Collins 516F-2 Power Supply RFI Analysis and Mitigation

1. Introduction and Scope

The Collins 516F-2 power supply was designed by Collins Radio Company in the early 1960s to serve as the primary DC and filament power source for the Collins S-Line series of amateur radio equipment — specifically the 75S-1/3/3B/3C receivers, 32S-1/3/3A transmitters, and 30L-1 linear amplifier^[1] — as well as the KWM-2 and KWM-2A transceiver models.^[2] While the 516F-2 was an elegant design for its era, decades of component aging, modern silicon rectifier retrofits, and contemporary electromagnetic environments have made it one of the most significant — and most overlooked — sources of elevated noise floor in a Collins HF station.

This document provides a comprehensive engineering analysis of every significant RFI emission mechanism within the 516F-2 and details proven mitigation strategies using ferrite cores, LC filter networks, shielding improvements, and grounding techniques. The objective is to achieve measurable noise floor reduction across the HF amateur bands (1.8–30 MHz) while preserving the historical integrity and safe operation of the equipment.

2. Collins 516F-2 Circuit Architecture Overview

Understanding the RFI emission mechanisms requires a thorough grasp of the 516F-2 circuit topology. The power supply provides multiple regulated and unregulated outputs from a single power transformer assembly.

2.1 Power Transformer and Rectification

The 516F-2 transformer provides multiple secondary windings. The high-voltage secondary delivers approximately 650V center-tapped AC, which is full-wave rectified to produce the +275V DC B+ supply. A separate secondary winding provides the bias supply voltage, rectified to approximately −90V to −130V for transmitter bias applications. The 6.3V AC filament supply is provided from a dedicated secondary winding and distributed to all connected equipment via the interconnect cables.^[2]

In the original design, Collins specified 5R4 vacuum tube rectifiers; later production used solid-state diodes. Many 516F-2 units in service today have been retrofitted with silicon diode replacements, using kits such as the Harbach Electronics SS-516 kit^[3] (instructions by John May, K6MAY) or the Radio Farm Projects SB-1 PCB^[4] designed by retired Collins Radio engineers Barry Buelow WØIY and KØDAS (also on GitHub). Collins engineers themselves recommended solid-state conversion in Service Information Letter SIL 1-76.^[5] While more reliable and efficient, silicon diodes introduce fast-recovery switching transients that are a primary source of conducted and radiated RFI.

2.2 Output Voltage Summary

2.3 Interconnect Cable System

The 516F-2 connects to associated equipment via multi-conductor cables carrying DC B+, bias, filament AC, control signals, and chassis ground. These cables act as both antennas for radiated RFI pickup and as conductors for conducted noise between units. The typical cable length of 18–60 inches places their resonant frequency within or near the HF amateur bands, making them efficient radiators at frequencies above 10 MHz.

3. RFI Emission Source Analysis

The 516F-2 generates RFI through five primary mechanisms, each requiring different mitigation approaches. The following analysis characterizes each source by frequency content, coupling path, and relative severity.

3.1 Silicon Rectifier Switching Transients

When the original 5R4 tube rectifiers (or selenium stacks in early units) are replaced with silicon diodes, the reverse-recovery characteristics change dramatically. A 5R4 vacuum rectifier has a soft recovery characteristic with transition times measured in microseconds. Silicon diodes (1N4007, 1N5408, etc.) exhibit reverse-recovery times of 2–5 μs with extremely fast di/dt transitions that generate broadband RF noise from below 1 MHz to well above 30 MHz.

Mechanism: During each half-cycle of the 60 Hz AC waveform, the conducting diode must transition from forward conduction to reverse blocking. During the reverse-recovery interval, a brief but intense current spike occurs as stored minority carriers in the diode junction are swept out. This spike generates broadband electromagnetic energy that couples both conductively into the DC output bus and radiatively from the transformer secondary winding and associated wiring.^[6]

KH6GRT documented the inclusion of RC equalizing networks across all high- and low-voltage diodes, plus an RC snubber across the high-voltage secondary in his 516F-2 equivalent build, specifically to address this transient noise mechanism.^[7]

3.2 Power Transformer Leakage Flux and Magnetostriction

The 516F-2 power transformer operates at 60 Hz but generates harmonics due to magnetic core saturation effects, particularly in aged units where lamination insulation has degraded. Magnetostriction — the physical dimensional change in core laminations as magnetic flux density varies — produces audible hum but also generates conducted harmonics on both primary and secondary windings.

