Chinese replacements of Soviet equipment during Vietnam War

I remember on this old US documentary called Wings, they interviewed Soviet advisers to North Vietnam during the war.
The Soviets said they would send anti aircraft and aircraft equipment to Vietnam via China, and when they arrived
they were surprised to find out that the Chinese kept all the original Soviet equipment and replaced them with inferior Chinese equivalents.
Anyone know exactly what anti aircraft or aircraft parts they referred to? they did not go into detail.