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)
To 50% increase,
Expansion maximum 12 dB upward
and 20 dB downward
To +12 dB (upward
Impact restoration only), program-
dependent
+0.5 dB 20 Hz – 20 kHz,
Frequency response
any setting
Dynamic range 107 dB
Total harmonic distortion (THD), no
0.15%
expansion
Intermodulation distortion (IMD) IHF or
0.1%, any setting
SMPTE
Equivalent input noise –90 dBV
Program-dependent,
Attack rates
optimized for each band
Linear expander program
-dependent, optimized;
Release rates
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.
 Model 3BX III Service Manual

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
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.

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 Model 3BX III Service Manual

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.

Approx.
Input Voltage Input Frequency Rms IC Test Point
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.
Pin Approx
Input Voltage Input Frequency Op-amp
Number 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

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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
a time 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.

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 Model 3BX III Service Manual

All photos:
All front controls @ maximum;
Rear trim (Impact Release Rate) centred
a & b photos: c photos;
300 Hz tone-burst @ 316 mV rms (447 mV peak), 3 kHz tone-burst @ 316 mV rms (447 mV peak),
128 cycles on, 172 cycles off, 0.2 s/div 128 cycles on, 172 cycles off, 20 ms/div

4.a.1 4.b.1 4.c.1

Top: Low-pass filter output, U702L, Top: Mid-band summer output, U701L, Top: High-pass filter output, U702L,
pin 1, or U702R, pin 7, 0.2 V/div; pin 7, or U701R, pin 1, 0.5 V/div; pin 7, or U702R pin 1, 0.2 V/div;
Bottom: Low-band rms-detector output, Bottom: Mid-band rms-detector output, Bottom: High-band rms-detector output,
U707, pin 7 (TP9), 0.1 V/div. U708, pin 7 (TP11), 50 mV/div. U709, pin 7 (TP13), 0.1 V/div.

4.a.2 4.b.2 4.c.2

Top: Low-band rms-buffer output, Top: Mid-band rms-buffer output, Top: High-band rms-buffer output,
U710, pin 7 (TP10), 1 V/div; U711, pin 7 (TP12), 1 V/div; U722, pin 7 (TP14), 1 V/div;
Bottom: Low-band impact-detector Bottom: Mid-band impact-detector Bottom: High-band impact-detector
output, U713, pin 5, 0.2 V/div. output, U714, pin 3, 0.5 V/div. output, U717, pin 5, 0.2 V/div.

4.a.3 4.b.3 4.c.3

Top: Output, low-band impact time- Top: Output. mid-band impact time- Top: Output, high-band impact time-
constants, U713, pin 1, 1 V/div; constants, U715, pin 7, 1 V/div; constants, U717, pin 1, 0.5 V/div;
Bottom: Low-band VCA control voltage, Bottom: Mid-band VCA control voltage, Bottom: High-band VCA control voltage,
U706L or R, pin 3 (TP3 or 4), 0.1 V/div. U705L or R, pin 3 (TP5 or 6), 0.1 V/div. U704L or R, pin 3 (TP7 or 8), 0.1 V/div.

Figure 4
4.a & b.4 4.c.4
Left or right output, 5 V/div Left or right output, 0.5 V/div

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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 (see Fig. 4)
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 (again see Fig. 4)
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
a typical LED display. The control
voltages from each of the three
sections of the Impact-Restoration
control are summed in 1/2-U719 to
produce 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

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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 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.

Figure 6, Power Supply

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.

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ALIGNMENT PROCEDURE

INPUTS OUTPUTS

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)

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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
0 dBV +541 mV dc ±50 mV
-10 dBV +1080 mV dc ±100 mV
-20 dBV +1620 mV dc ±150 mV

Wrong:

Right:

Figures 7a 7b 7c

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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
5 mV/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
1 kHz, +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

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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 (Fig. 8, below)
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
2 ms/div.
5.1.4 Adjust VR701L for minimum signal feed-through; see Fig. 8.

Wrong Figure 8 Right

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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.

– 13 –
 Model 3BX III Service Manual

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
2 ms/div.

– 14 –
 Model 3BX III Service Manual

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 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.

