Collins KWM-2 / KWM-2A
Sidetone Hum Fault Diagnosis — Injection Chain Analysis and Seven Ranked Candidates
Hum in the KWM-2 sidetone is a distinct fault pattern isolated to the CW transmit injection chain — not the main audio path, not the receiver, not the PA. Because the sidetone circuit is active only during CW transmit and is entirely separate from the receive audio, its failure modes are navigable by a single gateway test. This guide ranks the seven known injection chain candidates and provides the measurement sequence to reach a confident diagnosis without removing tubes or components unnecessarily.
The sidetone hum in a KWM-2 or KWM-2A is a fault with a confined diagnostic space. The sidetone injection chain is active only during CW transmit; it is substantially separate from the receive audio chain; and it is driven by the carrier oscillator sample rather than by the receive IF or detector. When a restorer encounters hum in the CW sidetone that is absent during SSB monitoring and absent during receive, the fault is in the injection chain, not elsewhere. This constraint dramatically reduces the number of components that require examination.
The hum most commonly reported in the KWM-2 sidetone is a steady tone at 60 Hz or 120 Hz that underlies the CW sidetone pitch, rising and falling independently of the SIDETONE level control. Understanding which frequency is present, and whether the hum varies with the SIDETONE control position, locates the entry point of the fault before a single component is touched. The seven candidates described here are ranked by community-documented frequency of occurrence for this specific fault pattern, not by general KWM-2 failure incidence.
This guide applies to both the KWM-2 and KWM-2A. Differences between the two variants relevant to the sidetone circuit are noted in the candidate descriptions. Consult the service manual edition matching your unit’s serial number range for the specific schematic and component designators.
Section 1 — The KWM-2 Sidetone Injection Architecture
Understanding where the sidetone signal comes from and how it reaches the operator’s ears is the foundation for the entire fault diagnosis. The KWM-2 sidetone is not a separately generated audio oscillator: it is derived from the transmit carrier itself, sampled at an appropriate point in the transmit chain and injected into the receive audio path at audio-frequency level.
How the Sidetone Is Generated
During CW operation, the KWM-2 transmits by keying the carrier oscillator. The carrier oscillator runs at an offset from the IF frequency, producing the CW output when mixed in the transmit chain. The sidetone injection takes a sample of this carrier activity and routes it through a detection or coupling path that produces an audio-frequency signal at the sidetone pitch. In the KWM-2 architecture, this audio signal is the beating of two oscillator products — or the envelope of the carrier being switched by the key — at a pitch determined by the CW pitch control and the BFO/carrier frequency relationship.
The resulting audio sidetone signal is routed through the SIDETONE level control, which adjusts the injection amplitude. The SIDETONE control wiper output is then coupled through an isolation capacitor into the audio amplifier stage at a point that allows the sidetone to be heard alongside (or in place of, depending on the mode switching) the receive audio. During CW transmit, the receive path is typically heavily attenuated or switched, leaving the sidetone injection as the primary source of audio from the transceiver.
The Injection Chain Components
The sidetone injection chain, from source to speaker, consists of:
- The carrier pick-off network — a resistor/capacitor combination that samples the transmit carrier signal and provides a low-level audio-frequency output. This circuit is powered from the B+ supply when the transmit mode is engaged.
- The sidetone coupling capacitor (input) — a small value capacitor (typically 0.001 µF to 0.01 µF) that passes the audio-frequency sidetone while blocking DC and RF. This capacitor is the most failure-prone element in the chain.
- The SIDETONE level potentiometer — a front-panel control that adjusts the sidetone injection amplitude from zero to maximum. The wiper output feeds the next coupling capacitor.
- The sidetone coupling capacitor (output) — a second isolation capacitor between the SIDETONE pot wiper and the audio amplifier grid. Also subject to leakage failure.
- The audio amplifier stage — the receive audio amplifier tube, whose grid accepts the sidetone injection. This stage is common to both receive audio and sidetone paths, and its B+ supply and tube condition affect both.
- The mode-switching relay or contacts — CW transmit mode engages switching that routes the sidetone injection; contaminated contacts in this path can introduce noise.
