Bleed over

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sndeng
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Bleed over

Post by sndeng » Mon Aug 21, 2017 10:29 pm

Here's the setup. 2 Gates one 1KW AM transmitters. One is at 1450 KHz the other at 1240 KHz. These are running into combiners to the same tower. Both should be doing 1KW output but the best they will do is 750 Watts each, that's it, max power output(they are licensed for 1KW). The 1450 is being fed by a older Orban AM processor. 1240 is being fed by a BWBroadcast DSPXmini-AM processor which I am not familiar with. These both have Marti R-10 receivers. At the studio we have a STL-10 transmitter for 1450 and a frequency agile model(forgot the brand name, STL-10 is being repaired) for 1240. Both studios use a Dynamax xm8 boards fed by Broadcast Tools switchers, no out board processing. The 1450 studio is running normal levels from the switcher and Dynamax board. The 1240 is where is the issue lies. The levels out of the switcher are at minimum, 2 bars on the peaks. The Dynamax board is set as low as it can go on the internal program level adjustments and the linear adjust cannot go above 50 or it bleeds onto 1450. The meters just wiggle a little bit. Going out of the board to the STL there is approximately 15 db resistive attenuation inline with it. I measured the signal with my peak reading scope and 1240 input to the STL was 75 mv, input to the 1450 STL was around 1.23 volts these are Pk-Pk. I tried several troubleshooting setups at the studio and found no distortion but very low levels. I went to the doghouse and attempted to adjust the BWbroadcast processor. My helper was back at the studio prepared to raise or lower the settings on and in the Dynamax mixer. I only adjusted the input and output levels. Initially the input was at +4dbu, output was +11dbu. I adjusted the output level down to -11dbu and the input to 0dbu. Hardly any output reported. I phoned my assistant and asked him to turn the linear adjustment to 75, all was fine. Then I asked him turn turn one of the program pots some the meter would read around 40. Bleed over was bad. I proceeded to try several different setting with the input and output but nothing helped. Major bleed over. I was not comfortable enough to make any processing or limiter adjustments. Turning the Dynamax board back to 50 and the internal programming pot to it's lowest level I still had some bleed over. When I finally set the input and output back to there original levels(+11dbu and +4dbu) it cleaned right up. The R-10 ,1240 receiver, is turned down as low as it can go and still give an audio output. Everything leading up to the processor is turned down as low as it can get. The only thing I have not check out are the Gates one transmitter and the combiners. Could the transmitter be causing this or possibly the 1240 combiner bandpass has moved? I have no information about the combiners. They were built by an engineer from another city. I stepped into a hornets nest with this one. I realize all levels must be balanced and set the same to keep the modulation index within perimeters for both. Any ideas or help would be greatly appreciated.

Update: All 4 860mfd caps have been replaced. One transmitter is now running 1000 watts, no issues. The other is only running at level 5 at 800 watts. The low drive led will come on at some time and, the automatic cutback led will light dropping it to level 4. Even though these come on it remains at 800 watts output. The owners happy with this, I'm not. We still have the issue of RF bleed over. I've tested the audio with headphones all the way to the STL and it's clean when turned up. Checked audio at the STL receiver, at the transmitter, and it too is clean. I called to get help with the setting on the BWbroadcast DSPXmini-AM processor. We turned the levels down for the clipper, limiter, input and output. Audio level was very low. Went back to the studio and started to turn up the levels on the Dynamax MX8 board. Turned the slide pot to 70 we got the RF bleed over in to the other transmitter(scratching). No adjustment to the internal levels or the slider would change this until I turned them back down. I even checked the audio up to the STL transmitter for the station that was getting the interference, it was clean. I haven't checked the audio quality into and out of the DSPXmini-AM processor. Pulling the XLR connectors, to get to the covers off the connections, would take it off the air and, the owner does not want that. I also, do not have access to the studios after 6 PM. This is not a good situation. The input level to the processor unit are turn as low as they can go 0db. What I see is everything seems to be pointing to the input to the processor unit. The input level is around 75 mv pk-pk. That's not much level but, as soon as you turn up the audio level at the studio is when the problem starts. Even though its causing this action the audio output of the transmitter is clean. I ended up turning the studio levels back down to where the needle is barely wiggling and turned up the processor levels at the transmitter. I'ts working with this setup. On the service monitor both transmitters show 100% modulation but, the audio levels feeding the processors are vastly different. Strange, anyone have any ideas on this??
Last edited by sndeng on Fri Sep 01, 2017 10:11 pm, edited 4 times in total.

