High Pitched Tone in FM Audio?

FM does it with frequency!
timinbovey
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High Pitched Tone in FM Audio?

Post by timinbovey » Thu Mar 02, 2017 11:50 am

You'll have to excuse me a bit, I've worked the past 30 years for the same AM/FM combo and my engineering work has been limited to equipment I know very well and now I'm having to work out of my comfort zone. We purchased a third station from a regional group that was dismantled this past winter.

It's a 100,000 watt FM with a Harris HT20CD transmitter. The station has had a high pitched tone in the audio for well over a year. We took control of the station with an LMA on January 1st and just closed on it this week. Listeners called to ask if now, with new ownership, we could finally get rid of the annoying tone on the air. It's been there a good long time.

The two engineers from the group that owned it have not been able to eliminate it, although I don't know what they may have done exactly. I know that before we closed on it they installed what they said was a "upgrade capacitor kit" for the Harris exciter, and they seemed to believe that this fixed it. it didn't.

They were feeding the exciter with an Orban 8200.

My limited trouble shooting can tell me that -- it's not coming in through the audio from the studio. If I remove all audio into the processor, the tone remains on the air. If I disconnect the processor from the exciter the tone goes away. of course this also means that any receiver we're listening to also changes to mono without the stereo generator/pilot tone in the processor feeding the exciter. Also, if we're listening off the air and switch the receiver to mono, the tone goes away.

This tone is very high pitch, which makes me think we're hearing the pilot tone. In fact, I myself cannot hear it as I have some hearing impairment and do not hear high frequencies. Although all my adult children and most of our staff can hear it fine. You don't notice it with music, but when a DJ is talking or there's a moment of silence it seems to be readily apparent. Yesterday I was up at the site with the owner who was listening in his truck and I came out to see if he could still hear the tone, he said yes, listen! Even with his truck radio turned up nearly all the way and my ear to the speaker I couldn't hear it! Even with the programming audio removed from the signal!

I switched the processor out for a new Orban 5500i. The audio sounds fantastic (it was only OK before) but the tone remains. So I'm ruling out a processor issue.

is there something obvious here I'm overlooking? Or where might I dive in? Again, I am not familiar with this transmitter, but have all the knowledge I've saved up from keeping a 40 year old RCA on the air, and a 50 year old Wilkinson, lol. I don't feel too bad because the two previous group engineers couldn't fix it -- then again i don't think there heart was in it.

Any advice/ideas much appreciated.

Tim

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Re: High Pitched Tone in FM Audio?

Post by Deep Thought » Thu Mar 02, 2017 11:55 am

Does it go away when you switch the stereo generator to mono?

Are you running any SCAs? RBDS?

Older Optimods were notorious for the power supplies having high frequency oscillations which would show up as birdies in stereo, but you say you changed out the processor already.

This should be showing up on the modulation monitor too if it is that audible. Check to make sure the AM noise is in spec.
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Re: High Pitched Tone in FM Audio?

Post by grich » Thu Mar 02, 2017 12:21 pm

Is the transmitter still running the Digit or Digit CD exciter?

If so, get ahold of service bulletins FM-535-WCF and FM-580-WCF from the Gatesair web site. Both bulletins address audio tones generated in the exciter caused by dried-up capacitors. Perhaps something was missed in the previous repair attempt.

Gregg

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Re: High Pitched Tone in FM Audio?

Post by timinbovey » Thu Mar 02, 2017 1:02 pm

The whistle was there long before the new processor, and remains following the new processor, so I'm eliminating the processor for now.

They were not running RDS and had the whistle for quite some time. The the new processor we are running static RDS but on or off that has no effect on the whistle.

AM noise is in spec.

I'm still believing it's going to be in the exciter -- as I said, I have no experience with this transmitter, but after reading about the service bulletins it sure sounds like that might be the issue.

How might one obtain a login for the Gatesair tech support area? I found the log in page, but no place to register. We've only actually owned this transmitter for about 2 days. The last Harris I worked on was the MW-5 at our AM which was replaced with a new Nautel a few years ago, and I never needed Harris support for that old bugger. I think most of the bugs had been worked out of that by the time I got involved with it.

Anyone perchance have these two service bulletins at hand that could send me a scan or pdf?

Tim

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Re: High Pitched Tone in FM Audio?

Post by Deep Thought » Thu Mar 02, 2017 1:11 pm

Given Gates Air's less-than-competent web presence, I would just call 'em.

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Re: High Pitched Tone in FM Audio?

Post by davek » Thu Mar 02, 2017 3:38 pm

Here you go :)
FM-535-WCF.pdf
(197.37 KiB) Downloaded 52 times
FM-580-WCF.pdf
(56.35 KiB) Downloaded 41 times

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Re: High Pitched Tone in FM Audio?