Section 2 — The Gateway Test: Confirming the Fault Is in the Injection Chain
If hum is present during receive: the fault is in the common audio chain, not the sidetone injection chain. This guide does not apply to this fault. Investigate the audio amplifier stage B+ bypass and the audio tube separately from the sidetone circuit.
If hum is absent during receive: proceed to Test B.
Test B: SIDETONE control test. Key CW transmit. Listen for hum in the sidetone. Now rotate the SIDETONE control slowly from its normal position toward minimum (fully counter-clockwise).
If hum tracks the SIDETONE control and approaches zero at minimum:
Hum enters before or at the SIDETONE pot. Investigate candidates F-01 and F-02 first.If hum persists at minimum SIDETONE control (does not track the pot):
Hum enters after the SIDETONE pot. Investigate candidates F-03 and F-04 first.Record which test result applies before reading Section 3. The two paths lead to different diagnostic priorities.
60 Hz vs 120 Hz: Frequency Identification
After completing the gateway tests, identify the hum frequency. This narrows the diagnosis to a specific mechanism before any component is disturbed.
- 60 Hz fundamental: the hum has a deep, low pitch. The mains power supply fundamental frequency is 60 Hz (North America) or 50 Hz (Europe/Australia). A 60 Hz hum in the sidetone indicates a coupling path from the heater supply, which operates at mains frequency on AC. The heater-to-cathode leakage mechanism (F-03) produces a 60 Hz signature. A grounding issue that allows the heater reference to enter the signal path also produces 60 Hz.
- 120 Hz fundamental: the hum has a slightly higher pitch than 60 Hz — twice the frequency. Full-wave rectification of 60 Hz AC produces 120 Hz DC ripple. A 120 Hz hum indicates ripple from the B+ supply is entering the sidetone or audio signal path. Inadequate B+ filtering (failed bypass electrolytics) or a leaky coupling capacitor passing B+ ripple are the 120 Hz mechanisms.
- Mixed or unclear: if both frequencies are present or the identification is unclear, use a calibrated audio frequency counter or oscilloscope. On the scope, 60 Hz produces a sine wave with 16.7 ms period; 120 Hz produces a distorted sine or saw-wave with 8.3 ms period. See the vk6ada.com.au HW-16 hum diagnosis for the scope-based measurement methodology.
Section 3 — Seven Fault Candidates: Injection Chain, Priority Order
The candidates below are ranked by community-documented frequency of occurrence for the specific KWM-2 sidetone hum fault pattern. Tier designation: Tier 1 (most common; investigate first), Tier 2 (common), Tier 3 (moderate probability), Tier 4 (consider if Tier 1–3 clear). The gateway test result from Section 2 is noted for each candidate; use it to skip candidates that your test has already ruled out.
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TIER
1
F-01Sidetone Coupling Capacitors — Dielectric Leakage and DC ContaminationGateway result that points here: hum tracks the SIDETONE control and approaches zero at minimum setting. This places the entry point before or at the pot, within the first coupling capacitor that precedes it.
The sidetone injection chain contains at least two small coupling capacitors (input and output sides of the SIDETONE pot). In the KWM-2, these were often waxed paper or mica types in the original build; some production runs used early film capacitors. All are now 60–70 years old. Waxed paper capacitors develop dielectric leakage with age and thermal cycling: the wax softens and migrates, leaving the paper dielectric saturated with conductive material that allows DC and low-frequency AC to pass through what should be a purely capacitive coupling.
When the first coupling capacitor (between the carrier pick-off network and the SIDETONE pot high-side input) develops leakage, the B+ DC or AC ripple from the pick-off network’s power supply enters the SIDETONE pot. This produces a voltage across the pot that tracks with the pot’s position — exactly what the gateway test observes. The hum at maximum SIDETONE control is the full ripple voltage reaching the audio stage; at minimum it is zero because no signal (including the hum) is being passed.
Test: measure DC voltage across the SIDETONE pot from wiper to ground with the pot at minimum and a DVM. Any measurable DC (more than a few millivolts) at this point with the pot at minimum confirms leakage in the input coupling capacitor. Replace both coupling capacitors around the SIDETONE pot with matched-value film types.
