Collins KWM-2/2A Carrier Balance Optimisation
Suppressed Carrier Level Directly Affects Transmit Audio Quality
The two-stage carrier suppression architecture, the balanced modulator null procedure, the first balanced mixer trim, and why the manual procedure is poorly explained — with a clear step-by-step approach that actually works
Carrier balance is not just a number on a meter — it is the single adjustment that most directly determines whether your KWM-2/2A sounds clean or sounds “rough” on SSB. A properly nulled balanced modulator produces a pure double-sideband suppressed-carrier signal at 455 kHz. When the carrier is insufficiently suppressed, it leaks through the mechanical filter and rides on top of your SSB signal as a coherent tone at the carrier insertion frequency. On the receiving end, this manifests as a harsh, buzzy quality to your audio that sounds like overmodulation but is actually carrier feedthrough.[1]
The KWM-2/2A uses a two-stage carrier suppression architecture that achieves a combined suppression of approximately 50 dB. The balanced modulator (CR1–CR4 diode quad) provides approximately 30 dB of carrier suppression through circuit balance. The Collins mechanical filter (FL1) then provides an additional 20 dB of carrier attenuation by placing the BFO carrier frequency approximately 20 dB down on the filter skirt.[2] Both stages must be working correctly to achieve a clean transmitted signal. If the balanced modulator null has degraded from 30 dB to only 15 dB — a common situation in an unserviced KWM-2 — the total suppression drops from 50 dB to only 35 dB, and the carrier becomes audible to receiving stations.
The KWM-2/2A balanced modulator is a diode-quad ring modulator formed by CR1, CR2, CR3, and CR4 (originally four matched 1N34A germanium diodes). Audio from the cathode follower V3A and BFO voltage from the beat-frequency oscillator are both fed to this ring. When the four diodes are perfectly matched and the resistive balance network is properly adjusted, the carrier (BFO) energy cancels at the output while the sum and difference products (upper and lower sidebands) pass through to IF transformer T1 and into the IF amplifier V4A.[2]
The key adjustment components are:
R2 balances the DC and resistive paths through the diode ring; C21 trims the reactive (capacitive) imbalance. The adjustment is interactive — touching one shifts the optimum of the other. A proper null requires iterating between R2 and C21 until the deepest possible null is found at the antenna output.[3]
Coupling Note — C25: In early KWM-2 units, C25 is 3 pF, which results in poor coupling between R2 and L4. Collins ASAB documentation recommends changing C25 to 6 pF (CPN 961-0122-000) to improve coupling. This change can improve the achievable carrier null depth and should be verified before attempting carrier balance optimisation.[4]
Understanding the suppression budget makes clear why both stages matter and why a degraded balanced modulator null sounds so much worse than you might expect:
| Stage | Mechanism | Nominal Suppression | Degraded (Typical Fault) |
|---|---|---|---|
Balanced Modulator |
Diode quad CR1–CR4 with R2/C21 null |
~30 dB |
~10–15 dB (aged/mismatched diodes) |
Mechanical Filter FL1 |
BFO placed 20 dB down on filter skirt |
~20 dB |
~20 dB (fixed — filter characteristic) |
Combined Total |
Balanced modulator + filter skirt |
~50 dB |
~30–35 dB (audible carrier on air) |
At 50 dB total suppression the carrier is inaudible to receiving stations under all normal conditions. At 35 dB it becomes audible on strong signals. At 30 dB or below, receiving stations will report your signal as having a “buzz,” “raspy tone,” or “rough audio” — symptoms frequently misdiagnosed as microphone problems, audio stage distortion, or ALC maladjustment.[1]
The instruction manual presents the carrier balance procedure as a terse list of steps buried in the alignment section without explaining the underlying logic. Here is the procedure with the reasoning made explicit, following the 9th edition manual sequence but with practical guidance the manual omits.[5]
⚠ Prerequisites: Before attempting carrier balance, ensure: (a) the transceiver has been warmed up for at least 30 minutes, (b) the PA bias is correctly set, (c) all paper capacitors in the balanced modulator and audio chain have been replaced, and (d) a dummy load is connected to the antenna output. Never perform this adjustment while radiating into an antenna.
