Look what took my 50kw off the air

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Look what took my 50kw off the air

Post by wtcmeng » Mon Jul 03, 2017 5:52 pm

My 4 tower DA site was off the air. Drove to the site and found VSWR faults on my 3DX-50 transmitter. I cleared the faults and tested the transmitter into the dummy load with no issues.

So I knew the problem was somewhere between the common point and the antenna system.
Upon inspection I found this little guy stuck between the lightning balls of tower # 1 providing RF a path to ground.

Who would think a little insect could take a 50kw station off the air!

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Deep Thought
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Re: Look what took my 50kw off the air

Post by Deep Thought » Mon Jul 03, 2017 9:34 pm

I once spent the better part of four hours troubleshooting an intermittent VSWR fault Nautel XR-12 only to find a toasted spider in the arc gap. I feel your pain. 8)
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Re: Look what took my 50kw off the air

Post by Shane » Mon Jul 03, 2017 11:39 pm

At KFAB, there were blowers installed and trained on the horn gap(s) where the feedline exited the brick doghouse on the non-D tower to blow the little stinkers out of the gap. Otherwise we were operating a 50kw bug zapper. The Continental Magnaphase (sp?) would interrupt carrier briefly so as not to feed the electrocution arc any longer than necessary. The blowers were wired to shut down for directional night operation.
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Re: Look what took my 50kw off the air

Post by RGORJANCE » Tue Jul 04, 2017 9:26 am

Waaay back in the dark ages of the late 50/s, I worked as a studio engineer at KVOO in Tulsa....50kw, 1170KHZ, 3 twr DA-N. I noticed on summer nights, the xmttr would momentarily trip off with the air monitor emitting a warning buzz. Called the xmttr eng and he said it was caused by June bugs flying between the arc gaps, and at midnight after sign-off, they would go to the towers with a brush to get rid of the bug dust on the arc gaps.

The xmttr was a western Electric 50kw linear, low level modulated, water cooled beast with a huge still for the cooling water. Also, part of the maintenance was to "walk in" after shutdown and do a feely-grabby on the components to check for abnormal heating issues. That rig was
reliable, but got lots better after they went 24 hours. "The good old days".


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Re: Look what took my 50kw off the air

Post by Tom Osenkowsky (RIP) » Tue Jul 04, 2017 6:03 pm

I was hired to represent a major transmitter in the capacity of a field service visit. No factory techs were available. The intermittent fault was in a new 3.5 kW FM transmitter. Yes, I know this is the AM section but the cause/solution is universally applicable. The problem was the tx would randomly dump off the air. The IPA would shut down leaving the tx with no drive. The exciter showed normal output and very low VSWR. No PA grid drive, no IPA current and (expectedly) high IPA voltage. The owner/engineer would wiggle around components in the PA section restoring normal operation.
When the Field Service Manager called me and described the problem I told him my thought was to feed the IPA output through my Bird 43 and Bird 100 W dummy load, move components to narrow down the culprit. We agreed on payment and I visited the station. It took me 20 minutes to ensure the plate, screen and bias voltages were disabled and the IPA and exciter would be the only energized portions. The owner slowly increased exciter power until I read 75W output from the IPA. I moved the wattmeter/DL and the output went to zero. The owner did not touch anything. I found I could make the problem appear at will by moving the meter.
What I found was one end of the RG-213 IPA out/PA matching network input was very sensitive to movement. Suspecting an improperly installed N connector I disassembled the connector to find a textbook installation. Staring down the barrel of the connector I saw a tiny sliver of brass making contact between the inner hot pin and ground ring! That's why wiggling things temporarily restored operation. Something so simple with such massive consequences. The brass was a leftover from the manufacturer. The connector shouldn't have made it to the field but it did. Also passed factory final test.

When I was 15 and a sophomore in HS dad worked for USS Chemicals in Danbury, CT. They had a contract with Sears to manufacture 30 gallon trash cans and covers of various colors, shapes, etc. USS did extrusion and injection molding. The machine that made the cans was down. The problem was traced to an electrical box that controlled the mold temperature. This was critical to making the product correctly. This was a Cincinnati machine but Cincinnati refused to send a replacement unless one of their technicians installed it. No techs were available. Dad mentioned that I was into electronics but was only 15. Well 15, 50 or 95 age wasn't an issue. FIX THE BOX was the order of the day. The contract deadline was looming.
I was given the box, hooked it up to the stove 220 VAC circuit breaker and connected the output to a 120 VAC space heater for a load. It ran fine for a couple of minutes then took down the breaker. The active device was a triac which was fairly new at the time (1970). It was press fit onto an aluminum plate 3/8" thick and about 6 inches square which acted as a heat sink. I was taught these were either pass or fail. Not this one. I did what I should have first done, an ohmmeter test. I found that by pressing on the upper aluminum plate I saw a short to the lower aluminum plate which was mounted to the case (grounded). There was a mica sheet insulating each of the plates.
I dismounted the triac plate and noticed a small black scorch mark on the mica. There was an arc caused by a tiny flick of solder no doubt a leftover from the factory. I removed the solder, cleaned the area, placed a small piece of mica from a TO-220 style transistor and patched with some heat sink compound. I tested the box, it worked perfectly! I returned the repaired box. They grabbed it from my hands, installed it and the cans were coming off the line.
I remember charging them $50 for the repair. My classmates were floored I was able to so simply make $50. I could have charged them $500 and they wouldn't have blinked. I save the Sears contract. A tiny piece of solder almost cancelled a high six figure contract.
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Re: Look what took my 50kw off the air

Post by W2XJ » Sat Aug 26, 2017 2:12 pm

That's the downside of the fast VSWR protection commonly found in transmitters designed in the last 50 years. There is the story about a legacy class A station where in the middle an important ball same vandels put a car battery jumper cable across the ball gap. The MW50 immediately shut down and would not come back on. The transmitter tech (site still manned) put their old legacy Westinghouse TX on air. It came up with strange readings and would not make full power until eventually all the readings popped back to normal. Later the jumper cables were discovered. The Westinghouse eventually melted the cables. There is a balanced between speed of VSWR protection and staying on air.

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Re: Look what took my 50kw off the air

Post by Bill DeFelice » Mon Aug 28, 2017 9:57 am

Years ago I had an MW-1 at a two-tower direction popping itself off the air intermittently. When I got to the site I noticed an entire row of plastic cable ties failed and a long length of the coax for the STL dish was flapping in the breeze. As the wire went further away from the tower leg the Harris tripped off. I put "old Betsy" on in it's place, an RCA BTA-1R. It didn't have a care in the world.
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Re: Look what took my 50kw off the air

Post by EJurich » Tue Nov 27, 2018 1:26 pm

Problem I once had was a transmitter dumping only when it rained. Problem was obvious, the johnny balls were installed one over the other. When it rained water formed across the balls. Repositioned and problem solved.

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