dbx 3BX III User manual

Model 3BX III
Three-Band
Dynamic-Range Expander
with Impact Restoration
SERVICE MANUAL
All dbx products are manufactured under patents in the US and abroad, and on all
dbx circuit designs dbx holds copyright in one or more of the following years:
1979-’85. “dbx” is a registered trademark of dbx, Newton, Mass. USA.

CONTENTS
SPECIFICATIONS (performance minimums)............................ 4
CIRCUIT DESCRIPTION.............................................. 1
SCHEMATIC CONVENTIONS .......................................... 1
SIGNAL PATH .................................................... 1
CONTROL-VOLTAGE (CV) PATH ...................................... 3
High-Band CV Path ............................................ 5
Low-Band CV Path ............................................. 7
Mid-Band CV Path ............................................. 7
LED DISPLAYS ................................................... 7
POWER SUPPLY and MUTE GENERATOR ................................ 8
ALIGNMENT PROCEDURE.............................................. 9
1. INSTRUMENTS REQUIRED ........................................ 9
2. INSPECTION and WARMUP ....................................... 9
3. POWER-SUPPLY TESTS .......................................... 9
4. CONTROL-CIRCUITRY TESTS ..................................... 9
5. VCA-SYMMETRY ADJUSTMENTS ................................... 12
6. VCA-GAIN ADJUSTMENTS ....................................... 13
7. PERFORMANCE TESTS .......................................... 14
MODEL 3BX III PRINTED CIRCUIT BOARD ASSEMBLY.................... 20
MODEL 3BX III MAIN ASSEMBLY..................................... 25

SPECIFICATIONS (performance minimums)
Expansion
To 50% increase,
maximum 12 dB upward
and 20 dB downward
Impact restoration
To +12 dB (upward
only), program-
dependent
Frequency response +0.5 dB 20 Hz –
20 kHz,
any setting
Dynamic range 107 dB
Total harmonic distortion (THD), no
expansion 0.15%
Intermodulation distortion (IMD) IHF or
SMPTE 0.1%, any setting
Equivalent input noise –90 dBV
Attack rates Program-dependent,
optimized for each band
Release rates
Linear expander program
-dependent, optimized;
impact restorer
adjustable
Maximum input and output 7 V
Notes
1) Specifications are subject to change without notice.
2) All data are for 20 Hz-20 kHz unless otherwise specified; line inputs
are driven by a source impedance of 1 k-ohms and outputs are loaded by
10 k-ohms in parallel with 1000 pF; all voltages are rms (root-mean-
square).
3) Dynamic range is defined as the difference between the maximum rms
output signal and A-weighted noise. All noise figures are A-weighted.
4) Frequency response figures are for pink noise (or music).
5) SMPTE IMD is measured with 60 Hz and 7 kHz mixed 4:1; IHF (difference-
tone) IMD is measured with 19 kHz and 20 kHz mixed 1:1; output 1 V.
6) Inputs and outputs have identical polarity.
7) All dbx home products are designed to be used with components whose
output impedance is less than or equal to 5 k-ohms. All units are
designed to drive loads of at least 5 k-ohms in parallel with 1000 pF or
less.

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CIRCUIT DESCRIPTION
SCHEMATIC CONVENTIONS
The 3BX III is a stereo unit, with two independent, identical signal
paths. We will refer here to the left channel only (channel 1). Almost
all of its components are identified by designations ending in L (e.g.,
C705L, R708L); the right channel’s component designations generally end
in R; and components common to the channels generally end in numbers
(e.g., C803, R801). Furthermore, components are coded according to their
function within the system. The 700 series indicates the signal path or
detector path (the detector section is common to both channels, so 700
components ending in a number are in this area, while those ending in L
or R are in the signal path). The 800 series indicates the power supply.
Finally, the L## series indicates the LED display (e.g., RL21, UL05).
SIGNAL PATH
Refer to Fig. 1, a block diagram of the signal path (note that L, R
suffixes are not shown and that test points [TPs] are for both channels,
left first.) Audio input signals first are buffered by 1/2-U701L and its
associated circuitry. The output of 1/2-U701L is TP1 (TP2, right
channel). The signal there should be identical to that at the input
except for a small attenuation (-0.83 dB) and a roll-off at 175 kHz.
Figure 1, Signal Path

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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
6dB/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.

