Repairing Omnibus F4 Pro without schematics

During last flight session a friend “landed” in the field, after next power cycle the camera video signal was only a black box. As first test we shortcircuit the VideoIN (from the camera) and the VideoOut(to the vtx) and we got a camera signal. Looks like the OSD got somehow broken.

So lets start repairing and look for the download of the F4 pro schematic. And then you realize: Shit! There is no schematic like for the good old cc3d.

Omnibus F4 Setups problems (because of having no information/ schematic)

That remembers me on a setup problem of a friend. He setuped the omnibus serials like this:

uart6- bidir fport (fport is special communiation protocol for frsky receivers created by collaboration between betaflight & frsky)
uart1 -gps modul
uart3 – tramp irc

Everything was setuped properly and worked except the frsky receiver.
Luckily we found this post in the inav project.

The problem here was that uart1 has inverters on uart1 tx/rx (Be aware on old omnibus versions this could be uart1). If you would have a schematic you could easily find that out [If you have it please mail it to me (: ]

But why causes the inverters troubles? Because fport requires bidirectional communication and the inverter works only one way. You can only disable inverter which is required if you use fport on a f4 flight controller (only stm32f3 flight controller can invert in hardware)

There are at least site with good pinout description:

https://www.unmannedtechshop.co.uk/omnibus-f4-pro-flight-controller-v3/

You have to look carefully, normally you find the pin-out for the normal omnibus F4 or a older revision.

Back to main topic:  OSD is not working anymore

First we took a closer look to the OSD ic, its a MAX7456.

Lets get the datasheet and the pinout:

Okay lets measure the power supply of it on pin 3: –> only 2.2V we found the problem. But what is the cause of that? Without schematics we are lost.

However we got luck! Someone had already similar problems and started with reversing and analyzing the Omnibus F4 board!

http://davidhkblog1.blogspot.com/2017/06/fixing-omnibus-f4-pro-over-heat-and.html

The problem is that the LDO U6 providing the 3.3V got somehow shorted. Lets measure again the voltage at the output of the LDO, still 2.2V

Lets took a look in the electronic box. Yeah – found a LDO MCP1824T-3302 LDO, lets check if pinout is ok:

Strike! i got the SOT-23-5 package and the pinout fits, so soldering old out new in and voilà we have a working OSD again!!

Thanks to the community for reversing and helping us, specially to davidhk blog! And to all sellers and producers of the great omnibus flight controllers: SHAME ON YOU! for holding us useful informations like the schematics back.