3.3 AC Mains Conducted Emissions

The 516F-2 generates conducted emissions back onto the AC mains through two paths: common-mode (noise currents flowing on both AC conductors referenced to safety ground) and differential-mode (noise currents flowing between the two AC conductors). Jim Brown, K9YC, provides extensive analysis of these coupling mechanisms and their mitigation in his comprehensive RFI tutorial.^[8]

3.4 Interconnect Cable Radiation and Coupling

The multi-conductor cables between the 516F-2 and associated Collins equipment carry DC, AC filament, and control signals. As K9YC emphasizes in his Ham’s Guide to RFI, wiring in any system acts as an unintentional antenna — any RF noise present on these conductors radiates directly into the near-field of the receiver and its antenna feedline. The 6.3V AC filament lines are particularly problematic because they carry significant 60 Hz current with harmonic content, connecting directly into the receiver chassis where they couple to sensitive RF front-end circuitry.^[9]

3.5 Grounding and Bonding Deficiencies

The original Collins station grounding scheme relied on chassis interconnection through the multi-pin power cables and a single ground bus. In many installations, the ground path impedance at HF frequencies is significant due to long ground leads, corroded connections, and lack of RF bonding between chassis. This creates ground loops that act as loop antennas for magnetic field pickup.

3.6 RFI Source Severity Summary

4. Ferrite Core RFI Suppression

Ferrite cores are the most cost-effective and non-invasive first line of defense against RFI in a Collins station. Jim Brown, K9YC, provides the definitive treatment of ferrite material selection and application in his comprehensive Ham’s Guide to RFI, Ferrites, Baluns, and Audio Interfacing (Rev. 7, 2019)^[10] and his New Choke Cookbook for the 160–10M Bands (2018).^[11] Proper material selection, placement, and winding technique are critical to achieving effective suppression across the target frequency range.

4.1 Ferrite Material Selection for HF Applications

Different ferrite materials exhibit peak impedance at different frequencies. For HF amateur radio RFI suppression (1.8–30 MHz), correct material selection is essential. K9YC’s measured data demonstrates that Fair-Rite #31 material is the best all-round performer to cover all HF bands and is clearly the primary choice at 5 MHz and below. Between 5 MHz and 20 MHz, #43 material has a slight edge (approximately 1 dB), and above 20 MHz #43 is preferred.^[12] The classic reference on ferrite dimensional resonance is E. C. Snelling’s 1969 text Soft Ferrites: Properties and Applications.^[13]

4.2 Ferrite Application Points on the 516F-2

4.2.1 AC Mains Input

Install a Fair-Rite #31 snap-on ferrite (2631803802 or equivalent, 1.020” ID) over the AC power cord as close to the rear panel entry point as possible. Wind 4–6 turns through the core if cable flexibility permits. A single pass provides approximately 100–200 Ω of impedance at 10 MHz; multiple turns increase this by the N² relationship. For best results, use a stack of two cores — one #31 and one #43 — for broadband suppression from 1 to 100 MHz.

4.2.2 DC B+ Output Cable

Install a #31 snap-on core directly at the 516F-2 rear panel output connector. If the cable can be accessed, wind 3–4 turns through a 1” ID toroid core (Amidon FT-140-31). This suppresses conducted rectifier switching noise before it reaches the receiver B+ input.

4.2.3 6.3V AC Filament Lines

The filament distribution lines are among the most critical RFI coupling paths because they connect directly into the receiver and transmitter chassis. Install #31 snap-on cores on the filament interconnect cables at both ends. The filament lines carry significant AC current (several amperes), so ensure ferrite cores are rated for the wire gauge and current involved.