Replacement: use 0.01 µF or the value from the service manual in a polypropylene or polystyrene film type. Do not use ceramic disc capacitors in this position; their microphonic characteristics will introduce additional noise.
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TIER
1
F-02Audio Stage B+ Bypass Capacitor — 120 Hz Ripple Into Signal PathGateway result that points here: hum does not track the SIDETONE control (persists at minimum), and the hum frequency is 120 Hz. This combination points to the B+ supply to the audio stage failing to filter rectifier ripple adequately.
The audio amplifier stage that receives the sidetone injection is powered from the B+ supply through a decoupling resistor and an electrolytic bypass capacitor. This capacitor filters the B+ ripple so that the plate supply to the audio tube is clean DC. When the electrolytic dries out (a near-universal failure in equipment of this age), its capacitance drops and its ESR (equivalent series resistance) rises, reducing its filtering effectiveness. The 120 Hz B+ ripple then appears on the plate supply of the audio tube, modulating the audio stage’s operating point at 120 Hz and producing the characteristic ripple hum in the audio output.
This fault affects both receive audio and sidetone equally, since they share the same audio stage. If the gateway test shows no hum during receive but hum during sidetone, this candidate is less likely — it implies the audio stage’s B+ is adequate for receive audio levels but not for the different loading during CW transmit. The loading difference between the two modes is small but real.
Test: measure the DC voltage on the audio stage plate with the unit in CW transmit mode and the hum present. Then measure the same point with the audio stage plate connected to the B+ supply through a 47 µF/450 V test capacitor temporarily bridged across the existing bypass cap. If the hum reduces significantly, the existing bypass capacitor is the fault.
Replacement: replace all electrolytic bypass capacitors in the audio stage B+ supply chain with modern 85°C or 105°C rated electrolytics of the same or slightly higher capacitance value. Do not increase capacitance dramatically — the decoupling time constant was chosen for a reason.
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TIER
2
F-03Audio Amplifier Tube — Heater-to-Cathode Leakage (60 Hz Signature)Gateway result that points here: hum does not track the SIDETONE control, and the hum frequency is 60 Hz (not 120 Hz). This combination strongly indicates heater-to-cathode leakage in the audio amplifier tube.
The heater filament in the audio tube operates from the 6.3 V AC heater supply at mains frequency (60 Hz). The heater is physically very close to the cathode inside the tube envelope — intentionally so, to heat the cathode efficiently. In a healthy tube, the heater is well-insulated from the cathode; in a tube that has aged or been overheated, the insulation between the heater wire and the cathode surface degrades. The 60 Hz heater voltage then couples capacitively or resistively into the cathode, which is part of the signal path. The result is a 60 Hz signal superimposed on the audio.
This fault is more common in tubes that have been operated at elevated heater voltage (above 6.3 V nominal, which occurs if the KWM-2’s heater supply transformer or wiring has an issue) or in tubes with long service histories. The audio tube in the KWM-2 is not a high-stress position, but decades of operation accumulate.
Test: substitute a known-good tube of the same type in the audio amplifier position. If the hum disappears or significantly reduces, the original tube has heater-to-cathode leakage. Confirm with a tube tester that has a heater-to-cathode leakage test function if available. Note that some tube testers do not test this parameter; substitution is the reliable test.
Note on tube type: verify the audio amplifier tube type from the service manual for your specific KWM-2 or KWM-2A serial number range. The audio stage tube may be a 12AU7 (dual triode) or another type depending on variant. Both sections of a dual triode tube may be affected; replace the complete tube, not just one section.
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TIER
2
F-04SIDETONE Level Potentiometer — Wiper Track Contamination and Open CircuitGateway result that points here: hum is variable in level as the SIDETONE control is rotated, but does not reduce smoothly to zero — instead it has a dead spot, a scratchy characteristic on rotation, or a point where the hum suddenly increases. These are symptoms of a contaminated or worn carbon track in the SIDETONE pot.