Model 3BX III Service Manual
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Finally, the signal passes to the switching circuitry. The switching
allows the 3BX III signal path (a) to come either before or after a tape
deck plugged into the tape jacks of the 3BX III, (b) to receive its
signal from either the tape deck or the source, and (c) to bypass the
circuitry.
CONTROL-VOLTAGE (CV) PATH
The 3BX III has two main purposes: first, to make loud signals louder
while making soft signals softer (upward and downward expansion), and
second, to emphasize musical transients (impact restoration). The three
rms detectors translate the signal level in each of the three bands into
voltages (at dc or nearly so) that indicate how loud the input signal is
in each of these bands. These voltages can then be processed and used to
control the gain of the VCAs in the 3BX III to accomplish its two
purposes. Choosing the correct time constants for the rms detectors in
the first place is critical to doing this job right. Then the rms-
detector signals must be processed to find the transients and produce
signals suitable for increasing the music’s impact. All of this
detecting and processing takes place in the CV-path section of the
3BX III.
Refer to Fig. 3, a block diagram of the CV path. The audio signals at
the outputs of each of the signal-path filters are connected to the
inputs of three rms-level detectors, one for each band (U707 is the low-
band detector, U708 the mid-band, U709 the high-band). For each band,
the left- and right-channel signals are summed at pin 1 of the rms-
detector IC. For a detailed explanation of this part, refer to the
booklet on the rms-detector IC.
Figure 3, Control-Voltage Path

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The output of these ICs (pin 7) is a dc voltage proportional to the dB
signal level at the output of the filter that feeds them. TP9 is the
low-band output, TP11 the mid-band, and TP13 the high-band. Table 1
shows the voltage and frequency required (simultaneously at the L- and R-
channel inputs) to cause (ideally) 0.00 mV at pin 7 of each rms-detector
IC. The exact calibration of these voltages is not critical, since
adjustments are provided in the stage following the rms detector.
Input Voltage Input Frequency Rms IC Test Point Approx.
Voltage
227 mV 100 Hz U707 TP9 0.00 mV
88 mV 1 kHz U708 TP11 0.00 mV
69 mV 10 kHz U709 TP13 0.00 mV
TABLE 1
Input levels and frequencies (both channels driven)
for 0.00 mV at the rms output
Along with the dc voltage is a small amount of ripple. Ideally, it will
be at twice the frequency of the input, with no fundamental. Trim pots
(VR704 in the low band. VR706 in the mid. and VR708 in the high) are
provided to allow this waveform to be adjusted for perfect symmetry (see
the Alignment Procedure, sections 4.1-4.3, p.9 and following). For
larger input signals the rms-IC output is positive, and negative for
smaller input signals; it varies by 6 mV for each decibel of input-level
change.
The time-constant of the rms detector is dependent on the capacitance
connected to pin 6. The larger the capacitor, the slower the time
constant. The mid-band and high-band rms-detector ICs have small
capacitors connected directly between pin 6 and ground (C731 in the mid
band and C741 in the high band). Also connected to pin 6 is a larger
capacitor (C720, C729, C740), which connects to an op-amp. This
capacitor and the circuit to which it connects form a “nonlinear
capacitor” with an equivalent “value” that changes with the signal
conditions. If there are problems in verifying the expansion timing (see
alignment procedure, section 7.2), check that this circuit is operating
correctly by probing the output of the associated op-amp (pin 1 of U710
in the low band, of U711 in the mid band, and of U722 in the high band).
The output of each op-amp should be a sinusoidal wave at twice the
frequency of the input signal. See Table 2 for appropriate frequencies
and levels.
Input Voltage Input Frequency Op-amp Pin
Number
Approx
Voltage (p-p)
1.0 v 50 Hz U710 1 60 mV
1.0 v 100 Hz U711 1 540 mV
1.0 v 100 Hz U722 1 510 mV
TABLE 2
Test conditions for the nonlinear-capacitor circuit

