sorry, but this is a very difficult subject.
Note, the receiving filter has a certain notch frequency which you can not change, and the tone generator should be tuned to that of course, by the calibration procedure.
If you write here it "doesn't work", I don't think somebody can just tell you what's the problem.
If there has already by done some changes to the internal settings, as you write, and "it didn't help", that's may be the cause of the whole problem. This settings are not to be done such that the needle goes up again, but such that it is by the specifications.
Unfortunately the English translations of the Russian manual, seem made by people who speak little English, but when you know what they are trying to tell, you can sure understand it.
The beginning is, you need to understand completely how the circuiz works at first. Once you are through that, you'll be able to do the calibration procedure, and after that, you can do the fault finding quickly. In that order. So when you want to "fix" off-calibration electronics, without know what you are doing, that's not going to work.