Collins S-Line Station Grounding & Bonding
RF Ground, Audio Ground & Chassis Ground Are Three Different Things
Incorrect bonding between the 75S-3, 32S-3, 312B-4, 30L-1, and 516F-2 is behind a significant proportion of hum, buzz, and RFI complaints — this guide explains the three ground domains, the interconnect cable ground paths, and the systematic approach to eliminating ground-related noise
A Collins S-Line station consists of up to six separate chassis — receiver (75S-3), transmitter (32S-3), station control/speaker (312B-4 or 312B-5), linear amplifier (30L-1), and power supply (516F-2) — interconnected by approximately a dozen cables carrying RF, audio, control, and DC power signals. Each cable has a ground conductor (usually the shield) that bonds its two chassis together. The result is a complex web of ground paths between chassis, with multiple parallel ground conductors of different lengths and impedances connecting the same pair of chassis.[1]
When any two chassis are bonded by multiple ground paths of different impedance, current flows through all of them — and the voltage drops across these paths are different. This means the “ground” voltage at one chassis is not the same as the “ground” voltage at another chassis. The voltage difference appears as hum (60/120 Hz), buzz (120 Hz + harmonics from rectifier switching), or RFI (broadband noise coupled from the 30L-1 or external sources). The symptoms are often intermittent because they depend on which cables are connected, in what order, and which contacts have oxidised.[2]
In the S-Line station, “ground” means three different things depending on context. Understanding which ground domain you are working in is the first step to diagnosing any hum, buzz, or RFI problem:
The green wire (or third prong) of the AC power cord. Its purpose is to protect the operator from lethal shock if a fault connects a live conductor to the chassis. The 516F-2 power supply connects the AC safety ground to its chassis. In a properly wired station, the 516F-2 chassis is the only point where the AC safety ground enters the S-Line system — all other chassis connect to ground only through the 516F-2 via the interconnect cables.[3]
⚠ Never defeat the safety ground. Removing the third prong from the 516F-2 power cord to “fix” a hum problem creates a lethal shock hazard. The 516F-2 contains 800V+ plate voltage — the safety ground is not optional.
The reference point for audio signals — microphone audio, line audio, speaker audio, and phone patch audio. In the S-Line, audio ground is carried by the shields of the audio interconnect cables between the 75S-3, 32S-3, and 312B-4. The audio ground is referenced to chassis at each unit, but the impedance of the chassis-to-chassis path through the cable shields determines whether hum currents flow in the audio ground path. Audio ground loops are the most common cause of 60 Hz hum in an S-Line station.[2]
The low-impedance return path for RF energy at the operating frequency. RF ground is carried by the coaxial cable shields between the 32S-3 antenna output, the 30L-1, the 302C-3 wattmeter, and the antenna tuner or feedline. At HF frequencies, the impedance of a ground conductor is determined by its inductance (not its resistance) — a few inches of wire that is perfectly adequate for audio or DC can present significant impedance at 28 MHz. RF ground loops between the transmitter, amplifier, and antenna system are the most common cause of “hot chassis,” RF feedback into the audio chain, and broadband RFI during transmit.[3]
Each interconnect cable in the S-Line station carries its own ground conductor. When all cables are connected, the total number of parallel ground paths between any two chassis can be surprisingly high:
| Cable / Connection | From | To | Ground Domain | Ground Loop Risk |
|---|---|---|---|---|
DC power cable (multi-conductor) | 516F-2 | 32S-3 + 75S-3 | Safety + DC return | Primary ground path — heaviest conductor |
Transceive cable (coax) | 75S-3 | 32S-3 | RF (crystal osc. injection) | Parallel path between RX/TX chassis |
Audio patch cables | 312B-4 | 75S-3 / 32S-3 | Audio ground | High risk — audio shields carry hum current |
Muting cable | 32S-3 | 75S-3 | Control / DC | Another parallel path between RX/TX |
ALC cable (coax) | 30L-1 | 32S-3 | DC (ALC voltage) | Adds 30L-1 chassis to the ground web |
RF drive cable (coax) | 32S-3 | 30L-1 | RF ground | RF current return — must be low impedance |
Antenna coax | 30L-1 | Antenna / tuner | RF ground | RF current return to earth/radials |
Speaker cable | 75S-3 | 312B-4 speaker | Audio ground | Low impedance — less loop risk |
External VFO cable | 312B-5 | 32S-3 J17 | RF + DC | Additional parallel path if 312B-5 used |
The Core Problem: Between the 75S-3 and 32S-3 alone, there can be four or more parallel ground conductors (DC power, transceive coax, muting cable, audio cables via 312B-4). Each carries a slightly different ground current. The voltage drops across these conductors are different. The difference appears as hum or buzz in the audio chain.
