Collins 75S-3 S-Line Receiver
AGC Collapse Fault Diagnosis — S-meter/Volume Interaction, AGC Bus Analysis, Root Cause Candidates

Section 1 — How the 75S-3 AGC System Works

Signal Path Overview

The Collins 75S-3 is a double-conversion superhetrodyne receiver. The incoming RF signal is first converted to a variable intermediate frequency (the first IF), then converted to the fixed 455 kc second IF where selectivity is provided by the Collins mechanical filter. Multiple tuned IF amplifier stages at 455 kc amplify the signal before it reaches the detectors. These IF stages are the AGC-controlled elements: their gain is reduced by the AGC system when a strong signal is received, and maximised when no signal is present.

The AGC Chain

The AGC system in the 75S-3 works as follows: a sample of the 455 kc second IF signal is taken from the IF strip and rectified by the AGC detector circuit to produce a DC voltage proportional to the received signal amplitude. This DC voltage is amplified by the AGC amplifier stage to produce adequate drive to overcome the bias on the IF stage control grids. The amplified AGC voltage is applied to the control grids of the AGC-controlled IF amplifiers, reducing their transconductance (and hence their gain) in proportion to the signal level. The S-meter is connected to the AGC bus through a calibrated resistive network, so S-meter deflection is directly proportional to AGC bus voltage.

The RF GAIN Control and AGC Interaction

The RF GAIN control in the 75S-3 operates independently of the AGC system. It is a manual attenuation that reduces the signal level entering the IF strip, and it acts before the AGC detector. In normal operation, the AGC compensates for RF GAIN changes: if you reduce RF GAIN, the received signal level drops, the AGC bus voltage relaxes (becomes less negative), the IF gain increases, and the net audio output stays approximately constant. The S-meter reading drops slightly because it reflects the RF GAIN’s effect on signal level upstream of the AGC detector, but the audio level remains stable.

When AGC has collapsed: reducing RF GAIN causes audio to drop proportionally, and the S-meter drops too — the two move together because neither is being compensated by a functioning AGC. This co-variation of S-meter and audio level with the RF GAIN control is the diagnostic signature of AGC collapse and the first test to perform.

Section 2 — The AGC Collapse Symptom Pattern

Characteristic Presentation

The 75S-3 with collapsed or severely degraded AGC presents with a distinctive constellation of symptoms. Not all will be present in every case — partial AGC collapse (AGC working but with reduced range) produces a milder version of the same pattern:

• Loud strong signals. Stations that should arrive at comfortable listening volume are painfully loud. On a busy amateur band, the receiver is uncomfortable to monitor at the normal operating volume control setting.
• Hypersensitive volume control. A small rotation of the AF GAIN control produces a large change in volume because the IF gain is not being held to a stable baseline by the AGC. The effective gain of the audio chain appears higher than expected.
• S-meter pins or saturates on moderate signals. A signal that should read S7 reads S9+40 dB. The S-meter consistently reads higher than expected for the signal strength, and on strong stations it pins hard right regardless of actual signal level.
• RF GAIN → S-meter coupling. Advancing the RF GAIN control moves the S-meter as well as the audio. In a healthy receiver, advancing RF GAIN does not significantly change the S-meter reading because the AGC compensates. This is the diagnostic signature.
• Poor strong-signal handling. On the same band as a strong local station, weak signals may be completely inaudible even with attenuating the RF GAIN, because the strong station is driving the whole IF strip into saturation rather than being compressed by AGC action.
• CW note pitch variation. On CW with collapsed AGC, the BFO zero-beat appears to shift on strong signals because the signal is overdriving the IF strip — the actual frequency is not changing, but the apparent pitch shifts as the IF tube operating points are driven out of their linear region.

What AGC Collapse Is Not

Several common 75S-3 faults produce superficially similar symptoms and should be ruled out before committing to the AGC chain diagnosis:

• Not an RF overload problem. Connecting an external attenuator at the antenna port and observing whether the symptoms reduce in proportion to the attenuation tests this. If a 20 dB pad restores normal behaviour, the problem is overload from a strong nearby transmitter, not AGC collapse.
• Not a mechanical filter bandwidth problem. A failed or high-insertion-loss mechanical filter produces weak signals across all modes, not loud signals or S-meter saturation.
• Not an audio volume control fault. A scratchy or carbon-tracked AF GAIN pot can produce erratic volume, but it does not affect the S-meter and does not produce the RF GAIN coupling signature.
• Not a power supply hum problem. Hum from failed filter capacitors produces volume-dependent or volume-independent audio contamination, not gain control loss and S-meter saturation.

