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Hum on WFAN 660?
Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660,
does anybody else? The R8B hears it except with DSB synch detection, so there's some relation between I and Q channels that cancels the hum. If I offtune 1 kHz and use CW, the 120Hz lines move up 1 kHz. Other stations don't have it. On the other hand I don't hear it on WFAN on other receivers, hence this query. But I don't get a good WFAN signal on other receivers either. The R8B is running on DC. -- Ron Hardin On the internet, nobody knows you're a jerk. |
Heh.
I remember CKNW here in Vancouver have 120Hz hum for two days. When driving to work one morning, there was a big BZZZZZT of 60 cycle on the channel and they were down for two hours for repair. -- Gregg *It's probably useful, even if it can't be SPICE'd* http://geek.scorpiorising.ca |
Does the hum go away at night ? Alot of IBOC testing above / below 660
-- Remove "zz" from e-mail address to direct reply. "Gregg" wrote in message news:T2uLb.192551$ss5.3060@clgrps13... Heh. I remember CKNW here in Vancouver have 120Hz hum for two days. When driving to work one morning, there was a big BZZZZZT of 60 cycle on the channel and they were down for two hours for repair. -- Gregg *It's probably useful, even if it can't be SPICE'd* http://geek.scorpiorising.ca |
Steve Stone wrote:
Does the hum go away at night ? Alot of IBOC testing above / below 660 I only hear WFAN at night (Central Ohio listening to NYC). -- Ron Hardin On the internet, nobody knows you're a jerk. |
Ron Hardin wrote: Steve Stone wrote: Does the hum go away at night ? Alot of IBOC testing above / below 660 I only hear WFAN at night (Central Ohio listening to NYC). -- Ron Hardin I can get WFAN almost any night here in Michigan, but I've never yet gotten it *strong*, and I, too, hear the "hum". It used to be a 50,000W station, but in August of 2002, it's output power dropped to 35,000W 'round the clock. Still, I feel it should come in stronger than it does. TOny ----== Posted via Newsfeed.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeed.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= 19 East/West-Coast Specialized Servers - Total Privacy via Encryption =--- |
"Tony Meloche" wrote in message ... Ron Hardin wrote: Steve Stone wrote: Does the hum go away at night ? Alot of IBOC testing above / below 660 I only hear WFAN at night (Central Ohio listening to NYC). -- Ron Hardin I can get WFAN almost any night here in Michigan, but I've never yet gotten it *strong*, and I, too, hear the "hum". It used to be a 50,000W station, but in August of 2002, it's output power dropped to 35,000W 'round the clock. Still, I feel it should come in stronger than it does. WFAN is a 50,000 station. It has not dropped power. They have a permit for a 35,000 watt auxiliary transmitter tuned to an auxiliary antenna (shorter) to use in emergencies and when they are doing work on the main tower. The auxiliary tower is not even 1/4 wave, so would, I believe, be non-conforming as a main antenna. |
In article ,
Ron Hardin wrote: Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660, does anybody else? The R8B hears it except with DSB synch detection, so there's some relation between I and Q channels that cancels the hum. If I offtune 1 kHz and use CW, the 120Hz lines move up 1 kHz. Other stations don't have it. On the other hand I don't hear it on WFAN on other receivers, hence this query. But I don't get a good WFAN signal on other receivers either. The R8B is running on DC. 10:11 PM - I'm listening to them on my R-2368 receiver and hear absolutely no hum at all (60, 120 or 180 Hz). Your receiver has a 50 kHz IF frequency. You sure your not getting an image in there 50 kHz away from 660 kHz (like WOR at 710 kHz)? Tim Brown |
Tim Brown wrote:
In article , Ron Hardin wrote: Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660, does anybody else? The R8B hears it except with DSB synch detection, so there's some relation between I and Q channels that cancels the hum. If I offtune 1 kHz and use CW, the 120Hz lines move up 1 kHz. Other stations don't have it. On the other hand I don't hear it on WFAN on other receivers, hence this query. But I don't get a good WFAN signal on other receivers either. The R8B is running on DC. 10:11 PM - I'm listening to them on my R-2368 receiver and hear absolutely no hum at all (60, 120 or 180 Hz). Your receiver has a 50 kHz IF frequency. You sure your not getting an image in there 50 kHz away from 660 kHz (like WOR at 710 kHz)? Tim Brown It's still there at 4:30am EST. It's not audible at all if you're hearing both sidebands but obvious in quiet phone call pauses (no music, not fan noise). Either