Government agents in California yesterday seized a $70,000 piece of sophisticated electronics equipment for enhancing airborne reconnaissance photos that had been exported illegally to the Soviet Union -- and then sent back to the United States for repairs, according to the Commerce Department

The seizure, which took place in Pasadena late yesterday, appeared to be another example of the government's continuing crackdown on the illegal movement of American technology out of the country.

"The system was seized to prevent return to the U.S.S.R. because it would enhance Soviet strategic technological capabilities and was originally illegally diverted," Theodore W. Woo, deputy assistant Commerce secretary for export enforcement, said in a statement.

The file-cabinet-sized device was manufactured by Comtal, a Pasadena-based subsidiary of Minnesota Mining and Manufacturing Co., which was not involved in the case, according to Comtal. Comtal sold it in 1979 to Vickers Ltd., a British aerospace company, which in turn shipped it to the Soviet Union, Woo said in an interview. Woo said Vickers' export license did not specify that the system was destined for Russian use.

"The re-exportation, or diversion, from Great Britain to the Soviet Union was illegal," Woo said in an interview.

The Soviets sent the system back to Comtal, through Vickers, for repair and upgrading earlier this year, according to Roy Brugman, international sales manager for Comtal. Vickers sent along an engineer, who mentioned to Comtal officials that the system had been used in Russia.

"At that point we just about dropped our pants," Brugman said.

"As soon as we were made aware this thing had been in Russia and had been sent back here for the upgrading and was going back to Russia, we said, 'Wait,' " Brugman added.

Comtal told Vickers it would not send the device back to Russia without Commerce Department approval, Brugman said.

After turning down the new export application, "the Commerce Department said, 'How the hell did it get to Russia in the first place?' " Brugman said.

Vickers then demanded that the system be sent to it anyway, but agents of the Commerce Department's office of export enforcement and the U.S. Customs Service seized the device yesterday afternoon, Brugman said.

Vickers officials could not be reached for comment.

Brugman said he had not yet informed Vickers that the device had been seized and "I'm not sure what the reaction from Vickers will be."

Woo said that such an attempt to take technology illegally out of the country "happens quite often."

Woo said the case is still under investigation.