Normally, signal can gets weak while charging not because physical connections between usb port and transmitter - it was weak cause standard transmitter drop strength to 80 when usb plug is present - Nokians wanted to avoid situation, when FM transmitter power may exceed regulatory domain.

Then, using jacekowski modified transmitter binary, we can use unlocked full range of frequencies +120 power without need for fmtx boost package, and power is NOT reduced when connected. This way, actual signal is stronger with USB cable, cause it act like resonating antenna (it's not directly connected to transmitter, but due close distance, it still boost signal up).

Still, if You're sure that Your transmitter worked fine in exact same circumstances before, and now You must put it 2 cm close to radio, there is something screwed @ hardware level, I think. Maybe really repair guys screwed something, and You should request fixing it back. Especially if You still have guaranty.