Mackie Onyx 1620: "Wet" Direct Outs Mod

This mod was found on the Mackie Tech-Forum at this address:
 
Thanks, Peter!
 
 
ONYX 1620 Post Fader Record Out Modification

This modification is for the first 8 channels of a 1620.

Essentially, it feeds the 8 mic channels to the Recording Outs and Firewire Outs, post HPF, post insert, post EQ, post fader and post mute.

Do not attempt this unless you have the appropriate soldering skills and a basic understanding of electronics!

The commercial mod I’ve read about incorporates the redundant pole on the EQ in/out switch, allowing you to maintain the pre mod status when the switch is out. I did not require that and wanted the signal post out on a constant basis, just as it is on a conventional analog desk. Besides which, I wanted to be able to use the HPF and inserts without necessarily having to engage the EQ. Therefore, this mod is permanent but I’ll show you later what needs to be done to do it the other way.

Make sure the mixer is disconnected from the mains.

Find a soft surface to work on (sponge rubber, bed, soft carpeted floor etc).

Place all the 1620’s faders at maximum.

Turn the desk over and gently place it with its control surface down. Make sure you have access to the back and both sides.

Remove the 8 screws on the bottom and 12 screws on the sides (6 each side). Remove the 2 larger recessed screws to the upper right anchoring the DB25 card. If you have a Firewire card installed you will need to remove the 2 recessed screws that anchor it as well. Place the screws in a safe place, preferrably 3 containers since they are all different.

Gently lift the bottom cover off and flip it to the left as if turning the page of a book. The power supply is mounted on the bottom cover and the wires from the IEC receptacle/switch on the back panel are connected to this. The bottom cover should lie flat, tightly beside the mixer proper.

In the upper right on the back panel, you will see the small PCB that is the DB25 (Recording Out) board. You will also see 2 multi-connectors, the one on the right being 15 pin and the one on the left being 13 pin. We are interested in the one on the right.

For ease of access and accuracy, on the back of the back panel, unscrew the 4 hex screws, 2 either side of the DB25 connectors and gently withdraw the board and flip it over so it rests on the main board and the blue caps are facing up.

Cut the first 8 wires (right to left) of the right hand 15 pin connector, midway between it and the other connector on the I/O board.

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Shrink some appropriately sized heatshrink tubing over each of the ends of the cut leads coming from the I/O board so as to avoid shorting and cable tie them together for neatness.

Strip back the ends of the 8 cut leads that run to the 15 pin multiconnector on the DB25 board about 3mm (1/8 “) and tin them making sure there are no blobs of solder. For safety’s sake, place a piece of paper under where you’re working to avoid any solder falling on the PCBs.

Replace the DB25 board back in position in the back panel with the previously removed hex screws and position the 8 tinned wires now facing down in such a way that they are pointing towards the front of the desk and can be easily soldered to later on.

Now, locate the point where the mute switches for each channel are attached to the main board. They’re just above the channel faders as you would expect and the pads on the board look like this:

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Measure, cut and prepare some twin core, high quality, shielded audio cable to connect between the mute switch terminals and the wires from the DB25 board. I used Belden 8761 because it has a foil shield and a drain wire that makes hookup easy in a fixed installation and offers outstanding shielding.

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Tin the two relevant pins of the mute switches (pole C) on the PCB. Strip and prepare the cables as shown above. Strip and tin the signal wires at the switch end back about 2 mm and the DB25 end about 3mm. Fit and heat some heatshrink tubing to both ends as seen in the photos to prevent shorting.

Very carefully, solder the hot and cold wires to the mute switch pins. Be careful and make sure that no stray solder bridges the pins or falls anywhere else on the board.

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Moving to the DB25 end, slip some appropriate heatshrink tubing over the 8 wires coming from the 15 pin multiconnector on the DB25 board. Solder the hot wire of the prepared cables to the previously stripped and tinned ends of the DB25 wires, slide the heatshrink tubing back over the splice and heat to shrink. Secure the cables with cable ties as seen in the photos below. The 3rd yellow cable tie from the bottom on each loom is also wrapped around the spacers that hold the I/O board in place to keep them secure.

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The mod is now complete other than putting the bottom cover back on. For safety’s sake, check and make sure that no leads from the power supply have worked their way free from the IEC receptacle/switch. If they have, re-attach them securely and fit the bottom cover in the reverse order of that described earlier for opening.

It must be stressed that if you ain’t up to this, for the love of Pete, don’t do it. It took me about 4 hours to perform but many, many days of consideration and preparation. It’s a wonderful thing and I don’t know why it wasn’t implemented initially.

The whole enchilada

The above description is how I wanted the recording/firewire outs to be but I realize others have different needs. For the sake of completeness, the following gives you an idea of what to do and from where to take the feeds, to have the Eq switch control the origin of the recording/firewire outs.

The Eq switch for each channel is located at the top of the main board and looks like this: [Photography is not one of my strong points :-)]

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All channels on the 1620 use a double pole switch to choose Eq in/out. Channels 9 - 16 use both poles (because they’re switching 2 channels at once). Channels 1 - 8, whilst using both poles, only need one pole. On the first 8 channels of a 1620, the poles of the Eq switch are wired in parallel on the PCB, therefore, merely doubling up the function.

To release one pole of the switch for our purposes, some simple cuts need to be made on the PCB. Please refer to the following diagram.

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For channels 1 - 8, carefully cut the PCB tracks, where shown, with an exacto knife or razor blade. Be extra careful when cutting between the lower switch pin and the 51K resistor. There isn’t much room and you don't want to damage the resistor. Because the tips/edges of those blades can break off at the most inconvenient moment, please wear some eye protection. After cutting all the tracks, double check with a multimeter set to ohms, that there truly is no connection between the left (A) and right (B) poles of the Eq switch. This process will be time consuming but important.

The 8 wires from the I/O board that were cut in the earlier description between the I/O board and the DB25 board and tied away, are now required and need to be connected to the Eq switch as shown in the above diagram.

It’s feasible that shielded cable is not required for the post Eq mod or for that matter, the post fader mod and the shields have been omitted from the diagram for ease of illustration. I just felt happier, in terms of crosstalk, using it for my application... not that I tried it the other way to see. If you choose to use shielded cable, please follow the directions above and make sure that the shields connect to each individual channel’s ground and that they’re only connected at one end.

I don’t intend to go into the complete details of the procedure but you do need to apply good wiring techniques. The diagram is fairly self explanatory.

So in summary, there are four types of mod that you can choose between:

Switchable In/Out

These methods allow the 1620 to operate in factory mode when the Eq switch is out and modified mode when the switch is in. The downside to this, if you see it that way, is that the Eq circuitry is in circuit even if you only need to use the HPF or Insert or both.

1. Post HPF/Insert/Eq/Fader/Mute, switchable via Eq switch. (Cutting of PCB tracks required)
2. Post HPF/Insert/Eq, Pre Fader/Mute, switchable via Eq switch. (Cutting of PCB tracks required)

Permanent

These methods allow the use of the HPF and inserts without necessarily having to place the Eq in circuit. The down side, if you see it that way, is that you can’t revert to the factory mode.

3. Post HPF/Insert/Eq/Fader/Mute, non switchable, as outlined in detail above. (No cutting of PCB tracks required)
4. Post HPF/Insert/Eq, Pre Fader/Mute, non switchable. (No cutting of PCB tracks required)

Mod 4 can be achieved by taking the feed from the middle left pin of the Eq switch (Pole A) directly to the appropriate channel’s wire of the 15 pin multi-connector going to the DB25 board, instead of from the mute switch.

Have fun,

Peter

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