The SIDETONE potentiometer in the KWM-2 is a carbon composition pot, now 60–70 years old. Carbon composition pots deteriorate in several ways: the carbon track oxidises at the ends (creating high-resistance regions that appear as dead zones near rotation limits), the wiper contact wears (creating intermittent contact that produces scratching noise and hum bursts), and the track can develop a high-resistance section anywhere along its travel (producing a flat region in the control response where rotation produces no change in level).
A partially open wiper contact is particularly problematic: when the wiper loses contact momentarily, the output side of the pot is left floating, and the high-impedance floating node picks up stray 60 Hz field pickup from the power transformer and other components. This produces hum that is dependent on SIDETONE control position (because the wiper contacts intermittently at specific rotation angles) and that may appear to be amplifier hum when it is actually electrostatic pickup from the open-circuit condition.
Test: disconnect the SIDETONE pot wiper output from the circuit (one end of the resistive element only — measure both ends of the element and the wiper before disconnecting). With an ohmmeter, slowly rotate the pot from end to end while measuring resistance from wiper to each end. Any discontinuities, dead zones, or resistance jumps confirm track or wiper failure. Normal carbon pot should show smooth, continuous resistance change proportional to rotation.
Replacement: match the pot’s resistance value and taper (audio or linear) from the service manual. A modern cermet pot of the same value is an acceptable substitute and will outlast the original carbon track by decades. Clean the replacement pot shaft with contact cleaner before installation.
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TIER
3
F-05TX Exciter Stage Screen Bypass Capacitors — Ripple-Modulated Carrier InjectionGateway result that points here: hum tracks the SIDETONE control (reducing toward zero at minimum), the frequency is 120 Hz, but the hum has a different character from a clean 60 Hz or 120 Hz tone — it may sound like a modulated buzz on the sidetone pitch rather than a pure hum underneath it, and it may vary with the SIDETONE pitch frequency, which would be unusual for a pure B+ ripple entry.
During CW transmit, the exciter stages in the KWM-2 transmit chain are active. These stages — the carrier oscillator buffer and the driver stages — are powered from the B+ supply and have screen bypass capacitors that filter the screen grid supply. If a screen bypass capacitor in an exciter stage has failed (open or high-impedance), the screen voltage becomes modulated by the B+ ripple at 120 Hz. This ripple-modulated screen voltage amplitude-modulates the carrier signal. When this AM-modulated carrier signal is the source for the sidetone injection pick-off, the sidetone itself is modulated at 120 Hz, producing the characteristic modulated-hum sound rather than a clean underlying hum.
This fault pattern is relatively uncommon compared to Tier 1 and Tier 2 causes but has been documented in KWM-2 units with unrestored exciter stages (where the screen bypass capacitors have not been replaced during a recap). It is distinguished from F-01 and F-02 by the modulated character of the hum: it sounds like AM modulation of the sidetone tone, not like a hum tone in parallel with the sidetone.
Test: identify all screen bypass capacitors in the exciter stages from the schematic (those associated with the carrier oscillator buffer and driver tubes). Measure ESR and capacitance on each. Elevated ESR (above 5 Ω for typical bypass electrolytic sizes) is the indicator of failure. Replace all suspect electrolytics in the exciter chain as a set rather than individually.
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TIER
3
F-06Mode-Switching Relay / Contact Contamination in CW TX PathGateway result that points here: hum is intermittent, position-dependent (varies with how the SIDETONE control is set but not in a predictable way), or appeared suddenly in a unit that was previously clean. Also consider this candidate if the hum has appeared or worsened after a period of infrequent use.
The KWM-2/2A uses switching to engage the CW transmit mode, routing the sidetone injection and reconfiguring the audio path. This switching may be implemented through a relay, through mode switch contacts, or through a combination. If the switching contacts in the CW TX path have developed contamination (oxide film, carbon deposits from arcing, or organic contamination from flux residue or environmental exposure), they introduce a resistive element in the injection path.
A contaminated switching contact in the sidetone injection path acts as a small-value resistor in series with the sidetone signal. By itself this does not introduce hum, but it can shift the impedance balance of the injection circuit in a way that increases the susceptibility to pickup of stray fields from the power transformer (which are always present inside the chassis). The result is hum that appears only in CW transmit mode and that may improve or worsen as the mode switch or relay contacts are exercised.