receive it SSB, or synch detect one sideband, or offtune until the signal almost distorts on AM, and I think you'll hear the hum. It's some difference between the upper sideband and lower sideband that gets modulated by hum; so if you don't hear them equally the hum starts to creep in, and at some point becomes noticeable. (So propagation at night can make the hum come and go if you're hearing both sidebands, by depressing one and raising the other.) Is there some form of stereo that produces sideband differences? WFAN is famous for ground loops in its audio and maybe they left one plugged in to the stereo difference channel or something. -- Ron Hardin On the internet, nobody knows you're a jerk. |
Ron Hardin schreef:
Tim Brown wrote: In article , Ron Hardin wrote: Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660, does anybody else? The R8B hears it except with DSB synch detection, so there's some relation between I and Q channels that cancels the hum. If I offtune 1 kHz and use CW, the 120Hz lines move up 1 kHz. Other stations don't have it. On the other hand I don't hear it on WFAN on other receivers, hence this query. But I don't get a good WFAN signal on other receivers either. The R8B is running on DC. 10:11 PM - I'm listening to them on my R-2368 receiver and hear absolutely no hum at all (60, 120 or 180 Hz). Your receiver has a 50 kHz IF frequency. You sure your not getting an image in there 50 kHz away from 660 kHz (like WOR at 710 kHz)? Tim Brown It's still there at 4:30am EST. It's not audible at all if you're hearing both sidebands but obvious in quiet phone call pauses (no music, not fan noise). Either receive it SSB, or synch detect one sideband, or offtune until the signal almost distorts on AM, and I think you'll hear the hum. It's some difference between the upper sideband and lower sideband that gets modulated by hum; so if you don't hear them equally the hum starts to creep in, and at some point becomes noticeable. (So propagation at night can make the hum come and go if you're hearing both sidebands, by depressing one and raising the other.) Is there some form of stereo that produces sideband differences? WFAN is famous for ground loops in its audio and maybe they left one plugged in to the stereo difference channel or something. -- Ron Hardin On the internet, nobody knows you're a jerk. It is more likely that the hum is provided by an electronic device generating a signal almost on 660 kHz. Stereo is not sideband split, and an AM TX cab never produce a different signal in one of its sideband: except for bandwidt. ruud |
Ruud Poeze wrote:
Ron Hardin schreef: Tim Brown wrote: In article , Ron Hardin wrote: Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660, does anybody else? The R8B hears it except with DSB synch detection, so there's some relation between I and Q channels that cancels the hum. If I offtune 1 kHz and use CW, the 120Hz lines move up 1 kHz. Other stations don't have it. On the other hand I don't hear it on WFAN on other receivers, hence this query. But I don't get a good WFAN signal on other receivers either. The R8B is running on DC. 10:11 PM - I'm listening to them on my R-2368 receiver and hear absolutely no hum at all (60, 120 or 180 Hz). Your receiver has a 50 kHz IF frequency. You sure your not getting an image in there 50 kHz away from 660 kHz (like WOR at 710 kHz)? Tim Brown It's still there at 4:30am EST. It's not audible at all if you're hearing both sidebands but obvious in quiet phone call pauses (no music, not fan noise). Either receive it SSB, or synch detect one sideband, or offtune until the signal almost distorts on AM, and I think you'll hear the hum. It's some difference between the upper sideband and lower sideband that gets modulated by hum; so if you don't hear them equally the hum starts to creep in, and at some point becomes noticeable. (So propagation at night can make the hum come and go if you're hearing both sidebands, by depressing one and raising the other.) Is there some form of stereo that produces sideband differences? WFAN is famous for ground loops in its audio and maybe they left one plugged in to the stereo difference channel or something. It is more likely that the hum is provided by an electronic device generating a signal almost on 660 kHz. Stereo is not sideband split, and an AM TX cab never produce a different signal in one of its sideband: except for bandwidt. ruud It's a mystery. I don't think it can be that though because it happens most when WFAN is strong, whereas a local signal would show up more at some point in its fading. Also a series of 120 Hz lines + harmonics show up on an audio spectrum around the carrier when it's detuned in SSB by a kHz or so. I have the feeling that they don't use straight AM transmitters anymore; something like SSB transmitters with carrier reinserted, fed into a broadband final. The high power part doesn't care what feeds it. Bandwidth limiting is done at audio frequencies, for instance. Well I can't experiment again until WFAN is again audible, sometime tonight. I can distinguish directionally very easily (I have an 8-element MW phased array) but have been unable to do anything but null away WFAN, winding up with weak Cuban stations rather than a surviving hum signal. -- Ron Hardin On the internet, nobody knows you're a jerk. |
