Collins KWM-2 / KWM-2A
Vietnam Modification (SCED 11b)

Section 1 — Theory of Operation

Original PTO Supply Circuit

In the unmodified KWM-2/2A, the PTO supply is derived from the 275 V B+ via a simple resistive divider. The 15 kΩ R73 connects from Pin 2 of J17 (the 275 V B+ line) to tie point A of turret E60. The 33 kΩ R131 connects from tie point A to chassis ground. The red/white PTO supply wire is taken from tie point A, giving a nominal PTO supply voltage of approximately:

  V(PTO) = 275V × 33k / (15k + 33k) ≈ 189 V  (nominal TX, B+ = 277V)
  V(PTO) = 301V × 33k / (15k + 33k) ≈ 207 V  (nominal RX, B+ = 301V)

  Measured frequency shift: ~3.1 Hz per volt × 17V = ~53 Hz T/R offset

Figure 1. Original resistive divider — unregulated PTO supply.

Modified Circuit — Zener-Regulated PTO Supply

The Vietnam Modification replaces R131 with a 150 V zener diode (CR12) referenced to chassis ground. R73 is retained as the current-limiting series resistor feeding the zener. An RF choke L42 (2–4 µH) is placed in series between the zener regulation point and the PTO supply line to prevent RF from the B+ supply from entering the PTO. Filter capacitors C278 and C279 are added to bypass high-frequency noise from the regulated supply node.

With the zener in place, the PTO supply is held at a constant 150 V regardless of whether the B+ rail is at 277 V (transmit) or 301 V (receive), provided sufficient current flows to keep the zener in regulation. The modified circuit:

  BEFORE MOD (unregulated):
  J17 Pin2 (B+) ──R73(15k)──[TIE PT A / E60]──R131(33k)──GND
                                    └─── PTO supply (≈190–207V, varies with B+)

  AFTER MOD (zener-regulated):
  J17 Pin2 (B+) ──R73(15k)──[NEW NODE]──┬──CR12(150V zener)──GND
                                         │  C278(0.05µF)──GND
                                         │
                                        L42(2-4µH)
                                         │
                              [TIE PT A / E60]──C279(0.02µF)──GND
                                    └─── PTO supply (150V, regulated)

Figure 2. Vietnam Modification (SCED 11b) circuit topology.

Section 2 — Parts List

Preferred Suppliers (Australia & International)

Section 2b — Safety Capacitor Replacement (Optional — Strongly Recommended)

What Safety Capacitors Are and Why the Originals Are Dangerous

The Collins 516F-2 power supply was designed in the late 1950s. Its mains input section includes small capacitors connected directly from each AC line conductor to the metal chassis. In the language of modern electrical safety standards these are Y-class positions — the most hazardous location in any mains-connected circuit, because a capacitor that fails short-circuit in a Y-class position places the full mains voltage directly on the equipment chassis.

In a properly engineered modern power supply, Y-class positions are fitted with Y2-rated safety capacitors certified to IEC 60384-14. These devices are manufactured with redundant dielectric construction and are specifically engineered to fail open-circuit rather than short-circuit when their dielectric breaks down. This fail-safe characteristic is the entire point of the Y2 rating — the capacitor simply stops working rather than connecting mains voltage to the chassis.

The original capacitors fitted to the 516F-2 are small ceramic disc or wax-paper types, typically 0.01 µF (10 nF). They were general-purpose bypass components chosen for their electrical characteristics, not for any safety certification. The IEC 60384-14 standard did not exist when the 516F-2 was designed. After 60–70 years of thermal cycling, vibration, and moisture absorption, the dielectric in these original capacitors has degraded. When they fail — and in equipment of this age, the question is when, not if — they are statistically likely to fail short-circuit. A short-circuit failure in a Y-class position connects one mains conductor directly to the chassis at 230 V AC (Australian mains).

  ORIGINAL CIRCUIT (Y-class position — general purpose cap):
  AC Line (Active) ──C15 (0.01µF general purpose)──┐
                                                     └── CHASSIS (earth)
  AC Line (Neutral) ──C16 (0.01µF general purpose)──┘

  FAILURE MODE — C15 fails short-circuit:
  AC Line (Active) ───────────────────────────────── CHASSIS at 230V AC
                            ↑
                    Operator contacts chassis + separate earth = ELECTROCUTION

  AFTER REPLACEMENT (Y2 safety class):
  AC Line (Active) ──C15 (10nF Y2 certified)──┐
                                               └── CHASSIS (earth)
  AC Line (Neutral) ──C16 (10nF Y2 certified)──┘

  FAILURE MODE — Y2 capacitor fails open-circuit:
  AC Line (Active) ──── [OPEN] ────────────────── CHASSIS unaffected
                           ↑
                   Radio operates normally (slightly less RFI filtering)
                   No shock hazard. Replace at next convenient opportunity.