Test: exercise the mode switch through CW TX mode twenty times in rapid succession and observe whether the hum level changes. If it does (temporarily better or worse), contact contamination is likely. Clean the mode switch contacts with a contact-rated cleaner (DeoxIT or similar); apply sparingly to avoid contaminating adjacent components. For relay contacts, exercise the relay coil with a 12 V DC signal while measuring contact resistance with an ohmmeter; any reading above 0.5 Ω for switch contacts indicates cleaning is required.
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TIER
4
F-07Injection Chain Ground Return IntegrityGateway result that points here: all Tier 1–3 candidates have been investigated and cleared, or the hum character does not match any of the above patterns — it is irregular, varies with chassis handling, or responds unexpectedly to scope probe placement.
The sidetone injection chain has a ground return path that may include one or more chassis grounds through which the signal return current flows. If any of these ground connections has become high-impedance (corroded chassis screw, poor solder joint at a chassis lug, or an internal wiring lug that has vibrated loose), the return current for the injection chain must flow through alternative paths. These alternative paths may include chassis sections that carry currents from other circuits — including the heater supply return — and the voltage developed by these currents across the impedance of the shared return path appears as hum in the injection circuit.
This is the most difficult candidate to diagnose and the least common, but it has been documented in KWM-2 units that have had previous non-standard service where chassis screws were not properly re-torqued or where ground lugs were resoldered without cleaning the chassis contact surface.
Test: use the scope probe ground clip directly on the audio output ground reference point while probing the SIDETONE pot wiper output. Any 60 Hz or 120 Hz signal visible at the wiper output when the scope ground is referenced to audio ground (rather than chassis ground) indicates a ground potential difference caused by a high-impedance ground return. Identify and resolder or re-torque all ground connections in the injection chain path, cleaning chassis contact surfaces with an abrasive pad before re-torquing.
Section 4 — Diagnostic Procedure: Ordered Step Sequence
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1
Gateway Test A — Mode isolation Set to receive mode. Listen to audio output with an antenna or 50 Ω dummy load connected. Record whether hum is present during receive. If yes, stop: the fault is in the common audio chain, not the sidetone injection chain. If no hum during receive, proceed.
-
2
Gateway Test B — SIDETONE control sensitivity Key CW transmit. Observe hum in headphones/speaker. Slowly rotate SIDETONE control to minimum. Record: does hum approach zero with the control at minimum (entry point is before the pot), or does hum persist at minimum (entry point is after the pot)? Note your result — it determines which candidates are highest priority.
-
3
Frequency identification — 60 Hz or 120 Hz While keying CW transmit, measure the hum frequency. Use an oscilloscope connected to the audio output through a series 10 µF capacitor (for AC coupling) or measure with a calibrated audio frequency counter.
16.7 ms period = 60 Hz; 8.3 ms period = 120 Hz.Record the frequency. This narrows the mechanism before any component is disturbed.
-
4
If hum tracks SIDETONE control (entry before the pot): test F-01 first Power off and discharge B+. Disconnect the high-side input of the SIDETONE pot from its coupling capacitor. Power on in CW TX mode. Measure DC voltage at the disconnected coupling capacitor output with a DVM. Any DC above 5 mV confirms leakage through the capacitor. Replace both coupling capacitors around the SIDETONE pot (input and output) with film types of the correct value from the service manual.
-
5
If hum persists at minimum SIDETONE and frequency is 120 Hz: test F-02 Power off and discharge B+. Locate the B+ bypass electrolytic for the audio amplifier stage. Temporarily connect a 47 µF / 450 V rated test capacitor (Nichicon or Panasonic, new) across the existing bypass cap using clip leads. Power on in CW TX mode. If hum reduces, the existing bypass cap has failed. Replace all electrolytic bypass capacitors in the audio stage B+ supply chain.
-
6
If hum persists at minimum SIDETONE and frequency is 60 Hz: test F-03 Power off and discharge B+. Substitute the audio amplifier tube with a known-good tested example of the same type. Power on in CW TX mode. If hum disappears, the original tube has heater-to-cathode leakage. Verify on a tube tester with heater-cathode leakage test if available. Do not return the original tube to service.