4.2.4 Bias Supply and Control Lines

Install small #31 snap-on cores on the bias supply lead and on T/R switching and relay control lines where they exit the 516F-2. Because bias current is low, multiple turns through a small toroid are practical and highly effective.

5. LC Filter Networks and Snubber Circuits

While ferrite cores provide excellent common-mode suppression, LC filter networks address differential-mode noise directly on the power supply output buses. The combination of ferrite common-mode chokes and LC differential-mode filters provides comprehensive noise suppression.

5.1 Rectifier Snubber Networks

The most effective way to suppress silicon rectifier switching transients at the source is to install RC snubber networks across each rectifier diode. As documented by Arturo Mediano in In Compliance Magazine, properly designed RC snubbers can reduce common-mode current by approximately 20 dB.^[17] Rudy Severns’ comprehensive technical paper for Cornell Dubilier, Design of Snubbers for Power Circuits, provides detailed snubber design methodology applicable to these circuits.^[18]

See also: Rohm Semiconductor RC Snubber Circuit application note for EMC noise countermeasures in switching power supplies.^[19]

5.2 Soft-Recovery Diode Replacement

Replacing standard silicon rectifier diodes with soft-recovery types significantly reduces switching transient amplitude. The IXYS/Littelfuse DSEI12-06A (600V, 14A, 35ns recovery) is specifically engineered for soft reverse recovery with low EMI/RFI.^[20] Even with soft-recovery diodes, the addition of snubber networks is still recommended for maximum noise reduction.

5.3 B+ Output Pi-Network Filter

The addition of a pi-network (CLC) filter at the B+ output terminal attenuates differential-mode switching noise. Crompton (W0UAL) documented the use of a MOSFET voltage regulator (designed by Clarence Watts, W7ASB) in his 516F-2 upgrade, which also serves to stabilize B+ and reduce output noise.^[21]

5.4 AC Mains Line Filter

A commercial AC mains line filter installed at the 516F-2 power cord entry point provides both differential-mode and common-mode suppression. Select an IEC-inlet style EMI filter rated for 6A minimum at 250V AC with >30 dB common-mode attenuation from 0.5 to 30 MHz. Suitable units include the Schaffner FN 2060 series, Schurter 5500 series, or Corcom equivalent.

5.5 Filament and Bias Supply Filtering

• Install 0.01 µF ceramic disc capacitors (100V min) from each filament line to chassis ground at both the 516F-2 output and the receiver/transceiver input connectors.
• Install a bifilar-wound choke (10–15 turns of #16 AWG bifilar wire on an Amidon FT-114-31 toroid) in series with both filament leads for common-mode rejection.
• For the bias supply: add a 10 kΩ series resistor followed by a 0.1 µF capacitor to ground. Minimal current draw makes this simple and highly effective.

6. Shielding, Grounding, and Station Layout

6.1 Station RF Ground Bus

Establish a single-point RF ground bus using heavy copper bus bar (minimum 1” wide copper strap or #4 AWG copper braid). Connect every piece of Collins equipment to this bus with the shortest possible copper strap connections. K9YC emphasizes that bonding all equipment together with short, fat conductors is fundamental to controlling common-mode noise currents.^[10]

6.2 Enclosure Shielding and Cable Improvements

• Ensure all 516F-2 chassis screws are tight with clean contact points. Apply EMI gasket tape along chassis seams.
• Install feed-through capacitors (1000 pF, 500V) at cable entry points for RF grounding at the shielding boundary.
• If original interconnect cables are in poor condition, replace with shielded multi-conductor cable, grounded at the 516F-2 end only.
• Maximize physical distance between the 516F-2 and receiver. Route antenna coax away from the 516F-2 and its cables.

