Download Excel Test Report Spreadsheet.

Power Supply

1. Run the unit for 5-10 minutes to stabilise.

2. Check the power supply rails. They should be +30VDC (± 0.5V) and -10VDC (± 0.5V).

‘Q’ Bias Adjustment

1. Set the controls as follows:

Control
Set to
Input fully CCW
Output fully CW
Attack fully CCW (switched to OFF)
Release fully CW
Compression ratio 20:1
Meter mode +4dB
Q bias trimmer fully CCW

 

2. Apply a 0.775Vrms (0dBu) 1kHz sine wave signal to the input.

3. Turn the input control CW until the VU meter reads +1VU on the meter.

4. Slowly turn the Q bias trimmer CW until a drop of 1dB occurs and the meter reads 0VU. The FET is now slightly in conduction.

 

Gain Reduction Meter Tracking

1. Set the controls as follows:

Control
Set to
Input middle
Output fully CCW
Attack fully CCW (switched to OFF)
Release fully CW
Compression ratio 20:1
Meter mode GR
VR54 trimmer 1/4 turn from fully CCW

 

2. With no signal applied, adjust VR55 so that the meter reads 0VU (gain reduction reading).

3. Apply a 0.245Vrms (-10dBu) 1kHz sine wave signal to the input.

4. Switch the meter mode to +4dB.

5. Turn the output level control CW until the meter reads 0VU.

6. Turn the attack control ON (CW) and observe the drop in the meter reading.

7. Adjust the input level control until -10dB is indicated on the VU meter.

8. Turn the attack control OFF (CCW) and re-adjust the output level control for 0VU meter reading if necessary.

9. Repeat steps 7 and 8 until the output drops 10dB whenever the attack control is turned ON.

10.Without adjusting the input or output controls, select the meter mode for GR.

11.Adjust VR54 until the GR meter reading reads -10dB when the attack control is ON.

12.Adjust VR55 until the GR meter reads 0VU when the attack control is OFF.

13.Repeat steps 11 and 12 until the results are consistent.

 

GR Meter Zero

1. VR55 may be adjusted slightly to zero the GR meter without affecting the other controls.

 

Signal Preamp Linearity

1. Set the controls as follows:

Control
Set to
Input fully CW
Output halfway
Attack fully CCW (switched to OFF)
Release fully CW
Compression ratio 20:1
Meter mode GR

 

2. Apply a 0.0245Vrms (-30dBu) 500Hz sine wave signal to the input.

3. Measure the THD of the output signal and adjust VR16 until the minimum amount of distortion is achieved.