Daewoo DV-K88 series, Service Manual

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16 CIRCUIT
3) The output AUDIO signal at the 1st pin through REC AMP is recorded on HEAD after being mixed with the 70KHz
AC BIAS signal.
At the same time, the output AUDIO signal is supplied to the 3rd pin and then filtered by 60KHz HPF, so only the
70KHz AC BIAS signal is passed.
The pure 70KHz AC BIAS signal is then compared to the voltage of 440mV at the comparator, so the AC BIAS
signal is controlled to maintain at 440mV.
4) The recording current is determined by the 440mV
rms
BIAS signal and the resistance between the 1st and the 3rd
pin.
* The method to set the recording current.
BIAS = 440mV
rms
÷
the resistance between the 1st and the 3rd pin.
ex) 440mV
rms
÷
1.8K OHM = 244µA
where, only the low error, G type resistor should be used here.
As a rule of thumb, the resistance between the 1st and the 3rd pin should range from 1.0 ohm to 2.2K ohm.
5) The conventional AUDIO circuitry uses a peaking COIL to enhance the high frequency region, but the LA71511M
uses the resistance of R/P HEAD.
Accordingly, The resistor and the capacitor is used to fit the frequency characteristics.
4. BIAS COIL
For normal operation of AUTO BIAS, the oscillating voltage of R/P HEAD is over 40V
p-p
, A/E HEAD is over 20V
p-p
,
and the FE HEAD is over 40V
p-p
.
Especially, the erasing current of F/E HEAD ranges from 130 to 280mA, and it should be 180mA when the HEAD
impedance is at the center.
1. SPECIAL FEATURES
BH7804K processes the conventional Hi-Fi audio. and additionally, contains special parts as the below.
(1) As
±
2 power source formula is adapted, decrease of coupling capacitor is possible. the REGULATOR circuit is
built in.
(2) Built in I 2 C bus control decoder circuit which has two lines formula, each mode for the inside of IC is set up by
serial data.
(3) Reverse characteristics system during recording and playback are used in both the PNR (Peak Noise
Reduction) processor and FM MODEM. And it can be simultaneous adjustment of carrier frequency and FM
deviation/playback level. And it can be independent adjustment of FM deviation/playback level. (The adjustment
of Lch and Rch is simultaneous.)
(4) Slope control FM switching noise correction circuit is built in.
(5) Auto adjust circuit of VCO and BPF making use of Fsc(4.43MHz) is built in. It is possible to adjust changelessly
without drift by board mounting stress and time.
BA7746FS contains the functions as the below
(1) Built-in VCA for easy adjustment of play back output level.
(2) Total gain 79dB of low noise Pre amp for play back mode.
(3) Built-in LPF to regulate input band width of recording amp.
(4) Recording amp has high output current for audio FM recording.
3-6. H
i
-F
i
Audio Signal Processing (BH7804K)