Squelch Circuit

The 1st AF amplifier is the first triode section of VI15. The second triode section of VI15 serves as a squelch tube to silence the receiver when no signal is being received. In fact, the second triode section controls a switch which opens or closes the circuit of the first triode section.

Note that the grid of the first AF amplifier can be shorted to ground through relay RL101. The relay, as shown, is in the no-signal condition and is shorting the grid of the first audio amplifier to ground. Note, too, that the current through the winding of the relay is the plate current of the first triode section of the audio and relay amplifier tube VI13. The grid of this triode section is connected directly to the plate of the squelch triode section of V115. In turn, the grid of the squelch triode is connected through the squelch rectifier circuit to the grid circuit of the 1st limiter tube.

With no signal, the grid of the first limiter draws no current and develops no bias. Consequently the squelch triode is not biased. This allows the squelch triode to conduct. Current flow through the plate load resistors R148 and R149 causes the plate voltage of the squelch triode to drop. This plate voltage is applied to the grid of the relay triode section of VI13. In the relay triode section the grid voltage is now below the cathode voltage. This is due to the fact that the cathode has high bias because of its connection between R163 and R141 which are part of a B4- bleeder circuit. The relay triode section does not conduct and the relay is de-energized.

However, when a signal is received, the first limiter develops bias, which is applied to the squelch triode section of VI15. The plate current of the squelch triode section is now reduced. As a result, plate voltage rises enough to raise the grid voltage of the relay section of VI13 above the cathode voltage. The relay section conducts and energizes the relay. This removes the ground from the grid of the 1st AF amplifier and allows the signal to pass through the audio amplifier stages.

The amount of grid current in the first limiter grid depends to a large extent on the overall gain of the preceding stages. This overall gain can be controlled for proper squelch operation by screwdriver adjustment of the potentiometer in the screen circuit of the high frequency IF amplifier. This potentiometer control is effective only when the squelch switch of the control panel is ON.

When the carrier operated relay, RL101 is energized, it lights a lamp on the front panel to indicate the presence of a carrier in the receiver circuits.

Carrier Operated Relay Schematic Diagram

Complete Circuit Diagram, Receiver R-19/TRC1 121

HIGH FREQUENCY I F ÛMPL

VIC4

LOW FREQUENCY F AMPL

V106 6SH7

1ST LIMITER

V107 6SH7

HIGH FREQUENCY I F ÛMPL

VIC4

1ST LIMITER

V107 6SH7

Squelch Seekic
1ST A F AMPL AND SQUELCH VI15 «SL70T
Philco 1256
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[input I

HIGH FREQUENCY I F 1MPL

L0» FREQUENCY

1ST A F AM PL AND SOUELCH V115 6SL7GT

[input I

HIGH FREQUENCY I F 1MPL

L0» FREQUENCY

1ST A F AM PL AND SOUELCH V115 6SL7GT

SI04 P 9 1MULTI CH*WNEL|

Complete Circuit Diagram, Receiver R-19/TRC1 121

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