D.C. mayoral aide Joseph P. Yeldell failed to disclose to the Small Business Administration that he had recently obtained more than $21,000 in cash at a time when he was contending he could not repay an SBA-guaranteed loan, court testimony yesterday indicated.

Yeldell had realized the $21,222.12 in cash in June 1976 after a series of intricate bank transactions stemming from a loan made secretly to Yeldell by millionaire developer Dominic F. Antonelli Jr., according to witnesses called by federal prosecutors in Yeldell's and Antonelli's bribery and conspiracy trial.

In a financial statement dated July 5, 1976, and filed with SBA, Yeldell listed total cash assets amounting to only about $1,900. The financial statement was entered as evidence in U.S. District Court yesterday. No explanation was given yesterday by prosecution or defense lawyers for Yeldell's failure to note the recently obtained cash in his SBA statement.

At the time, Yeldell was bargaining with SBA officials over possible compromise terms for repaying part of an SBA-guaranteed loan on which Yeldell and his associates in a now-defunct travel agency venture had defaulted. According to testimony by an SBA official yesterday, Yeldell and his associates then owned SBA about $75,000.

Yeldell, then head of the D.C. Department of Human Resources, had offered to repay SBA a total of $25,000 over a 10-year period as a compromise settlement of the debt, Neal R. Weaver, the SBA official, testified. Other SBA officials were pressing Yeldell for a $35,000 settlement, Weaver said.

Through yesterday's testimony, the prosecution appeared to be seeking to demonstrate to the jury that Yeldell benefited substantially from his allegedly corrupt relationship with Antonelli and that the two men had concealed their personal financial ties from government officials and the public.

Yeldell, now on unpaid leave from his job as a top mayoral aide, and Antonelli, a parking and real estate executive, have been charged with conspiring to arrange a lucrative DHR lease for a two-story office building at 60 Florida Ave. NE that was eventually purchased by a partnership controlled by Antonelli. In return, Antonelli is alleged to have given Yeldell a secret $33,000 loan after helping him get an earlier $21,500 loan from a bannk in which Antonelli is a major stock-holder.

So complex was the series of bank transactions described by prosecution witnesses yesterday that the prosecutors sought to show the jury a specially prepared chart to illustrate how part of Antonelli's $33,000 loan was allegedly converted to about $21,000 in cash for Yeldell after it was initially used to pay off three other bank loans that together amounted to more than $40,000. The prosecutors were blocked, at least temporarily from displaying their chart because of objections raised by defense attorneys.

In essence, the prosecution contended that the $33,000 was first used to pay off two loans from McLachlen National Bank and Madison National Bank. Then, according to testimony yesterday, Yeldell was reimbursed by checks from his business partners in the travel firm, knows as Entrepreneur Travel Associates Inc., for their shares of two outstanding debts, the Madison loan and a loan from Industrial Bank of Washington.

Afterward, the testimony indicated, Yeldell used the checks from his business associates to pay off the Industrial Bank debt. The result of these transactions left Yeldell, the testimony showed, with two checks totaling $21,222.12, which he asked his DHR chauffeur and errand runner, Joseph B. Turner, to cash at a bank for him.

Turner recalled his trips to the bank for the jury yesterday, saying that a bank official "called me over to the teller and they started counting the money out. They put it in an envelope and I took it back to Mr. Yeldell."

In his financial statement to SBA, Yeldell listed as cash assets only three checking and savings accounts totaling $1,895.19. He described his net worth as a negative $87,415.19 indicating that his debts at the time far exceeded his bank accounts, the value of his home and his other assets.

In other testimony yesterday, Herbert J. Weichert, a senior vice president at Madison National Bank, provided further evidence yesterday of apparent help by Antonelli to Yeldell in arranging bank loans. Weichert testified that he approved a series of $4,000 loans to Yeldell and six associates in 1976 after inquiries about such loans by Antonelli, a Madison director and stockholder.

Weichert asserted, however, that he approved the loans on their own merit and not because of Antonelli's influence. "These people all had good credit; in fact, it was excellent," Weichert testified. "Mr. Antonelli did not ask me to make these loans."

Immediately after Weichert testified, the prosecution disclosed that FBI laboratory tests, relying on infrated photographic techniques, had showed that what appeared to be Antonelli's initials (DFA) had been written by Weichert on each of the seven loan applications and then blocked out with a black felt-tipped pen. The FBI reported that the first initial was clearly "D" and the last initial was "A." The middle initial was described as either "F," "T" or "J."

Weichert had previously acknowledged that he had written something on the applications and then blacked it out, but he claimed he had "no recollection" of what he had written or what brought him to delete it. Often, he said, such markings might indicate who had recommended a loan applicant or offered to guarantee a loan. But he added, "I don't know why I would just put a person's initials there."