In article ,
Ron Hardin wrote: Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660, does anybody else? The R8B hears it except with DSB synch detection, so there's some relation between I and Q channels that cancels the hum. In article Ron Hardin wrote: It's still there at 4:30am EST. It's not audible at all if you're hearing both sidebands but obvious in quiet phone call pauses (no music, not fan noise). It's some difference between the upper sideband and lower sideband that gets modulated by hum; so if you don't hear them equally the hum starts to creep in, and at some point becomes noticeable. (So propagation at night can make the hum come and go if you're hearing both sidebands, by depressing one and raising the other.) WFAN's transmitter is diplexed into a common antenna with WCBS's (880 kHz) transmitter. Part of the diplexing arrangement includes impedance matching networks that are supposed to equalize the antenna load impedance reflected towards the transmitter output over the sideband bandwidth. I suppose they could be slightly out of adjustment, causing asymmetrical sideband response. One side effect of asymmetrical response would be the generation of AM-to-PM. At the joint transmitter site are older Continental transmitters that function as backup for the regular Harris DX50 transmitters. Maybe they have one of the Conti's on occasionally? Most of these high power transmitters run off of 480V 3-phase power. Ripple frequency with a straight 3-phase rectifier would be 180 Hz, not 120 Hz. A popular supply arrangement is the 3-phase 6-pulse rectifier which outputs a 360 Hz ripple frequency. 120 Hz ripple frequency would most likely originate in either an audio or low level RF stage powered from a conventional 2-phase rectifier. I'll take another listen tonight. |
Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660,
does anybody else? I've been hearing a background hum on WFAN for a LONG time, but only when listening in "forced" AM Stereo. When listening in mono, the hum disappears. Normally, AM Stereo receivers rely upon a 25 Hz "pilot tone" that is transmitted by an AM Stereo station, which tells the receiver to switch into stereo mode. However, unlike FM Stereo, this pilot tone is NOT necessary to receive AM Stereo, so some receivers -- particularly from Sony -- operate in "forced" AM Stereo mode, which receives _all_ AM signals in stereo mode, regardless if a 25 Hz pilot tone is received or not. Thus, with my Sony receiver, I can listen to WFAN (and other "mono" stations) in "forced" AM Stereo mode, and in this mode, I hear the 120 Hz background hum. However, the hum is "out-of-phase", indicating that it is being received in the L-R "stereo difference" component of the signal. When the receiver is switched to mono, the L-R component is not received, and WFAN's hum disappears. I once asked WFAN's engineer about this hum, and they said WFAN was not transmitting it, and that it was being caused by my reception conditions, such as electrical interference. However, I hear this hum from WFAN, and ONLY from WFAN, 24 hours a day, even when there's a local power failure and thus no electrical interference can possibly be generated! WFAN _was_ an AM Stereo station until about two years ago, when they reverted to mono. But, it IS possible that their C-Quam AM Stereo exciter is still installed and operational, but merely is being fed with mono audio and has its 25 Hz pilot tone generator switched off -- which, for all intensive purposes, means WFAN is transmitting a conventional "mono" signal. However, if that AM Stereo exciter ITSELF is the causing this 120 Hz hum, it could be introducing it into WFAN's signal in the manner that I described -- only "forced" AM Stereo listeners will be able to hear it, while mono listeners won't hear the hum. Also, listeners with sophisticated communications receivers may also hear it in Independent Sideband (ISB) mode, which operates in a similar manner to AM Stereo reception and can pick up the stereo difference component transmitted by AM Stereo stations (although not in a manner which provides correct left-to-right stereo separation in the received audio). Thus, with this kind of receiver, you may also be able to hear this mysterious "WFAN hum". |
