Recently I purchased a DBX_3BX on EBAY and it came with the service manual. After a couple of weeks I finally figured out the schematic and the adjustments in the service manual. First I noticed the schematic and the manual didn't jive. The IC chip numbers were not correct for the measurements that were to be made.
When I recapped my DBX-3BX then I figured out what was wrong with the schematic.
Here is my calibration scheme!
Set the transistion control fully left at 1.0.
RMS chip adjustments.
Input a Low frequency 100HZ signal on both right and left channels.at -15dBV or .178 volts AC RMS.
Check the voltage at C10 (4.7uF NP cap)(right side) capacitor input for symmetry and adjust R195 for a balance in signal output.
Check the voltage at C20 (4.7uF NP cap)(right side) capacitor input for symmetry and adjust R243 for a balance in signal output.
Input a Mid frequency 1KHZ signal on both right and left channels.at -10dBV or .316 volts AC RMS.
Check the voltage at C10 (4.7uF NP cap)(right side) capacitor input for symmetry and adjust R230 for a balance in signal.
Check the voltage at C20 (4.7uF NP cap)(right side) capacitor input for symmetry and adjust R163 for a balance in signal.
Input a High frequency 1KHZ signal on both right and left channels.at -10dBV or .316 volts AC RMS.
Check the voltage at C10 (4.7uF NP cap)(right side) capacitor input for symmetry and adjust R214 for a balance in signal.
Check the voltage at C20 (4.7uF NP cap)(left side) capacitor input for symmetry and adjust R179for a balance in signal.
Then adjust the transition level for each frequency (Low, Mid,and Hi).
R58 is the low freq. level at 100HZ input and -15dBV (0.178 VAC RMS) input this should be adjusted for 0 volts.
R76 is the mid freq. level at 1KHZ input and -10dBV (0.316 VAC) input this should be adjusted for 0 volts.
R98 is the high freq. level at 10KHZ input and -27dBV (0.045 VAC) input this should be adjusted for 0 volts.
R98 is the high frequency transition level adjustment and this is where I disagree and think the highs should be dialed down a bit. Instead of zero volts I have adjusted mine slightly above this by turning the pot CCW while listening to an album and reducing the enhancement. Set the Expansion control to 1.5 and adjust to your liking.
Voltage Controlled Amplifier Level Adjustments
Set the input to 1 VAC for left.
Low freq(100hz) adjust R69 for 1 VAC output
Mid freq(1000hz) adjust R90 for 1 VAC output
High freq(10000hz) adjust R109for 1 VAC output
Then the right channel.
Low freq(100hz) adjust R70 for 1 VAC output
Mid freq(1000hz) adjust R91 for 1 VAC output
High freq(10000hz) adjust R110for 1 VAC output
Distortion is the next test and usually is fine after doing these adjustments correctly. My results were better than the 0.03% published.
