Millionaire Dominic F. Antonelli Jr. and his longtime business partner, John W. Lyon, repeatedly guaranteed bank loans for former D.C. mayoral aide Joseph P. Yeldell, even though Yeldell occasionally failed to make payments on time, a federal jury was told yesterday.

Lyon, a partner of Antonelli in numerous real estate and parking ventures, was called as a prosecution witness yesterday in Yeldell's and Antonelli's bribery and conspiracy trial to testify about his efforts to help Yeldell out of his personal financial difficulties.

Yeldell's financial difficulties, according to court testimony and other records, stemmed largely from his establishment of a private travel agency, known as Entrepreneur Travel Associates Inc., which eventually defaulted on bank loans and collapsed. Yeldell set up the firm in 1971 along with a group of his friends.

Much of Lyon's testimony centered on a series of short-term loans, amounting to about $21,500, that he and Antonelli had helped Yeldell obtain from Madison National Bank a bank in which Antonelli is a major stockholder and director. Lyon said he and Antonelli had both personally guaranteed the bank's loans to Yeldell - an arrangement that would make them liable for the debts if Yeldell defaulted.

Frequently, Lyon testified, he was notified either by the bank or by Antonelli that Yeldell had failed to meet payment deadlines on his loans. "I called Mr. Yeldell two or three times and requested that he bring the note current." Lyon told the jury. "Mr. Antonelli sat on the loan committee of the bank and I'm sure it was embarrassing to horn."

The prosecution contends that Antonelli's help in obtaining and guaranteeing Yeldell's loans from Madison was part of an allegedly corrupt relationship between the powerful city official and the wealthy real estate, parking and financial executive.

Defense lawyers have argued that Yeldell's assistance from Antonelli and his associates was wholly "aboveboard" and legitimately designed to help out a fellow businessman confronted with financial troubles.

"Signing notes (of indebtedness) for people is not something new either for me or Mr. Antonelli or for both of uas as a team," Antonelli's business associate, Lyon, testified yesterday under questioning by Antonelli's main defense lawyer, Edward Bennett Williams. "We did it because we wanted to help somebody who had a problem.

Lyon, a blunt-spoken man with closely cropped hair who was dressed to a plaid sport jacket and yellow shirt, told the U.S. District Court jury that the bank loans and guarantees arranged by him and Antonelli stemmed from a talk the three men had in late 1973.

"I set up an appointment, and Nick (Antonelli) and I and Joe Yeldell met in Mr. Antonelli's office," Lyon said. Yeldell, he said, described his travel firm's financial difficulties and asked from help in obtaining a $21,500 loan. "We told him we'd see if we could help him."

The upshot, Lyon testified was that either Antonelli or he - Lyon said he could not recall clearly who took the initiative - went to Madison National Bank and arranged the loan for Yeldell.

Lyon portrayed Yeldell's meeting with Antonelli as having come about in a casual, almost imprompty manner. He said that Yeldell had first discussed his financial troubles with Larry A. Campbell, one of Lyon's business associates. Campbell is general manager of Excavation Construction Inc., a firm headed by Lyon. The talk between Campbell and Yeldell took place, Lyon said, at a 1973 meeting that had dealt mainly with trucking industry problems.

Lyon quoted Campbell as telling Yeldell, "Give Jack (Lyon) a call and we'll see if we can help you." Lyon agreed to arrange the meeting with Antonelli and the Madison bank loan followed shortly afterward.

The Madison loans are cited in a federal indictment as evidence of an alleged conspiracy in which Antonelli is accused of later secretly providing Yeldell with a separate $33,000 loan in exchange for Yeldell's help in securing a lucrative city government lease for a partnership controlled by Antonelli. Lyon, who is not accused of any wrongdoing, testified yesterday that he is a limited partner in the same real estate partnership.