Section 3 — The 75A Series V11 Parallel

Restorers familiar with the older Collins 75A receiver series — the 75A-1, 75A-2, 75A-3, and 75A-4 — will recognise the AGC collapse symptom pattern immediately. In the 75A series, the equivalent fault is typically attributed to the AVC (Automatic Volume Control) amplifier tube, often referred to as V11 in community documentation. When V11 has reduced emission, the AVC drive to the IF stage control grids is insufficient and the receiver runs at elevated gain, producing the same loud-signal, S-meter-saturation pattern seen in the 75S-3.

Design Differences That Matter for Diagnosis

While the failure mechanism is analogous, the 75S-3 AGC system differs from the 75A’s AVC system in ways that affect the diagnosis:

• Linear vs delayed AGC. The 75A series uses a delayed AVC characteristic — the AVC holds off until the signal is large enough to warrant gain reduction. The 75S-3 uses a more responsive linear AGC that begins controlling gain at lower signal levels. This means partial 75S-3 AGC collapse is visible at lower signal levels than partial 75A AVC collapse.
• CW keying circuit. The 75S-3 (in some variants) includes an AGC keying circuit for CW operation that briefly defeats the AGC during key-down periods to prevent the receiver from responding to the transmitter’s own CW burst. This circuit uses additional components not present in the 75A series and can contribute to AGC anomalies specific to CW mode. If the collapse is observed primarily on CW and less on SSB or AM, the keying circuit should be examined separately from the main AGC chain.
• More IF stages to control. The 75S-3’s 455 kc IF strip typically has three or more AGC-controlled stages. The AGC amplifier must drive all of these simultaneously. A marginal AGC amplifier tube may provide adequate drive for partial gain control but insufficient drive to compress the full available IF gain, producing the partial collapse pattern (S-meter overshoots but doesn’t pin; audio is louder than expected but still usable).

Section 4 — Gateway Test: AGC Bus Voltage Measurement

Before any tube substitution or component replacement, make this single measurement. It takes three minutes and answers the most important question: is the AGC bus responding to signal level at all? The answer determines which sub-category of AGC fault you are dealing with and which candidates to address first.

The Measurement Procedure

Connect a high-impedance DVM (input impedance >10 MΩ) on the DC voltage range to the AGC bus. The AGC bus test point is identified in the Collins 75S-3 Service Manual alignment section; it is the node from which the AGC voltage is distributed to the IF stage control grids. Allow 15 minutes warm-up. Take three readings:

• Condition A — no antenna, RF GAIN at maximum: the AGC bus should sit near 0 V or a small negative value. A large negative voltage (more than −2 V) with no signal indicates the AGC system is self-biasing incorrectly — a leaky AGC filter capacitor or an incorrect resistance value in the AGC bias network.
• Condition B — antenna connected, active HF band, RF GAIN at maximum: as signals arrive, the AGC bus should fluctuate negatively in response to signal level. You should see voltage change visibly as signals come and go. On a busy band the AGC bus should vary between approximately −1 V and −6 V or more depending on signal strength. Compare against the service manual specification for the specific variant.
• Condition C — inject a known strong signal (signal generator, −40 dBm, 14 MHz, 1 kHz AM modulation): observe the AGC bus voltage. It should drive substantially negative in response to the injected carrier and return toward 0 V when the generator is switched off.

Section 5 — Seven Root Cause Candidates in Priority Order

Candidates #1 through #4 address the AGC amplifier chain between the AGC detector and the IF stage control grids. Candidates #5 through #7 address adjacent circuits that modify the symptom pattern without causing the primary collapse. Verify all component designators against the Collins 75S-3 Service Manual for the specific variant before any measurements.

• #1
  AGC amplifier tube — the 75S-3 equivalent of V11 in the 75A series FIRST ACTION — MOST PROBABLE

  The AGC amplifier is a single tube stage that amplifies the small DC voltage produced by the AGC detector into adequate drive for the IF stage control grids. This is the highest-leverage point in the AGC chain: a tube that has lost half its emission produces roughly half the AGC drive, which may be just sufficient to prevent the receiver from completely saturating but not enough to compress strong signals adequately. This produces the partial collapse pattern: S-meter that reads 3–5 S-units high, audio that is consistently louder than comfortable, but a receiver that still functions on weak signals.