This is the primary carrier suppression adjustment. You are minimizing the BFO carrier energy that leaks through the diode ring to the modulator output.
Why the Manual Procedure is Poorly Explained: The manual presents R2 and C21 as separate adjustments, but it does not adequately emphasize that they are interactive and must be iterated. It also does not explain that the RF voltmeter at the antenna output measures the combined effect of both the balanced modulator null and the first balanced mixer null — which is why Step 2 (below) is necessary to distinguish between the two.
After the balanced modulator is optimally nulled, the 9th edition manual directs you to trim the first transmit mixer (V5, 12AT7) balance. The mixer is also a balanced circuit, and carrier leakage through this stage adds to the total residual carrier at the output.
Key Distinction: An RF voltmeter at the antenna output does not differentiate between carrier leakage from the balanced modulator and carrier leakage from the first balanced mixer. If you achieve a sharp null at R2/C21 but the overall reading remains high, the problem is in the mixer stage, not the modulator. Conversely, if R2/C21 cannot produce a sharp null at all, the problem is in the modulator diodes or the modulator balance network — not the mixer.[3]
| Fault | Symptom | Cause | Fix |
|---|---|---|---|
No sharp null at R2 |
RF voltage at output cannot be reduced below ~1V regardless of R2/C21 position |
One or more 1N34A diodes (CR1–CR4) open, shorted, or severely mismatched |
Replace all four diodes with matched 1N4454 (or 1N270). Replace as a set[6] |
Null at extreme of R2 range |
Null occurs at one end of R2 rotation only; insufficient adjustment range |
Drifted fixed resistors in the balance network; drifted R2 pot (>250Ω) |
Measure and replace drifted resistors; verify R2 pot value |
Distorted audio at balanced modulator input |
Audio waveform severely clipped before reaching the diode ring |
Failed 1N34A diodes clipping the audio signal; one open, one shorted in the ring[6] |
Replace all four diodes CR1–CR4. Verify clean audio on oscilloscope at T1 input |
Carrier suppression degrades after warm-up |
Good null when cold; carrier rises after 15–30 minutes |
Temperature-sensitive diode mismatch; PSU ripple (C264 aging) |
Replace diodes with matched set; add/replace C264 (4 µF → 22 µF per ASAB)[4] |
High residual after good modulator null |
R2/C21 produce a sharp null but overall output >0.5V |
First balanced mixer (V5) out of balance; drifted R24 or cathode resistors; weak 12AT7 |
Adjust R24; try a new 12AT7 in V5; measure/replace cathode resistors[3] |
Power supply ripple carrier |
Buzz or hum on transmitted signal; not correctable with R2/C21 |
Insufficient filtering on T+275V supply; C264 missing or dried out |
Install/replace C264 per ASAB 1016; verify electrolytic caps in 516F-2[4] |
The 32S-1 and 32S-3/3A transmitters use an identical balanced modulator architecture with a diode quad ring (CR1–CR4), a resistive balance pot, and a capacitive trim. The carrier suppression procedure is functionally the same: null the balanced modulator with MIC GAIN at OFF, then trim the first balanced mixer, iterating between the two adjustments.[7]
In the 32S-1, the ALC rectifier is a 6AL5 tube rather than silicon diodes. The balanced modulator diodes in the 32S-1 are also 1N34A germanium types and are subject to the same aging failure modes as the KWM-2. The 32S-3/3A uses silicon diodes for ALC rectification but retains the same balanced modulator diode topology for SSB generation.[8]
- Collins Collectors Association — KWM-2/2A Transceiver Equipment Profile. Balanced modulator description, two-stage carrier suppression (30 dB modulator + 20 dB filter skirt), BFO crystal frequencies Y16/Y17 matched to FL1 passband. collinsradio.org — KWM-2/2A
