So I finally figured out why I was losing the overlay on my MinimOSD - It was browning out under load due to voltage fluctuations (See attached graph of the APM VCC graph). The rest of the components seem to be able to cope with the fluctuations just fine but the MinimOSD can't... For testing it's temporarily powered from an ESC (Only using the positive) and appears to work without dropping the overlay.
So, my next question is, is there a way to increase the output of the APM to prevent these 'low' values? The reading at idle from the telemetry port is circa 4.8v, shouldn't it be closer to 5? I verified that the feed into the APM is 5.3v which I believe is correct. I can't check this under load however.
Could there be a faulty component somewhere, low quality / too long wires on my Telemetry / OSD Y cable? Is there anything I can check or replace, or is this simply caused by running the APM, GPS, Telemetry and Receiver all off the power module?
As always, any assistance would be greatly appreciated.
I think more important that your OSD problems is that with those Vcc values you are risking browning out your APM, with the resultant unpleasant fall from the sky. Your schematic looks fine though. You are correct that there is a ground loop, but it's likely not a problem, and if it did act up it would most likely manifest itself merely as interference on your video signal.
It does seem like your PM is outputting low, so perhaps there's an issue with it. This would be my guess. It's not like your load is too great for it. The PM is capable of putting out 2.5 amps, but your APM fuse will blow at 500 mA. It has the capability to hold its output pretty stiffly. You can't change the PM's output without changing a surface mount resistor on its board.
It may be that your MinimOSD is the canary in the coal mine and just happens to be the first to go if your Vcc gets too low. But your APM can't be far behind.
Were it me, I'd put JP1 back in, disconnect the PM and try running your APM with the ESC you mentioned (temporarily). If your MinimOSD comes back and your Vcc pops back up, you likely have a flaky PM and need to replace it. Not great, but better to find out about it on the ground.
Attached is a diagram showing my current system layout....
I realise there is a ground loop, but am not sure where best to remove it? I also presume that the loop is not related to the OSD brown out as I did test it with the APM -> OSD ground removed...
***This is my first quad build, so at least I've not blown anything up... for now :-) ***