Model 3BX III Service Manual
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The rms-detector outputs connect to inverting buffer stages (2/2-U710,
2/2-U711, and 2/2-U722) with gains of 9. At these buffers, individual dc
voltages from trim pots VR705, VR707 and VR709 are added to the rms-
detector output voltages, and a single dc voltage from the Transition-
Level control (VR710 on the front panel) is added to all three stages.
The trim pots allow the outputs of the rms detectors to be calibrated to
specific references (see the Alignment Procedure, sections 4.1, 4.2 and
4.3), and the front-panel control allows the entire system’s unity-gain
point (no upward or downward expansion) to be adjusted by the customer to
match the levels in his or her stereo system.
TP10 is the low-band rms-buffer output, TP12 the mid-band, and TP14 the
high-band. The CV path now splits in two, differently for the high- and
low-bands from the mid-band. First the high band…
High-Band CV Path
TP14 connects directly to the high-band-expansion control (VR715) on the
front panel. This is one section of a three-gang pot (the other sections
are labelled VR713 and VR711) which controls the amount of rms-detector
signal that eventually reaches pin 3 of the high-band VCA IC. Setting
the pot for more expansion causes more of this signal to be allowed
through. At 50% expansion, a 100 mV change at the rms output (TP13)
causes a negative 50 mV change at pin 3 of the VCA (TP7). When the
signal at TP14 is positive, the gain of the high-band VCA will be either
negative in dB or 0, depending on the position of the expansion control.
When TP14 is negative, the gain will be positive or 0. The wiper of the
expansion control connects to the positive input of a summer stage
2/2-U719) whose output is sent on to the VCA.
TP14 also connects to the impact detector (U720 and associated
circuitry), which differentiates the rms-detector-output waveform and
clips off the negative-going portions of it. Its output looks like a
sharp positive-going spike every time a sudden increase in input-signal
level takes place. This positive-going spike will cause the gain of the
high-band VCA to increase (how much it increases depends on the setting
of the Impact-Restoration control). Note that because the impact
detector clips off the negative portions of the control signal, the
impact restorer never causes negative gain (unlike the Expansion
section). See Fig. 4, next page, for typical waveforms in the impact-
restoration part of the CV path (note that column ‘a’ is low, ‘b’ is mid,
‘c’ is high). The output of the impact detector connects to a time-
constant circuit (1/2-U717, 2/2-U717), which stretches this spike out for
atime determined by the setting of the rear Impact-Release-Rate control
(VR723).
The output of the time-constant circuit is directly connected to the
impact-level control (VR716) on the front panel. This, too, is one
section of a three-gang pot (the other sections are labelled VR714 and
VR712), and it controls the amount of impact-control signal that reaches
pin 3 of the high-band VCA. The wiper of the pot connects to the impact-
disable circuit (Q703, driven from 1/2-U721), which turns off the impact-
control signal at low signal levels. This prevents record-surface noise
and other small signals from being raised in volume by the impact-
restoration circuitry.