Figure 9c

7.1.6.3 Observe that the Impact display lights at least 10 out

of the 12 LEDs.

– 15 –
 Model 3BX III Service Manual

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
1 V/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.

– 16 –
 Model 3BX III Service Manual

Figure 10b
7.2.4.3 Observe that the mid-band display lights at least to
the 4th red LED during the burst and returns to the 1st red LED
between bursts.
7.2.5 High-Band Test
7.2.5.1 Set the sine-wave oscillator and the tone-burst
generator as in 7.1.6.1. As in the low-band test, the 20 dB
figure is critical to achieving results consistent with the
pictures presented here. Set the scope to 2 V/div and
2 ms/div.
7.2.5.2 Observe the output on the scope. Verify that the wave
shape is substantially as shown in Fig. 10c, and then confirm
for the other channel.

Figure 10c
7.2.5.3 Observe that the high-band display lights at least to
the 5th red LED during the burst and returns to the 1st red LED
between bursts.
7.3 Expansion Tracking
7.3.1 Set the unit as in 7.2.1
7.3.2 Connect the sine-wave oscillator to the left and right inputs
of the unit.
7.3.3 Connect the output of one channel to an AC voltmeter.

– 17 –
 Model 3BX III Service Manual

7.3.4 Low-Band Tests

7.3.4.1 Set the oscillator to 100 Hz at 0 dBV (1 V rms).
7.3.4.2 Adjust the Transition-Level control for 0 dBV at the
3BX III output.
7.3.4.3 Reduce the oscillator output to –10 dBV (316 mV rms).
7.3.4.4 Verify that the 3BX III output level decreases to
-15 dBV (178 mV rms) ±1 dB.
7.3.4.5 Reduce the oscillator output to –20 dBV (100 mV rms).
7.3.4.6 Verify that the 3BX III output level decreases to
-30 dBV (31.6 mV rms) ±2 dB.
7.3.4.7 Repeat for the other channel.
7.3.5. Mid-Band and High-Band Tests
Repeat the steps in 7.3.4, but use 1 kHz for the mid-band and
10 kHz for the high-band.
7.4 Distortion
7.4.1 Set the unit as follows:
Expansion Minimum
Impact Restoration Minimum
Transition Level Mid-Point
Pre/Post Pre
Source/Tape Source
Bypass Out (not set)

7.4.2 Connect the sine-wave oscillator to the unit’s left and right
inputs.
7.4.3 Connect the output of one channel of the 3BX III to a
distortion analyser or THD meter.
7.4.4 Low-Band Distortion
7.4.4.1 Set the oscillator to 100 Hz at 0 dBV (1 V rms).
7.4.4.2 Verify that the THD is less than 0.15%.
7.4.4.3 Repeat for the other channel.
7.4.4.4 Now set the Expansion and Impact-Restoration controls
to their maximum positions.
7.4.4.5 Adjust the Transition-Level control for 0 dBV at the
3BX III output.
7.4.4.6 Verify that the THD is less than 0.25%.
7.4.4.7 Repeat for the other channel.
7.4.5 Mid-Band Distortion
Repeat the steps in 7.4.4 using 1 kHz. The first (minimum-
position) THD reading should be 0.15%, and the second 0.2%.
Don’t forget to do the second channel.

– 18 –
 Model 3BX III Service Manual

7.4.6 High-Band Distortion

Repeat the steps in 7.4.5 using 10 kHz at –10 dBV (316 mV
rms).
7.5 Noise
7.5.1 Set the unit as in 7.4.1
7.5.2 Disconnect all sources from the 3BX III and connect 1 k-ohm
resistors from the input terminals to the input ground (RCA, or
phono [pin], plugs with 1 k resistor terminations are ideal).
7.5.3 Connect one channel output of the 3BX III to the input of the
rms-responding AC voltmeter.
7.5.4 Verify that the noise level is less than –85 dBV unweighted
or –90 dBV A-weighted. (0 dBV is 1 V rms.)
7.6 Muting Check
7.6.1 Set the unit as in 7.4.1
7.6.2 Connect the sine-wave oscillator, set to 100 Hz at 0 dBV (1 V
rms), to both inputs of the unit.
7.6.3 Connect the oscilloscope to the output of one channel of the
unit, with its front controls turned down. Set scope to 0.5 V/div
and 100 ms/div.
7.6.4 Power the unit up and down a few times to confirm that the
output-mute circuitry is working, as shown in Fig. 11.
7.6.5 Check the other channel, too.