Figure 4. Y-class capacitor failure modes — original vs. Y2-certified replacement.

The X-Class Capacitor — Line to Line

The 516F-2 mains input also typically includes an X-class position capacitor connected directly across the two AC line conductors (Active to Neutral). An X2-class failure mode is an open-circuit — less immediately dangerous than a Y-class short because the chassis is not involved. However, X-class capacitors can occasionally fail to a resistive or partial short that causes overheating, and after 60 years of service replacement is prudent. Fit an X2-certified 275 VAC type. Verify the original value from your service manual before ordering.

Critical Safety Requirement: X/Y Class Certification is Non-Negotiable

Capacitor Positions in the 516F-2

Identification — How to Recognise the Original Capacitors

The original C15 and C16 in the 516F-2 are typically small ceramic disc capacitors approximately 5–8 mm diameter, marked 0.01, 103, or .01 µF. They may be orange, brown, or tan coloured, often with brown epoxy coating, and typically have a slightly domed or flat disc profile. They are wired directly from the AC input terminal strip — one to each line conductor — to a grounded chassis lug or terminal point. Some units have them mounted directly on the AC terminal strip; others have them tucked alongside the mains transformer primary leads. If you cannot find them, check both places and consult the schematic.

An aged capacitor that has been running hot will show telltale signs: brown discolouration around the base, cracked or crazed epoxy coating, or a slight pungent smell of scorched insulation. Any of these is a strong indicator of imminent failure and makes immediate replacement mandatory rather than merely recommended.

Installation Notes

Y2 safety capacitors are physically larger than the original ceramic disc types — typically 10×9 mm body with 5 mm lead spacing in the Kemet PHE840 series, versus the original 5–8 mm disc. Leads will need to be trimmed and the component seated carefully. The lead material on modern safety capacitors is tinned copper and solders easily to the original 516F-2 terminal strip. No special technique is required beyond standard through-hole soldering practice.

Polarity is not relevant — Y2 and X2 capacitors are non-polarised AC devices. Orientation in the circuit does not matter. Dress the leads so the capacitor body cannot vibrate against adjacent wiring, and apply heat shrink to the leads at the terminal connections for mechanical security as you would for any other mains-connected component.

Heat Shrink Tubing — Specifications & Suppliers

Section 3 — Pre-Work QA Checks

Before beginning the installation, complete the following pre-work checks. These establish a baseline and identify any pre-existing issues that could affect the modification or test results.

• Service manual available: Confirm you have the KWM-2/2A 9th Edition (or later) manual with the SCED 11b schematic note on Sheet 1 and page 7-6.
• Schematic confirmed: Locate R73, R131, C204, J17, turret E60, tie point A, J26 on the schematic before touching the chassis.
• Mains power removed: Mains lead physically unplugged. Wait a minimum of 5 minutes before opening chassis.
• B+ discharged: Using a 10 kΩ / 10 W bleeder resistor connected from the B+ test point to chassis ground, verify B+ has fallen to below 10 V before proceeding.
• Baseline frequency noted: Before disassembly, power the radio on a dummy load, allow 15 minutes warm-up, and note the receive frequency for a stable reference signal (e.g., WWV 10 MHz). Note the frequency offset observed when keying into transmit. This is your baseline T/R offset. Typical pre-mod value: 40–60 Hz.
• Baseline B+ voltages measured: With HV-rated meter: record B+ in receive (expected ≈ 295–305 V) and B+ in transmit (expected ≈ 270–285 V) at J17 Pin 2.
• PTO supply voltage measured: Record voltage at tie point A, turret E60, in receive and transmit. Expected: ≈190–210 V range with pre-mod variation of 15–20 V.
• Parts kit inspected: Verify all required components per Section 2 are on hand before opening chassis, including heat shrink tubing: at minimum HS-1 (red, 3.0 mm/3:1, 600 V) for CR12 cathode — this is non-negotiable. Do not start if any part is missing.
• Tools prepared: Confirm you have: temperature-controlled solder iron (700°F / 370°C tip), 60/40 or 63/37 solder, solder wick, needle-nose pliers, ESD-safe component tray, HV-rated digital multimeter.
• J26 identification: Physically confirm J26 is the spare (unused) RCA jack on the rear panel before removal. Verify it has no wiring attached.