Before investigating cable ground loops, ensure that every chassis in the station has clean, low-resistance internal ground connections. Oxidised chassis ground points are the single most common cause of ground-related noise in vintage Collins equipment — and the most frequently overlooked.[4]
Inside each chassis: Tighten every screw that secures a ground lug, shield can, tube socket ground ring, or terminal strip to the chassis. Pay particular attention to the IF transformer can grounds, the bandswitch shield can grounds, and the BFO enclosure ground (the BFO can ground was documented as causing “three weeks of troubleshooting” in a 75A-4 — the same problem occurs in the 75S-3). Apply a star washer under each ground screw to bite through the cadmium plating oxide. Clean the mating surfaces with contact cleaner.
Between chassis: Bond all S-Line chassis together with a heavy (#10 AWG or larger) copper ground bus. This bus should be as short as possible — ideally, all chassis are stacked or arranged side by side so the bus is less than 2 feet total. Connect each chassis to the bus using a ring terminal and a chassis screw (not a cabinet screw). The bus provides a low-impedance equalising path that keeps all chassis at the same ground potential regardless of which interconnect cables are connected.[3]
Every interconnect cable connector in the S-Line station is a potential high-resistance ground junction. The PL-068 plugs used on Collins multi-pin cables are brass, which oxidises to a dull tarnish over decades. RCA plugs and jacks on the audio cables develop similar oxidation. Coaxial connector centre pins and shields accumulate corrosion.
Polish every PL-068 plug contact with 000 steel wool or a fine 3M abrasive micro-pad (purple grade). Clean every RCA plug and jack with DeoxIT D5. Clean every coaxial connector (BNC, SO-239, PL-259) centre pin and shell. Clean the Collins microphone jack contacts. A single high-resistance connector contact can introduce enough voltage drop to produce audible hum.[4]
If hum or buzz persists after Steps 1 and 2, use this systematic disconnect procedure to identify which cable is carrying the offending ground current:
Start with only the 75S-3 receiver powered, no transmitter connected, no 312B-4. Listen for hum with headphones (more revealing than a speaker). If hum is present with only the receiver powered, the problem is internal to the receiver — check for leaky capacitors in the audio chain, particularly the Black Beauty coupling caps documented in the companion 75S-3 Known Issues guide.
Add one cable at a time, listening for hum after each addition. Connect the DC power cable first (this establishes the primary ground path). Then the transceive cable. Then the muting cable. Then the audio cables to the 312B-4. Then the ALC and RF drive cables to the 30L-1. The cable whose addition introduces or increases the hum is carrying the ground loop current. This is the cable where intervention is needed — either by improving the bonding (Step 1) or by breaking the loop.
The RF ground is a separate concern from audio hum. At HF frequencies, the impedance of the ground path between the transmitter, amplifier, and antenna must be as low as possible. A few feet of wire that is adequate for 60 Hz audio has significant inductive reactance at 28 MHz.