Power on Figure 11 Power off

– 19 –
 Model 3BX III Service Manual

MODEL 3BX III PRINTED CIRCUIT BOARD ASSEMBLY

REFERENCE
DESCRIPTION PRODUCT CODE DBX P/N QTY.
DESIGNATION
LED CONNECTOR CORD ASSEMBLY, M” LEADS 510ACCNL31ULA 310472 5
POWER LED CONNECTOR CORD ASSEMBLY SIOACC9L7SULA 320215 1
CONNECTOR RED, ASSEMBLY 510ACZZ243CFA 320216 1
HEX NUT M3 x N-P S-ZMC 510BN11CL30NSZ 310626 2
PAN READ RIVET 43 x 4.5 QCK NYLON-B SIOBRP3045QNB 310467 2
PAN HEAD SCREW M3 x 8 S-ZMC 510BSPP3008NZ 310628 3
CND WASHER M3 x 6 x .2T BS-NI 510BWT30602BN 310660 1
C701L, 701R CERAMIC CAP. 100pF RAD DC 50V, 10% NPO OR S2L 510CCGB101JOT 121258 2
C709L, 709R, CERAMIC CAP. 33pF RAD DC 50V, 10%, NPO 51OCCGB330JOT 121174 4
777, 778
CBOI-804 ELECT CAP 470uF RAD 35V, +50/-10% SIOCKAF471ALX 127490 4
C719, 728, ELECT CAP 10uF RAD 25V. +50/-10% 510CEVE100ALX 127203 6
738, 805, 806,
808
C720 ELECT CAP 22uF RAD 25V, 10% (MAY BE NP) 510CEVE220ALX 127264 1
C729 ELECT CAP 3.3uF RAD 25V, 10% 51OCEVE3R3ALX 127100 1
C776L, 776R ELECT CAP 4.7uF RAD NP, 25V, 20% 510CEVE4R7RAN 127161 2
C710-15, 721, ELECT CAP 1uF 50V, 20% 510CEVGOIOALX 127084 34
722, 730, 732,
739, 740, 742,
747-751, 753,
755-757, 760-
767, 769, 770,
775, 807
C706L, 706R, ELECT CAP 1uF RAD NP 50V, 10% 510CEVCO1ONAN 127087 4
707L, 707R
C774 ELECT CAP 1.5uF RAD 50V, 20% 510CEVG1R5ZMN 177088 1
C717, 718, CERAMIC CAP 1000pF 50V 10% Y5E 51OCKGB102KBT 121432 6
726, 727, 736,
737
C733, 735, FILM CAP 1uF RAD 50V, 5% 510CQVB104JTW 123149 5
745, 746, 752
C754, 758 FILM CAP .047uF RAD 50V, 5% 510CQV8473JTN 123095 2
C702L, 702R FILM CAP .068uF RAD 50V, 5% 510CQV8683JTN 123123 2
C708L, 708R, FILM CAP .33uF RAD, 50V, 5% 510CQ48334JEH 123213 8
716, 723, 725.
731, 741, 768
C734, 759 FILM CAP .01uF RAD 50V, 5% 510CQ8B103JTN 123025 2
C704L, 704R, FILM CAP 3300pF RAD 50V, 5% 510CQ85332JTN 123350 5
705L, 705R,
744
C703L, 703R, FILM CAP .033uF RAD 50V, 5% SIOCQ8R333JTN 123075 4
724, 743
C771 FILM CAP 5600pF RAD 50V, 5% SIOCQ88682JTN 123098 1
HEAT SINK 2335389 R81071-004 SIONS736ADOOI 310629 1
TERMINAL SIONW20155001 310630 20
SHORT JUMPER 10 mm SIOMW4OICX006 110085 183
BUSHING ASSEMBLY 2666116 TB-30012675136 R80010-019 51OWBR2666116 310679 4 PR
SIONIRR2675136

