Wednesday, 26 March 2014

Transformer Panel Wiring Complete

After fitting the large output transformers to the panel, the next step was to wire them up. First you need to wire the links on both the primary and secondary windings to set the correct ratio (2:1 in this case). Two links are required on the primary as the two windings are wired in parallel. They are the red and black wires shown on the picture below. The secondary needs only a single link to wire the two windings in series. This was done with yellow wire. The primary sides are connected to unbalanced amplifier outputs so these need a single core screened cable terminated in a two pole Molex KK connector to plug into the backplane PCB. These are the grey cables you can see.The secondaries are balanced and use a twin core screened cable (black cables). Only the hot and cold are connected to the transformer. The screen is not connected at the transformer end but only at the XLR end. The XLRs are not connected yet. They will be connected after the panel has been mounted in the mixer.

The four cable pairs on the right are the the direct outs of the four channel amps. The four on the left are the master bus L/R and AUX1/2 send bus outputs which come from the two Twin Line Amps fitted next to the four channel modules.

I have a simple convention I use to identify the hot and cold wires in the Van Damme twin screened cable I use. The two wires have white and blue insulation so I make the white hot  and the blue cold for obvious reasons.

Sunday, 23 March 2014

If Only..

I am not a fan of mechanics but there is the odd occasion when things go just right. Tonight I needed to drill the 16 holes for the VTB2291 transformers mentioned in the last post. I discovered I could set them two inches apart and start 3 inches from one end of the panel. Fortunately the ears of the transformer have slots rather than holes so you don't need to be dead on with the positioning of the holes - got to be a good thing with my mechanical skills. I marked it all out with a ruler and a Sharpie and whacked each intersection with my trusty hole punch. I then placed a transformer at each position to make sure all the punch marks lined up with the slots in the transformer mountings and then drilled sixteen 4mm holes. I de-burred them on both sides with my old three eighths inch drill and attached the transformers using a locking nut on each to make sure they held firm. Job done in less than an hour. If only all mechanics were that straightforward. Here's the result, ready to be wired up:

Saturday, 22 March 2014

Quart Into A Pint Pot

The other day I completed wring up the mic and line inputs and the pre-fader unbalanced insert points including the wiring to the four channel faders themselves. The next job was to wire up the direct outs. These are fed from OUT2 of the channel amp, via a VTB2291 transformer to the direct output XLR connectors at the rear. As there are four channels there are four transformers. There are also the two AUX send outputs and the two main bus outputs that need output transformers for a total of eight. Initially, I intended to mount these eight transformers on the bottom of the Rackz enclosure towards the back. The main sub-rack is plenty high enough to fit the transformers in and there are some convenient ventilation slots in the bottom of the enclosure that could double up as fixing holes for the transformers. However, I had not allowed for the mass of wiring I had just put in. Here is a picture of the bottom of the enclosure looking through from the back:

You can clearly see there is no room to fit the transformers on the right hand side because of all the wiring and when the AUX send and returns and the main outs and the 2 track playback all get wired in there will be no room on the left either. Clearly the transformers can no longer fit there. So where to fit them?
Above is a view of the partially completed rear panel. The mic, line and insert connectors are annotated. Above the connector panels is a good sized black panel marked 'X'. This panel can be removed and behind it you can see the top of the rear of the sub-frame:

The bottom of the panel is 80mm from the sub-rack and the top of the panel is 130mm from the sub-rack. This should leave plenty of room to fit the transformers to the rear of this panel. The only downside is that the channel output transformers are now quite near the channel input transformers.Fortunately they are at 90 degrees to each other so hopefully there will be no magnetic feedback! Here is how the transformers will fit onto the panel.

Now all I have to do is drill 16 holes on the steel panel, bolt on the transformers and wire them up. Did I mention I hate mechanics?

Monday, 10 March 2014

Looks a Bit Like a Mixer

It's funny how sometimes you end up doing some things twice. Recently I wired up the PSU to the sub-rack and fitted them both into the 19 inch Rakz box. For the heater wiring I used heavy gauge mains cable. Nice and thick, low voltage drop but not exactly flexible. Thing is, you do need some flexibility in the power wiring so you can move things about for wiring up and testing other parts of the mixer. So you make the power cables somewhat longer than they need to be to provide this flexibility which is just what I did. The trouble is they do not sit very neatly in the bottom of the mixer because the heater wiring is so stiff. What you really need is some high current but flexible wire for the heaters.

This evening I was starting to wire up the XLRs at the back of the mixer and I was looking for the cable to do this. In this search I came a cross some Van Damme loud speaker cable that I had forgotten I had purchased. Examining it showed it had really thick cores, ideal for heater wiring, but the cable as a whole was really flexible. It seems that in a moment of lucidity some time ago I had ordered a few metres of this cable for just this purpose. So that is how I ended up re-wiring the heaters this evening. Here is a picture showing the sub-rack removed from the main chassis and laid on its back on top of the VU meter bridge:

The black and red twisted pair is the HT supply, the blue and white is the phantom power and the thick blue cable is the new heater cable. Next I replaced the sub-rack into the chassis in order to check that the cable curled up neatly and it did.

Lastly I connected to the mains to make sure the supply worked and lastly I fitted all the finished modules and ran a burn in test to make sure the power supply does not get too hot. In fact it just gets luke warm. At last it is beginning to look like a real mixer.