dbx 3BX III, Руководство по обслуживанию

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Model 3BX III Service Manual
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In the LED display, current sources (QL01 through QL08) of approximately
7 mA each provide constant currents to each series-connected string of
six LEDs regardless of the number of LEDs on at any one time. The LM339
comparator sections (UL01 through UL12) have open collector outputs that
shunt the LED supply currents to ground when they are “low” and allow the
currents to pass into the LEDs when they are “high.” This turns off all
the LEDs “below” the last comparator whose output is low. Any LEDs
between the current source and the uppermost low comparator will be
illuminated.
The comparators compare the CV signals with a set of predetermined
voltages set by resistor strings across the supply voltage. For the red
LEDs, the comparator goes high and an individual LED turns on when the CV
negatively exceeds the fixed voltage at the comparator’s other input.
For the yellow LEDs, the CV must positively exceed the fixed voltage to
turn on an LED. Because of the series connection, if any one LED is
open, all LEDs in that string will go out as soon as the comparator at
the bad LED goes high. An open in a comparator output will cause its LED
to light as soon as the one “above” it is lit; a short in a comparator
output will prevent any LEDs from lighting “below” that comparator.
POWER SUPPLY and MUTE GENERATOR
Refer to Fig. 6, a block diagram of the power supply. The secondary of
the power transformer produces approximately 37 Vac rms (the normal range
is 31 to 43 V), which is full-wave rectified by the bridge rectifier
(D801-D804) and smoothed by electrolytic capacitors (C801-C804) to
produce approxi18 Vdc (the normal range is 17 to 24 V). Three-
terminal regulators reduce this voltage to a co12 Vdc, which is
used to supply most of the circuitry. The unreg18 Vdc is used to
supply current to the LED strings.
Also in the power-supply section is the muting generator (Q803, etc.).
Refer to the main schematic for details. The output of this circuit (the
collector of Q803) is normally -18 V. However, it produces a short
positive pulse when power is first applied or removed. The pulse on
turn-on lasts longer than the time required for the supplies to
stabilize, and the pulse after turn-off lasts longer than the time
required for the supplies to collapse. These pulses turn on the muting
FETs (Q801 and Q802), shorting the audio output to ground during the On
and Off transients.
Figure 6, Power Supply