Model 3BX III Service Manual
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All photos:
All front controls @ maximum;
Rear trim (Impact Release Rate) centred
a&bphotos: c photos;
300 Hz tone-burst @ 316 mV rms (447 mV peak),
128 cycles on, 172 cycles off, 0.2 s/div
3kHz tone-burst @ 316 mV rms (447 mV peak),
128 cycles on, 172 cycles off, 20 ms/div
4.a.1 4.b.1 4.c.1
Top: Low-pass filter output, U702L,
pin 1, or U702R, pin 7, 0.2 V/div;
Top: Mid-band summer output, U701L,
pin 7, or U701R, pin 1, 0.5 V/div;
Top: High-pass filter output, U702L,
pin 7, or U702R pin 1, 0.2 V/div;
Bottom: Low-band rms-detector output,
U707, pin 7 (TP9), 0.1 V/div.
Bottom: Mid-band rms-detector output,
U708, pin 7 (TP11), 50 mV/div.
Bottom: High-band rms-detector output,
U709, pin 7 (TP13), 0.1 V/div.
4.a.2 4.b.2 4.c.2
Top: Low-band rms-buffer output,
U710, pin 7 (TP10), 1 V/div;
Top: Mid-band rms-buffer output,
U711, pin 7 (TP12), 1 V/div;
Top: High-band rms-buffer output,
U722, pin 7 (TP14), 1 V/div;
Bottom: Low-band impact-detector
output, U713, pin 5, 0.2 V/div.
Bottom: Mid-band impact-detector
output, U714, pin 3, 0.5 V/div.
Bottom: High-band impact-detector
output, U717, pin 5, 0.2 V/div.
4.a.3 4.b.3 4.c.3
Top: Output, low-band impact time-
constants, U713, pin 1, 1 V/div;
Top: Output. mid-band impact time-
constants, U715, pin 7, 1 V/div;
Top: Output, high-band impact time-
constants, U717, pin 1, 0.5 V/div;
Bottom: Low-band VCA control voltage,
U706L or R, pin 3 (TP3 or 4), 0.1 V/div.
Bottom: Mid-band VCA control voltage,
U705L or R, pin 3 (TP5 or 6), 0.1 V/div.
Bottom: High-band VCA control voltage,
U704L or R, pin 3 (TP7 or 8), 0.1 V/div.
4.a & b.4 4.c.4
Left or right output, 5 V/div Left or right output, 0.5 V/div
Figure 4

Model 3BX III Service Manual
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Finally, the output of the impact-disable circuit connects to the summer
stage (2/2-U719) previously mentioned, where both the expansion and
impact-restoration CVs are combined before being sent to the VCA. Trim
pots (VR721 and VR722) are provided at the output of this summer to allow
the CVs sent to each VCA (left and right channel) to be offset slightly,
in order to account for differences in VCA-gain offsets (see Alignment
Procedure, section 6, p.13).
Low-Band CV Path
This path is nearly identical to the high-band, except that the time-
constants used throughout are generally much longer, as is appropriate
for low frequencies. The signal at TP10 (the output of the buffer
following the low-band rms output) splits into two paths: one to the
expansion control (VR711) and one to the impact detector. The output of
the impact detector goes through a circuit to lengthen the decay, and
then to the impact-disable circuit. (The impact-disable controller is
common to all three bands.) The impact-control signal and the expansion-
control signals are combined in 1/2-U718 and then sent to the low-band
VCAs. Trim pots VR717 and VR718 are provided to set the VCA gains
precisely.
Mid-Band CV Path
This path also closely resembles the high-band, except for different
time-constants (midway between the low- and high-band ones) and the
addition of a mid-band expansion time-constant circuit. This circuit is
placed between the output of the buffer following the mid-band rms
detector (TP12) and the front panel expansion control (VR713). Its
purpose is to prevent the mid-band CV from dropping too fast after an
input signal goes away (toward lower signal level indication, which at
this point is toward positive voltages). This circuit closely resembles
the impact time-constant circuit; it performs a similar function but does
not provide a user-adjustable release rate.
LED DISPLAYS
Refer to Fig.
5, a block diagram of
atypical LED display. The control
voltages from each of the three
sections of the Impact-
Restoration
control are summed in 1/2-
U719 to
pro
duce a signal representative of
the average of the impact CVs.
This signal is sent to the LED
-
display section. In addition, the
CVs from each of the three sections
of the Expansion-
Level control
(VR711, VR713, and VR715) are sent
(separately) there.
Figure 5, LED Display
(see Fig. 4)
(again see Fig. 4)

Model 3BX III Service Manual
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Inthe LED display, current sources (QL01 through QL08) of approximately
7mA 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 approximately +18 Vdc (the normal range is 17 to 24 V). Three-
terminal regulators reduce this voltage to a constant +12 Vdc, which is
used to supply most of the circuitry. The unregulated +18 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