-
7
SIDETONE pot test — if control behaviour is erratic or hum is position-dependent Power off and discharge B+. Disconnect the SIDETONE pot wiper from the circuit. Measure resistance across the pot from wiper to each end while slowly rotating through full range. Any irregularity, dead zone, or jump in resistance greater than 5% of full-scale at any position confirms track or wiper failure. Replace the pot with a modern cermet type of the same resistance value and taper.
-
8
Screen bypass inspection — if hum has modulated character (not pure tone) Power off and discharge B+. Locate the screen bypass capacitors on the transmit exciter stages from the service manual schematic. Measure each with an ESR meter or capacitance meter. Replace any electrolytic with ESR above 5 Ω or capacitance below 50% of rated value. Replace as a set in the exciter chain rather than individually.
-
9
Verification after each repair After each component replacement, reassemble sufficiently to operate in CW TX mode and listen for the hum. Record whether it is eliminated, reduced, or unchanged. If unchanged, the replaced component was not the cause; continue with the next candidate. Do not assume that replacing multiple components simultaneously has definitively identified the cause — replace one group at a time where possible, so that the responsible component can be confirmed rather than guessed.
Section 5 — Injection Chain Architecture and Fault Entry Points
┌──────────────────────────────────────────────────────────────────────────┐
│ COLLINS KWM-2/2A SIDETONE INJECTION CHAIN — FAULT ENTRY MAP │
└──────────────────────────────────────────────────────────────────────────┘
SIGNAL SOURCE: Transmit carrier oscillator / BFO beat product (CW pitch)
──────────────────────────────────────────────────────────────────────────
[TX Carrier / BFO]
│
[Screen bypass caps on exciter stages]
│ ← F-05 enters here if screen bypass has failed
│ (modulated carrier amplitude, 120 Hz AM on sidetone pitch)
│
[Carrier pick-off resistor/capacitor network]
│
│ B+ supply to pick-off network
│ ← F-01 enters here: B+ ripple passes through leaky
[Sidetone coupling cap — INPUT] input coupling cap into pot
│ ← measure DC here to test F-01
│
[SIDETONE level potentiometer ─── wiper]
│ ← F-04 enters here: wiper contamination, open track
│
[Sidetone coupling cap — OUTPUT]
│ ← second coupling cap; also test for F-01 leakage
│
[Mode switch / CW relay contacts]
│ ← F-06 enters here: contaminated contacts, high
│ resistance path, stray pickup on floating wiper
│
[Audio amplifier grid injection point]
│
┌─────────────────────────────────────────────────┐
│ AUDIO AMPLIFIER STAGE (common to RX and TX) │
│ │
│ Tube heater → cathode leakage │
│ ← F-03 enters here: 60 Hz heater AC │
│ │
│ B+ plate supply via decoupling resistor │
│ → Bypass electrolytic │
│ ← F-02 enters here: 120 Hz B+ ripple │
│ if electrolytic has dried │
└────────────────────────────────┬────────────────┘
│
[Audio output — to speaker / headphones]
│
← F-07: ground return impedance in injection
chain; stray 60/120 Hz pickup on high-Z
floating node due to open ground return
──────────────────────────────────────────────────────────────────────────
GATEWAY TEST RESULTS → CANDIDATE PRIORITY MAP
──────────────────────────────────────────────────────────────────────────
Hum tracks SIDETONE control + 120 Hz → F-01 (leaky input coupling cap) FIRST
Hum tracks SIDETONE control + 60 Hz → F-01, then F-04 (pot wiper ground)
Hum independent + 120 Hz → F-02 (audio B+ bypass cap) FIRST
Hum independent + 60 Hz → F-03 (audio tube heater-cathode) FIRST
Hum position-erratic, varies on turn → F-04 (pot track/wiper)
Hum sounds modulated (not pure tone) → F-05 (exciter screen bypass)
Hum appears after period of disuse → F-06 (mode switch contacts)
Hum survives all above, scope shows → F-07 (ground return impedance)
ground potential difference
──────────────────────────────────────────────────────────────────────────
COMPARISON: SIDETONE HUM vs RECEIVE AUDIO HUM
──────────────────────────────────────────────────────────────────────────
┌────────────────────────────────┬────────────────┬────────────────────────┐
│ Observation │ Sidetone hum │ Receive audio hum │
│ │ (this guide) │ (different diagnosis) │
│ ───────────────────────────── │ ──────────────│ ───────────────────── │
│ Present during CW TX │ YES │ Often YES (shared) │
│ Present during SSB monitor │ No │ YES │
│ Present during receive │ No │ YES │
│ Tracks SIDETONE control │ Often YES │ No (not in chain) │
│ Fault location │ Injection │ Audio/B+ supply │
│ │ chain │ (common path) │
└────────────────────────────────┴────────────────┴────────────────────────┘
If hum is present in BOTH receive audio and sidetone:
investigate the common audio stage (F-02, F-03) first —
these candidates affect both paths equally.
