
μ
PC1876
22
Data Sheet S11666EJ4V0DS00
5. ADJUSTMENT PROCEDURE
Precise alignment of the dbx decoder is absolutely critical for optimum performance. Where possible, the alignment
should be performed after the
μ
PC1876 is mounted in the chassis and with the video system active.
5.1 Stereo VCO Adjustment
Perform this adjustment with no signal applied.
(1) Set the STEREO VCO FREE-RUN MONITOR SWITCH pin “H” by connecting it to the V
CC
.
(2) Measure the frequency of the STEREO LED DRIVER/STEREO VCO FREE-RUN MONITOR pin with a frequency
counter, and adjust the variable resistor connected to the STEREO VCO SETTING pin for a measured frequency
of 15.734 kHz
±
50 Hz.
(3) Set the STEREO VCO FREE-RUN MONITOR SWITCH pin unconnected.
5.2 Filter Adjustment
Perform this adjustment with the MATRIX MUTE pin “L” and the STEREO VCO FREE-RUN MONITOR SWITCH
pin unconnected.
(1) Short the capacitor across the PILOT DISCRIMINATION FILTER (PD1 and PD2) pins by connecting these pins
together directly.
(2) Apply a 15.734 kHz sin wave signal to the COMPOSITE SIGNAL INPUT pin at a level greater than 30 mV
r.m.s.
(100 mV
r.m.s.
is recommended).
(3) Adjust the variable resistor connected to the FILTER ADJUSTMENT pin so that the AC output level at the RMS
OFFSET ABSORPTION pin is minimized. This signal is best monitored through a band-pass filter (15.734 kHz).
A recommended circuit is shown in
7. MEASURING CIRCUIT
.
(4) Disconnect the short circuit across the PILOT DISCRIMINATION FILTER (PD1 and PD2) pins after adjustment.
5.3 Separation Adjustment
Perform this adjustment with the MATRIX MUTE pin “L”, the STEREO VCO FREE-RUN MONITOR SWITCH pin
unconnected, and the F-MONAURAL SELECTION pin “H”.
Verify that a L+R signal (100% modulation, 300 Hz, without noise reduction, pilot signal off) results in approximately
150 mV
r.m.s.
at the COMPOSITE SIGNAL INPUT pin, and that a pilot-only signal results in approximately 30 mV
r.m.s.
at the COMPOSITE SIGNAL INPUT pin.
(1) Apply a composite signal (30% modulation, 300 Hz, L-only, with noise reduction) to the COMPOSITE SIGNAL
INPUT pin.
(2) Adjust the variable resistor connected to the WIDE-BAND RMS SETTING pin so that the output at the R CHANNEL
OUTPUT pin is minimized.
(3) Apply a composite signal (30% modulation, 3 kHz, L-only, with noise reduction) to the COMPOSITE SIGNAL
INPUT pin.
(4) Adjust the variable resistor connected to the SPECTRAL RMS SETTING pin so that the output at the R CHANNEL
OUTPUT pin is minimized.
(5) Repeat steps (1) and (2).
Caution Be sure to perform step (5).