How can I refer to two tables in ArcPy's CalculateField_management?

I tried to refer to the different table's fields just like I saw here. And it doesn't work.

There are two tables in my example. I have created table "c":

I generated a summarized table (called ttt) with right click on the field "helysz" and summarize. Then I added a field "hely", and copied the newly created "helysz" field's content into that, just for sure, for different field names. Then I added the ossz field to the second table "ttt". After this I linked the c's helysz field to the secondly created table's hely field. The table c's fields from the second OBJECTID field are from the linked table. They have appeared only after the linking. (ArcGIS shows the first one in this view).

This is the second table "ttt":

I wrote this ArcPy to go through the hely field with CalculateField_management and find those certain values from the original helysz field, then work with the values what are in one record with the found helysz.

ArcPy Code

The error message I get:

raise e arcgisscripting.ExecuteError: ERROR 000539: Invalid field D:/JAN17ALG/joo/Felsotarkany_energetika_kiserleti.gdb/c.helysz Failed to execute (CalculateField).

Why this doesn't work?