A Montgomery County District Court judge has convicted two men of illegally selling an alcoholic beverage to a minor, and 18 other clerks and liquor store owners have agreed to do 24 hours of public service to erase charges that they sold alcohol to a 17-year-old youth working as a police undercover agent.

After hearing evidence in three separate cases, Judge Thomas A. Lohm on Thursday acquitted one man, Thomas R. Meyers, a clerk at the Tipsy Mouse in Gaithersburg, of illegal alcohol sales and convicted two others.

One of the two convicted was Forrest Feegle, 58, of Rockville, who was charged with selling alcohol to a minor, Martin Kilpatrick, 17, after police observed Feegle sell Kilpatrick a bottle of beer at the Stone Street Pub in Rockville. The judge placed Feegle on probation and agreed to erase the charge from Feegle's court record if there are no further violations of the liquor sales law.

The other man convicted was Lawrence A. Haight, an employe at the Belby Discount Beer and Wine Store in Rockville.

The remaining 18 persons who were charged with illegally selling alcohol to a minor last month chose to voluntarily do public service work for 24 hours rather than be prosecuted. Under the arrangement, charges against them are dismissed and any record of the charges against them are removed from court files once the public service has been completed.

All of the charges, which occurred on the weekend of Nov. 12-13, came in the county's first major crackdown on violations of a law that raised the drinking age from 18 to 21 last July 1.