KWM-2/2A sidetone injection chain fault entry map. Component designators vary between KWM-2 and KWM-2A production runs and between contract years; verify all component values and locations from the service manual edition applicable to your unit’s serial number before undertaking any component replacement. The injection chain architecture shown here is schematically representative; the exact circuit topology should be confirmed from the Collins service manual.
References and Notes
- Collins Radio Company, KWM-2 Operating and Service Instructions and KWM-2A Operating and Service Instructions. Available through the Collins Collectors Association at collinsradio.org and the Virtual Collins Radio Museum (wa3key.com). The service manual schematic and the component designators for the sidetone injection chain are the authoritative reference for all measurements described in this guide. Variant differences between the KWM-2 and KWM-2A (particularly in the audio section tube types and supply decoupling values) require that the correct edition be used for each unit’s serial number range.
- Mike Peace VK6ADA, Heathkit HW-16 Audio Hum Fault Diagnosis, vk6ada.com.au. The companion fault diagnosis guide documenting the 60 Hz vs 120 Hz frequency discrimination methodology using an oscilloscope, including the scope setup procedure for measuring mains-frequency hum components in audio circuits. The methodology for period measurement (16.7 ms for 60 Hz, 8.3 ms for 120 Hz) and the use of a series coupling capacitor for AC measurement are documented in that guide. While the HW-16 fault is in the receive audio path rather than the sidetone injection chain, the measurement methodology is identical.
- Collins Collectors Association, KWM-2 Technical Notes and Service Bulletins, collinsradio.org. Factory service bulletins covering known KWM-2 and KWM-2A production issues and component updates. Review all applicable service bulletins for the specific serial number range before assuming a sidetone hum is a wear-related failure — some service bulletins address sidetone circuit modifications that may already have been applied to the specific unit. The bulletin cross-reference at vk6ada.com.au/collins-service-bulletin-index/ indexes all available bulletins by topic.
- Collins S-Line reflector and CCA forum community documentation, archived 2000–2026. The candidate fault rankings (Tier 1 through Tier 4) reflect the frequency of occurrence documented in community restoration reports, service forum discussions, and restoration logs for the specific symptom pattern of sidetone-only hum in the KWM-2/2A. Community members who have documented successful sidetone hum diagnoses in the CCA forum and Collins reflector archives have consistently identified sidetone coupling capacitor leakage (F-01) and audio stage B+ bypass failure (F-02) as the leading causes, consistent with the age-related failure pattern of paper and electrolytic capacitors in equipment of this vintage.
- Mike Peace VK6ADA, Collins KWM-2A Failure Prevention Kit, vk6ada.com.au. The comprehensive failure prevention reference for the KWM-2A, covering the general failure mode spectrum of the transceiver. The sidetone hum fault is distinct from the common failures documented in the FPK (mechanical filter degradation, PTO coupling wear, carbon resistor drift) but shares the common root cause of electrolytic capacitor degradation with the FPK’s capacitor replacement recommendations. A KWM-2A that has had a full preventive capacitor replacement per the FPK guidance is significantly less likely to develop sidetone hum from capacitor failure (F-01, F-02) than an unrestored unit.