Collins 51S-1 / 1A / 1F / 1AF / 1B — AM, CW & SSB Receiver RFI Mitigation Strategies

The single largest source of RFI ingress in any vintage receiver is conducted common-mode noise riding on the coax shield from the antenna feedline. The 51S-1 antenna input is 50 Ω unbalanced at J1,[4] with the signal immediately entering impedance-matching transformer L30. A toroid-wound common-mode choke installed at J1, before L30, stops conducted RF from the shack environment — SMPS hash, LED driver noise, PC USB interference — from entering the receiver while passing the differential antenna signal with negligible insertion loss.

Wind 8–10 turns of RG-316 (or equivalent small-diameter coax) through a Fair-Rite Type 31 toroid (p/n 2631801202, 1.4″ OD). Type 31 (MnZn, μ ≈ 1500) provides 400–1000 Ω of common-mode impedance across 1–30 MHz.[8] For MW and low-band work below 2 MHz, substitute Type 75 or Type 77 mix for better low-frequency permeability. A split ferrite clamp (Fair-Rite 0431164281) at the antenna entry provides additional suppression without rewiring.

The RF amplifier following the double-tuned input network (L33/L32/L31, C40, L69/L68/L67, C71, turret wafers A1–A5) sets the noise figure for the entire receiver. The tuning slugs of L32 and L68 are mechanically ganged to the main tuning control.[4] Factory screen bypass capacitors on V1 are often marginal by current standards, and supply lead inductance allows HF hash to reach the screen grid directly, raising the receiver noise floor.

Add a Fair-Rite Type 43 ferrite bead (100 Ω at 100 MHz, p/n 2643000201) on the screen grid B+ lead immediately adjacent to the socket pin, followed by a 0.01 µF / 630 V ceramic disc cap to chassis. Repeat on the plate load resistor B+ lead. Type 43 (NiZn, μ ≈ 850) provides suppression from 10 MHz upward — the range where IF strip spurious responses and oscillator harmonics radiate into the front end.[9] Do not use Type 61 or Type 67 here; those mixes are optimised above 50 MHz and provide insufficient impedance at HF.

The IF strip contains the mechanical filter, AGC-controlled IF amplifiers, the Q-multiplier, and the product detector — all sharing a common B+ bus. Ripple and high-frequency hash on this bus modulates IF gain stages and raises the noise floor, particularly on weak SSB signals. The first local and first line AF amplifiers obtain input signal from either the SSB/CW preamplifier Q1, or from AM detector CR15, selected by the EMISSION switch S2.[10] The original wax-and-paper bypass capacitors are almost certainly deteriorated; replacement alone provides measurable noise floor improvement.

On each IF stage B+ feed, add a Fair-Rite Type 43 ferrite bead (1/4″ lead, 600 Ω at 25 MHz) immediately at the tube socket B+ pin, followed by a 0.1 µF / 630 V film cap (Vishay MKP or equivalent) to chassis. Replace all original wax-and-paper bypass capacitors. For mechanically filtered stages, add a 10 Ω / 2 W carbon film resistor in series with the B+ feed to each IF transformer primary to form a passive RC decoupling section.

The BFO (V15) generates the product detector injection signal at approximately 500 kHz. Poorly bypassed BFO B+ leads allow 500 kHz injection signal to couple into the audio chain, producing a low-level heterodyne tone particularly audible on SSB with AGC released. Microphonics from a loose BFO circuit can masquerade as broadband noise. The BFO coupling capacitor to the product detector should be a high-stability silver-mica or C0G ceramic type if previously serviced with a ceramic disc replacement.[11]

Wrap the BFO compartment with 3M 1181 copper foil tape, bonded to chassis with conductive adhesive or a solder lug. Add Fair-Rite Type 43 beads on V15 plate and screen leads. Ensure V15 has a snug tube shield with firm contact on its retaining ring. Verify the BFO coupling capacitor is silver-mica or C0G; replace if a standard ceramic disc type is found.

