Codan NGT 2010 RF Technical & Service Manual page 120

NGT—Technical description (2010)
The output of the forward detector D1 is also applied to V8 via an averaging detector
circuit formed by R40, R43 and C147. When the average of the signal exceeds the peak
detector circuit V7, V8 takes control and reduces the output power (this occurs on single
tones such as Tune mode, CW (Morse) and in some data transmissions).
When the handset PTT is active, V11 is switched off, which disables the average ALC
circuit. In these circumstances, the transceiver produces full PEP on a single tone. This is
for measurement reasons only.
The peak output collector swing is rectified by D202 and applied to the transistor V10.
V10 conducts and limits the collector peak to 42 V to prevent damage to the output
transistors.
The battery voltage is monitored by V5. When the supply voltage (A rail) drops to
approximately 12 V, the voltage at the base of V5 falls and causes V5 to conduct. This
changes the threshold of the forward power ALC detector. The ALC detector then starts
to reduce the output power and since the control is linear, a further reduction in supply
voltage continues to lower the output power.
If the heatsink temperature exceeds 80°C (176°F), the PTC resistor R262 rapidly
increases its resistance. This causes V13 to conduct and thus reduces the reference
voltage R19. This lowers the output power and prevents the heatsink temperature from
exceeding 100°C (212°F).
The relay protection circuit is formed by IC5/B and associated components. It
simultaneously monitors the output transistor collector swing and forward detector
output of D1. If a relay failure results in no signal at the antenna output, the output of
IC5/B goes high. This lowers the gain of the input amplifiers V200 and V201 and thus
reduces the drive to the output transistors to a safe level.
The ALCs control the output from the summing amplifier IC5/A. This output passes to
IC201/B, which inverts the direction of the control signal to drive the variable resistor
FETs V202 and V203 in the gain stage (see
86
page 81, Gain control
NGT Transceiver System Technical Service Manual
stage).