Abell A720T, Service Manual

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A720T (VHF) Service Manual V1.0
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5.3.2 First frequency mixer
The signal from the RF amplifier is mixed with the first oscillator signal from the PLL frequency synthesizer circuit at
the first mixer (Q502) and generates the 1st IF signal of 58.05MHz.The first if signal is further eliminated by a
two-stage single-crystal filter (F500).
5.3.3 Intermediate Frequency Amplifier (IF AMP)
First intermediate frequency amplified through Q503, then go into chip U500(FM process chip)
。
Signal will mix
with second local referenced signal and generates a 450KHz second IF signal, which will go through a 450KHz filter
to filter out unwanted spurious signal before it is amplified and and discriminated.
5.3.4 AF Amplifier
In the U500, the audio signal demodulated by frequency discrimination is de-emphasised by IC501(A,B) high-pass
filter and then IC501(C, D) low-pass filter. The processed AF signal passes through an AF volume control and is
amplified to a sufficient level to drive a loud speaker by an AF power amplifier (U306).
5.3.5 Noise Suppression Circuit
Part of the audio signal from 13 pin of U500 will go through U500 again, use filter and amplifier to amplify noise
and then Rectify to generates a DCV with corresponding noise level. The DCV go through A/D port of micro
processor. U300 will detect the input voltage value to see if whether it exceed or lower than the preset value, then
decide the output voice through speaker.
To output voice through speaker, micro processor(U300) will send a high level signal to AF_MODE control line:
open Q302 through Q303 to supply power, open the Audio Power Amplifier (U306).