Bad hum

AM Radio discussion. Directional arrays are FUN!
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Bad hum

Post by sndeng » Wed Jul 26, 2017 3:53 pm

I just started back, 2 days ago, with the company after 22 years. There are no diagrams to help with connections, nothing. I'm retired,69, and on full VA disability but, they asked me to come back. A lot changed. Went from 1 AM and 2 FM to 2 AM and 4 FM. The 2 AM transmitters are identical at 1KW output going through combiners(built by the previous engineer) and shunt fed to the same single tower(ND). Not sure of the model of the transmitters, haven't been to the doghouse yet. At only 220 KHz apart it's super touchy. Ones at 1240 the other is at 1450. The 1240 station has a hum that comes and goes. I've tried tracking it down in the few days since I started but, no luck. Mixer is a Dyamax MX8L, Broadcast tools ACS 8.2 switcher and a Audioscience BOB 1024. If you shut down the system to the mixer, no output from the Ethernet computer, the hum is horrendous. I can't leave it in this condition to troubleshoot it naturally. The previous engineer installed this and is no help. I've tried ground isolation, changing from balanced to unbalanced on several connections to no avail. They(station owner) even changed out the Marti STL receiver at the transmitter. I haven't checked the Marti transmitter yet(possible problem?). This has been on going for several years. I just walked into the fire. Any help would be greatly appreciated.

Tom Osenkowsky (RIP)
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Re: Bad hum

Post by Tom Osenkowsky (RIP) » Wed Jul 26, 2017 5:38 pm

Does this sound like 120 Hz power supply hum? An open power supply capacitor or is it hum as in an open shield on an audio cable? I would start with these answers first. A scope on the audio outputs section by section may be revealing. While it could also be caused by RF from one of the other transmitters it will take time to turn things off one at a time to isolate the source. After that you will have eliminated some of the possibilities narrowing down the suspect. Tedious, yes but probably the best start.
A second or third pair of eyes would be helpful.

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Re: Bad hum

Post by lewiscallaway » Wed Jul 26, 2017 7:24 pm

Are any transmitters in the building with the mixer and switcher?

It's pretty basic I know, but I like to just plug in headphones starting at the source and then work my way down the transmission path until I hear the hum, then I step back a piece of equipment and start troubleshooting. That's how I got into this crazy industry, by finding the source of a hum at a local class A station!

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Re: Bad hum

Post by ChuckG » Wed Jul 26, 2017 9:49 pm

I haven't checked the Marti transmitter yet(possible problem?).

You bet it is. - if you have a Marti STL 10 transmitter, the power supply is a common source of hum/buzz in the audio.
The factory supply runs hot, the caps eventually fail, the plug connector and pins fail, sometimes the PC board itself gets brittle and fails.

The fix is to tear it all out- transformer, rectifier board, regulator, and install a 15V 4A switching power supply. Connect AC, B+ (red wire on the current supply) and ground...that's it. Takes about 20 minutes, costs about $20.
If you hear clean audio going into the STL transmitter but hum coming out of the receiver... there ya go.
Chuck Gennaro
Central Wisconsin

Kent T
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Re: Bad hum

Post by Kent T » Mon Aug 07, 2017 3:56 pm

Yes, those Marti STL 10 power supplies were always a frequent hum cause and common failure items. That previous post recommends exactly what I did to the ones I maintain, they've been vastly more reliable ever since.

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Re: Bad hum

Post by Kelly » Tue Aug 08, 2017 12:01 pm

As low-tech as it sounds, the tried and true troubleshooting always pays. I would:

Start at the transmitter site. Disconnect the audio input from the STL to the transmitter and look for the noise to go away. Goes away?
Then go to the studio and disconnect the audio from the transmitter STL input. Noise gone yet? If not, then as folks have said, it's your STL.
If the noise goes away by disconnecting the STL input, then keep working your way back to the audio console. Make sure everything audio is wired balanced.

My guess is that your automation PC is using the unbalanced sound card connections to the console and creating a ground loop. Sometimes a quick way to fix that is go to your local hardware store and buy one of those 3 pin to 2 pin AC ground lifts for two prong outlets. Plug your automation PC power cord into this and float the ground. Hum gone?
Skype:kellyalford Twitter: @KellyAlford

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Re: Bad hum

Post by sndeng » Mon Aug 21, 2017 8:46 pm

Found the STL-10 only had approximately 280 mW of output power. Replaced it with a agile unit and all is well in that department. Thanks for all the help. There's a whole lot more wrong with this setup. Details in the next post,"bleedover.

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