Section 4 — Installation Procedure

• 1
  Open the chassis top cover. Remove the KWM-2/2A top cover. Locate the PA cage at the rear right of the chassis. The working area is immediately in front of the PA cage: J17 (the rear panel connector carrying the 275 V B+) and turret E60 (the terminal strip where tie point A is located).
• 2
  Identify original components. Locate and confirm:

  • R73 — 15 kΩ / 2 W resistor, from J17 Pin 2 to tie point A on E60
  • R131 — 33 kΩ / 2 W resistor, from tie point A on E60 to chassis ground
  • C204 — 0.01 µF capacitor, from J17 Pin 2 to ground (leave in place — do not disturb)
  • Red/white PTO supply wire at tie point A on E60
• 3
  Photograph before touching. Take a clear photograph of the J17/E60 area showing R73, R131, C204, and the PTO supply wire before any component is touched. This is your reference image if any issue arises.

• 4
  Remove R131. Unsolder and remove R131 (33 kΩ / 2 W) from both ends. Both connection points — tie point A on E60 and the ground point — must be clean and free of solder bridges after removal. Discard R131.
• 5
  Disconnect R73 from tie point A. Unsolder the E60 tie point A end of R73 only. The other end of R73 (at J17 Pin 2, alongside C204) remains attached and undisturbed. Lift the free R73 lead clear of the chassis; it will form part of the new node in step 8.
• HS
  Apply heat shrink HS-3 — R73 relocated free lead (recommended). Slide a 15 mm length of 3.0 mm / 2:1 polyolefin heat shrink (600 V rated, orange or yellow) onto the free lead of R73. Leave it slid back toward the resistor body while you make the solder connection at step 8. Once the joint is cool, slide the sleeve to cover from the R73 body to 4 mm past the solder joint and shrink with a heat gun. The orange/yellow colour serves as a permanent live-rail marker on this lead for future reference during maintenance.
• 6
  Remove J26. Unscrew and remove the spare J26 RCA jack from the rear panel. This provides both a mounting aperture and a convenient mechanical attachment point for CR12. Confirm no wiring is attached to J26 before removal.

• 7
  Mount CR12 at J26 location. Mount the 150 V zener CR12 (1N3011 or NTE5161A) at the J26 hole on the rear panel. The cathode (banded end) is connected to the new floating node (step 8). The anode connects to chassis ground. Use a nylon standoff or sleeve to insulate the body of CR12 from the chassis panel — the cathode must not short to chassis.
• HS
  ⚠ Apply heat shrink HS-1 (critical) and HS-2 (if required) — before making cathode connection.

  • HS-1 — CR12 cathode lead: Before routing the cathode wire to the new node, slide a 20 mm length of 3.0 mm / 3:1 polyolefin heat shrink (600 V rated, red) onto the cathode lead. Position it so it will cover from the diode shoulder to 4 mm past the eventual solder joint when slid into final position after soldering. Do not shrink yet — leave it slid back toward the diode body while you make the solder connection. Once the joint is cool, slide the sleeve into final position covering the full exposed cathode lead and shrink with a heat gun at 120–150°C. Verify after cooling that no bare metal is visible at either end of the sleeve.
  • HS-2 — CR12 body (conditional): Measure the clearance between the CR12 body and the J26 panel rim. If clearance is less than 3 mm, slide a length of 6.0 mm / 3:1 polyolefin heat shrink (red) cut to the diode body length plus 4 mm each end over the CR12 body before mounting. Shrink it in place with the diode unmounted, then install CR12 through the panel hole. The sleeve will provide a dielectric barrier between the diode body and any incidental contact with the metal panel rim.
• 8
  Create the new regulation node. Connect together at a free-floating point (or an installed terminal strip lug):

  • The free end of R73 (disconnected from tie point A in step 5)
  • The cathode lead of CR12 (routed via Teflon wire from J26 area)
  • One end of L42 (the RF choke)
  • One plate of C278 (0.05 µF; other plate to chassis ground)

  Use Teflon-insulated wire to route the CR12 cathode wire alongside the PA cage to the J17 area. Keep the lead short; avoid routing over hot components.
  
  ⓘ Apply heat shrink HS-4 — Teflon wire solder joints (recommended; required for mobile/vehicle use). Before making either solder joint on the Teflon interconnect wire, pre-thread a 12 mm length of 3.0 mm / 2:1 polyolefin heat shrink (clear or red) onto the wire at each end. After each joint cools, slide the sleeve over the tinned conductor stub and 5 mm of wire insulation and shrink in place. Repeat at both ends of the wire. This protects the joint from mechanical stress if the wire is disturbed during future maintenance and prevents the tinned stub from contacting adjacent metalwork.