That's the most compelling argument I've seen to explain this...when Ron mentioned
that the modulation was in the Q channel, you had to know that it wasn't coming from conventional amplitude modulation. The only other explanation I can come up with, building on this one, is that this transmitter's exciter has developed a phase-modulated hum. Keeping in mind that all transmitters have exciters which operate from low voltage power supplies, and those supplies are usually powered with single-phase 120 VAC, a 'lytic that went south could possibly explain it without having to resort to an abandoned but still connected stereo exciter. You wouldn't hear the hum, as already established, if you were dead-on tuned, since the RF exciter's amplitude, even if it was affected by the problem, is clipped by the next class C stage the RF path encounters, but you'd see the sidebands in a spectrum analyzer, and you could hear all but the quietest of these when you tuned through the center. If I were the engineer in charge, I'd be embarrassed to have to admit the latter, but probably even more embarassed to admit that maintenance on the rig was so bad that this situation could occur. After all, it's detectable with a simple tune-through...is this to say that the station's engineers NEVER tune into the station? When I ran a transmitter plant, it was an expected job function to LISTEN to the station for frequent quality checks, even if it wasn't the preferred format for the engineer. And from WBRW says, this was mentioned to the CE a long time ago....if he was doing his job, he'd have looked into it. I couild understand if this was a 1kW daytimer, but this is a Class I Clear, after all. And what quasi-natural interference, in anyone's experience, isn't amplitude modulated? And they say that deregulation is good for broadcasting! -- For direct replies, take out the contents between the hyphens. -Really!- "WBRW" wrote in message om... Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660, does anybody else? I've been hearing a background hum on WFAN for a LONG time, but only when listening in "forced" AM Stereo. When listening in mono, the hum disappears. [...] I once asked WFAN's engineer about this hum, and they said WFAN was not transmitting it, and that it was being caused by my reception conditions, such as electrical interference. However, I hear this hum from WFAN, and ONLY from WFAN, 24 hours a day, even when there's a local power failure and thus no electrical interference can possibly be generated! WFAN _was_ an AM Stereo station until about two years ago, when they reverted to mono. But, it IS possible that their C-Quam AM Stereo exciter is still installed and operational, but merely is being fed with mono audio and has its 25 Hz pilot tone generator switched off -- which, for all intensive purposes, means WFAN is transmitting a conventional "mono" signal. However, if that AM Stereo exciter ITSELF is the causing this 120 Hz hum, it could be introducing it into WFAN's signal in the manner that I described -- only "forced" AM Stereo listeners will be able to hear it, while mono listeners won't hear the hum. [...] |
In article ,
Tim Brown wrote: In article , Ron Hardin wrote: Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660, does anybody else? The R8B hears it except with DSB synch detection, so there's some relation between I and Q channels that cancels the hum. In article Ron Hardin wrote: It's still there at 4:30am EST. It's not audible at all if you're hearing both sidebands but obvious in quiet phone call pauses (no music, not fan noise). It's some difference between the upper sideband and lower sideband that gets modulated by hum; so if you don't hear them equally the hum starts to creep in, and at some point becomes noticeable. (So propagation at night can make the hum come and go if you're hearing both sidebands, by depressing one and raising the other.) WFAN's transmitter is diplexed into a common antenna with WCBS's (880 kHz) transmitter. Part of the diplexing arrangement includes impedance matching networks that are supposed to equalize the antenna load impedance reflected towards the transmitter output over the sideband bandwidth. I suppose they could be slightly out of adjustment, causing asymmetrical sideband response. One side effect of asymmetrical response would be the generation of AM-to-PM. At the joint transmitter site are older Continental transmitters that function as backup for the regular Harris DX50 transmitters. Maybe they have one of the Conti's on occasionally? Most of these high power transmitters run off of 480V 3-phase power. Ripple frequency with a straight 3-phase rectifier would be 180 Hz, not 120 Hz. A popular supply arrangement is the 3-phase 6-pulse rectifier which outputs a 360 Hz ripple frequency. 