OK, so what you are saying is you were cited for making an illegal U-Turn against a no U-Turn sign... Or at least that is what you can be sure the officer will testify to, and your defense is you disagree that your U-Turn was made at the intersection, instead, you insist that it was an illegal U-Turn in a business district...
Without seeing the sign, best I can say is if it is reasonable to infer that the prohibition mandated by the sign is in reference to a U-Turn at the intersection, then it makes little difference if it followed the MUTCD recommendation to the tee.
There is probably a good reason why the sign was located where it is. And here is what the MUTCD says about "location":
Page 55 of the 2012 California MUTCD, Paragraph 11a:
11a The figures shown in the California MUTCD are typical or example applications of the traffic control devices to illustrate their use and manner. Criteria for position, location, and use of traffic control devices in the figures are furnished solely for the purpose of guidance, understanding and information, and are not a legal standard. Engineering judgment must be used to apply these guidelines to the typical or example applications, or adjust them to fit individual field site conditions. The California MUTCD is not intended to be a substitute for engineering knowledge, experience or judgment.
You should also read Section 1A.09 Engineering Study and Engineering Judgment it reiterates the same idea:
Standard:
02 This Manual describes the application of traffic control devices, but shall not be a legal requirement for their installation.
You can also refer to the actual definition of "Engineering Judgement" which you'll find on page 81 paragraph 64:
Engineering Judgment—the evaluation of available pertinent information, and the application of appropriate principles, experience, education, discretion, provisions, and practices as contained in this Manual and other sources, for the purpose of deciding upon the applicability, design, operation, or installation of a traffic control device. Engineering judgment shall be exercised by an engineer, or by an individual working under the supervision of an engineer, through the application of procedures and criteria established by the engineer. Documentation of engineering judgment is not required.
And the point I wanted to highlight there is that engineering judgment need not be documented. So even if you were to obtain a copy of the engineering report to look for the reason why it was posted where it was posted, in this case you are not likely to find much but even if you did, there is no "error" that was committed by not following the MUTCD because the MUTCD is merely a guide.

Quoting
pixelfreak
So do I have a case here?
Not even a hint of one! But you are free to try whatever ideas you can come up with. Just keep in mind that assuming you are eligible for traffic school and can automatically choose to take that option at the arraignment, the court is no under any obligation to allow you that option after a trial.