Tuesday, 28 January 2014

Channel Testing

Now we have some channels fully built, it is time to test and calibrate them. The picture above shows the set up used to do this. It shows the main sub-rack from the rear. You can see the three motherboards into which the modules plug, each motherboard taking two modules. Towards the bottom of each motherboard you can see the power bus wiring that links the heater and HT voltages between motherboards which are made using heavy duty terminal blocks. All the signal connections to and from each module are made to Molex connectors soldered to the rear of the motherboards. Connections from these to other parts of the mixer are made using crimp connection Molex plugs (see earlier entry on crimping).

For module testing we first need an input connection. This comes from the XLR connector labelled 'Input' which connects to a 3 way Molex crimp connector that can be plugged into the mic or line input Molex on the motherboard. The output comes from another Molex crimp connector to a VTB2291 output transformer and then to the XLR labelled 'Output'. These two are all you need to test the first amplifier stage in a channel. You plug the input into either the mic or line input and take the output from OUT1 of the module.

In normal operation the OUT1 signal goes to the channel fader and the return from the fader is connected to the Fader input on the motherboard. This input is then connected internally to the input of the EQ section of the channel and the EQ output goes to the input of the second amplifier (IN2) and the channel output comes from OUT2. So for testing the whole module you need the additional connections shown to and from a fader and the VTB2291 is connected to OUT2 instead of OUT1. Thanks to Tony Reeves of  MTR for the bargain 100mm throw faders at just £2 each

I started testing the Helios channel module and the first stage worked fine. However, with the fader connected, the signal drops a lot when the EQ is switched in. Clearly there is a fault on the EQ board that needs to be fixed.

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