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Grain, yes; computers, nyet. US manufacturers don't expect President Reagan to follow his lifting of the grain embargo against the Soviet Union with an easing of the ban on high-technology sales.

Says a representative of one such firm: Reagan is facing political pressure to act tough toward the Kremlin, doesn't want to give any more ammunition to his critics, and may even tighten the trade ban rules.

Also, say computer builders, trade with the Russians was never par ticularly profitable, so the industry isn't pushing for a lifting of the embargo.m