– 20 –
 Model 3BX III Service Manual

REFERENCE
DESCRIPTION PRODUCT CODE DBX P/N QTY.
DESIGNATION
D701-727, 807 SILICON DIODE MA150, OR 1N4148 510QDSMA150XN 140031 28
D805, 806 SILICON DIODE SR1K-4LF OR 1N4003 510QDSSR1K4AP 140022 2
D801, SILICON DIODE W02M 5l0QDSW02MXXG 140008 1
U712, 720 IC, OP-AMP, LM301AN 510QQM00301AL 146003 2
ULOI-12 IC, QUAD COMPARATOR, LM339K 510QQM00339A& 146271 12
U701L, 701R, IC, OP-AMP, LP353N 510QQM00353NL 146241 5
702L, U702R,
703
V721 IC, DUAL COMPARATOR, LM393N 510QQM00393A& 146273 1
U704L, 704R, IC, VOLTAGE CONTROLLED AMPLIFIER, UPC125282 510QQM01252AA 146732 6
705L, 705R,
706L, 706R
111707-709 IC, RMS LEVEL DETECTOR, UPC1253H2 510QQM01253AA 146742 3
U710, 711, IC, DUAL OP-AMP, 4558P 510QQM04558A& 146061 10
713-719, 722
U801 IC, VOLTAGE REGULATOR, +12V, UA7812 510QQM07812CJ 146361 1
V802 IC, VOLTAGE REGULATOR, -12V, UA7912 510QQM07912BJ 146362 1
Q803 PNP TRANSISTOR 2SA1015 Y 510QTA1015XAT 142162 1
QL01-LO8 PNP TRANSISTOR 2SA1020 Y SIOQTA102OXAT 142171 8
Q701-703 NPN TRANSISTOR 2SC1815 Y 510QTC1815XAT 142172 3
Q801, 802 PET TRANSISTOR 2SKI70 510QTX0170XAT 142173 2
R746, 760, 774 RESISTOR, CARBON FILM 1/4W 1K ohm 5% 51ORD25PJIO2X 054102 3
R701L, 701R, RESISTOR, CARBON FILM 1/4W 10K ohm 5% SIORD2SPJIO3X 054103 16
727L, 727R,
728L, 728R,
729L, 729R,
779, 780, 786.
787, 796, 797,
801, 805
R7B1L, 7B1R, RESISTOR, CARBON FILM 1/4W 100K ohm 5% SIORD25PJ104X 054104 10
7E4-7E6,
R702L. 702R,
733L. 733R,
790
R7F7, 747, RESISTOR, CARBON FILM 1/4W 1M ohm 5% 51ORD2SPJIOSX 054105 6
773, 803, 804,
806
RL01, L13, RESISTOR, CARBON FILM 1/4W 120 ohm 5% 510RD25PJ121X 054121 8
L23, L35, L45,
L57, L67, L76
R7F4 RESISTOR, CARBON FILM 1/4W 1.2M ohm 5% 51ORD2SPJI2SX 054125 1
R777 RESISTOR, CARBON FILM 1/4W 130K ohm 5% SIORD25PJ134X 054134 1
RL06, L07. RESISTOR, CARBON FILM 1/4W 1.5K ohm 5% 51ORD25PJ152X 054152 11
L28, L29, L51,
L50, L51, L70,
L71, L85,
L7D4, L7D5
R7A9, 7C2, 798 RESISTOR. CARBON FILM 1/4W 15K ohm 5% 510RD25PJ153X 054153 3
R7A3, 7A4, RESISTOR, CARBON FILM 1/4W 150K ohm 5% 51ORD2SPJ154X 054154 12
7B5, 7B6, 7C7,
7C8, 721L,
721R, 722L,
722R, 723L,
723R