The station ground bus from Step 1 serves both purposes — it equalises chassis potentials at audio frequencies (eliminating hum) and provides a low-impedance RF return path (reducing hot chassis and RF feedback). For the RF ground specifically, the bus should connect to a good earth ground — a ground rod, a cold water pipe, or a dedicated RF ground system. The connection from the ground bus to earth should be as short and direct as possible, using wide copper strap rather than round wire (strap has lower inductance per unit length than round wire of the same cross-section).[3]
RF Feedback Test: If you experience “hot mic” (RF burns on the microphone), distorted transmit audio, or erratic ALC behaviour during transmit, the RF ground path from the transmitter/amplifier to earth is inadequate. These symptoms are RF current flowing through the audio ground path because the RF ground path has too much impedance. Improving the RF ground — shorter, wider, more direct connection to earth — is the correct fix, not adding bypass capacitors to the audio chain.
| Mistake | Why It’s Wrong | Correct Approach |
|---|---|---|
Removing the 516F-2 safety ground to fix hum | Creates lethal shock hazard — 800V+ plate voltage on exposed chassis | Bond chassis together with heavy bus; find and fix the actual ground loop |
Running a long wire to a distant ground rod | At 28 MHz, a 20-foot wire has significant reactance — it is not a “ground” | Keep the earth ground connection as short as possible; use wide strap not wire |
Grounding each chassis to a separate earth point | Creates ground loops between the earth points via the soil and the interconnect cables simultaneously | Single-point earth ground via the ground bus; all chassis bond to the same bus |
Ignoring oxidised PL-068 plug contacts | A high-resistance ground connection is worse than no connection — it creates a voltage divider with noise | Clean all connector contacts systematically before troubleshooting |
Assuming internal chassis grounds are solid | After 60 years, cadmium-plated screw joints develop high-resistance oxide films | Tighten and clean all internal ground screws; use star washers |
Adding bypass capacitors to mask hum | Treats the symptom, not the cause; can introduce instability or new ground paths | Find and fix the source of the ground loop current |
- Collins Collectors Association — RX For Your Collins. Master index including: “S-Line Grounding and Contact Cleaning.” collinsradio.org — RX For Your Collins
- Brown, Jim K9YC. Power, Grounding, Bonding, and Audio for Ham Radio — Safety, Hum, Buzz, and RFI. Comprehensive presentation on ground loops, bonding vs. grounding, single-point vs. mesh grounding, audio ground vs. RF ground, and practical noise elimination. audiosystemsgroup.com — Grounding (PDF)
- OnAllBands. Ham Radio Grounding and Bonding and RFI. Bonding philosophy: keeping equipment at the same voltage; managing ground loop currents rather than eliminating ground loops; local bonding with short, wide conductors; distinction between bonding (voltage equalisation) and grounding (earth reference). onallbands.com — Grounding and RFI
- W5GW. Vintage Equipment at W5GW — S-Line Station Restoration. Ground loop suspected in 32S-1 causing erratic grid drive; resolved by loosening and retightening screws on PA RF cage, tube sockets, and bandswitch shield cans; oxidised connector contacts causing intermittent operation. w5gw.com — S-Line Restoration
- K9ZW. Collins S-Line Station. Practical documentation of S-Line station cabling complexity: approximately a dozen cables in the KWM-2A station, five to six more in the 32S/75S station; multiple complex cables carrying RF, audio, control, and DC power; PL-068 brass plug oxidation. k9zw.wordpress.com — S-Line Station
- W2PA. Collins S/Line Restoration Notes. 75S-3B + 32S-3 + 516F-3 station: complete restoration including connector cleaning, cable inspection, inter-unit cabling, and cabinet speaker wiring. w2pa.com — S-Line Restoration
- Nobile, John N6ZP. Grounding & Bonding for the Radio Amateur Station (TEAC presentation). Distinction between AC safety, lightning protection, RF ground, and audio ground; system approach to station grounding. TEAC — Station Grounding (PDF)
Jim Brown, K9YC — For the definitive presentation on ham radio grounding and bonding that provides the theoretical framework for understanding ground loops in any multi-chassis station.
Collins Collectors Association (CCA) — For the “S-Line Grounding and Contact Cleaning” article in the RX For Your Collins library.
W5GW — For documenting the real-world S-Line ground loop troubleshooting experience where loosening and retightening internal ground screws resolved erratic transmitter behaviour.
K9ZW — For documenting the practical complexity of S-Line station cabling and the PL-068 connector oxidation issue.