Model 3BX III Service Manual
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ALIGNMENT PROCEDURE
1. INSTRUMENTS REQUIRED
Audio-frequency sine-wave oscillator with 50 ohm output impedance (Kron-
Hite 4200A or equivalent)
10 MHz oscilloscope capable of 2 mV/div sensitivity (Philips PM 3233 or
equivalent)
DC voltmeter capable of accurately measuring 1 mV (digital preferred,
Fluke 8060A or equivalent)
RMS-responding AC voltmeter (Fluke 8060A or equivalent)
Tone-burst generator (GenRad 1396A or equivalent)
2. INSPECTION and WARMUP
2.1 Inspect the unit to be tested and verify that all internal
interconnect cables are properly installed.
2.2 Confirm that the voltage-selector switch is in the proper position.
2.3 Connect the unit to a source of rated AC voltage, turn it on, and let
it warm up for 10 minutes.
3. POWER-SUPPLY TESTS
3.1 Verify the following power-supply conditions:
Probe location Test Condition Tolerance
3.1.1 U801, pin 3 +12 Vdc +600 mV
3.1.2 U802, pin 3 -12 Vdc +600 mV
3.1.3 Plus(+) end of C803 +18 Vdc +6 V/-1 V
3.1.4 Power indicator illuminated
If these conditions are not met, troubleshoot and correct the problem
before attempting calibration.
4. CONTROL-CIRCUITRY TESTS
4.1 Low-band rms calibration
4.1.1 Set the controls as follows:
Expansion Maximum
Impact Restoration Minimum
Transition Level Minimum
Pre/Post Pre
Source/Tape Source
Bypass Out (i.e., not set)
INPUTS OUTPUTS

Model 3BX III Service Manual
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4.1.2 Connect the sine-wave oscillator to both inputs of the unit
under test. Set it to 20 Hz and an output level of +10 dBV
(3.16 V). Connect the scope to the output of the low-band rms-
detector buffer at TP10 (U710, pin 7) and set it to 2 mV/div (AC-
coupled) and 10 ms/div.
4.1.3 Adjust VR704 for best rms symmetry, as shown in Fig. 7a.
4.1.4 Reset the oscillator to 100 Hz, +10 dBV (3.16 V).
4.1.5 Also connect a digital voltmeter to TP10.
4.1.6 Adjust VR705 for 0.00 Vdc ±10 mV. Verify that no more than
one LED in the low-band gain-change display is lit.
4.1.7 Reduce the oscillator’s output level in 10 dB steps and
verify that the voltage at TP10 changes as follows:
Input Level TP10 Voltage Tolerance
+10 dBV 000 mV dc ±10 mV
0dBV +541 mV dc ±50 mV
-10 dBV +1080 mV dc ±100 mV
-20 dBV +1620 mV dc ±150 mV
Figures 7a 7b 7c
Wrong:
Right:

Model 3BX III Service Manual
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4.2 Mid-band rms calibration
4.2.1 Set the controls as in 4.1.1.
4.2.2 Connect the sine-wave oscillator to both inputs. Set it to
100 Hz, +10 dBV (3.16 V). Connect the scope to the output of the
mid-band rms-detector buffer at TP12 (U711 pin 7) and set it to
5mV/div (AC-coupled) and 2 ms/div.
4.2.3 Adjust VR706 for best rms symmetry, again as shown in
Fig. 7b.
4.2.4 Reset the oscillator to 1 kHz, +15 dBV (5.62 V).
4.2.5 Also connect a digital voltmeter to TP12.
4.2.6 Adjust VR707 for 0.00 Vdc ±10 mV. Verify that no more than
one LED in the mid-band gain-change display is lit.
4.2.7 Reduce the oscillator’s output level in 10 dB steps and
verify that the voltage at TP12 changes as follows:
Input Level TP12 Voltage Tolerance
+15 dBV 000 mV dc ±10 mV
+5 dBV +541 mV dc ±50 mV
-5 dBV +1080 mV dc ±100 mV
-15 dBV +1620 mV dc ±150 mV
4.3 High-band rms calibration
4.3.1 Set the controls as in 4.1.1.
4.3.2 Connect the sine-wave oscillator to both inputs. Set it to
1kHz, +10 dBV (3.16 V). Connect the scope to the output of the
high-band rms-detector buffer at TP14 (U722 pin 7). Set the scope
to 2 mV/div (AC-coupled) and 0.2 ms/div.
4.3.3 Adjust VR708 for best rms symmetry, Fig. 7c.
4.3.4 Reset the oscillator to 10 kHz, -2 dBV (794 mV).
4.3.5 Also connect a digital voltmeter to TP14.
4.3.6 Adjust VR709 for 0.00 Vdc ±10 mV. Verify that no more than
one LED in the high-band gain-change display is lit.
4.3.7 Reduce the oscillator’s output level in 10 dB steps and
verify that the voltage at TP14 changes as follows:
Level TP14 Voltage Tolerance
-2 dBV 000 mV dc ±10 mV
-12 dBV +541 mV dc ±50 mV
-22 dBV +1080 mV dc ±100 mV
-32 dBV +1620 mV dc ±150 mV

