“Buzz-Buzz” Interference Resolved!

MMRA logoAn unusual interference problem plaguing the Minuteman Repeater Association’s 146.61 repeater in Marlborough since the spring of 2004 has been solved, according to MMRA’s DeMattia, K1IW.

The two-second on, one-second off buzzing noise would sometimes be present for hours, days, even weeks at a time. There were also times when it would disappear for hours, days, or weeks. MMRA members invested countless hours tracking down the source. They found it was emanating from the vicinity of a house in a residential neighborhood about one-quarter mile from the repeater.

Limited cooperation on the part of the homeowner hindered their ability to determine what in the home was actually causing the interference. “At the time that we were investigating this, we noticed one other detail which turned out to be quite important,” notes DeMattia. “A similar sounding interference, weaker in strength, was coming out of another home in the same neighborhood. This led us to suspect that perhaps it was some sort of utility problem.” K1IW and company reported the interference to all three utility companies, cable, telephone, and power. Although each of the utilities visited the neighborhood, they could not find any problem.

The MMRA team eventually discoverd six different sources of this strange interference. Three homes in Marlborough and three in Northborough were identified, where each source appeared to be on different frequencies in the range of 141 to 152 MHz.

“One of the last homes discovered turned out to be a neighbor of mine in the same neighborhood,” says DeMattia. “I paid him a visit one day and he was very cooperative. Inside his home, the signal was more of a continuous buzz than intermittent.” K1IW disconnected power to each circuit in his home one at a time; the interference changed from the constant buzz back to the intermittent one. In the kitchen, the group found that a cordless phone charger was causing the interference. When the phone was removed from the cradle, the noise would stop. When they returned the phone to its cradle it started again.

The MMRA crew eventually identified the problem: a parasitic oscillation, tuned by the junction capacitance of the charging indicator LED and the leads that connect it to the circuit board of the charger.

DeMattia says the ARRL is going to report the problem to Motorola and it will hopefully be fixed in future production units.

The Minuteman, Volume 34, Number 3, January 2005

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