– 21 –
 Model 3BX III Service Manual

REFERENCE
DESCRIPTION PRODUCT CODE DBX P/N QTY.
DESIGNATION
R751 RESISTOR, CARBON FILM 1/4W 1.5M ohm 5% 51ORD2SPJISSX 054155 1
RL02, L14, RESISTOR, CARBON FILM 1/4W 1.6K ohm 5% 51ORD25PJ162X 054162 8
L24, L36, L46,
L58, L68, L77
R703L, 703R, RESISTOR, CARBON FILM 1/4W 16K ohm 5% SIORD2SPJ163X 054163 6
704L, 704R,
708L, 708R
RL75 RESISTOR, CARBON FILM 1/4W 160K ohm 5% 21ORD25PJ164X 054164 1
R7D8 RESISTOR, CARBON FILM 1/4W 1.8K ohm 5% 51ORD25PJ182X 054182 1
RL03, L15, RESISTOR, CARBON FILM 1/4W 18K ohm 5% 51ORD25PJ183X 054183 8
L25, L37, L47,
L59, L69, L78
R745, 758, 772 RESISTOR, CARBON FILM 1/4W 20 ohm 5% 51ORD25PJ200X 054200 3
RL08, L18, RESISTOR, CARBON FILM 1/4W 2K ohm 5% 51ORD2SPJ202X 054202 6
L30, L40, L52,
L62
R7A7, 7A8, RESISTOR, CARBON FILM 1/4W 20K ohm 5% SIORD2SPJ203X 054203 10
7B9, 7C1, 7D2,
7D3, 734L,
734R, 735L,
735R
R7F1 RESISTOR, CARBON FILM 1/4W 2M ohm 5% 51ORD2SPJ205X 054205 1
R731L, 731R, RESISTOR, CARBON FILM 1/4W 220 ohm 5% 51ORD25PJ221X 054221 4
732L, 732R
R807 RESISTOR, CARBON FILM 1/4W 2.2K ohm 5% 51ORD25PJ222X 054222 1
R7F3, 737. RESISTOR, CARBON FILE 1/4W 22K ohm 5% SIORD25PJ223X 054223 4
749, 764
R7E8 RESISTOR, CARBON FILM 1/4W 220K ohm 5% 51ORD25PJ224X 054224 1
R792 RESISTOR, CARBON FILM 1/4W 2.2M ohm 5% 51ORD25PJ225X 054225 1
R7D7 RESISTOR, CARBON FILM 1/4W 27K ohm 5% 51ORD2SPJ273X 054273 1
RL09, L19, RESISTOR, CARBON FILM 1/4W 3K ohm 5% 51ORD25PJ302X 054302 16
L20. L31. L41,
L42, L53, L63,
L64, L72, L73.
L74, 7C3, 7D6,
756, 770
R712L, 712R, RESISTOR, CARBON FILM 1/4W 30K ohm 5% SIORD25PJ303X 054303 7
713L, 713R,
714L, 714R,
802
RL04, L16, RESISTOR, CARBON FILM 1/4W 330 ohm 5% SIORD2SPJ331X 054331 6
L26, L38, L48,
L60
R7E1-7E3 RESISTOR, CARBON FILM 1/4W 3.3K ohm 5% 51ORD25PJ332X 054332 3
R7D9, 730L, RESISTOR, CARBON FILM 1/4W 33K ohm 5% 51ORD2SPJ333X 054333 5
730R, 783, 793
R759 RESISTOR, CARBON FILM 1/4W 360K ohm 5% 51ORD25PJ364X 054361 1
R7A2, 7B4, 7C6 RESISTOR, CARBON FILM 1/4W 390 ohm 5% 51ORD2SPJ391X 054391 3
R724L, 724R, RESISTOR, CARBON FILM 1/4W 3.9K ohm 5% 51ORD25PJ392X 054392 6
725L, 725R,
726L, 726R
R7A1, 7B2, RESISTOR, CARBON FILM 1/4W 39K ohm 5% 51ORD2SPJ393X 054393 9
7B3, 7C4, 7C5,
736, 748, 763,
799