  Consult the service manual to identify the AGC amplifier tube position. Remove it, test on a calibrated tube tester. Test the control grid current (gm) as well as plate current emission: a tube with acceptable plate emission but reduced gm produces inadequate voltage amplification even though it tests “good” on simple emission tests. Replace with a NOS tested type. Recheck the AGC bus voltage after substitution before any further diagnosis.

  Also check: the AGC amplifier tube socket for silver contact oxidation or carbon tracking between adjacent pins. Clean with 99% IPA if there is any evidence of contamination. A partially oxidised socket contact raises the effective cathode resistance and reduces the tube’s available transconductance regardless of the tube’s own condition.

• #2
  AGC filter capacitors — attack and decay time constants HIGH PROBABILITY — ESPECIALLY IF AGC BEHAVIOUR IS ERRATIC OR MODE-DEPENDENT

  The AGC time constants are set by RC networks that determine how quickly the AGC responds to a new signal (attack) and how slowly it releases after the signal disappears (decay). These networks use small electrolytic or film capacitors. After 60 years, these capacitors fail in two ways that affect the AGC symptom pattern differently:

  Open circuit: the AGC time constant goes to zero (attack and decay become instantaneous). Symptoms: very fast S-meter pumping, AGC that responds to individual syllables on voice signals (producing a “breathing” quality to SSB), and CW signals where the AGC attacks on each dit and releases between them (severely degraded CW copy at high speeds). On CW specifically, an open AGC filter capacitor sounds quite different from a collapsed AGC amplifier.

  Leaky (partial DC conduction): the leakage introduces a DC offset onto the AGC bus, biasing the IF stage control grids into a fixed-gain state. Symptoms: AGC bus voltage that is consistently negative even with no antenna connected (Condition A in Section 4 returns −2 V to −4 V instead of near 0 V). This is the cleaner diagnostic: if the Condition A reading is substantially negative, suspect a leaky AGC filter capacitor before suspecting the amplifier tube.

  Service: identify all capacitors in the AGC time constant network from the schematic. Measure each out of circuit. A capacitor that reads significant resistance on a DVM’s high-resistance range or shows other than near-open-circuit on the DC range has leaked dielectric and must be replaced with a polypropylene or NP0 ceramic type.

• #3
  AGC detector circuit — diode or tube rectifier stage SECONDARY — IF AGC BUS SHOWS NO RESPONSE TO ANY SIGNAL (FULL COLLAPSE)

  The AGC detector rectifies the 455 kc IF signal sample to produce the DC control voltage that drives the AGC chain. Depending on the 75S-3 variant and production year, this may be a silicon or germanium diode, or a tube-based detector sharing a tube with other functions. A failed AGC detector produces complete AGC collapse: the AGC bus does not respond to any signal level, the bus reads near 0 V regardless of signal strength (Condition C in Section 4 produces no change).

  Distinguishing a failed AGC detector from a failed AGC amplifier: if the AGC amplifier is the fault, the AGC bus reads near 0 V but a small signal is present at the amplifier’s input (measurable with an AC-coupled DVM or scope). If the AGC detector is the fault, the amplifier input itself is near 0 V AC regardless of signal level. Probe the amplifier input node with an AC-coupled DVM on the 3 V AC range while injecting a strong signal. Presence of AC voltage confirms the detector is producing an AGC signal that the amplifier is failing to amplify; absence confirms the detector is not producing a signal.

• #4
  Carbon composition resistors in the AGC bias and distribution network SECONDARY — MEASURE WHEN AGC BUS READS STABLE BUT INCORRECT VOLTAGE

  The AGC bus voltage is distributed to the IF stage control grids through a resistor network that sets the operating point range and limits the maximum AGC voltage applied to any individual tube. These resistors are carbon composition types subject to upward resistance drift over decades at operating temperature. Drifted resistors change the effective AGC operating point in two ways: they reduce the AGC voltage that reaches the IF stage control grids relative to what appears at the AGC bus (increasing the grid resistors reduces IF stage response to a given AGC voltage), and they change the idle bias on the IF stages if they are also part of the grid bias network.

