Cadet II RGB encoder ground question

#1

I’ve just finished building a DIY Cadet II RGB encoder, and my build doesn’t currently work correctly. I have a V1.1 board that I bought a week ago. I started checking voltages, and found the +5V output from the 7805 was no good (~1.2V). Then I traced all my soldering, and found that the common (ground) pin on the 7805 wasn’t connected to ground on the euro power connector. I couldn’t see any issue with my soldering, so I referred to the (hi-res) photos I took of the board before soldering it, and I can’t find any connection between the euro power connector ground (pins 3 through 8) and the main ground net on the board! Is this expected?

I looked at the schematic and the +5V regulation section is a bit different to what I’m used to seeing - the 7805 has some small capacitors next to it (presumably internal to the 7805 part and just there for information?) and a little ground icon connected to the pins common to those two little capacitors, which looks like it is then connected to the main ground plane.

#2

P.S. I wasn’t using patch leads for my testing, just bare wires. Maybe it works for other people because when they patch RGB signals in via the panel jacks there is a path to supply ground via that patch lead? On the PCB that I have the euro power connector’s ground pins only connect to C2 and C6 (as per the schematic).

#3

There are a lot of things that connect to ground in that module, so I’d be very surprised if the only things connected to ground on your PCB are C2 and C6. Typically, ground is a whole plane (often the back of the board), and the ground plane connections are very easy to miss when looking at the board because they’re little connections from the filled plane to the pads.

Interestingly, in the schematic, it looks like the eurorack power header ground isn’t connected to the ground net the way it is in the other Cadet schematics. I’m not sure what EDA software LZX uses, so it might connect to that net in some way that’s not on the schematic.

It looks like the sync header’s ground connects to the ground net, so maybe that’s the ground connection on this module? Do you have the Cadet II plugged into a Cadet I via the sync headers? It needs to be in order to work since the Cadet II needs the sync. (And maybe it needs the ground too, I guess?)

#4

Rather than a cadet 1 I am using a DIY sync generator that I designed and built on veroboard, using an LM1881 for csync and a CD4538 for blanking. That board worked fine with my DIY veroboard layout of the cadet 2 circuit (until the AD724 overheated and failed since I had failed to include the power pin caps.)

I tested various other grounded component legs on the cadet 2 and they’re all connected to the ground plane just fine. It sounds like your interpretation of the schematic is the same as mine, and the euro connector and C2/C6 might indeed be missing a connection to the ground plane on the PCB though.

#5

Looks like you’re correct! C2/C6 and the EuroRack connector are missing a GND connection on the C2 PCB. And Joe is totally on the ball here with his theories. This is a bit of a nonissue in terms of correct operation for most, because you’re always going to be using this module with the 14-pin sync ribbon, and will receive system power and sync ground through the power ribbon and sync ribbon on those modules (which is explains why we never noticed this.) Still bad practice though of course, because you want those return ground connections going back through the ribbon with the power connections. Here’s an easy wire lead kludge for your C2:
Capture

I’ll make sure we clean this up in the next revision.

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#6

THANKS!! This was driving me insane. I don’t always use a full sync ribbon, and I couldn’t figure out why it wasn’t working sometimes. Hallelujah!

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#7

Update, it works. I’m a little embarrassed that I didn’t figure this out myself. It takes a village!