• 9
  Connect L42 output to tie point A. Connect the other end of L42 to tie point A on E60 — the same point where the red/white PTO supply wire is already attached. Do not disturb the PTO supply wire connection.
  
  ⓘ Apply heat shrink HS-5 — L42 leads (recommended). Pre-thread a 10 mm length of 2.4 mm / 2:1 polyolefin heat shrink onto each lead of L42 before soldering. After the joint cools at each end, slide the sleeve over the lead and shrink. Fine choke leads are vulnerable to fracture from repeated flexing; the sleeve protects the solder bond from mechanical stress when the top cover is replaced.
• 10
  Install C279. Connect one plate of C279 (0.02 µF / 500 V) to tie point A on E60 (alongside the PTO supply wire and the L42 output). Connect the other plate to chassis ground. C279 bypasses the regulated PTO supply output to ground, filtering residual noise from the zener.
  
  ⓘ Apply heat shrink HS-6 / HS-7 — C278 and C279 hot leads (conditional). Inspect both capacitors after mounting. If either hot lead (the lead connected to the 150 V node, not the ground lead) is free-standing with more than 10 mm of exposed conductor, slide a length of 3.0 mm / 2:1 polyolefin heat shrink (600 V rated, orange) cut to cover the exposed lead plus 4 mm past the solder joint, and shrink in place. The ground leads do not require heat shrink.
• 11
  Dress all leads. Dress and cable-tie all new leads so they are clear of the PA cage, away from hot surfaces, and cannot vibrate against the chassis. All Teflon wire should be routed tight to the chassis floor. No new wiring should cross the PA tank circuit area.

• 12
  Final visual inspection (pre-power).

  • Confirm R131 is fully removed and its former pads are clean
  • Confirm R73 free end is secured at the new node with no cold joints
  • Confirm CR12 cathode is insulated from chassis; anode is solidly grounded
  • Confirm L42 installed between new node and tie point A
  • Confirm C278 from new node to ground; C279 from tie point A to ground
  • Confirm C204 and the PTO supply wire at tie point A are undisturbed
  • Confirm no solder bridges, no component touching PA cage, no loose strands
  • Confirm HS-1: CR12 cathode lead fully covered by red heat shrink — no bare conductor visible at either end of the sleeve
  • Confirm HS-2 (if applied): CR12 body sleeve in place; diode body cannot contact panel rim
  • Confirm HS-3 (if applied): R73 relocated lead sleeved in orange/yellow from body to joint
  • Confirm HS-4 (if applied): Both Teflon wire solder joints sleeved
  • Confirm HS-5 (if applied): Both L42 lead joints sleeved
  • Confirm HS-6/7 (if applicable): Free-standing cap hot leads sleeved in orange
  • Photograph completed modification for records

Section 5 — Quality Assurance Plan

Perform all resistance checks with power fully removed and B+ discharged. Use the ohms range of a digital multimeter.

Section 6 — Post-Installation Test Plan

• 1
  First power-on with observation. Apply mains power to the 516F-2 power supply. Do not key the radio. Observe for 30 seconds: no unusual arcing, no smoke, no burning smell, mains fuse intact. If any anomaly is observed, immediately switch off and return to Section 5.
• 2
  B+ verification (receive mode). Using HV-rated meter, measure B+ at J17 Pin 2 in receive mode. Expected: 295–305 V. Record actual reading. Compare with baseline from Section 3 pre-work check.

• 3
  Measure PTO supply voltage — receive. Measure voltage at tie point A on E60 in receive mode. Expected: 149–151 V. This confirms CR12 is conducting and regulating. If >160 V, CR12 is not in regulation (check cathode connection); if <140 V, CR12 may have too little current — check R73 integrity.
• 4
  Measure PTO supply voltage — transmit. Key the transmitter briefly (2–3 seconds into dummy load). Measure voltage at tie point A during transmit. Expected: 148–152 V — within 2 V of receive reading. Pre-mod value was approximately 190 V (receive) and 173 V (transmit) — the modification should show near-identical readings in both modes.

• 5
  Allow 15-minute warm-up. Run the radio in receive on a known stable reference signal (WWV 10 MHz, or a nearby beacon). Allow 15 minutes for thermal stabilisation before measuring T/R offset.
• 6
  Measure T/R frequency offset. Using a second receiver with 1 Hz resolution, or a frequency counter at the antenna port on low power, compare receive frequency to transmit frequency. Key into transmit for 3 seconds and observe any pitch shift. Expected post-mod: <10 Hz T/R offset. Typical result: 3–8 Hz. If offset >20 Hz, check zener voltage stability between modes.

