Before you apply power for the first time, check the following:
- Make sure the components on the PCBs don't short. The layout is very tight. Check components arount the TO-3 heatsink carefully.
- Make sure you have good contact between all metal parts of the enclosure and mounting brackets. Check resistance between pin 1 of both XLRs and chassis. You should read less then 0.5 Ohm. If not, remove the paint more thoroughly. This is very important - if you have bad chassis connections anywhere, your unit will be much more sensitive to noises and may even oscillate at high gain settings.
Apply power and check the power LED. If it doesn't light up, something is wrong. Turn off the unit immediately and check the PCBs for shorts and correct components.
If you see the LED light up, check the unit to ensure there's no suspicious smell or smoke. In case you see or feel something, turn off the module immediately.
Hopefully, you didn't make any mistakes and the preamp is working fine. Here's the testing procedure that we suggest:
- Make sure all the switches are disengaged (levers down). Set the Trim control fully clockwise.
- Insert the 40 dB resistive pad between the line output of the signal generator and mic input of your 1290 preamp.
- Inject 1 kHz 0 dBu test tone to the input of the unit. Make sure the unit is passing clean and not distorted audio.
- Measure the current consumption of the module and adjust RV1b to read 130mA.
- Inject 40 Hz 0 dBu test tone. Measure THD and adjust RV1b for minimum distortion.
- Turn the Gain switch counter-clockwise and check that each step increases output by 5 dB.
- Test the Trim control by turning it fully counter-clockwise. Check that output level changes when you turn the knob. Return the Trim control to the fully clockwise position.
- Inject white noise 0 dBu test tone to the unit input. Put spectrum analyzer at the output and make sure that it shows flat line from 20 Hz to 20 kHz.
- Turn on the Phantom switch. Measure voltage between pins 1 and 2, and between pins 1 and 3. Voltages should be equal. Connect the condenser microphone and turn on the phantom power. Make sure the microphone is working.
- To check the Phase switch you will need to record a short piece of audio and observe the waveform at full zoom in your DAW. Make sure your unit ouputs signal in phase when the switch lever is down, and out of phase when the switch lever is up.
If the unit passes all these tests, it's time to put it to good use - making records!
Enjoy your Neve 1290!