Over-the-air Audio Levels Drop Suddenly For No Apparent Reason

[dx7]

A quick question from a programming guy who's filling in until our new contract engineer starts --

Any idea why the over-the-air audio level at our small market C3 FM station will get quieter for about 15-20 seconds or so then come back to normal, then lower again..over and over 'round the clock? This started a few days ago.

In other words, people can hear us on the radio at normal volume, then the level drops causing listeners to have to turn up their radios, only to have it normalize again, etc etc. This happens during a consistent program source.

The VU meter on the console doesn't drop when it happens and the volume level on the STL TX doesn't drop. We're using a Vorsis VP-8 processor ahead of the STL and no settings have been changed.

We have a new solid state transmitter and all appears normal at the TX site. It's all worked fine 'til now.

Would appreciate any ideas you have. Thanks in advance.

 

[Doctor_Technical]

Intermittent problems / failures can be tough for anyone to catch. You're doing your new CE a favor by trying to log as much detailed info as possible.

You mentioned that the "volume level" on the STL TX doesn't drop. Good observation -- is this an analog composite STL, or a digital box?

The reason I ask is to help you narrow down where in the air chain the problem occurs (audio processor, STL, exciter, transmitter, other). Do you have any kind of modulation monitor? One trick is to set the monitor where it displays the pilot level. Depending on your equipment layout, the pilot dropping at the same time as the audio (or not) will give you an additional clue.

15 seconds is not enough time to dial up a remote control and ask a transmitter site "what's wrong?". Can you convince someone to stay out at the site, keep a radio on and just watch the meters for something out of kilter? Won't be fun; you may have to apply liberal amounts of what we used to call "transmitter fluid" -- comes in 6-packs and cases.

 

[boiseengineer]

Exercise all the BNC connectors on the STL & exciter. Found several bad crimps on factory BNC plugs in transmitters that way.
Muting circuits on several different brands of STL receivers have done that to me. Cycling the STL transmitter temporarily fixed it until they could be accessed on the 7500' mountain with 6' of snow & fixed.

 

[ChiefEngineer]

Found some bad bnc connectors at the studio recently. In your case check the transmitter site.

The STL audio is not dropping at the studio and this is where your processor is? Sitting at the transmitter site is the only way to catch it. Most sites these days have no speakers and no good radio. Make sure there is something to overcome blower noise while someone listens.

The best way to see what is happening is to have a mod monitor. But, if you have one at the studio see if the stereo pilot level goes from 10 percent to 5 percent or some variation. This is a way to verify that the level is definitely changing without going to the site.

This would isolate the problem in the stl and the exciter. Then go to the site and see if the level changes on the stl receiver. if so, this is probably what is bad. If not watch the exciter input level.

Remember they call this the air chain and each component links the console to the transmitter. In checking connectors remember that some transmitters have the exciter built in. Checking these connectors next to 10 thousand volts is life or death. Once you know the area of the problem let someone who knows about the Jesus stick fix it.

With temperature change capacitors can go bad. Also, it may be too cold. All that hot blower air vented out of the building needs to be sent back in for winter. Unless you're in Florida.