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Fitting the NET Audio 303 MK3 Boards to your Quad 303.

Remove the bottom and top covers to expose the chassis and internal circuit boards.

Carefully de-solder each wire from the 303 amplifier circuit boards so that there are two sets of nine wires as per the photo above.

Each channel has either a white or red stripe on the wires but the colours are the same for each channel.

If you have the NET Audio 303 power transistor conversion MJ15003/MJ15004 (recommended for best sound quality) and have connected them as per this photograph then to check that the wiring of these is correct. They should measure as in the list below:-

On either left or right channel using a diode or resistance checker you can test for correct connection as described on this page 
0.45V is a typical reading on a diode checker or would be low resistance on an analogue multimeter. 
Open Circuit should be high resistance on a multimeter or overload on a diode checker - i.e. no current is flowing. 
0R in the table represents very low resistance, less than 1 ohm and is effectively the resistance of the connecting wire. 
-ve is the black lead on the tester and +ve is the red lead on the tester. 
The colour specified is the wire colour being tested, the number in brackets is the wire number reading from let to right on each amp channel.
O/C is Open Circuit - no or very high resistance.

Red (1) -ve to main PSU caps positive terminal +ve = 0R
Red (2) -ve to Purple (3) +ve = 0.45V
Purple (3) +ve to Black (4) -ve = 0.45V
Green (5) -ve to Speaker capacitor positive terminal +ve = 0R
Blue (6) +ve to Orange (7) -ve = 0.45V
Orange (7) -ve to Brown (8) +ve = 0.45V
Brown (8) -ve to Middle transistor case on the rear heatsink +ve = 0R
Brown (9) +ve to speaker negative terminal -ve = 0R
Red (2) +ve to Black (4) -ve = O/C
Blue (6) +ve to Brown (8) -ve = O/C

Now check each transistor case to the heatsink bolt holes (the ones that bolt the heatsink to the chassis) and make sure no current is flowing otherwise one of the transistors is shorted to the heatsink and the short should be removed before connecting the NET Audio MK3 circuit boards.

If the above tests are correct on each channel then it is OK to solder in the new NET Audio 303 Circuit Boards.

On the NET Audio 303 MK3 boards the solder terminals are marked with numbers - the wires should be soldered into the holes so that each coloured wire is in its correct hole:-

Hole 1 = Red
Hole 2 = Red
Hole 3 = Purple
Hole 4 = Black
Hole 5 = Green
Hole 6 = Blue
Hole 7 = Orange
Hole 8 = Brown
Hole 9 = Brown

Hole 10 = Input Screen Wire
Hole 11 = Input Signal Wire (White or Red)

When the boards are soldered in they should look like this:-

Make sure that VR1 (blue pot in the middle of the board) is turned clockwise until it clicks. It is a 25 turn pot so may take quite a few turns. Now adjust VR2 to its middle position. 

Check all your wiring again and look for solder blobs etc - lightly clean the board with a dry toothbrush or similar on the track side to make sure all invisible solder residue has been removed that may cause a short circuit..

Now switch the 303 on. Then make sure the PSU voltage is 67V by measuring on the pin 1 and pin 9 of one of the amplifier boards. A standard Quad PSU board's output voltage will drift as it warms up so it is best to leave the amplifier on for a few minutes before adjustments are made. A NET Audio PSU board is set to the correct voltage when tested and should not need adjustment.

Secondly adjust the standing output voltage on the output of the amplifier. This should be set to 33V by adjusting VR2 and measuring the voltage on pin 5 of the amplifier board to the chassis of the 303. This is either the green/white or green/red wire to ground.

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 R19 and R20 which needs checking (the two large 0.33 ohm resistors in the middle of the board near the wiring harness). Measure the voltage across these two resistors on the channel which you are adjusting. The voltage should be adjusted to 14mV across the points X and X marked on the photo below directly onto the conductors from the resistors not at the wire connections. If the voltage is not correct then adjust VR1 anticlockwise on the PCB until it has reached 14mV on points X to X, let it settle and warm up over 10 minutes or so continuously checking and adjusting to keep it at 14mV across the resistors. The pot will require around 9 to 12 turns before the bias will start to increase so be carefull when adjusting the pot and monitor the reading on the voltmeter which is measuring the voltage between X and X.

The multi turn pots are delicate - the 303 boards are supplied with minimum bias settings. Only turn the pot anticlockwise until the bias point has been reached. If the pot is turned clockwise at the start point too many times it may damage the pot.

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.

When the bias pot is adjusted in a clockwise direction (looking from the parts side of the board) it will decrease the bias current. If you require minimum bias current then the bias pot should be turned to its fully clockwise direction looking from the component side of the board until the pot adjuster goes stiff. This is about twice the stiffness it requires to adjust it normally. Be carefull not to overturn the adjuster at the end points as it can damage the pot. The boards are supplied tested and adjusted to minimum bias so only positive (anticlockwise) adjustment is required when the bias is being set for the first time.