User Manual

2. Amplifier Control Circuit board Assembly 30C1829G2: Figs 1 and 2.
Finally, when the interlock chain is complete, the +12V is applied to the
solenoid of the power supply primary contactor through J2-10, and the cooling
fans and power supply are all turned on. The DS3 LED marked EXT 2 is lighted, as
is the optodiode in U3A. The output active low from U3A at pin 15 informs the
remote control via J5-15 that the EXT 2 interlock is intact.
The interlocked +12V is also available at J3-6 so it can be used for special
on-site control functions.
The amplifier AGC system (overdrive protection) is based on an RF attenuator
located at the input stage of the amplifier, and this is controlled by DC voltage
supplied from RF detectors which sample the RF output from the amplifier. If the
output rises, the DC voltage increases, and this increases the amount of
attenuation, thus the output is maintained at a constant level. The majority of
the AGC processing is done by analog op-amp circuits in the Metering Board, but
the initial threshold setting is done in the pin attenuator board, potentiometer
R5. These simply provide an adjustable reference bias voltage to the AGC
circuit, which adjusts the power output inversely according to this bias voltage.
In the event of a VSWR that exceeds a preset amount, the AGC voltage becomes
modified a little to reduce the amplifier output by an amount proportional to the
reflected signal. This "VSWR Cutback" permits the amplifier to remain on the air
at reduced power if the antenna should gradually accumulate a layer of ice.
The AGC voltage and modifications to it from VSWR, are summed in U2A which is
basically a buffer amplifier that also provides a telemetry output to the remote
control system through J5-3.
Forward and Reflected meter calibration is done with potentiometers on the
Metering Board.
A 50 ì A meter mechanism that is fitted with a scale 0-125%, is connected to J2-9
and J2-7. The + terminal of the meter connects to J2-9.
Although F1 is drawn in Figure 2 as a fuse, in reality it looks like any ordinary
disc ceramic capacitor, but it is a current limiting device similar to a
thermistor having an extreme positive temperature coefficient. In normal
operation it maintains a low resistance, until it gets hot from too much current
and then suddenly switches to a high resistance state so that the current it is
able to pass is only a very small amount. This small amount of current is
sufficient to keep it warm enough that it remains in its high resistance state.