Output transformer T4 uses negative feedback to V14A cathode to reduce line amplifier distortion.[10] The 600 Ω balanced line output is inherently well protected, but the local speaker/headphone output is an unbalanced path that acts as an antenna for conducted RFI. RF rectification in semiconductor junctions in connected external equipment can feed noise back into the audio chain through the headphone jack — a mechanism well documented by W8JI and others.[7]

On the local audio output, fit a small common-mode choke wound on a Fair-Rite 2643625702 (Type 43, 0.5″ OD dual-hole balun core), 5–6 bifilar turns. Add 0.01 µF / 50 V C0G caps from each audio lead to chassis after the choke. Add 100 Ω series resistors at the phones jack to prevent RF rectification. For the 600 Ω balanced line output, verify the TB1 transformer centre-tap ground solder joint is solid; a cracked joint is a common source of hum on this series.[12]

The 51S-1 draws approximately 125 watts from the AC line with a conventional mains transformer. The 50–400 Hz power frequency specification reflects the receiver’s original military and aviation heritage, where power quality is highly variable.[4] Modern shack environments deliver an AC line rich in switching noise from LED drivers, variable-speed motor controllers, and switching power supplies — all sources entirely absent when the 51S-1 was designed in 1960.

Install a commercial AC line filter (Schaffner FN2010-6-06 or equivalent 6 A / 250 V EMI filter) at the power entry point, or construct an equivalent using X2 differential capacitors (0.1 µF) and a common-mode inductor wound on a Fair-Rite Type 31 toroid (200 µH, 3 A). Bond the filter case directly to the 51S-1 chassis with a short braided strap. Physically separate the filtered and unfiltered power leads inside the chassis.[8]

The Collins 70K-7 permeability-tuned oscillator operates on a regulated low-voltage supply. Any ripple or transient on this supply causes the characteristic frequency hopping reported in community discussions.[13] Phase noise on the PTO directly determines the receiver’s reciprocal mixing performance on strong adjacent signals. Diodes CR501 (series) and CR502 (Zener) are the first suspects in any frequency instability complaint. The 70K-7 uses a permeability-tuned coil mechanism; the core slug must be clean and the coil form crack-free.

Replace CR501 with a 1N4148 and CR502 with a 1N5230B or BZX79C5V1 Zener if either tests soft or leaky. Add a 10 µF / 35 V low-ESR electrolytic cap directly across the PTO B+ supply close to the tube socket. For maximum stability, add a further RC filter — 10 Ω in series, 47 µF across the output — ahead of the existing Zener regulation. This mod is consistent with the diagnostic approach documented in the EDAboard 51S-1 community forum.[13]

The 51S-1 perforated wrap-around cabinet increases ground impedance compared to a solid-steel chassis. At audio frequencies, multiple return current paths form loops that intercept magnetic flux from the power transformer, manifesting as 60 Hz and 120 Hz hum. Green oxidation on aluminium chassis lugs is common on receivers of this era and adds measurable impedance. A restoration by Grayson Evans (WN4GVM/now KJ7UM) documents this as a first-priority maintenance item on the 51S-1.[14]

Identify the chassis point physically closest to the power transformer secondary centre tap and designate it the star ground. Run a braided copper strap (minimum 3/4″ wide, as short as physically possible) from the audio ground bus to this star point. Tie the RF ground bus in the front-end area to the star point via a separate short, wide braid — not through the perforated chassis. Torque all original chassis lug screws and clean oxidised contact surfaces with IPA before reassembly.

The high-frequency crystal oscillator V2B operates on all bands; frequency is determined by one of sixteen crystals on the turret wafer selected by the MEGACYCLES control. The 17.5 MHz oscillator V3B operates only for the 2–7 MHz range. Radiation from these oscillators into the RF stages produces internal birdies. In the 0.2–2.0 MHz range, the 51S-1 instruction book itself notes internally generated spurious whistles at 333 kHz, 666 kHz, 1000 kHz, 1500 kHz, and 2000 kHz.[2]

Wrap the turret assembly perimeter with 3M 1181 copper foil tape, ensuring shield continuity across all joints. Bond the foil to the turret frame with silver conductive epoxy or a machine screw through a solder lug. Add individual Fair-Rite Type 43 ferrite beads on V2B and V3B plate and grid leads at the tube sockets. Do not add beads on the crystals themselves — the crystal series resistance interacts with the load and can shift frequency or cause oscillation instability.[9]