• 7
  Audio quality check. Receive a strong SSB signal. Audio should be clear without hum or buzz. Compare to pre-mod audio quality. Any new hum on receive suggests C278 or C279 connection issue — check bypass capacitor grounds.
• 8
  Transmit ALC and power check. Tune to a clear frequency, key with carrier (TUNE mode or CW). Verify PA output power is consistent with pre-mod baseline. The modification does not affect PA output; any significant change in power suggests an accidental ground in the PTO supply path.
• 9
  All band check. Operate briefly on each amateur band (80, 40, 20, 15, 10 m). Confirm PTO tracks correctly through all bands. Confirm no audio artifacts or instability at band edges.

Section 7 — Test Results Record

Section 8 — Troubleshooting

Section 9 — Ferrite Enhancement for HF Noise Reduction

Why the Zener Generates HF Noise

At 150 V, CR12 operates in the avalanche breakdown regime rather than the quantum-tunnelling (Zener effect) regime, which applies only below approximately 5–6 V. Avalanche breakdown is driven by impact ionisation in the depletion region — a highly energetic, stochastic process that generates broadband white noise across HF and beyond. Higher breakdown voltages sustain larger avalanche cascades and produce proportionally more noise per unit of shunt current than low-voltage reference diodes.

This noise couples into the PTO via two mechanisms. The first is direct supply-rail amplitude modulation, which C278 and C279 address. The second, more subtle mechanism is phase noise: the PTO’s measured voltage sensitivity of 3.1 Hz/V means that even a few millivolts of HF noise on the 150 V rail produces small but real frequency modulation of the oscillator. On a spectrum analyser this appears as an elevated close-in noise floor — a widening of the PTO spectral line that degrades reciprocal mixing performance on receive.

Why L42 Alone Is Insufficient

L42 is a wire-wound inductor. It presents reactive impedance (Z = 2πfL) at HF, which reflects energy back toward the zener rather than absorbing it. At the nominal 3 µH value, L42 provides only approximately 57 Ω impedance at 3 MHz and 190 Ω at 10 MHz — modest suppression. More significantly, the combination of L42 with C278 (50 nF) resonates at approximately 411 kHz, and L42 with C279 (20 nF) resonates at approximately 651 kHz. Both resonances are below the HF band; at these frequencies the circuit provides good attenuation, but at HF frequencies where the avalanche noise is most relevant to receive performance, the attenuation is limited.

  L42 (3µH) + C278 (50nF) → resonance ≈ 411 kHz  (good LF rejection)
  L42 (3µH) + C279 (20nF) → resonance ≈ 651 kHz  (good LF rejection)

  At 3.5 MHz:  Z(L42) ≈ 66Ω  — HF noise largely passes through
  At 14 MHz:   Z(L42) ≈ 263Ω — improving, but reactive/reflective
  At 28 MHz:   Z(L42) ≈ 528Ω — reasonable, still reflective

  Ferrite bead (Material 31) in same position:
  At 3.5 MHz:  Z ≈ 200–400Ω  — and LOSSY (dissipates rather than reflects)
  At 14 MHz:   Z ≈ 500–800Ω  — near peak loss frequency
  At 28 MHz:   Z ≈ 400–600Ω  — still lossy across full HF range

Figure 3. Comparative impedance — L42 wire-wound choke vs. ferrite bead Material 31 across HF.

The critical difference is that ferrite beads are resistive-dominant at their target frequencies — they dissipate HF energy as heat rather than reflecting it. Reflected energy from a reactive choke does not vanish; it can find other coupling paths into sensitive circuit nodes. A lossy ferrite bead has no sharp resonance peak and eliminates this risk.

Recommended Ferrite Placements

Ferrite Material Selection

Expected Improvement in Practice

On a stock, well-aligned KWM-2, the improvement from ferrite addition will likely be inaudible in normal operating conditions. The dominant noise sources on receive — thermal noise in the first RF amplifier, atmospheric noise, and man-made interference — are orders of magnitude larger than the zener’s contribution to the noise floor.

The most likely scenario where ferrite suppression produces a measurable result is on the quietest bands in low-noise conditions (15 and 10 metres at solar minimum), and specifically in terms of close-in reciprocal mixing. A cleaner PTO supply reduces the PTO’s own phase noise contribution, which directly determines how well the receiver handles strong signals on adjacent channels. For operators who use the KWM-2 seriously — DX operating, quiet rural locations, or where the radio is connected to a high-performance antenna — this is a worthwhile marginal gain.

For field use, contest use, or where adjacent-channel performance is not a priority, ferrite addition is optional. The Vietnam Modification is complete and fully functional without it.