High Pitched Tone in FM Audio?

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Re: High Pitched Tone in FM Audio?

Post by W2XJ » Tue Mar 07, 2017 9:09 pm

Start with the basics first. I had this exact problem years ago on an analog unit. Turns out a three terminal regulator was oscilating due to a failed bypass cap. Check all the various power rails first with a scope and look for oscillation.

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Re: High Pitched Tone in FM Audio?

Post by BroadcastDoc » Wed Mar 08, 2017 9:07 am

grich wrote:
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.

Seconded (or thirded!). Had the exact same issue on a Digit exciter. Did the service bulletins and the whistle disappeared.
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Re: High Pitched Tone in FM Audio?

Post by JRB » Wed Mar 08, 2017 12:32 pm

Most likely in the exciter. In BE FX-30/50 it's a cap in the power supply, it's a bypass cap. There is a service bulletin for this. In a Harris Digit can also do this, it was posted earlier in the thread. There is a service bulletin for this too. Both make a kit that provides instructions and the parts to fix it. Also, with the BE not only do you get the whistle, you get spurs as a bonus.

If you don't have an aux transmitter expect the repair to take about two hours from start to finish.

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Re: High Pitched Tone in FM Audio?

Post by Shane » Wed Mar 08, 2017 10:24 pm

Welcome, John. So if you do have an aux xmitter does it take longer or less time? :roll:
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Re: High Pitched Tone in FM Audio?

Post by W2XJ » Thu Mar 16, 2017 10:52 pm

As previously stated, look for power supply oscillations.

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Re: High Pitched Tone in FM Audio?

Post by timinbovey » Wed Mar 22, 2017 3:47 pm

I thought I should follow up --

Went out this last early Sunday morning. GatesAir sent me the capacitor kit and replacement circuit board cover at no charge (!)

The service bulletin calls for replacing 7 (maybe 8) electrolytic caps on the multiplexer board. And also wants you to either drill a total of 10
holes in the cover for increased ventilation, or switch out the cover for a new one supplied with the holes already in it. I didn't have to do the
hole drilling routing as the modified cover was also provided.

Remember now, this is my first foray into this transmitter -- we just closed the deal March 1st. So I wasn't sure what to expect.

After shutting down the transmitter I went to pull the exciter out of the rack (it's on rails) and it wouldn't pull out more than half an inch. Turns out the top cover had ZERO screws in it, so it was up slightly preventing me from pulling the unit out. I slipped my pocket knife in and was able to hold down the cover enough to slide the exciter out. ZERO screws in the top cover.

The multiplexer board is on the inside of the top cover, which is hinged. Easy enough. About half the nuts that hold the multiplexer board cover on were missing. I removed the circuit board and headed to my shop (it's only about 10 minutes away, and this site isn't really a good spot to be working on boards -- with my old man eye sight I need good lights and a magnifier). Once I got to the shop I was readily apparent that these capacitors had been replaced before. Clearly by someone with poor soldering skills. In fact, one cap had one lead that wasn't soldered at all. It LOOKED like it was soldered, but the lead was loose. I noticed some brown discoloring of the circuit board from heat.

Worth noting. When I opened up the exciter it was awfully hot in there. I had of course shut things down before pulling it out and opening it up.

Reinstalled the board, routed the coax leads correctly (they were all over, none under the nylon retainers where they belonged) Closed it all up (couldn't find the screws for the cover around anyplace). Fired it all up and it sounded fine. I couldn't tell if the whine was gone as I've never been able to hear it. Old man ears and too many years of loud headphones kinda wiped out my high end hearing.

Stuck around for a half hour or so scoping out the rest of the building, keeping an eye on everything. All seemed well. Left and got about 5 minutes from the site when it went off the air. Returned. Turned it back on, all appeared fine. Lasted about 15 minutes. Off again. Pulled the exciter out again, things were awfully hot. Saw the the PLL unlock was blinking. I suspected things were too hot. I blew on the heat sinks behind the front panel and everything popped right back on! Ran a few minutes, went out again, and even the slightest blowing on it would cool it enough to start up again. Cooling fan was NOT running. My guess is it was running slow before I got there and just didn't start up again. As I was completely unfamiliar with this unit I didn't know if the fan might be thermostatically controlled of if it runs continuous (continuous is the answer).

So to keep it on the air it needed air flow. I ran home, grabbed a small cheap desk fan, a mic stand with a gooseneck, and duck tape. You can see where this is going. Properly positioned the little desk fan gives more than enough airflow to keep things cool. So, since Sunday morning we're on the air with the exciter pulled out, the top cover open, and the fan blowing.

New fan is on the way. High pitched whine is gone, as determined by the station owner, my daughter and her husband, and two listeners who had emailed right after the news was out that the station had new owners begging us to get rid of that "horrible whistling noise".

I find out after the fact, that a couple weeks before, our owner and our IT guy were out there doing other things, and noticed the transmitter was running with the exciter pulled out of the rack. My guess is they were having cooling issues.

Now I wonder what sort of other bad work lurks inside. The previous engineers said they did NOT to the cap replacement on this board -- which I'll believe as there have been several engineers for this station over the past few years. They DID however claim they did some other fix that was supposed to stop the whine (that didn't) that they said involved "replacing a capacitor and a coil".

Anyway, the whine is gone, and a new fan will be in by next week!

I don't suppose anyone knows off hand what size screws go in the top cover?

Sorry for the long story!


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Re: High Pitched Tone in FM Audio?

Post by kcbooboo » Wed Mar 22, 2017 5:18 pm

If the holes in the top cover are counter-sunk, then it probably wants phillips flat-head screws. If not, then ordinary phillips round-head or pan-head screws with a split-ring lockwasher and a flat washer. The thread size will need to be determined by trying some other screws of known size, but possibly #4-40 or #6-32, 1/4 to 3/8 inch long depending on how thick the top cover is and what the screws might hit on the inside. If I recall, the Harris DexStar used a boatload of #4-40 by 3/8 screws.

I'm not sure if you ever mentioned exactly what model exciter you were working on, but GatesAir should certainly be able to take a better guess than I did as they probably have a full parts list including hardware.

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Re: High Pitched Tone in FM Audio?

Post by Kelly » Thu Mar 23, 2017 7:51 am

Now that you've got the exciter issues in hand, you may want to check for AM noise next. If you don't have one available already, see if you can get your hands on a newer Belar or decent modulation monitor that measures AM noise. That transmitter-exciter combination all tuned up, should see AM noise in the -63db range. My guess is with the past deferred maintenance, you'll probably find the AM noise is at unacceptable levels. As you probably know, but is frequently neglected, AM noise can be really destructive to the overall signal quality.
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Re: High Pitched Tone in FM Audio?

Post by timinbovey » Mon Mar 27, 2017 3:24 am

Back out this Sunday morning to replace the cooling fan, to replace my ic stand and desk fan that had been keeping it on the air all week.

Fan replacement was straightforward, didn't even have to remove the exciter from the rack, easily accessible from the back of the transmitter.

Transmitter build date was 2004, manufacture date for the failed fan was 2014! Bought our replacement from GatesAir -- $116! Digikey had one listed for $34, different manufacturer, but it was out of stock and they told me 8 weeks....

For the record the screws for the top cover are 6-32, 3/8" with #6 lock washers. (Just in case someone else ever needs to know).

I could get the old fan to turn with a bit of coaxing it would turn, slowly. I imagine when I shut down to do the capacitor job the fan just wouldn't start up again.

BTW, AM noise, OK, checked on my Inovonics 531N


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