The prosecution's case against Jamie A. Scarborough, who was acquitted in October of masterminding an alleged murder-for-hire plot to kill her estranged husband, lost a key piece of evidence because a Fairfax County police tape recorder failed to work, according to the county prosecutor.

County Commonwealth's Attorney Robert F. Horan Jr. said yesterday the tape recorder failed to record a conversation last May 30 between Mrs. Scarborough and Betty M. Holler. Holler testified at Mrs. Scarborough's trial that she helped arrange the Jan. 31 slaying of George H. Scarborough at his estranged wife's request.

The conversation occurred, Horan said, soon after Holler, 37, gave police a statement asserting her involvement in the slaying. That 33-page statement was ruled admissible yesterday as evidence at Holler's forthcoming trial.

Police attached a tape recording device to a telephone and also listened in on extensions as Holler called Scarborough Horan said.

The tape recorder managed to record only what Holler said to Mrs. Scarborough, Horan said; what Mrs. Scarborough said on the phone was mostly garbled. "Two detectives were listening to the conversation, but they didn't take any notes," the prosecutor said.

Fairfax police have since replaced the faulty tape recorder, Horan said.

Mrs. Scarborough, accused of using an intermediaty to hire two men to kill her husband for $7,000, was acquitted of a capital murder charge Oct. 27.

Horan said after the trial that the jurors apparently "believed the defendant over Betty Holler." During the trial, Mrs. Scarborough's attorney, Guy O. Farley, argued that there was no substantial evidence to link Mrs. Scarborough to a murder-for-hire plot.

Farley could not be reached for comment last night.

Two 21-year-old Clinton men, Charles D. Stewart and James T. Clark Jr. have been convicted of murdering George Scarborough. Clark was sentenced to death last week; Stewart is awaiting sentencing.

Holler faces trial on a capital murder charge next Tuesday.