vacuum tube
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Re: Bleed over

Post by vacuum tube » Tue Aug 22, 2017 7:53 am

The red flag here is both transmitters limited to 750 watts. Are the transmitters folding back due to VSWR? If you have a dummy load put it on one transmitter at a time and see if they make power on the dummy load, or turn one transmitter off and see if you are able to make power with the transmitter still on the air. If you can then that would indicate an antenna or combiner problem or possible transmitter spur issue. This sounds like RF is getting into something it shouldn't. RF could also be getting into the audio chain probably at the transmitter site, but the thing that stands out to me is the limited power output on both transmitters.

w9wi
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Re: Bleed over

Post by w9wi » Tue Aug 22, 2017 8:42 am

IMHO you need to break this down step by step. I suspect you may have more than one problem.

I think what I would do... is *turn off* the 1240 transmitter. Turn up the levels on the 1240 audio and see if it still bleeds over.

If it still bleeds over with the 1240 transmitter turned off... you have a problem in the audio chain. Take some kind of monitor -- a set of headphones, whatever -- and listen to the audio on the input of the 1450 transmitter. Do you hear the bleedover there? Listen again at the input of the processor. If you have bleedover coming out of the processor but not going in, the problem is the processor. If you have bleedover going into the processor, listen to the input of the STL. Eventually you're going to find a device that has clean 1450 audio going in and bleedover coming out. That's the device that's causing your problem. (or the wiring to that device)

If it *doesn't* bleed over with the 1240 transmitter turned off... then as "vacuum tube" says you have an RF problem. If it worked at one time then you probably have a loose wire somewhere at the transmitter building, probably in the audio wiring.

My gut feeling is the inability to get past 750 watts is a separate problem. What happens when you *try* to increase power?
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ncradioeng
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Re: Bleed over

Post by ncradioeng » Tue Aug 22, 2017 9:42 am

Look for a broken or loose ground strap(s) around the combiner and tower - it acts like a bad load on the transmitters and things are getting eat up with RF. This can be verified with the transmitters off and listening to the STL receiver and processor audio outputs - if they are clean then that may be it.

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Deep Thought
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Re: Bleed over

Post by Deep Thought » Tue Aug 22, 2017 11:01 am

A couple thoughts:

1) The "combiners" are just traps set for the other station''s frequency. At this frequency spacing you've probably got three parts in each, one coil and two capacitors on the 1230 KHz side and two coils and one capacitor on 1450 KHz side. Once set these do not normally require maintenance unless damaged by lightning or some physical shock. However, if they have not been checked since you got there something may be wrong with one of them which will affect both stations. An easy test is to look at the signal coming back to the "other" transmitter with just one of them on. If it is more than a couple volts the traps need adjustment. Also, do the stations run at 1 KW without modulation? How about with one on and the other off?

2) You say you have two STLs, one for each station. Are the transmitters combined into one antenna? Have the receivers been checked for proper tuning? It is hard to read that monoblock of text above but it almost seems like the problem is in the STL itself, but you really haven't given enough information to guess one way or another.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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kkiddkkidd
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Re: Bleed over

Post by kkiddkkidd » Tue Aug 22, 2017 2:08 pm

And since these are Gates1 transmitters, check the +/-60khz spurs. If the transmitters are over 6 or 7 years old, they may be suffering Blue Cap of Death symptoms... Now you could have 6 carriers to filter rather than 2.
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sndeng
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Re: Bleed over

Post by sndeng » Tue Aug 22, 2017 4:49 pm

Thanks so much for the suggestions.

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NECRAT
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Re: Bleed over

Post by NECRAT » Wed Aug 23, 2017 5:31 pm

sndeng wrote:
Tue Aug 22, 2017 4:49 pm
Thanks so much for the suggestions.
Please, when you do find the solution, take a couple of minutes and let us know what you found...
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