Post by grich » Thu Mar 02, 2017 3:39 pm

I found both bulletins on my computer. Attached.
Attachments
FM-580-WCF_DIGIT_DIGIT-CD_PA_Self_Osc_C29.pdf
(60.34 KiB) Downloaded 25 times
FM-535-WCF_DIGIT-CD_ DIGIT_A4_Upconverter_caps.pdf
(191.74 KiB) Downloaded 31 times

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Re: High Pitched Tone in FM Audio?

Post by grich » Thu Mar 02, 2017 3:40 pm

davek wrote:
Thu Mar 02, 2017 3:38 pm
Here you go :)

FM-535-WCF.pdf
FM-580-WCF.pdf
You beat me to it! :D

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Re: High Pitched Tone in FM Audio?

Post by davek » Thu Mar 02, 2017 3:54 pm

grich wrote:
Thu Mar 02, 2017 3:40 pm
davek wrote:
Thu Mar 02, 2017 3:38 pm
Here you go :)

FM-535-WCF.pdf
FM-580-WCF.pdf
You beat me to it! :D
Must have been the coffee I had for breakfast :lol:

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Re: High Pitched Tone in FM Audio?

Post by KPJL FM » Fri Mar 03, 2017 8:31 am

Check for a ground loop on the composite, also. Although that usually causes a low hum, there may be a feedback happening somewhere.
Is the Digit fed AES or composite, Try changing the input? Try using a different exciter? Try using a larger hammer :lol: ?
Lotsa luck, let us know what fixes this.
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Re: High Pitched Tone in FM Audio?

Post by Kelly » Mon Mar 06, 2017 9:32 am

It's probably the exciter, (known problems with the Digit).
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NavyBOFH
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Re: High Pitched Tone in FM Audio?

Post by NavyBOFH » Tue Mar 07, 2017 12:15 pm

I've been watching this forum for a while but decided to finally make an account in hopes to also give another solution...

We run some HT20CDs, a handful of HT30CDs, and a pair of new HPX models. Out of them, we noticed one of our HT30CDs having the same issue.

We replaced the capacitors under the bulletin listed above, but came to find out we were having more issues as well since the noise did not disappear. I do not know the specifics of your site, but the "offending" transmitter I am mentioning for us was in an environment where (when it failed) it was sucking in 85 degree air, putting out a 140-145 stack temp and a 100-110 degree cabinet temp. Because of that, the RG-213 jumpers inside the cabinet had essentially baked themselves and the dielectric insulator was starting to ooze out of some ends. We noticed specifically the IPA to PA jumper (right behind a blank panel going into the PA tuning portion), was dried out to the point that when we actually went to replace it, it cracked in half. We ended up replacing all the RG-213 cables in the cabinet because of the issue and the whine disappeared.

This is a transmitter that has been on the air without issue for 10+ years until this showed up - so I would also add that to your items to inspect. The cables we replaced were not frequency specific and GatesAir told us to "just get them as close as possible". We used the OEM RG-213 cable, which I believe was Carol cable - and Amphenol connectors.

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Re: High Pitched Tone in FM Audio?

Post by Lee_Wheeler » Tue Mar 07, 2017 1:00 pm

Fascinating that they developed an audible whine with the problem in the RF stage. Those IPA to PA cables have cooked on my FM25k rigs (I have 2 pair, 4 transmitters) but I never noticed an audio problem. The problem manifested itself as purely an RF problem where a transmitter would dump for no apparent reason. I finally was standing in front of one of them when you could actually see the glow inside the cabinet when the RF burned through the coax.

Since both sets are combined I couldn't replace just one cable and I had to replace them in pairs. Gates Air was zero help other than suggesting I call Pasternack. I wound up getting them for a few bucks less through RF Specialties.

...Lee

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Re: High Pitched Tone in FM Audio?

Post by grich » Tue Mar 07, 2017 1:58 pm

I have also ran into the rotten jumper issue on the HT10. In my case, the IPA was erratic. The outer jacket of the RG213 had cracked off and the exposed braid was shorting to the IPA regulator heatsink. The only thing handy was a short piece of RG8 with N connectors left over from a Marti antenna install. The transmitter was happy with that cable, and they're still running with it.

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Re: High Pitched Tone in FM Audio?

Post by NECRAT » Tue Mar 07, 2017 8:15 pm

I know of a station that had this exact issue. High pitched buzzing tone when the FM was silent.

AM/FM pair, FM was a HT20CD, with a co-located AM. The engineer eliminated the noise by removing the exciter from the transmitter and mounting it in a nearby rack.

Not sure more on the specifics, honestly, it was a very long time ago he showed me the fix in person.
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