  Measure all carbon composition resistors in the AGC network out of circuit. Replace any more than 10% above nominal with 1% metal film resistors. This is most productive when the AGC bus voltage reads correctly under the Section 4 tests (i.e., the AGC chain is producing the right voltage at the bus) but the IF gain is still not being properly controlled — suggesting the problem is in how the AGC voltage reaches the IF stage control grids, not in how it is generated.

• #5
  IF amplifier tube grid connections — AGC voltage not reaching the controlled stages CHECK IF AGC BUS IS CORRECT BUT IF GAIN IS STILL UNCONTROLLED

  If the Section 4 gateway test confirms that the AGC bus is producing a correct and varying voltage but the receiver still shows the collapse symptom pattern, the fault is in the delivery path from the AGC bus to the IF tube control grids. Two mechanisms:

  Open decoupling capacitor on an IF stage grid: each IF amplifier stage has a small bypass or decoupling capacitor on the grid supply line. If this capacitor opens, the AGC voltage can no longer reach that tube’s control grid, and that stage runs at maximum gain regardless of AGC bus voltage. The affected stage produces the overload pattern; other stages continue to be controlled normally. Identify which stage is the culprit by measuring DC voltage on each IF tube’s control grid pin: the correctly controlled stages will show a significant negative voltage on strong signals; the stage with the open decoupling capacitor will show near-zero grid voltage regardless of signal level.

  Open or high-resistance grid resistor: the resistor connecting the AGC bus to an IF tube’s control grid prevents RF leakage back onto the AGC bus. If this resistor has gone open circuit, the AGC voltage cannot reach that grid. Measure out of circuit and replace with metal film if found open or significantly high.

• #6
  CW AGC keying circuit (75S-3B and 75S-3C with noise blanker variants) CHECK IF COLLAPSE IS OBSERVED PRIMARILY OR EXCLUSIVELY ON CW

  The 75S-3B and some later 75S-3C variants include a CW AGC keying circuit that briefly disables the AGC during the key-down period to prevent the receiver from responding to the transmitter burst and deafening itself during sending. This circuit uses additional switching components not present in the basic 75S-3 and 75S-3A. If the keying circuit develops a fault that holds the AGC defeat condition permanently — rather than only during actual key-down — the result is AGC collapse that is present at all times, not just during transmit.

  The distinguishing diagnostic: disconnect the external keying line at the rear panel connector. If AGC behaviour improves after disconnecting the keying input, the fault is in the external keying circuit (the transmitter keying output, or a wiring fault between the 32S-3 and 75S-3). If AGC remains collapsed with the keying input disconnected, the fault is inside the receiver. Consult the 75S-3B/75S-3C service manual section on the CW keying circuit for the specific component path to check.

• #7
  S-meter calibration network — not a cause of collapse, but confuses the diagnosis VERIFY SEPARATELY IF S-METER READS WRONG WHILE AGC FUNCTIONS CORRECTLY

  The S-meter is connected to the AGC bus through a calibration resistor network. Drift or failure in this network does not cause AGC collapse — it does not affect the AGC bus voltage or IF gain control. However, it can make the S-meter read incorrectly even when the AGC is functioning well, leading to a misdiagnosis of AGC collapse when the real issue is only S-meter calibration.

  Distinguishing factor: if the S-meter reads too high or incorrectly but the RF GAIN coupling test (Section 2) shows normal decoupling between RF GAIN and S-meter reading, the S-meter calibration network is the issue, not the AGC chain. Verify by measuring the AGC bus voltage directly (Section 4): if the AGC bus voltage varies correctly with signal level but the S-meter needle does not track proportionally, the fault is in the S-meter circuit, not the AGC system. Replace or adjust the S-meter calibration resistors per the service manual.