- WA3KEY. KWM-2/2A Transceiver — Circuit Description. Diode quad balanced modulator CR1–CR4, audio/BFO feed, IF amplifier V4A, mechanical filter FL1, balanced mixer architecture. wa3key.com — KWM-2/2A
- Antique Radio Forums. KWM-2A Carrier Balance. Discussion of R2/C21 null procedure, first balanced mixer R24 adjustment, cathode resistor drift diagnosis, 12AT7 tube substitution. October 2015. antiqueradios.com — KWM-2A Carrier Balance
- VK3KCM. KWM-2/2A Service Information. Compiled ASAB service changes: C25 coupling improvement, C264 ripple filtering, R122 audio feedthrough suppression, carrier suppression and power supply ripple interaction. angelfire.com/de/vk3kcm — KWM-2 Service
- Collins KWM-2 and KWM-2A Instruction Book, 9th Edition (January 1978). Section 4: Alignment — balanced modulator carrier null procedure, first balanced mixer adjustment, ALC zero, bandpass alignment. Collins Radio Company / Rockwell International. collinsradio.org — KWM-2/2A Manual Section 4 (PDF)
- Gamma-Five.com. My Collins KWM-2 — Restoration and Repair. Diode quad failure diagnosis: two of four 1N34A diodes failed (one open, one shorted) causing severe audio clipping at balanced modulator input. Replacement with 1N4454 equivalent restored clean audio. gamma-five.com — KWM-2 Restoration
- Collins Collectors Association — 32S-3 Equipment Profile. Balanced modulator and mixer circuit description, dual time-constant ALC, RF inverse feedback. collinsradio.org — 32S-3
- Collins Reflector / QTH.net. 32S-1 Grid Problem — ALC Rectifier Evolution. 6AL5 tube ALC rectifier in 32S-1 vs. silicon diodes in 32S-3; C142 leakage; balanced modulator diode aging across S-Line family. December 2005. qth.net — Collins Reflector
- Borowski, Gerd DJ7HS. My Collins KWM-2. V3A cathode charging transient through balanced modulator producing RF output spike on T/R switching; C264/CR10/R122 modification sequence; balanced modulator feed path analysis. CCAE — DJ7HS KWM-2 (PDF)
- Collins Collectors Association — RX For Your Collins. Master index including: “KWM-2 Keying & VOX Issues/Solutions & Tutorial,” “Undocumented ALC Changes in KWM-2,” and related S-Line technical articles. collinsradio.org — RX For Your Collins
- VK3KCM. KWM-2/2A Additional Service Information. Resistor drift table (R4, R7, R148, R169, R175), VOX relay transient fix, C267 calibrator trimmer range issue, AGC time constant discussion. angelfire.com/de/vk3kcm — KWM-2A Info
- Electronics Notes. Collins KWM-2 Vintage Ham Radio Transceiver. Complete circuit overview, tube complement, double-conversion architecture, balanced modulator and mixer topology. electronics-notes.com — KWM-2
Collins Collectors Association (CCA) — For maintaining the KWM-2/2A equipment profile, the “RX For Your Collins” technical article library, and the archived ASAB service information that documents the carrier suppression architecture and known service issues.
Gerd Borowski, DJ7HS — For the detailed oscilloscope analysis of the V3A cathode transient through the balanced modulator, demonstrating how the T/R switching path produces carrier feedthrough independent of the R2/C21 null adjustment.
VK3KCM — For compiling and hosting the KWM-2/2A service information derived from Collins ASAB letters, including the C25 coupling improvement and C264 ripple suppression changes that directly affect carrier balance performance.
Gamma-Five.com — For documenting the diagnostic pathway from “rough transmit audio” through oscilloscope tracing to the balanced modulator diode quad failure, with before/after confirmation of the 1N4454 replacement.
Antique Radio Forums contributors — For the carrier balance troubleshooting thread that clearly identifies the distinction between balanced modulator null and first balanced mixer null — a point the manual does not adequately explain.