CD Rack Format Zynthian – Part 2

I’ve recently updated my “CD Rack” box to a horizontal black box, still shaped for my “CD Rack” format modules. But as it is black, and I’ve a black TX816, I’ve rebuilt my CD Rack format Zynthian with a black, PCB front panel too.

• Original Design Notes: CD Rack Format Zynthian.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers and single board computers, see the Getting Started pages.

The Panel

Once again I used KiCad to build the PCB using the front silkscreen for the labelling. This is the same approach I used for my TX816 panel as described here: MiniDexed TX816 – Part 3 – Panel Design.

Here are some of the key design notes:

• As with my original panel, this is designed for use with the Waveshare 4″ HDMI touchscreen.
• I’ve kept the layout largely the same as my original panel too.
• I’m making use of my Zynthian IO PCB.
• I’m going to use holes with pads to link them to a common GND plane under the solder mask.
• I had the choice of mounting the display set-back to the panel, to hide the sides, or mounting it flush. I went with flush, but it does mean you can see the connections down the right hand side.

Some specific dimensions:

• The rotary encoders and audio jack socket need a 8.4mm (M8) hole with pad.
• The TRS MIDI jack sockets need a 6mm hole with pad.
• The four mounting holes are 2mm radius holes.
• The power switch is a 6mm radius hole.
• The LED is a 2mm mounting hole.
• The four screen mounting holes are 2.5mm mounting holes.
• The screen cutout itself is 127x57mm.

In terms of practical use, there were a couple of issues though:

1. The power switch hole wasn’t quite big enough. I really needed a 12.5mm diameter hole, not 12mm, so that had to be expanded slightly.
2. The LED hole was too small for the LED I was using so again that had to be expanded with a needle file slightly to allow it to fit.
3. The screen mounting holes were slightly in the wrong place and too small, so I had to expand them with a needle file and make them slightly elongated away from the screen. See photo below for the “before” and “after”.

One other quirk, I had to ensure that all the pads and holes were connected to GND so they linked up to the zone fill. The easiest way to do that was to include them in a simple schematic.

Errata

The different hole sizes is part of tailing a panel to the components I have, but there is one actual error that never occurred to me until I start wiring up the MIDI sockets. MIDI IN sockets are not meant to be connected to GND, only MIDI OUT is connected. This means that the screen/shield in MIDI cables are grounded at the OUT side only.

However, all my MIDI TRS sockets have metal casings and I’ve used pads connected to the GND zone of the PCB. This means all the casings are connected to GND…

It’s not a major issue, but it does break the typical GND separation between MIDI instruments so if anyone was doing this kind of thing properly that could lead to audio noise problems.

But it will do for my messing around.

Find it on GitHub here.

Closing Thoughts

I had five PCBs built with black solder mask and white silkscreen to match my TX816.

If I was doing this properly, I’d re-spin the board with the adjusted hole positions and sizes and do something about that MIDI IN GND connection.

But for my own messing around, I really like the look of this, especially when next to my TX816 🙂

I just need to decide if I use the same knobs or find some different ones! And I might yet still make some kind of simple bezel for the outside of the display.

Kevin