– 22 –
 Model 3BX III Service Manual

REFERENCE
DESCRIPTION PRODUCT CODE DBX P/N QTY.
DESIGNATION
R761 RESISTOR, CARBON FILM 1/4W 390K ohm 5% 51ORD2SPJ394X 054394 1
RL22, L44, L66 RESISTOR, CARBON FILM 1/4W 430K ohm 5% SIORD25PJ434X 054434 3
718L, 718R, RESISTOR, CARBON FILM 1/4W 47 ohm 5% 51ORD2SPJ470X 054470 6
719L. 719R,
720L, 72OR
R7F6, 762, RESISTOR, CARBON FILM 1/4W 470 ohm 5% 51ORD2SPJ471X 054471 7
778, 781, 785,
788, 795
RL11, L12, RESISTOR, CARBON FILM 1/4W 4.7K ohm 5% 51ORD2SPJ472X 054472 6
L33, L34, L55,
L56
R707L, 707R, RESISTOR, CARBON FILM 1/4W 47K ohm 5% 51ORD2SPJ473X 054473 4
775, 791
RL10, L32, L54 RESISTOR, CARBON FILM 1/4W 470K ohm 5% 51ORD2SPJ474X 054474 3
R744, 757, 771 RESISTOR, CARBON FILM 1/4W 560K ohm 5% 51ORD25PJ564X 054564 3
RL21, L43, L65 RESISTOR, CARBON FILM 1/4W 6.2K ohm 5% 51ORD25PJ622X 054622 3
R7E7, 776, RESISTOR, CARBON FILM 1/4W 68K ohm 5% 51ORD25PJ683X 054683 4
784, 794
RL05, L17, RESISTOR, CARBON FILM 1/4W 750 ohm 5% 51ORD25PJ751X 054751 12
L27, L39, L49,
L61, L79-L84
R7F2 RESISTOR, CARBON FILM 1/4W 7.5K ohm 5% 51ORD25PJ752X 054752 1
R740, 752, 766 RESISTOR, CARBON FILM 1/4W 750K ohm 5% 51ORD2SPJ754X 054754 3
R741, 753, 767 RESISTOR, CARBON FILM 1/4W 82K ohm 5% SIORD25PJ823X 054823 3
R743, 755, 769 RESISTOR, CARBON FILM 1/4W 9.1K ohm 5% SIORD25PJ912X 054912 3
R742, 754, 768 RESISTOR, METAL OR CARBON FILM 1/4W 22M 5% SIDRGQANJ226K 054226 3
R789 RESISTOR, METAL OR CARBON FILM 1/4W 3M 5% 510RCQANJ305K 054305 1
R738, 739, RESISTOR, METAL OR CARBON FILM 1/4W 3.3M 5% SIORGQANJ335K 054335 4
750, 765
R782 RESISTOR, METAL OR CARBON FILM 1/4W 4.7M 5% 51ORGQANJ475K 054475 1
VR723 TRIM POT 47KB H1022C 51ORPBNB47301 070115 1
VR701L, 701R, TRIM POT 50K 1/2W TM8K 51ORPJNB50309 070068 19
702L, 702R,
703L, 703R,
704-709, 717-
722, 724
R715L, 715R, RESISTOR, METAL FILM 1/4W 33.0 ohm 1% OR 33.2 ohm 510RQBCF0330X 013329 6
716L, 716R, 1%
717L, 717R
R705L, 705R, RESISTOR, METAL FILM 1/4W 10.0K 1% 510RQSCF1002X 011002 8
709L, 709R,
710L 710R
711L, 711R
R706L, 706R RESISTOR, METAL FILM 1/4W 20K 1% 510RQBCF2002X 012002 2
R7A5, 7A6, RESISTOR, METAL FILM 1/4W 511 1% 510RQBCFSIIOX 015110 6
7B7, 7B8, 7C9,
7D1
VR710 ROTARY POT (FRONT PANEL) SINGLE SELECTION 10K-B 51ORVNA103B31 (170283 1
16M/M 1AX-1G
VR711, 712 ROTARY POT (FRONT PANEL) TRIPLE SELECTION 10K-B SIORVSA103B01 070284 2
16M/M 1AX-2G K164B0
S701 PUSH BUTTON SWITCH ASSEMBLY 5105POSYFXO2A 250094 1
ZZ704 NYLON SHOULDER WASHER 510VF164DNO03 310443 2