7. Equipment-Specific Implementation Notes

7.1 Collins S-Line Station (75S-3 / 32S-3 / 516F-2)

The S-Line configuration uses the 516F-2 to power both receiver and transmitter simultaneously. The complete factory service modification history is documented in the Electric Radio Magazine Collins S-Line/KWM Service Modification Compendium — a 260-page indexed archive of all known Collins factory service bulletins and modifications.^[22]

7.2 Collins KWM-2 / KWM-2A with 516F-2

The KWM-2/2A transceiver shares receiver and transmitter circuitry on a common chassis, making it particularly sensitive to power supply noise. Ernst F. Schroeder, DJ7HS, documented the sensitivity of the product detector and BFO circuits to filament-coupled noise in his detailed KWM-2 technical analysis.^[23] Apply all S-Line modifications with particular attention to filament line filtering. The KWM-2A’s mechanical filter IF provides better selectivity but does not reject power supply noise coupling into the IF amplifier B+ supply.

For a complete walkthrough of the WØIY SB-1 kit installation on a KWM-2 station 516F-2, see Chris M1ABK’s practical installation blog post.^[24]

7.3 Collins 30L-1 Linear Amplifier

While the 30L-1 has its own internal power supply, ensure it is connected to the station ground bus with its own short copper strap. Install #31 ferrite cores on the 30L-1 AC power cord and exciter interconnect cable. The same mitigation principles — RC snubbers across internal rectifiers, pi-network output filter — apply equally to its internal rectifier circuits. See also W8ZR’s complete 516F-2 rebuild documentation for transformer specifications that apply to equivalent rebuilds.^[25]

8. Expected Performance Improvement

9. Broader Ham Shack RFI Reduction

Beyond the 516F-2 itself, the following additional measures will further lower the noise floor in a Collins station:

• LED Lighting: Replace LED bulbs in the shack with incandescent or high-quality filtered LED bulbs. Install AC line filters on LED lighting circuits.
• Switching Power Supplies: Use linear supplies wherever possible. Install #31 ferrite cores on all switching supply power cords.
• Ethernet: Use shielded STP Cat6 cables with #31 ferrite cores at both ends. Remove powerline network adapters (HomePlug, MoCA) if present — these are a major HF noise source.
• Antenna Feedline: Install a 1:1 common-mode choke at the antenna feedpoint and at the shack entry point using stacked #31 and #43 toroids. See K9YC’s Choke Cookbook for measured designs.^[11]
• AC Mains: Run a dedicated circuit to the shack. Install a whole-shack EMI filter at the shack entry point.
• KF7P Ferrite Products: Chris KF7P provides practical measurement data and pre-wound ferrite choke products for RF applications.^[16]

10. References and Acknowledgments

This document draws upon the work of numerous engineers, authors, and contributors in the Collins restoration community and the EMC/RFI field. Their contributions are acknowledged with gratitude.

Jim Brown, K9YC — Audio Systems Group, Inc. His Ham’s Guide to RFI, Ferrites, Baluns, and Audio Interfacing and New Choke Cookbook are the definitive references for amateur radio RFI mitigation. k9yc.com

Barry Buelow, WØIY & KØDAS — Retired Collins Radio engineers who designed the 516F-2 SB-1 PCB modification kit (GitHub). Radio Farm Projects.

Harbach Electronics, LLC — SS-516 Solid State Conversion Kit. Instructions by John May, K6MAY.

Crompton, W0UAL — 516F-2 upgrade with MOSFET regulator (Clarence Watts, W7ASB design).

KH6GRT — 516F-2 equivalent build with snubber and equalizing network documentation.

W8ZR — Complete 516F-2 rebuild documentation including custom Heyboer transformer specifications.

Ernst F. Schroeder, DJ7HS — KWM-2 technical modifications and analysis. CCAE archive.

Chris, M1ABK — Practical installation walkthrough of WØIY SB-1 kit.

Fair-Rite Products Corp. — Ferrite material datasheets: #31 Material, #43 Material. fair-rite.com

Palomar Engineers & KF7P — Ferrite products, application notes, and RFI kits. palomar-engineers.com / kf7p.com

Rudy Severns & Arturo Mediano — Snubber circuit design methodology (Cornell Dubilier) and EMI measurement techniques (In Compliance Magazine).

Collins Collectors Association (CCA) — Archive of original Collins factory documentation, service bulletins, and instruction books. collinsradio.org

Electric Radio Magazine — Collins S-Line/KWM Service Modification Compendium. ermag.com