Model 3BX III Service Manual
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4.4 Impact-display adjustment
4.4.1 Set controls as follows:
Expansion Maximum
Impact Restoration Middle
Transition Level Minimum
Pre/Post Pre
Source/Tape Source
Bypass Out (i.e., not set)
4.4.2 Connect a clip lead from the junction of R704, R705, and D727
to ground. Short the inputs to ground.
4.4.3 Adjust VR724 so the first impact-restoration LED is just off.
Remove the clip lead.
4.5 Impact-release-rate control (initial adjustment)
4.5.1 Connect the dc voltmeter to the wiper of VR723 (the impact
release-rate control, on the rear panel).
4.5.2 Set VR723 so that the voltage measures -6.75 Vdc.
5. VCA-SYMMETRY ADJUSTMENTS
5.1 VCA-Symmetry Adjustment, Low Band, Left Channel
5.1.1 Set the controls as follows:
Expansion Minimum
Impact Restoration Maximum
Transition Level Maximum
Pre/Post Pre
Source/Tape Source
Bypass Out (i.e., not set)
5.1.2 Short the main inputs to ground and connect a sine-wave
oscillator to TP3 (U706L, pin 3). Set the oscillator for 100 Hz,
100 mV rms.
5.1.3 Connect the scope to the 3BX III left-channel output. Set
the vertical sensitivity to 20 mV/div and the horizontal to
2ms/div.
5.1.4 Adjust VR701L for minimum signal feed-through; see Fig. 8.
Wrong Figure 8 Right
(Fig. 8, below)

Model 3BX III Service Manual
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5.2 VCA-Symmetry Adjustment, Mid-Band, Left Channel
Follow the procedure in 5.1 but go to TP5 and VR702L.
5.3 VCA-Symmetry Adjustment, High Band, Left Channel
Again follow the procedure in 5.1 but go to TP7 and VR703L.
5.4 VCA-Symmetry Adjustment, Low band, Right Channel
Follow the procedure in 5.1 but go to TP4 and VR701R and change
outputs (left to right).
5.5 VCA-Symmetry Adjustment, Mid-Band, Right Channel
Follow the procedure in 5.4 but go to TP6 and VR702R.
5.6 VCA-Symmetry Adjustment, High Band, Right Channel
Follow the procedure in 5.4 but go to TP8 and VR703R.
6. VCA-GAIN ADJUSTMENTS
6.1 Set the controls as in 5.1.1
6.2 Connect the sine-wave oscillator to the left and right inputs of the
3BX III.
6.3 Left-Channel Adjustment
6.3.1 Set the oscillator for 100 Hz at 0 dBV (1 V rms) ±0.1 dB.
6.3.2 Connect the left-channel output of the 3BX III to an AC
voltmeter.
6.3.3 Adjust VR717 for unity gain, i.e., an output of 0 dBV (1 V
rms) ±0.1 dB.
6.3.4 Set the oscillator for 1 kHz at 0 dBV (1 V rms) ±0.1 dB.
6.3.5 Adjust VR719 for unity gain.
6.3.6 Set the oscillator for 10 kHz at 0 dBV (1 V rms) ±0.1 dB.
6.3.7 Adjust VR721 for unity gain.
6.4 Right-Channel Adjustment
Repeat 6.3 above, but observe the right-channel output and make the
unity-gain adjustments at VR718 (at 100 Hz), VR720 (at 1 kHz), and
VR722 (at 10 kHz).
6.5 Verify that the gains have been set correctly by sweeping the
oscillator from 20 Hz to 20 kHz and observing the output amplitude vs.
frequency of both channels. It should be ±0.5 dB.