Section 6 — Recommended Diagnostic Procedure

• 1
  Confirm the AGC collapse symptom: RF GAIN → S-meter coupling test With receiver powered and tuned to an active HF band, RF GAIN at 50% rotation, note the S-meter reading on a moderate signal (target: S5–S7 range). Now rotate RF GAIN from 50% to 100% (maximum). Observation: in a healthy receiver, the S-meter reading should not change significantly (the AGC compensates for the increased RF gain). A movement of more than 2 S-units confirms AGC collapse is present. Record the S-meter position at both RF GAIN settings as a baseline.
• 2
  Powered: AGC bus voltage — three-condition measurement (Section 4) Identify the AGC bus test point from the service manual. Measure DC voltage: (A) no antenna, RF GAIN max — note reading and compare against Section 4 criteria; (B) antenna connected, active band — observe that voltage varies with signal; (C) signal generator injected at −40 dBm — note maximum negative voltage reached. Record all three readings. They determine which candidate group to address first.
• 3
  Power off: AGC amplifier tube — remove and test on calibrated tester (Candidate #1) Remove the AGC amplifier tube. Test emission AND gm (transconductance). A tube that tests above 70% of rated emission but with reduced gm may produce inadequate AGC drive. Compare against the service manual operating point for the specific tube type and circuit conditions. Substitute with a NOS tested specimen. Restore power and repeat Step 2 to assess improvement in the AGC bus voltage range. If the full Condition A to Condition C range is restored, the tube was the cause. If range is still limited, continue to Step 4.
• 4
  Power off: AGC filter capacitors — out-of-circuit measurement (Candidate #2) Identify all capacitors in the AGC time constant network from the schematic. Lift one lead of each, measure resistance on the DVM’s highest resistance range. Any capacitor showing measurable resistance (i.e., not effectively open-circuit on the resistance range) is leaky and must be replaced. Also check for open-circuit capacitors: a capacitor that reads high resistance on the capacitance range or does not charge when a DC test voltage is applied has failed open. Replace all AGC filter capacitors with polypropylene film or NP0 ceramic types of the original value.
• 5
  Power off: AGC bias resistors — out-of-circuit measurement (Candidate #4) With the capacitor condition resolved, measure all carbon composition resistors in the AGC network. Replace any more than 10% above nominal with 1% metal film. Also check the grid resistors on each AGC-controlled IF tube: these should measure at their nominal value. Open circuit or dramatically high resistance on a grid resistor confirms Candidate #5 (delivery path failure) and identifies the specific IF stage affected.
• 6
  Powered: IF stage grid voltage check (Candidate #5) — if AGC bus is correct but gain uncontrolled If Steps 2–5 confirm that the AGC bus is producing the correct voltage range but the receiver still shows collapse symptoms: with a strong signal applied, measure DC voltage at the control grid pin of each AGC-controlled IF tube. Each controlled stage should show a significant negative voltage (the AGC bias) when a strong signal is present. A stage showing near-zero grid voltage when all other stages show correct negative voltage has a delivery path fault — identify the specific open or high-resistance grid resistor or decoupling capacitor at that position.
• 7
  CW-specific collapse: check keying circuit connection (Candidate #6, 75S-3B/3C only) If the collapse is primarily observed on CW mode: disconnect the external keying line at the rear panel. Test CW reception with the keying line disconnected. If AGC restores to normal with the keying disconnected, trace the keying line back to the 32S-3 transmitter or external key for the fault. If collapse persists, consult the CW keying circuit section of the 75S-3B/3C service manual for the specific internal switching path.
• 8
  Powered: RF GAIN coupling retest and S-meter calibration verification After all identified faults are corrected, repeat the Step 1 RF GAIN coupling test. The S-meter should now remain stable (<1 S-unit variation) as RF GAIN is advanced from 50% to maximum with a steady signal at the antenna. If the S-meter is now stable but reads incorrectly for the signal level, proceed to S-meter calibration (Candidate #7 and service manual alignment section). Note: S-meter calibration should not be adjusted until the AGC chain is confirmed functioning correctly.

Section 7 — AGC Chain and Fault Map

  ┌──────────────────────────────────────────────────────────────────────────┐
  │   COLLINS 75S-3 AGC SYSTEM — SIGNAL FLOW AND FAULT LOCATIONS            │
  │   All component positions require verification from the 75S-3 Service   │
  │   Manual for the specific variant before probing or measurement          │
  └──────────────────────────────────────────────────────────────────────────┘

