dbx 3BX III, Руководство по обслуживанию

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Model 3BX III Service Manual
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This signal is applied to a 4.21 kHz, two-pole high-pass filter (2/2-
U702L), a 210 Hz, two-pole low-pass filter (1/2-U702L), and a summing
stage (2/2-U701L). The summer subtracts the outputs of the high-pass and
low-pass filters from the output of the buffer, forming a band-pass
filter with cut-offs at about 4.21 kHz and 210 Hz. Its roll-off will be
6 dB/octave (with some peaking at each corner frequency), while that of
the high-pass or low-pass filters is 12 dB/octave. Fig. 2 shows the
frequency responses of these filters. Note that in their pass-bands the
high-pass and low-pass filters are unity (0 dB) gain.
Figure 2, Low, Mid, and high Filters, 10 dB/division
The outputs of each filter are applied to the inputs of voltage-
controlled amplifiers (VCAs), one each for the low (U706L), mid (U705L),
and high (U704L) ranges. The gain of each VCA is independently
controlled by a voltage derived from the rms-level detector for each band
and the front-panel control settings. This circuitry is described in the
Control-Voltage Path section, next page.
The VCA’s gain in decibels is proportional to the voltage at pin 3 of
each IC. Ideally the gain is 0 dB when the voltage is 0.00 mV; gain
trims in the control-voltage (CV) path (see section 6 in the Alignment
section) allow for a small adjustment. When pin 3 is positive, the gain
in dB is negative (less than unity); when pin 3 is negative, the gain in
dB is positive (greater than unity). Also under ideal conditions,
variations in pin 3 voltage will not cause any variations in the dc level
at the output of a VCA, but in the 3BX III, symmetry adjustments (VR701L
for the low band, VR702L for the mid, VR703L for the high) are provided
to compensate for non-ideal performance. (See the Alignment Procedure,
sections 5.1 through 5.6, p.12 and following.) For a detailed
explanation of this part, refer to the booklet on the VCA IC.
The outputs of all three VCAs are connected to a single summer stage,
2/2-U703 (in the right channel this stage is 1/2-U703). This op-amp
converts the VCA-current output to a voltage signal and recombines the
three bands into one. Note that the signal at pin 6 of U703 (pin 8 of
the VCAs) is a current, not a voltage, which means that there won’t be
very much voltage at pin 6 of U703 even if the VCA is working properly.
By the way, a relatively large signal at this pin (more than 10-20 mV)
usually indicates a fault with U703.
The output of 2/2-U703 goes through an RC-coupling stage (R731L, C776L,
R732L) before connecting to the FET-based muting circuit (Q802 in the
left channel, Q801 in the right channel). These FETs are turned on for a
short time whenever power is applied or turned off, to attenuate the
output during power-up and power-down transients.