Tag Archives: tuning

Electronics

Cap Exploded in an ATU

I got called from a small AM station to help them out with their power problem. Their “first-gen” solid state Nautel transmitter was outputting a wimpy 400w output as it had backed itself off on on high SWR. A quick look at the transmitter showed some gross neglect as everything caked in dirt the problem wasn’t obvious. I followed the raised transmission line out to the antenna tuning unit (ATU). The ATU is generally some combination of impedance matching network(s). My attention was immediately drawn to the black “goo” that had run down the lower capacitor and it appears the two capacitor had been removed from the circuit.  Apparently there was a lightning storm and the antenna spark gap failed; the capacitor took one for them team. According to the “Chief Engineer” (and DJ, and sales guy, and station manager) the “insulator” below up. He replaced the capacitor with one he found in a back room… after a discussion about determining where the old “insulator” went so I could grab a p/n (found it’s way to the round file) I told him I’d have to do the math and figure out the original value. They never ended up having me come out to fix it and eventually they were fined by the FCC for various reasons including operating below licensed power and eventually all their of their stations went dark.

In the photo note half of the old capacitor in the cabinet.

This is what happened when someone that doesn't know what they are doing changes an "insulator".
This is what happened when someone that doesn’t know what they are doing changes an “insulator”.