Model 3BX III Service Manual
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7. PERFORMANCE TESTS
7.1 Impact-Restoration Magnitude and Timing
7.1.1 Set the controls as follows:
Expansion Minimum
Impact Restoration Maximum
Transition Level Maximum
Pre/Post Select Pre
Source/Tape Source
Bypass out (i.e., not set)
7.1.2 Connect the sine-wave oscillator to the tone-burst
oscillator, and connect the tone-burst oscillator output to the
left and right inputs of the 3BX III.
7.1.3 Connect the oscilloscope to the output of one channel of the
3BX III. Connect the scope sweep-trigger input to the gate or
trigger output on the tone-burst oscillator.
7.1.4 Low-Band Test
7.1.4.1 Set the oscillator to 100 Hz at 0 dBV (1 V rms) and
the tone-burst oscillator to produce approximately an 80 ms
burst (8 cycles) followed by approximately 500 ms (50 cycles)
of at least 20 dB attenuation. Set the scope to 2 V/div and
20 ms/div.
7.1.4.2 Observe the output on the scope. Verify that the wave
shape is substantially as shown in Figure 9a. Then verify for
the other channel.
Figure 9a
7.1.4.3 Observe that the Impact display lights at least 10 out
of the 12 LEDs.
7.1.5 Mid-Band Test
7.1.5.1 Set the oscillator to 1 kHz at 0 dBV (1 V rms) and the
tone-burst oscillator to produce approximately an 8 ms burst
(8 cycles) followed by approximately 100 ms (100 cycles) of at
least 20 dB attenuation. Set the scope for 2 V/div and
2ms/div.

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7.1.5.2 Observe the output on the scope. Verify that the wave
shape is substantially as shown in Fig. 9b, and confirm for
the other channel.
Figure 9c
7.1.6.3 Observe that the Impact display lights at least 10 out
of the 12 LEDs.
Figure 9b
7.1.5.3 Observe that the Impact display lights at least 10
out of the 12 LEDs.
7.1.6 High-Band Test
7.1.6.1 Set the oscillator to 10 kHz at 0 dBV and the tone-
burst oscillator to produce approximately a 3.2
ms burst
(32 cycles) followed by approximately 100 ms (1000
cycles) of
at least 20 dB attenuation. Set the scope for 2
V/div and
0.5 ms/div.
7.1.6.2 Observe the
output on the scope. Verify that the
wave shape is substantially as shown in Fig.
9c; confirm for
the other channel.

Model 3BX III Service Manual
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7.2 Expansion Magnitude and Timing
7.2.1 Set the unit as follows:
Expansion Maximum
Impact Restoration Minimum
Transition Level Middle
Pre/Post Pre
Source/Tape Source
Bypass Out (i.e., not set)
7.2.2 Connect the oscillator and tone-burst generator as in 7.1.2
and 7.1.3 above.
7.2.3 Low-Band Test
7.2.3.1 Set the sine-wave oscillator and the tone-burst
generator as in 7.1.4.1. (In this test, the 20 dB attenuation
figure is critical to achieving results consistent with the
pictures presented here.) Set the scope for 1 V/div and
50 ms/div.
7.2.3.2 Observe the output on the scope. Verify that the
shape is substantially as shown in Fig. 10a; confirm for the
other channel.
Figure 10a
7.2.3.3 Observe that the Low-Band display lights at least to
the 4th red LED during the burst and returns to the 2nd yellow
LED between bursts.
7.2.4 Mid-Band Test
7.2.4.1 Set the sine-wave oscillator and the tone-burst
generator as in 7.1.5.1 except for 32 cycles on instead of 8.
As before, the 20 dB figure is critical to achieving results
consistent with the pictures presented here. Set the scope to
1V/div and 10 ms/div.
7.2.4.2 Observe the output on the scope. Verify that the wave
shape is substantially as shown in Fig. 10b, next page, and
then verify for the other channel.