– 23 –
 Model 3BX III Service Manual

REFERENCE
DESCRIPTION PRODUCT CODE DBX P/N QTY.
DESIGNATION
ZZ703 THERMALLY CONDUCTIVE TRANSISTOR INSULATOR 510VS223RH002 310417 2
WIRE JUMPER, 22 AWG, YELLOW 310184 A/R
WIRE JUMPER, 22 AWG, GREEN 310183 A/R
WIRE JUMPER, 22 AWG, BLUE 310185 A/R
WIRE JUMPER, 22 AWG, GREY 310188 A/R
WIRE JUMPER, 22 AWG, WHITE 310180 A/R
WIRE JUMPER, 22 AWG, BLACK 310181 A/R
WIRE JUMPER, 22 AWG, ORANGE 310187 A/R
WIRE JUMPER, 22 AWG, RED 310182 A/R
WIRE JUMPER, 22 AWG, BROWN 310186 A/R
WIRE JUMPER, 22 AWG, VIOLET 310189 A/R
SHIELDED WIRE, 2 CONDUCTOR 51OWWX845JXJJ 320121 A/R
CN704 LED WIRING JUNCTION JACK (8 PIN) SIOYJFOBS028Z 280222 1
CN701-703 LED WIRING JUNCTION JACK (10 PIN) 510YJPIOS044Z 280223 3
CN705-706 LED WIRING JUNCTION JACK (6 PIN) 510YJFO6S035Z 280224 2
J701-704 RCA (PIN) DUAL JACK 510YJP02S026U 280232 4
Y801 CONNECTOR CORD ASSEMBLY 510ACCNL76ULA 320042 1
Y802 CONNECTOR CORD ASSEMBLY SIOACCNL77ULA 320043 1
Y803 CONNECTOR CORD ASSEMBLY 510ACCNL78ULA 320044 1
Y804 CONNECTOR CORD ASSEMBLY 510ACCNL79ULA 320045 1
Y805 CONNECTOR CORD ASSEMBLY 510ACCNL8OULA 320046 1
Y806 CONNECTOR CORD ASSEMBLY 510ACCNL81ULA 320047 1
LED LN222RP, RED 510QLBLN222RN 140202 30
LED LN422YP, YELLOW 510QLBLN422YN 140203 18
LED CASE 510VB721SW002 210217 4

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 Model 3BX III Service Manual

