accessed through the inspection hole described in the Collins service notes: “3/4” diameter, 2½” from the top cover, 1½” front, exposing the relay contacts” for burnishing. Use a fine contact burnishing tool (not abrasive) or a DeoxIT-dampened strip of card stock to clean the contacts.
• 6
  Crystal Plug-in Unit and Crystal Selector Switch S1 — Missing or Intermittent Band Segments The KWM-1’s frequency coverage from 14 to 30 Mc is provided by a plug-in crystal unit containing up to 10 crystals selected by rotary switch S1 on the front panel. Two distinct failure modes affect this system. First, the plug-in crystal unit may have been lost, damaged, or have missing crystals — a KWM-1 without a complete crystal set is not full-coverage. Second, both the plug-in connector contacts and the S1 wafer contacts develop oxide film after 65 years of service, causing intermittent or completely absent signal on one or more band segments. The symptom is no transmit output on specific 100 kc segments while adjacent segments work normally. DeoxIT D5 on the S1 contacts (all wafer positions, cycling through all crystal positions at least 20 times) is the first treatment. Inspect the plug-in connector for bent pins or oxidised contacts before attempting alignment.
• 7
  Triple-Section HV Filter Electrolytic Capacitor — Hum on Audio and B+ Collapse The KWM-1’s internal B+ filtering uses a large triple-section electrolytic capacitor: 40/40/40 µF at 250 V DC, mounted in a twist-lock can (Collins #138-1764-00). After 65 years this capacitor is virtually certain to have degraded. Failure modes: increased ESR producing 120 Hz ripple on the B+ rail (the root cause of service bulletin SB-1E “tweets” in audio and SB-1D hum in the speaker); partial or complete short circuit causing fuse blowing or over-stressed other components. The Collins email list community documents this part specifically: “Antique Radio carries the can type filter caps. As long as it fits in the 3 chassis slots you can use a larger value cap and/or one of higher voltage. AR has part number C-EC40X3-525, a 40/40/40 at 525 DCV.” A higher-voltage replacement (400–525 V DC) provides additional margin over the original 250 V specification. The twist-lock mounting must be matched or adapted for modern replacement units.
• 8
  C146 10 m Load Trimmer Short — All-Band No-Output Failure Capacitor C146 is the 10-metre load trimmer in the pi-L output network of the KWM-1. The Collins mailing list community documents a specific and misleading failure: “Check 10 meter load trimmer C146 for short. This cap affects all bands. Inspection hole: lower right hand side.” A shorted C146 does not simply remove 10 m coverage — it takes down transmit output on all bands, producing a complete no-output symptom that appears to be a PA or driver failure rather than a specific trimmer capacitor. The trimmer can be accessed through the inspection hole described. Measure C146 for short-circuit with a DMM on low-resistance range (power off, B+ discharged) as the first step in any all-bands no-output diagnosis. If C146 is shorted: remove it, restore full output on all remaining bands, and replace with a high-voltage trimmer at the correct capacitance value.
• 9
  Emission Switch S4 Multi-Section Contact Oxidation — Mode-Selective Failures The emission switch S4 has seven sections (S4A through S4G) simultaneously switching PA screen voltage, BFO state, bias circuits, audio muting, microphone input, ALC grounding, and the low-voltage supply enable. With all seven sections operated in parallel, a dirty or high-resistance contact on any one section can produce a completely unexpected symptom: no transmit in SSB while CW works; audio hum present only in CW mode; VOX not working while PTT operates normally. S4G, mounted on the rear of the physical switch, enables all low-voltage supplies — an intermittent S4G contact can cause the entire radio to appear completely dead at random. Clean all S4 sections with DeoxIT D5, cycling through all positions (OFF/SSB/CAL/CW/TUNE/LOCK KEY) at least 20 times per section before deeper fault diagnosis.
• 10
  6146 PA Finals Exhausted — Mismatched Pair and Parasitic Suppressor Condition The two 6146 triodes (V23/V24) operate in parallel and must be matched for equal plate current contribution. The Collins community documents the matching procedure: “Match 6146s via resting/static plate current measurement, mike keyed. Disconnect one plate cap at a time after shorting to ground. Match to within 10 mA or less, with the top cover secured.” An unmatched pair produces unequal heat dissipation, potentially causing one tube to fail prematurely while the other still tests good. Additionally, the parasitic suppressor coils L15/L16 have documented sensitivity: “Low output on 10 metres is sometimes caused by the parasitic suppressor coils L15 and L16 having their turns too closely spaced. Separating them too far can result in parasitic oscillations.” Inspect L15/L16 for physical distortion during restoration; the winding spacing is set at the factory and must not be arbitrarily changed.

