Grundig Service Manuals
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Yeah, I knew it was for the 700, but I was hoping on that site that it would also have information for the 750. I should have stated that in a clearer manner in my original post.
Grundig 3097 User Manual
You are right, it is difficult to find service information for these radios. It makes me wonder if they really want a person to be able to service their equipment. It did not used to be this way. It seems, though I may be incorrect, that the equipment made in China.they would rather have you toss it than repair it. A year or so ago, I acquired some customer return Grundig-branded S450 'field radios' for an extremely low price. About a dozen of them, I think. Like the 750, service information is non-existent.
The situation is even worse for these because they are built specifically under contract for Eton and have no direct Asian-branded equivalent anywhere. I was able to repair several of them by module swapping. Those were the easy ones. With those that were left, I analysed the internals. What I found was that all of the ICs were Chinese clones of off-the shelf Japanese components in which the patents had expired. Thus, it was possible to pull the data sheets on the individual ICs and study how they work. From that, I was able to repair a couple of the radios with no other service information.
This may be the route that the OP will have to go. I would expect that a schematic would have appeared somewhere online for the Tecsun version by now (the 750 came out in 2008) but I was not able to find one by searching. I also acquired some ultra-cheapo Grundig-Branded M400 pocket radios from the same vendors.
Grundig Service Manual Download
These radios were made in both analog and digitally synthsized versions and both use off-the-shelf components in their designs. The QA of all these radios are really bad. All sorts of weird problems have been reported out-of-box and the return rate is high, with returns being necessary two or three times for some buyers. I don't think that the Chinese have the capability to produce complex custom ICs for their internally designed stuff yet. Most of their stuff seems to be very derivative of earlier technologies. For guys like us, that is actually a good thing because it means it is still possible to get some components for them and possibly gain some knowledge of the inner workings of the radios that facilitates a successful repair.