Fixing Omnibus F4 Pro over-heat and shorted LDO problem … by David HK June 2017

Recently I have two Omnibus F4 Pro over-heat and then shorted LDO problem, the 2^nd one only after a few flights and last for just over a month.  I also saw other people in the Omnibus F4 Pro rcgroups discussion group have similar problems.

For the 1^st board I totally have no idea what was the problem and how to go about investigating it.  Then when the 2^nd board had similar problem, I re-read the discussion group again (I read it once for my 1^st F4 Pro build), and came across a few people have similar issues, either drop out of the sky, intermittent no video, or after landed, cannot armed anymore, I encountered all these scenarios.

It appears that all these are due to a shorted LDO (Low DropOut) voltage regular.  In the F4 Pro board, there are three such LDOs, each takes 5V in, produces 3V out.

There are 2 in front of the board, U4 (marked A in my picture below, for providing external 3V power output) and U6 (marked B, for providing 3V power to the OSD), and one at the back of the board U5 (marked C, to power the MCU).

Here is the 5pins layout of the LDO.  The far left pin at the other end of the VOUT is not used, as I’ve tear-up the PCB solder joint for this particular pin of 2 LDO when I tried to remove it L, but the board still working after I replaced and soldered the other 4 pins.

This picture was uploaded by teralift in the discussion group

To see if you have a shorted LDO, without connecting any battery, use a DMM to check for the continuity between a GND pin (eg those in the ESC rail) and the VIN pin.  If you have 1 LDO shorted this way, then all 3 LDOs will appear as shorted when measuring by DMM, including 2 nearby white resistors are also appeared as shorted.

The only way to find out which LDO is shorted, is to remove one LDO at a time, then check for continuity again on the other LDOs & resistors.  Also you can directly measure the GND and VIN pins of an LDO once it is removed, which will be 100% sure if it is shorted or not.

There could be more than 1 shorted, but I’m not sure, as in my case, both of my boards only shorted 1 LDO, but each board shorted a different LDO.  One has shorted U4, and the other has shorted U5.

This is how small these LDO are: :)

To remove a LDO, it is easier to use a very hot iron, rather than a heat-gun, as heat-gun can cause other damage in the surrounding area, while hot iron damage is local and easier to control.  Here are the steps I used to remove then solder back a new LDO.

1.       Paste some flux on to the 5 pins of the LDO to be removed

2.       Turn your iron to 400+ C, the hotter the better, as you want to quickly melt the solder without prolonging heating the PCB area.

3.       Use the hot iron to touch the 2 pins side first, this will soften the existing solder joints, so when you tear it up in the next step, hopefully you are not tearing up the PCB joint as well.

4.       Then touch the 3 pins side, when you see the solder is melted, use your iron to tilt up the LDO, it should come off very easy as the heat from the 3 pins will quickly transfer to the other 2 pins as well.

5.       Now the old LDO is removed, exam the 5 solder points left behind, if they are intact, good, if they are not, I think so long you have the GND, VIN, and VOUT, then you should still be okay.

6.       Add more flux, then use the hot iron to smooth out the joints solder that left behind, if there is not enough solder left, you need to add a bit of solder to the joints for the next step, as when we solder the new LDO, we just use the iron to heat up the pins, and if there is enough solder in the joint, the solder should just ‘flow’ upward towards the pin and the hot iron when a hot iron is placed there (solder always travel towards the heated point).

7.       Now add more flux (it serves 2 purposes, 1 is obvious, to lower the melting temperature of the solder, 2 is to use it as a glue to keep the new LDO pins in alignment when you place it on the PCB before soldiering

8.       Now the new LDO pins are in alignment of the soldering points, use one finger to hold the LDO in place (I found using my finger is easier to control instead of a tweezers, as you can control how much force to use without dislodging the LDO position).  Need to be careful with the hot iron and your fingerJ.

9.       You should heat up the 3 pins side first, as it is easier, make sure you’ve touched each pin, otherwise 1 of the pin may not get a good contact and you may not notice it until your quad is in the air!

10.   After all 5 pins are touched with the hot iron, and you can see the solder have been pull up from the PCB (see my picture above, these are after I’ve replaced the 3 LDO), then you are good.

11.   Check continuity again, if still have continuity, continue to replace the next LDO. 

12.   Remember to clean up all the flux using Isopropyl alcohol and a brush (I used an old toothbrush)

13.   When no more continuity, try to connect the board to a PC, if no status green light, then you need to solder 2 wires to the VBAT+ and GND pad to provide power to the board via this way.  I used a 7.5V 2S spare battery to power up my board this way for testing. 

14.   Once you have green status light, or your board is still able to take the 5V from the USB, you can connect the board to your PC, connect to betaflight, while keeping the batter connected to provide power to the board if necessary.

15.   One of my boards can still take the 5V power directly from the USB port, the other one I need to add a batter via the VBAT+ & GND pad in order to power up the board.

You can buy these LDO from ebay, just search LG33 on eBay.  They are very cheap, a few dollars for 20 – 50 pieces.

So there you go, a shorted F4 Pro board is fixed for just a few cents!  Happy flying… 

20171008 update:

Have been flying the fixed FR Pro board a few times, and using 4s, but lately after a few crashes, the board is dead again, unable to arm, unable to connect to bfc in PC, but the green light is on. Oh well, a good experience in fixing it, time to move on to another board...

Not sure if the omnibus f4 v3 (non pro) will fare any better?