The Montgomery County Police Department closed the books on its own sex scandal yesterday iwth the announcement that a hearing board has upheld charges that a police sergeant has sexual relation while on duty.

The finding against the sergeant -- which carries a $500 fine and makes him ineligible for promotion until next June -- brings to 21 the number of officers found to be involved in the sex scandal that surfaced last spring.

The department's internal investigation of the matter began following allegations by two women, one of them a 26-year-old Montgomery County nurse who said she had had sexual relations with 25 officers while they were on duty.

In announcing the end of the probe, Police Chief Bernard D. Crooke also said that rules governing the police department's public participation ride-along program -- on which many of the sexual encounters reportedly took place -- have been tightened.

Citizens will now be eligible to join police on patrol only once every six months. The times have been changed from 6 p.m. to midnight to 10 a.m. to 10 p.m.

Following an investigation by the department's Office of Internal Affairs, 20 officers, ranging from patrol officer to sergeant, accepted recommended fines of between $300 and 500.

Four, including the sergeant who has now been fined, refused to accept punishment recommended by Crooke and sought exoneration. Two of those were acquitted by an appeals board made up of three police officers, Charges against a third were dismissed by Crooke on grounds of insufficient evidence.

The sex scandal became public knowledge only when police officials were alerted by a reporter for the Montgomery County Journal to whom one of the women allegedly involved has told her story. At the time the department has a policy -- since changed -- of releasing quarterly reports from its internal affairs office.

Now, internal investigations into charges of misconduct are handled secretly. Crooke said yesterday: "Anything major would be released, (but) I would not want to alarm the community or the department over something that has not been substantiated."