Richard C. Sennett, former assistant director of the U.S. Bureau of Engraving and Printing, was fined $500 and placed on a year's unsupervised probation yesterday in connection with his earlier plea of no contest to a federal conflict of interest charge.

Sennett, now vice president of engineering for ABN Development corp. a subsidiary of American Bank Note Co., participated in a July 28, 1977, meeting concerning the modification of printing presses leased from American Bank Note. Sennett, 52, left the bureau the next day to go to work for ABN.

However, U.S. District Court Judge John H. Pratt said Sennett's offense was a "technical violation" and that there was no indication of "malice" or "evil intent" on Sennett's part. Two other former Bureau of Engraving officials who attended the July 1977 meeting wrote Pratt that they knew at the time that Sennett was about to go to work for ABN.

Defense lawyer James L. Lyons told Pratt that a $1,000 fine should be the maximum penalty imposed in the case and the judge reduced that to $500. The judge dismissed two other charges that Sennett had lied to a grand jury investigating the case.

Similar conflict and false declaration charges against Sennett's one-time boss, James A. Conlon, the former director of the bureau, had been thrown out by U.S. District Court Judge Louis F. Oberdorfer.

The Bureau of Engraving, at 14th and C streets SW, prints money, postage stamps, food stamps and other government documents.