DBX-3BXIII-de-sm维修电路原理图.pdf
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1、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, Newt
2、on, Mass. USA. RadioFans.CN 收音机爱 好者资料库 RadioFans.CN 收音机爱 好者资料库 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 SUPPL
3、Y 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 M
4、ODEL 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 (
5、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)
6、 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 defi
7、ned 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 mixe
8、d 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 Serv
9、ice Manual 1 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 channels
10、 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
11、 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
12、 (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
13、 for a small attenuation (-0.83 dB) and a roll-off at 175 kHz. Figure 1, Signal Path Model 3BX III Service Manual 2 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
14、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 s
15、hows 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
16、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 VCAs gain in decibe
17、ls 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 p
18、in 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
19、) 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 rig
20、ht 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 wont be very much voltage at pin 6 of U703 even i
21、f 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
22、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 3 Finally, the signal passes to the switching circuitry. The switching allows the 3BX III signal p
23、ath (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 mak
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