120 Hz ripple frequency would most likely originate in either an audio or low level RF stage powered from a conventional 2-phase rectifier. I'll take another listen tonight. Okay. I set the receiver to USB and tuned in WFAN this evening and can now hear the "hum". The frequency sounds more like 125 Hz, not 120 Hz, which is significant. Assuming that they still have their CQUAM exciter patched into the transmitter, my guess is that we are hearing the 5th and other odd harmonics of a 25 Hz stereo pilot square wave. This would be phase modulating the carrier, which is why it is not audible when receiving the station with a conventional envelope detector. One way to deal with this would be to make a really clean recording of some "hum" infested program material, burn it onto a CD-R, and mail the CD and a -nicely- written letter to the corporate director of engineering at Infinity. I'll wager you, it will get fixed really fast! |
Tim Brown wrote:
I'll take another listen tonight. Okay. I set the receiver to USB and tuned in WFAN this evening and can now hear the "hum". The frequency sounds more like 125 Hz, not 120 Hz, which is significant. Assuming that they still have their CQUAM exciter patched into the transmitter, my guess is that we are hearing the 5th and other odd harmonics of a 25 Hz stereo pilot square wave. This would be phase modulating the carrier, which is why it is not audible when receiving the station with a conventional envelope detector. One way to deal with this would be to make a really clean recording of some "hum" infested program material, burn it onto a CD-R, and mail the CD and a -nicely- written letter to the corporate director of engineering at Infinity. I'll wager you, it will get fixed really fast! Here's some short .wav recordings from around 5am (apparently ``The Continent'' Carlin does overnights on weekends, nice career arc; luckily he has a speaking style that produces a lot of dead air) http://rhhardin.home.mindspring.com/HUMUSB.WAV (31k) synch det USB (4:52am EST) http://rhhardin.home.mindspring.com/HUMDSB.WAV (17k) synch det DSB (4:54am EST) http://rhhardin.home.mindspring.com/HUMLSB.WAV (27k) synch det LSB (4:56am EST) http://rhhardin.home.mindspring.com/HUMAM.WAV (28k) AM (4:59am EST) http://rhhardin.home.mindspring.com/HUMAMU.WAV (27k) AM detuned to 662.400 (5:08am EST) I see lines at 120 240 360 480 600 720 840 1080, the lines above 480 being faint, with ``birding'' software that has a granularity of about 20Hz, but in particular it doesn't seem to have any lines that would reflect a low frequency around 20 Hz. These recordings all went through a Timewave 599zx DSP to get the AGC to jack up dead air loudness. The lines are all visible in any program material but the ear doesn't notice them when something else is there too, so much. -- Ron Hardin On the internet, nobody knows you're a jerk. |
Tim Brown wrote:
WFAN's transmitter is diplexed into a common antenna with WCBS's (880 kHz) transmitter. When did they start sharing the antenna? Was there ever a time when the three major radio networks (NBC, CBS, ABC) in NYC each had there own antenna? -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Sure. For one, WABC has "it's own" antenna at Lodi, New Jersey.
-- ---------------------------------------------------------------------------- -- Don Forsling "Iowa--Gateway to Those Big Rectangular States" "starman" wrote in message ... Tim Brown wrote: WFAN's transmitter is diplexed into a common antenna with WCBS's (880 kHz) transmitter. When did they start sharing the antenna? Was there ever a time when the three major radio networks (NBC, CBS, ABC) in NYC each had there own antenna? -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
That's the most compelling argument I've seen to explain this...when Ron
mentioned that the modulation was in the Q channel, you had to know that it wasn't coming from conventional amplitude modulation. Here's my recording of the "WFAN hum", as received today on a battery-powered Sony SRF-A100 receiver in wideband AM Stereo mode: ftp://ftp.amstereoradio.com/uploads/wfan-hum.mp3 This was received in C-Quam-compatible mode, but switching to ISB (Kahn) mode yielded no audible difference. A spectrum analysis of the L-R component reveals it to be a 120 Hz hum with multiple harmonics extending well above 1 kHz... so it's more of a "dirty" hum, akin to that of an audio device with inadequate filtering of its DC power supply. |
I HAVE A HUM??