  ANT → [RF AMP] → [1ST MIXER] → [1ST IF] → [2ND MIXER] → [455 kc IF STRIP]
                                                                     │
                             ┌───────────────────────────────────────┤
                             │  455 kc IF SAMPLE (taken from IF)     │
                             ▼                                       ▼
                    [AGC DETECTOR]        [AGC-CONTROLLED IF AMP ×3 or more]
                    Diode or tube         Control grids receive AGC bias
                    rectifier             (Candidate #5: check each grid)
                    (Candidate #3)
                             │
                             ▼
                    [AGC AMPLIFIER] ◄─── THIS IS THE V11 EQUIVALENT
                    Single tube stage     (Candidate #1: replace first)
                    Amplifies detector    Test gm not just emission
                    output to drive IF    (Candidate #6: CW keying
                    stage grids           circuit also connects here
                             │            in 75S-3B/3C)
                             │
                   ┌─────────┴────────────────────────────┐
                   │  AGC BUS                              │
                   │  ← measure here for gateway test      │
                   │  (Section 4: conditions A, B, C)      │
                   │                                       │
          [AGC FILTER CAPS]              [AGC RESISTOR NETWORK]
          Time constant R/C              Distribution to IF grids
          (Candidate #2)                 (Candidate #4: carbon comp)
                   │
                   ├────────────────────────► [S-METER CALIBRATION]
                   │                          Resistive network
                   │                          (Candidate #7: not a
                   │                          collapse cause, only
                   │                          affects S-meter reading)
                   ▼
          → IF stage control grids ← AGC bias applied here
            (each stage via grid resistor + decoupling cap)

  ─────────────────────────────────────────────────────────────────────────
  GATEWAY TEST INTERPRETATION
  ┌────────────────────────────────────────────────────────────────────────┐
  │  RF GAIN → S-meter moves together:  AGC chain broken. Begin diagnosis. │
  │  AGC bus A→C range > 4 V:           Good detector+amplifier. Check     │
  │                                      delivery to IF grids (Cand. #4,5) │
  │  AGC bus A→C range 1-3 V:           Marginal AGC amplifier. Replace    │
  │                                      tube first (Candidate #1)         │
  │  AGC bus A→C range < 0.5 V:         Full collapse. Detector or amp    │
  │                                      has failed. Begin at #1 then #3   │
  │  AGC bus Condition A >> 0 V:         Leaky AGC filter cap (#2) likely  │
  └────────────────────────────────────────────────────────────────────────┘
  ─────────────────────────────────────────────────────────────────────────
  75S-3 vs 75A SERIES — AGC SYSTEM COMPARISON
  ┌────────────────────────────────────────────────────────────────────────┐
  │  Feature              │  Collins 75S-3         │  Collins 75A-3/4      │
  │  Control system name  │  AGC                   │  AVC                  │
  │  AGC type             │  Linear                │  Delayed              │
  │  AGC amplifier role   │  Same as V11 in 75A    │  V11 (AVC amplifier)  │
  │  CW keying circuit    │  Yes (B/C variants)    │  No                   │
  │  IF stages controlled │  3 or more             │  2–3                  │
  │  Gateway measurement  │  AGC bus DC voltage    │  AVC line DC voltage  │
  │  First tube to try    │  AGC amplifier         │  V11 replacement      │
  │  S-meter source       │  AGC bus               │  AVC line             │
  └────────────────────────────────────────────────────────────────────────┘
  ─────────────────────────────────────────────────────────────────────────
  MODE-SPECIFIC AGC COLLAPSE PATTERNS
  ┌────────────────────────────────────────────────────────────────────────┐
  │  Mode   │  Collapse present on this mode?  │  Most likely candidate    │
  │  SSB    │  Yes, all candidates             │  #1 AGC amplifier tube    │
  │  CW     │  Yes, + possible keying issue    │  #1 first, then #6        │
  │  AM     │  Yes, all candidates             │  #1 then #2 filter caps   │
  │  CW only│  Yes on CW, normal SSB/AM        │  #6 CW keying circuit     │
  │  S-meter│  Reads wrong, AGC seems ok       │  #7 S-meter calibration   │
  │  Breathing│  Fast pump on voice, CW        │  #2 open AGC filter cap   │
  └────────────────────────────────────────────────────────────────────────┘

AGC system signal flow and fault location map — Collins 75S-3. All component designators require verification from the Collins 75S-3 Service Manual for the specific variant. The AGC bus gateway measurement in Section 4 is the required first step before any component work.

Section 8 — Verification Tests

RF GAIN Coupling Test — Pass Criterion

AGC Range Verification

S-meter Calibration Check

CW AGC Behaviour (75S-3B/3C)