(4) The accurate voltages for each pin

During seek:

When the voltage of pin 11 is HIGH 5 V, transistor N is ON and transistors N3, N4, N6, N7, N8 and N9, OFF.

Pin 18 is the stop detector output when a carrier is present. When N10 is ON, pin 18 is LOW, V18

6 ON, and when N10 is

OFF, pin 18 is HIGH, V18

During stop:

When the voltage of pin 11 is LOW and the voltage of pin 18 is HIGH, transistor N5 is OFF, and transistors N3, N4, N7,

N8, N9, P1 and P2, ON. The SD cut-off circuit for N8 and N9 are ON. Since N9 is ON, no current flows through the stop

When the microcontroller pulls pin 18 to ground, N5 and N7 turn OFF, and N3, N4, N6, N8, N9, P1 and P2, ON. As a

result, N2 is also turned OFF and the amplifier load for fr = 900 kHz is R1. The amplifier load for fr = 900 kHz is R1 // R2.

The AGC ON level is 10 dB lower for R1, R1//R2 than for other frequencies.

16. IF block AGC time constant switching circuit

These tuners incorporate various AGC time constant switching circuits that serve to prevent incorrect stops during seek operations.

A summary of these circuits is provided below.

(1) Problems with IF AGC response, such as incorrect stops during seek operations

1 Incorrect stops (or overshoots) due to S-meter oscillation caused by transient response in IF AGC (secondary filter).

2 Overshoots of adjacent stations or weak signal stations due to the time required to discharge the electric charge that has

built up in the IF AGC capacitor when a seek is initiated while the device is receiving a strong signal

(2) Countermeasures

Although this explanation is for the LA1137N, it is identical for the LA1136N, except that the pin numbers are different.

Countermeasure for problem 1:

When a high signal on pin 11 is detected during a seek operation, change the time constants as indicated below.

Countermeasure for problem 2:

AGC capacitor discharge times can be reduced by tracking the S-meter output voltage and rapidly discharging the AGC

capacitor connected to pin 14 as shown below. This circuit is interlocked with the MUTE signal on pin 11 so that it operates

only during seek.

It also reduces the AGC time constant during seek to one-eighth of its normal value.

During receive

Second-order filter

First-order filter

t=1k

Ω× 4.7 µF

During seek

Unit (resistance:

Ω, capacitance: F)

LA1136N, 1136NM, 1137N, 1137NM

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