BERNIE.................................... Ruud Poeze wrote: Ron Hardin schreef: Tim Brown wrote: In article , Ron Hardin wrote: Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660, does anybody else? The R8B hears it except with DSB synch detection, so there's some relation between I and Q channels that cancels the hum. If I offtune 1 kHz and use CW, the 120Hz lines move up 1 kHz. Other stations don't have it. On the other hand I don't hear it on WFAN on other receivers, hence this query. But I don't get a good WFAN signal on other receivers either. The R8B is running on DC. 10:11 PM - I'm listening to them on my R-2368 receiver and hear absolutely no hum at all (60, 120 or 180 Hz). Your receiver has a 50 kHz IF frequency. You sure your not getting an image in there 50 kHz away from 660 kHz (like WOR at 710 kHz)? Tim Brown It's still there at 4:30am EST. It's not audible at all if you're hearing both sidebands but obvious in quiet phone call pauses (no music, not fan noise). Either receive it SSB, or synch detect one sideband, or offtune until the signal almost distorts on AM, and I think you'll hear the hum. It's some difference between the upper sideband and lower sideband that gets modulated by hum; so if you don't hear them equally the hum starts to creep in, and at some point becomes noticeable. (So propagation at night can make the hum come and go if you're hearing both sidebands, by depressing one and raising the other.) Is there some form of stereo that produces sideband differences? WFAN is famous for ground loops in its audio and maybe they left one plugged in to the stereo difference channel or something. -- Ron Hardin On the internet, nobody knows you're a jerk. It is more likely that the hum is provided by an electronic device generating a signal almost on 660 kHz. Stereo is not sideband split, and an AM TX cab never produce a different signal in one of its sideband: except for bandwidt. ruud |
Yeah, that's what my ears tell me, too....it's not a ground loop, in my
opinion...sounds just like the buzz I get when I plug a DC-powered radio into a battery eliminator, before the eliminator's loosely-coupled transformer-powered DC supply's come up to full filtering. The only other thing I could think it is (which you've pretty much eliminated when you said you see it even in the midst of a power failure (I assume that was in August?) is that I've seen it where the AC line can get intermodulated somehow with a station, which is localized around only one station or so. Even that, however, is amplitude modulation, so it's doublenixed as a possible explanation. -- For direct replies, take out the contents between the hyphens. -Really!- "WBRW" wrote in message om... That's the most compelling argument I've seen to explain this...when Ron mentioned that the modulation was in the Q channel, you had to know that it wasn't coming from conventional amplitude modulation. Here's my recording of the "WFAN hum", as received today on a battery-powered Sony SRF-A100 receiver in wideband AM Stereo mode: ftp://ftp.amstereoradio.com/uploads/wfan-hum.mp3 This was received in C-Quam-compatible mode, but switching to ISB (Kahn) mode yielded no audible difference. A spectrum analysis of the L-R component reveals it to be a 120 Hz hum with multiple harmonics extending well above 1 kHz... so it's more of a "dirty" hum, akin to that of an audio device with inadequate filtering of its DC power supply. |
Ron Hardin wrote:
Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660, does anybody else? Maybe their cooling Wfans on the transmitter are noisy? ;-) |
Maybe their cooling Wfans on the transmitter are noisy? ;-)
Or maybe their transformerless "All-American Five" transmitter has its line cord plugged in the wrong way! :-) (Casting an appreciative glance towards my Hallicrafters S-38C....) |
In article ,
Robert Casey wrote: Ron Hardin wrote: Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660, does anybody else? Maybe their cooling Wfans on the transmitter are noisy? ;-) When I tuned in tonight (Monday evening 1/13/04), I noticed an apparent decrease in the "hum" level. At least the 120 Hz component of it appears to have become attenuated and what I hear now are the upper remaining harmonics - sounds sort of like a little roto rooter whirring in the background. I was having a good deal of difficulty hearing the hum during my previous post last week when I identified the hum frequency as 125 Hz. There was a Patriots football game in progress and there were precious few moments to hear any hum at all (the Pats won the game and I lost the hum). So I guess I was "out of bounds" on my frequency estimate and will concede to Ron's measurements were he has identified the hum as being composed of numerous 60 Hz harmonics. That whole bit about the stereo exciter pilot tone was a bit of a stretch, anyway. The spec sheet for the Harris 3DX-50 transmitter identifies the prime power supply as a 12-pulse SCR controlled rectifier. This is exactly the type of rectifier that is capable of generating the large number of 60 Hz harmonics noted in Ron's measurements. If the 3-phase ac input power becomes unbalanced, then lower frequency 60 Hz harmonics will appear in the ripple spectrum. There is no spec for incidental PM due to power supply ripple, but the IQM spec for 95% 1 kHz modulation is -36 dB, which is a lot higher than I would have expected, indicating that the potential for phase mod definitely exists. Granted, they are using more vintage DX-50s than the quoted model, but I would assume that the power supply configurations are similar in nature. I've listened to WCBS on 880 kHz (LSB), and don't hear any hum at all. They are colocated with WFAN and are both using the same ac power line feed, so why hum happens on transmitter and not the other is a mystery to me. T. Brown |
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