Saturday, 1 February 2014

Bus Testing

Using the channel testing technique described in the previous post, I soon identified a problem with the first Helios channel I tested. It was OK when the EQ was bypassed but when it was switched in, the level dropped by about 30dB. Clearly something was not right. A quick inspection of the PCB near where the signal is input to the EQ revealed a simple wiring error. I corrected this and the EQ worked as expected.

So, with the channel testing set up and working and with a working channel to play with, it was time to start testing the buses. To do this, all that was necessary was to wire up the buses to carry the mixed signal to the mix amplifiers, and to connect the Left and Right mix bus to the mix amplifiers via the master volume control. (There was no need to do this for the AUX sends as their master controls are already wired up in the AUX send module). The picture above shows the new test set up. The input, output and channel fader are as before. The added bus wiring and the master (stereo) level control are indicated on the picture above.

Before adding the stereo master level, I checked the AUX send buses. It was a good job I did because I soon discovered I had wired the AUX send module to the L&R master buses instead of the AUX send buses. Changing the connections of two wires in the AUX send module fixed this. I then connected the master level control and tested the stereo bus. The Smart Pan worked exactly as expected and I measured the Pan mid drop as 4.3dB, very close to the calculated value. In fact it all went very smoothly.

As part  of building the test set up, I had to do quite a lot of crimping and I have to say that, with practice, it becomes quite easy. You need to use the right sort of wire and I found that lightly tinning stranded wire was a great help but other than that it is becoming quite straightforward. I might at some point in the future revise the motherboard slightly to change the bus connectors from Molex to IDC for two reasons. First, it is easier to make a cable of an accurate length using IDC and secondly, it would allow me to include a ground wire between each pair of bus wires, thus reducing bus to bus capacitance and hence reducing high frequency cross talk. As yet I have no reason to suspect there is a high frequency cross talk problem but so far I have only been making tests at 1KHz.

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