Philips LFH 0388, Service Manual

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LFH 0388/LFH 0488/LFH 0588
14
Automatic gain control
Depending on the level of output at the final amplifier IC 2-3, AGC-voltage is produced via
C 1, TS 15 and C 5 which in turn controls the impedance of TS 3.
The built-in microphone is automatically disconnected by SK 7 (socket switch), when an
external microphone is connected to socket SO 2.
All markings in mode RECORD only.
On model LFH 0588 the timing is controlled by the microprocessor IC 201.
End of letter marking (LFH 0588)
The marking oscillator, consisting of IC 1d and R 12/C 11, generates 40 Hz.
By pressing SK 4 once ( ) the marking sequence starts.
IC 201-2:1, Marking oscillator: 1 (for 650 msec.). Via C 10 and harmonic filter R 45, C 36, R 37
the marking signal on IC 1-3 arrives at the input of the final amplifier IC 2-7.
The speech signal is muted during marking by TS 1 (TS 17:1 and IC 201-2:1, via R 67 and
R 55, TS 1:1).
End of letter marking (LFH 0488)
The marking oscillator, consisting of IC 1d and R 12/C 11 generates 40 Hz. As long as the
marking button (SK 4) is held depressed, the oscillator is switched on (IC 1-1:1).
The marking signal at IC 1-3 arrives at the input of the amplifier IC 2-7 via C 10 and harmo-
nic filter R 45, C 36, R 37.
The speech signal is muted during marking by TS 1 (SK 4:1, via R 55 transistor TS 1:1).
Priority marking (LFH 0588)
By pressing SK 4 twice ( ) during a 700 msec. interval the priority marking sequence
starts.
A 1500 Hz marking signal (325 msec.) is added additionally to the 40 Hz marking signal
(650 msec.) and recorded (IC 201-28: via R 69 to input of IC 2-7).
The »Voice Activated Recording« circuitry is switched on by SK 70.
The output signal of the record amplifier at IC 2-1 arrives at the amplifier TS 70 via R 70,
C 71. Being rectified by D 70/C 73 the speech signal arrives at the switching circuitry
TS 71/TS 72.
At an operating level of - 15 dB/1 kHz, transistor TS 72:1 and TS 73:0.
If the signal level at the microphone input is < -15 dB for longer than approx. 2 sec., TS 72
becomes not conductive; TS 73:1 and the tape transport will be stopped by D 1. (TS 13:0,
TS 14:0, TS 16:0)
During that stop period, D 72 will not allow C 15 to be charged (for immediate start of the
tape drive as soon as speech signal is back again).
D 10 prevents activating of the VAR-circuitry at the end of tape position by any interference
voltages or speech signals (IC 1-11:0, signal shorted out via D 10).
LFH 0588/0488 only:
By TS 2 the LED D 6 will extinguish at the end of tape (TS 73:1, TS 2:1, D 6:0).
Marking
LFH 0588/0488
VAR circuitry