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transistor temperature increases. This bias feedback system keeps transistor quiescent current stable
independent of transistor junction temperatures. These thermistors also feed a comparator that removes drive
when transistor temperature becomes unsafe.
The second set of thermistors, R1, monitor heatsink temperatures. Voltage from thermistor R1 regulates fan
speed, increasing airflow as the heat sink warms.
Combiner CMB
Each PAM-600 delivers a rated maximum of 600 watts peak envelope power into a 50-ohm load. Each PA
module feeds a 50-ohm high power combiner CMB that isolates the two outputs. A 50-watt 100-ohm power
resistor dissipates signal level errors between the PA modules.
Voltage step down transformer T1 senses voltage across combiner dump resistor R1. T1 is located on the
2KWF lowpass filter board. This voltage, representing PA combiner unbalance, appears on the front panel
multimeter as a “PAB” (power amplifier balance) indication. PA unbalance sample voltage also feeds a
comparator on the CB1 control board. This comparator disables the PA in the event the power amplifiers
become significantly unbalanced, and illuminates the PA front panel light.
2KWF Lowpass Filter Assembly
The output of the high power combiner goes into the 2KWF lowpass filter assembly. Power enters through a
directional coupler consisting of T2, C28, 98 and 99, and R2. This system detects power amplifier termination
errors. Any significant error will trip the power amplifiers off. A comparator on the CB1 control board monitors
directional coupler termination errors. Such errors normally come from selecting the wrong filter for the
exciter’s operating band, or having a poor load SWR on the amplifier.
5-pole lowpass filter 2KWF further reduces harmonics. Harmonic levels are significantly below levels
mandated by FCC rules governing amateur radio service.
The CB1 control board contains all protection comparators (U1) and latches (U3). In the event of an operational
fault, latches lock out the amplifier and illuminate a warning light on the front panel. CB1 also contains bias,
relay sequencing, and fan speed controls.
Frequencies above 21.5 MHz are disabled in units sold in the United States and possessions.
SWR
The SWR board is a standard 50-ohm directional coupler. It samples line current and line voltage, vectorially
summing them before detection. The resulting voltages represent forward and reflected power, or mismatch of
the ideal 50 ohm load.
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