MODEL 3BX III MAIN ASSEMBLY

REFERENCE
DESCRIPTION PRODUCT CODE DBX PM QTY
DESIGNATION
1-A ESCUTCHEON 510ME9SEAA024 290693 1
1-B BUTTON/LED GUIDE 510VF132SBOO4 210213 6
2, 3, 4 VR KNOB ASSEMBLY 510AVKNOB*175 380211 3
B15 PAN HEAD RIVET M3 x 4.5 QCK NYLON-B 510BRP3045QNB 310467 8
B14 THIN HEAD RIVET M2.4 x 5.5 STD AL 510BRU2455XAJ 310648 2
B2 BIND HEAD SCREW(+) M3 x 12 S-ZMC 510BSPB3012NZ 310649 4
B1 BIND HEAD SCREW(+) M4 x 10 S-BLK 510BSPB401ONB 310650 6
B4-6 BIND T.T. SCREW(+) M3 x 5 S-BLK S-T 510BTPBOO5ONB 310651 7
B7-12 BIND T.T. SCREW(+) M3 x 5 S-ZMC S-T 510BTP83005TZ 310437 13
B3 FLAT T.T. SCREW(+) M3 x 5 S-ZMC S-T 510BTPS3005TZ 310653 2
B16 FLAT L WASHER M3 x 7 x 0.5T S-NI 510BWM30705SN 310654 2
B17 IT LOCK WASHER M3 x 6.5 x 0.5T S-BLK 510BWU30655SB 310655 1
B13 IT WASHER M4 x 8.5 x 0.5T S-BLK 510BWU40855SB 310656 2
6 REAR PANEL 510MB951SE010 290695 1
17 BRACKET SW A 51OML464SLOOI 290696 1
16 BRACKET SWITCH 2255095 RB0010-011 51OML544SDO04 290554 1
19 COVER SWITCH 510MS625AA002 290698 1
5 CHASSIS 510MU984SLOO1 290699
8 COVER 510MU985SM001 290700 1
20 SERIAL NUMBER PLATE 510MVSSX10**1 290701 1
22 FOOT BASE ONLY 51ONBR9227577 380217 4
22 FOOT CUSHION ONLY 51ONBR2675132 380217 4
12 VOLUME CONTROL BUSHING 510VF177DBOO1 210219 3
15 POWER SWITCH BUTTON EXTENDER 510VK321SBOOI 210097 1
11 PUSH KNOB 510VN220SH004 210221 6
27 SPONGE SPACER 510VS336RF001 210222 3
25 SHEET TRANS 510VS667X8001 310664 1
B18 WASHER A4 510VS704VF002 310659 2
18 LED PCB HOLDER 510VS74ISBOO1 210230 1
AC01 AC CORD ASSEMBLY (UL/CSA AND PX UNITS ONLY) 510ACAC029ULA 320087 1
AC01 AC CORD ASSEMBLY (EUROPEAN UNITS ONLY) 510ACAC171EEA 320133 1
col CAPACITOR CERAMIC 1000pF RAD AC 125V OR 4700pF 51OCKDU102KBM 121539 1
UL/CSA
LD01, LD04 LED LN222RP RED 510QLBLN222RN 140202 2
LD02, 03, 05, 06 LED LN322GP GREEN 510QLBLN322GN 140204 4
SW1 POWER SWITCH (UL/CSA AND PX UNITS ONLY) 510SPO1AAW02A 250072 1
sol VOLTAGE SELECTOR SWITCH HSW0573-01-310 (UL/CSA AND 51OSS010226AJ 250096 1
PX UNITS ONLY)
PT1 POWER TRANSFORMER (110/220V, UL/CSA AND PX UNITS 510TPL41U001K 230035 1
ONLY)
ACB1 POWER CORD BUSHING SR3P-4 (UL/CSA AND PX UNITS 510VM27ONBOO1 310403 1
ONLY)
ACB1 POWER CORD BUSHING (EUROPEAN UNITS ONLY) 510VM27ONBOO4 310469 1
ZZ1 SOLDERLESS CONNECTOR CE-230 (UL/CSA AND PX UNITS 510ZZZ0000154 280089 1
ONLY)
PT1 POWER TRANSFORMER (220V EUROPEAN UNITS ONLY) 510TPL415001K 230036 1
SW1 POWER SWITCH (EUROPEAN UNITS ONLY) 510SPO1AASI7N 250073 1
POWER SWITCH COVER (EUROPEAN UNITS ONLY) 510VB443SBOOI 210091 1
RACK MOUNT ADAPTOR 51OML543AA003 290703 2

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Model 3BX III Service Manual

SCHEMATIC

– 26 –
 Model 3BX III Service Manual

CIRCUIT BOARD, COMPONENT SIDE (TOP)

– 27 –
Model 3BX III Service Manual

CIRCUIT BOARD, NON-COMPONENT SIDE (BOTTOM)

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Model 3BX III Service Manual

LED SCHEMATIC, PCB, PINOUTS

– 29 –
Model 3BX III Service Manual

MECHANICAL ASSEMBLY, EXPLODED VIEW

– 30 –
 Model 3BX III Service Manual

NOTES
Miscellaneous notes on servicing the dbx 3bx III Audio Expander
===============================================================

Removing the control knobs

--------------------------
Remove the case top.
Unplug the 5 LED connectors from the right hand side of the front panel.
Using a 1.5mm hex ball driver, untighten the Allen screws holding each knob to
the shafts.
(Note: for the right-most knob, you’ll need to align the grub screw with the
hole in the metal shield surrounding the pot shaft.)
Pull the knobs out from the front panel, taking care not to lose the plastic
‘grommet’.

Audible hum
-----------
If the unit produces audible hum in the output, it’s likely that some of the
power supply filter capacitors have dried out or leaked. It is recommended you
replace all 4 capacitors, even if some seem OK, since they’re all of a similar
age and likely to go the same way if not already. You’ll need 4 capacitors
470uF/35V with radial leads on 5mm pitch. If any of the capacitors have leaked
electrolyte, take care to clean all traces of corrosion from the PCB and its
components. A mild ammonia solution will assist in dissolving copper sulphate
(blueish) residues, followed by distilled water washing and thorough drying.

Daniel Ford
15 January 2006

– 31 –
 Model 3BX III Service Manual

NOTES

– 32 –
3853C-600307
Printed in USA