Section 3 — Kit Component Reference

Section 4 — Pre-Operational Safety Protocol

Visual Inspection Checklist

• Crystal plug-in unit: is it present? How many of the 10 crystal positions are populated? Confirm before planning coverage testing.
• Triple-section electrolytic: with power off, inspect for electrolyte seepage, bulging, or discolouration. Any distress: replace before power-up.
• C146 10 m load trimmer: measure resistance to ground with power off (should be very high). If shorted: do not attempt to tune the amplifier until C146 is replaced.
• L15/L16 parasitic suppressors: inspect coil winding spacing. Do not adjust; just verify the coils are not physically distorted.
• Mechanical filter FL1: inspect externally for corrosion on end-cap seams, damage to transducer connections, or physical shock damage.
• All tubes: reseat by gentle removal and reinsertion; inspect for cracked envelopes, missing shields, or obviously burned structure.
• Service bulletin status: note any visible modifications from service bulletins before powering up.

Section 5 — Circuit Modifications

  COLLINS KWM-1 SIGNAL PATH — TRANSMIT AND RECEIVE

  TRANSMIT:
  MIC → V19 (audio) → V18A (cathode follower) → CR1-CR4 (ring balanced modulator)
      → FL1 (455kc mechanical filter, LSB select) → V6 (1st TX mixer, +VFO 3.445-3.545Mc)
      → 3.9-4.9Mc → V3 → V4 (EXCITER TUNE, gang-tuned) → V5 (2nd TX mixer, +crystal HF)
      → output frequency (14-30Mc) → V2 (driver) → V23/V24 (2x 6146 PA)
      → Pi-L network (PLATE TUNE / LOADING) → K1 antenna relay → antenna

  RECEIVE:
  antenna → K1 relay → preselector → V4 (shared RX/TX amp, EXCITER TUNE)
           → V5 (1st RX mixer, +crystal) → V6 (2nd RX mixer, +VFO)
           → 455kc → FL1 (mechanical filter, selectivity)
           → V13, V14 (fixed IF amps) → V15 (product detector, +BFO via V18B)
           → V16, V17 (audio amps) → speaker

  CRITICAL SINGLE POINTS:
  ┌────────────────────────────────────────────────────────────────┐
  │ FL1: fails on both TX (sideband) and RX (selectivity)         │
  │ CR1-CR4: carrier leakthrough affects TX SSB quality           │
  │ C146 SHORT: kills ALL BANDS transmit output (not just 10m)    │
  │ R148 DRIFT: reduces screen V → low power especially HF bands  │
  │ S4G: intermittent low-V supply kill → total radio death       │
  └────────────────────────────────────────────────────────────────┘

Figure 1. Collins KWM-1 transmit/receive signal path and single-point failure summary.

Section 6 — Installation Sequence

• 1
  Documentation, crystal inventory, and service bulletin download Obtain KWM-1 service manual (2nd edition or later) and all five service bulletins from collinsradio.org. Count crystals in plug-in unit. Download CCA PTO service document.
• 2
  Visual inspection: FL1, C146, triple-section cap, L15/L16, R148, S4G (K-001 through K-005) With power off and B+ discharged: inspect mechanical filter, measure C146 resistance to ground, inspect filter cap, measure R148. Check all tubes are seated and the crystal plug-in unit is fully inserted.
• 3
  Replace triple-section filter cap, R148, and C146 if faulty (K-002, K-003, K-004) Replace triple-section can cap with 40/40/40µF at 400–525V. Replace R148 with 820Ω/5–7W. Replace C146 if shorted. These are mandatory before power-up.
• 4
  PTO service (K-006) Remove PTO, clean lead screw and bearings, apply new synthetic grease, measure frequency run-out, zero kHz dial after correction.
• 5
  Relay cleaning, switch cleaning, and tube reseating (K-007, K-008, K-009) Clean TX/RX and VOX relay contacts through inspection hole. Apply DeoxIT to all S4 sections and S1 crystal selector. Clean crystal plug-in connector. Reseat all tubes.
• 6
  Service bulletin implementation (MOD-3) Implement all outstanding service bulletins. Priority: SB-1B (spurious, legal requirement) first; then SB-1C (transient); then SB-1D and SB-1E (audio quality).
• 7
  First Variac power-up and B+ verification Connect 516F-1 to Variac. Raise mains from 0 to full over 10 minutes. Verify B+ at specification. All tubes lit. No burning smell. Receive audio present.
• 8
  Mechanical filter passband verification (K-010) Verify FL1 passband with signal generator and scope or spectrum analyser at 455 kc. If filter is degraded: source a replacement before proceeding with alignment.
• 9
  Balanced modulator null, BFO crystal, 6146 matching, and neutralisation (MOD-1, MOD-2, K-009) Adjust carrier null per MOD-1. Match 6146 pair per MOD-2 procedure. Neutralise PA with top cover on. Verify CR1–CR4 diode quality.
• 10
  Full alignment and all-crystal-segment verification (MOD-4) Complete ALC zero, RF gain, S-meter zero, dial calibration using V1 calibrator and WWV. Verify all 10 crystal segments for transmit output and receive sensitivity. Document baseline performance.

Section 7 — Verification Tests

Balanced Modulator Carrier Null

C146 Pre-Transmit Short Check

All-Crystal-Segment Power Output

Receive Sensitivity All Bands