Aaron Cohen resigned as chairman of National...

Aaron Cohen resigned as chairman of National Technical Systems, a Calabasas engineering services company, to concentrate on running United Education & Software Inc., where he is president and chief executive.

National Technical President Jack Lin said the company was “extremely reluctant” to accept Cohen’s resignation, and had begun searching for his successor.

Lin also is chairman of United Education, an Encino operator of trade schools that formerly was part of National Technical. National Technical sold 40% of United Education to the public in 1981 and divested its remaining stake in 1985.

In recent months, United Education has struggled with numerous operating problems at its subsidiary, which processes government-backed student loans. The company said last week it would shed that business and take a pretax write-down of $9.5 million in its fiscal third quarter, which ended Oct. 31.