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Quad 303 Setup ProcedureOn a standard Quad 303, there are five potentiometers which need adjusting to set the amplifier up for correct operation. These control the power supply voltage, the amplifier output standing voltage and the amplifier bias current. If you have just completed the complementary output stage modification then with the amplifier turned off, turn the bias pot RV101 so that it is at maximum resistance so that the amplifier will power up with no bias set i.e. maximum resistance from the base to emitter of TR107. Be very carefull that this is set to maximum resistance and not minimum otherwise the amplifier output stage may be destroyed. Maximum resistance on all 303s from serial number 115001 onwards is when the pot is turned fully anticlockwise looking from the component side of the board. If the amplifier has not been modified then the bias pot can be left as it is and you can continue from this point. Then set up the PSU voltage by adjusting the pot on the PSU board (RV200) until the output voltage is 67V. A standard Quad PSU boards output voltage will drift as it warms up so it is best to leave the amplifier on for 1/2 an hour before adjustments are made. Secondly adjust the standing output voltage on the output of the amplifier. This should be set to 33.5V by adjusting RV100 and measuring the voltage on pin 5 of the amplifier board. This is either the green/white or green/red wire. Please see diagram link at the bottom of the page. The bias is the standing current which passes through the output transistors when the amplifier is idling. It is the current through, or the voltage across R124 and R125 which needs checking (the two large 0.3 ohm resistors in the middle of the board near the wiring harness). Measure the voltage across one of these two resistors on the channel which you are adjusting. The current should be around 15mA through each resistor (about 5mV across the resistor or across the points marked in the photo below X to Z or X to Y). If it is not correct then adjust RV101 on the PCB until it is, let it settle, warm up over 10 minutes or so continuously checking and adjusting to keep it at 5mV across one of the resistors. Please make these adjustments in accordance with your experience as care is required with these measurements and adjustments as it can cause faults if done incorrectly. The bias can be set by adjusting pot RV101 untill the correct voltage is present between point X and Y or between points X and Z on top or underneath the board. When the bias pot is adjusted in a clockwise direction (looking from the solder side of the board) or an anticlockwise direction (looking from the component side of the board) will decrease the bias current. If you require minimum bias current then the bias pot should be turned to its fully anticlockwise direction looking from the component side of the board. If your 303 appears to be different from the unit shown you may have a pre 11500 serial number. Basic rule - If RV101 is adjusted so that maximum resistance is measured across the two pins at the flat end of the pot (the pot is D shaped) then this will be minimum bias on all known models of the 303.
Quad 303 amplifier schematic can be found here. The amplifier should now be ready to power up. Replace the lids and screws, reconnect to your Hi Fi. Should you have a fault, equivalent transistors for the 303 can be found here.
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