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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.

Summary of Contents for 3BX III

Page 1: ...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...

Page 2: ......

Page 3: ...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...

Page 4: ...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...

Page 5: ...ates 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...

Page 6: ...gnment 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 cond...

Page 7: ...oltages 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 cri...

Page 8: ...nput 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 cap...

Page 9: ...sion 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 a...

Page 10: ...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 Lo...

Page 11: ...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 differe...

Page 12: ...h 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 an...

Page 13: ...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 SUPP...

Page 14: ...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 on...

Page 15: ...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 Conne...

Page 16: ...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...

Page 17: ...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 outp...

Page 18: ...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...

Page 19: ...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...

Page 20: ...r 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 di...

Page 21: ...AC voltmeter 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...

Page 22: ...ion 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 Co...

Page 23: ...f 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 t...

Page 24: ...776R ELECT CAP 4 7uF RAD NP 25V 20 510CEVE4R7RAN 127161 2 C710 15 721 722 730 732 739 740 742 747 751 753 755 757 760 767 769 770 775 807 ELECT CAP 1uF 50V 20 510CEVGOIOALX 127084 34 C706L 706R 707L 7...

Page 25: ...2162 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 CAR...

Page 26: ...5 51ORD25PJ221X 054221 4 R807 RESISTOR CARBON FILM 1 4W 2 2K ohm 5 51ORD25PJ222X 054222 1 R7F3 737 749 764 RESISTOR CARBON FILE 1 4W 22K ohm 5 SIORD25PJ223X 054223 4 R7E8 RESISTOR CARBON FILM 1 4W 22...

Page 27: ...0K 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 M...

Page 28: ...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 J...

Page 29: ...217 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...

Page 30: ...Model 3BX III Service Manual 26 SCHEMATIC...

Page 31: ...Model 3BX III Service Manual 27 CIRCUIT BOARD COMPONENT SIDE TOP...

Page 32: ...Model 3BX III Service Manual 28 CIRCUIT BOARD NON COMPONENT SIDE BOTTOM...

Page 33: ...Model 3BX III Service Manual 29 LED SCHEMATIC PCB PINOUTS...

Page 34: ...Model 3BX III Service Manual 30 MECHANICAL ASSEMBLY EXPLODED VIEW...

Page 35: ...re 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...

Page 36: ...Model 3BX III Service Manual 32 NOTES...

Page 37: ...3853C 600307 Printed in USA...

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