PLEASE HELP ME TO REPAIR DM800HD

Okay, thus the U6 3.3 and 5v outputs by ohms are wrong, as also the U7 2.6v output.
Both are feed by the 12v circuits which has an good ohm value, at least not shorted.

The Cpu works with the 1.2/2.6/3.3 voltages, thus it seems to me a bit illogical that it could be a issue with the cpu.
Nevertheless, when there are more than two wrong ohm values in the circuits, it's often a issue with the cpu.

At least it seems to me not easy to repair this box if you do not have the right tool which can help to find the problem.

But I think, if you still want to continue, do at least not power up the motherboard as long the ohm values on these circuits are wrong.
Assuming that you have measure then correctly and based on the other values it seems to me done fine.
Than the the best way to finding out from which circuit this accursed is to isolate the Dc to Dc outputs from the circuits by removing some zero ohm smd resistors.

As for the U6, the 5v output, that is the R109.
After it removed, check this output circuit by ohms, the U6 side and the output circuit side.
Place it back if you have the idea that all is fine, thus is makes at least no big ohm value difference on the 5v circuit.
The other output, the 3.3v side is the smd zero ohm resistor R118.
More about the U6, see here...

As for the U7, you need to remove the R283, the 000 ohm resistor for isolation this output from the circuit.
Again, when removed, check the ohms output from the output on both sides.
More about this, see here...

But still, since you have an strange ohm value to the 5/3.3/2.6 voltage circuit, it's a bit unknown to me of what it could be.
I've never experience this strange measurements between the 2.6 circuit and the two others.

 

When you measure the wrong value, at least much lower than normal in this case to the removed R109 U6 side, nd you have a similar at the other R118 side, then I could think of a issue with the U6.
If you have it only with the R118 side, then I think of a diode issue D1, D2 or one of the ceramic smd caps the C90, or C93.

In any case, when it's on one side of these Dc to Dc circuit, and it's a non shorted issue, then it's often a diode issue.
When it's a fully shorted, it's often due a defective (ceramic) cap.

But to know if it's caused due the U6, you need also remove the other zero resistor, but only if you still measure on these side the wrong ohm value.

 

Assuming that you measure the 139ohm value on the u6 side, same sort of situation on the u6/5v side, then it could be the u6, or the diodes D2/3, but it's a bit strange that you have it on both sides.

Hopefully you have this type motherboard, with these dc to dc ic's, and not another newer type, otherwise may the ohm values I had shown not be suitable.

 

Hmm okay, then in that case it could indeed be the U6, but to be sure, and when you still have the zero ohm resistors disconnected on them, check then the U7 2.6v if it still has the Pin 02- 422ohm instead of the normal ohm value 1069 ohm (1k).
Because if it's caused due the u6, then the u7 should have a normal ohm value.