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--- kg7vfo:start [2025/12/28 18:51] – [4. Testing Objectives (Cold State)] admin
+++ kg7vfo:start [2026/01/13 15:52] (current) – external edit 127.0.0.1
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   * resistor bypass switch
   * 240V operation
+====== Technical Analysis: FM Mode Implementation ======
-The presence of an FM setting on the mode switch is a significant clue that narrows the origin of this amplifier considerably. While most amateur amplifiers only featured SSB and CW, an FM option reveals several key things about the unit's design and intended market:
+The presence of a dedicated **FM** position on an HF amplifier of this era is highly unusual and provides specific insights into its design and origin.
-. Class C Bias Capability
+===== 1. Technical Implications =====
-The Technical Shift: In FM mode, an amplifier does not need to be "linear" because FM is a constant-envelope signal.
+  * **Efficiency and Bias:** The FM setting likely switches the **4CX250B** tubes from Class AB (linear for SSB) into **Class C**.
+  * **Duty Cycle:** Class C operation is non-linear but much more efficient, allowing the tubes to handle the 100% duty cycle (constant carrier) of FM without overheating.
+  * **Voltage Regulation:** In FM mode, the screen grid voltage and plate bias are adjusted to maximize saturation rather than linearity.
-Efficiency: This setting likely switches the 4CX250B tubes from Class AB (linear for SSB) to Class C.
+===== 2. Market Identification =====
+  * **Export Market Focus:** While FM was rare for US hams on HF in the 1970s, it was popular in the European and South American "Export" markets for 10-meter and 11-meter operations.
+  * **Pride Fingerprint:** This specific mode sequence (**AM-1, AM-2, SSB, CW, FM**) is a definitive characteristic of late-production **Pride Electronics** units.
+  * **Competitive Differentiation:** Brands like **Sonar** and **Hy-Gain** (Afterburner) typically lacked a dedicated FM tap on their HF linear units.
-The Result: Class C is much more efficient and allows the tubes to run cooler while producing more power, which is ideal for the 100% duty cycle of FM transmissions.
+===== 3. Relation to 100-Watt Output Fault =====
+  * **Switch Failure:** If the Mode switch has carbon tracking or mechanical wear, the unit may be "latched" into a low-bias or **TUNE** state internally.
+  * **Logic Conflict:** If the bias relay does not engage the "High" voltage/current state when moving from FM to SSB, the output will hit a hard ceiling regardless of drive levels.
-. Market and "Export" Origins
+===== 4. Diagnostic Notes (Cold Testing) =====
-The CB/11-Meter Connection: In the late 1970s, FM was not a common mode for hams on HF, but it was becoming popular in the "Export" CB market (especially in Europe and South America).
+  * **Task:** Verify continuity across the Mode switch wafers for each setting.
+  * **Focus:** Inspect the bias resistors connected to the FM/AM-1 positions for signs of thermal drift.
-Pride’s Strategy: Including FM on a unit labeled "AMATUER" was a way for Pride Electronics to appeal to a global market that used FM on the 11-meter and 10-meter bands.
-VHF/UHF Potential: Some 4CX250B amplifiers were designed for 2-meter (VHF) use where FM is the primary mode; however, the presence of an 80M–10M Band Switch confirms this is an HF unit with specialized FM circuitry added.
+===== AM-1 vs. AM-2 Operational Differences =====
+  * **AM-1 (Low Power):** Designed for a "dead key" carrier of ~100W; used for tuning or low-drive AM.
-. Connection to Your 100-Watt Issue
+  * **AM-2 (High Power):** Enables full plate current for maximum RF swing; requires high airflow.
-The Relay/Switch Logic: If the mode switch is internally damaged or carbon-tracked, the amplifier might be physically stuck in the FM or AM-1 bias state.
+  * **Switching Logic:** Changes the bias voltage on the 4CX250B grids and may switch plate voltage taps.
-Bias Bottleneck: If the unit thinks it is in a low-power "Tune" or "Low-Bias FM" mode, it will provide a very "stiff" ceiling on your output, resulting in that persistent 100-watt reading regardless of drive.
-. Comparison to Other Brands
-Sonar and Hy-Gain: While Sonar and the Hy-Gain "Afterburner" series were professional, they rarely included a dedicated "FM" position on their HF amplifiers.
-Pride Signature: This specific "Mode" layout—AM-1, AM-2, SSB, CW, FM—is a definitive fingerprint of Pride Electronics' later, most versatile production runs.