TM 11-5820-921-40-1
Note
After completion of steps d.(8) through d.(12), the active elements of the
control circuit are in condition C (see table 2-5). The dc-to-dc converter
can be restarted by changing any of the frequency select switches on the
receiver-transmitter front panel (except the 100 Hz switch) or by turning
off the power momentarily. This generates a TUNE START pulse (30
millisecond ground) which resets overload latch 1A2AR1 at pin 4. When
the power amplifier is again keyed, the dc-to-dc converter will restart. If
the overload is still present, the overload latch 1A2AR1 will latch again
and converter oscillation will stop.
2-140. The filaments of output amplifier tubes 1A14V1 and 1A14V2 are supplied with a regulated fila-
ment voltage of about +25.5 Vdc. When the primary supply voltage (+26.5 Vdc nominal) drops below
+25 Vdc, the filament voltage to tubes 1A14V1 and V2 also drops, with a 1 volt difference. The filament
regulator for 1A14V1 consists of voltage regulator 1A2VR5, transistor 1A2Q2 and regulator transistor
1A14A6Q4. The filament regulator for 1A14V2 consists of voltage regulator 1A2VR6, transistor 1A2Q1,
and regulator transistor 1A14A6Q5. The series transistors 1A14A6Q4 and Q5 are fastened to the heat
sink (on chassis 1A14) for cooling.
2-142. Static Power Inverter Assembly 1A12 convert the +26.5 Vdc (Nominal) voltage to a 400 Hz square
wave, about 20 volts in amplitude. The inverter assembly consists of three main circuit groups.
2.
The next two circuit groups are identical amplifiers. One amplifier consists of transistors 1A2Q2,
Q3, Q4, Q5, Q6, Q12, and Q13 and diodes 1A2CR1 and CR2. The other amplifier consists of
transistors 1A2Q7, Q8, Q9, Q10, Q11, Q14, and Q15, and diodes 1A2CR3 and CR4. When the
two amplifiers receive a pulse from the oscillator, one amplifier turns ON while the other one
stays turned OFF. On the next pulse from the oscillator, both amplifiers change state (i.e., the
amplifier that was ON now turns OFF while the amplifier that was OFF now turns ON). The
output of either amplifier, as seen at 1A12TP2 or 1A12TP3, is a square wave whose amplitude is
5 volts less than the line voltage applied to the inverter.
2-144. The filtering and cooling circuits are described in subsequent paragraphs 2-144 through 2-147.
2-145. FILTERING
2-147. COOLING
2-148. The majority of power amplifier components and assemblies are conduction and/or convection
cooled. Heat is transferred by conduction from the main heat sources (i. e., tubes 1A14V1 and V2, Driver
Module 1A11, and Dc-to-Dc Converter 1A14A2) to the heat sink. The heat sink is cooled by forced air
blowing over the fins located outside of the sealed power amplifier enclosure. High speed air flow past the
fins of the heat sink is provided by axial-flow 400 Hz blower 1A14B1. The blower is powered by a 400 Hz
inverter (1A12). The inverter is powered by +26.5 Vdc that is switched ON by the normally open contacts
of thermostat switch 1A14A3S2 when the power amplifier temperature reaches 55C.
