BE FM 25T Wandering Readings

FM does it with frequency!
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fmrfman
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Joined: Sun Aug 17, 2014 1:43 am

BE FM 25T Wandering Readings

Post by fmrfman » Sun Dec 10, 2017 1:28 am

Gents:

I have a BE FM 25T with wandering readings when observing from R/C. I started out with a Broadcast Tools WVRC-8 Plus. The wandering was random and tolerable (slight variance). I recently upgraded to Burk ARC Touch Plus and the readings have gone ballistic! Forward power jumps from 100% to 104% from one sample to the next. Amps and plate volts not so much but definitely not normal.

In a nutshell the front panel readings on the transmitter ain't dancing around like my R/C readings are!

Someone suggested a cap across the inputs to the R/C but of course that didn't help. Burk support told me to make sure sample leads were only grounded on one end and after disconnecting it at the Burk nothing changed.

Bad sample board at the transmitter?

Mark

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RGORJANCE
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Re: BE FM 25T Wandering Readings

Post by RGORJANCE » Sun Dec 10, 2017 6:09 am

Thinking out loud here.......I believe I have seen this before on the Burk ARC 16''s when the voltage sample from the xmttr is too high. Typically, I like to see the sample voltages at or below the 2 volt range. I think there is a jumper in the xmttr somewhere that will allow you to select the voltage. Check out your sample voltages at the R/C with a dvm to see what they are running.

Fossil

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Deep Thought
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Re: BE FM 25T Wandering Readings

Post by Deep Thought » Sun Dec 10, 2017 12:19 pm

And once you've done what Fossil suggests, you might consider installing a simple two resistor voltage divider on the sample with a capacitor from the midpoint to ground where it feeds to the Burk. It is much better to have a higher voltage sent to the r/c and then lower it there than to lower the sample, which effectively raises its susceptibility to RFI. I had to do this with a POS "Sicon" system which was installed at a two AM station site before any of the readings would make sense.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

Lee_Wheeler
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Location: Kansas City

Re: BE FM 25T Wandering Readings

Post by Lee_Wheeler » Mon Dec 11, 2017 3:11 pm

I had a bunch of trouble with the Arc Plus readings on the Continental 816s looking like a modulation monitor but in the last firmware and software revs there is an option to increase the sample time which settled the readings down tremendously. I think I gave them a 2 second averaging time on the troublesome transmitters.

...Lee

kcbooboo
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Re: BE FM 25T Wandering Readings

Post by kcbooboo » Tue Dec 12, 2017 7:10 am

It doesn't take much RF floating around to upset the analog readings. I have a site with a 1kw AM TX in the adjacent rack and the tower about 50 ft away, and EVERY analog input has a 10uF 25V electrolytic cap across its screw terminals. That at least made the readings usable. A divider/filter as recommended above would be a better solution although it would be messier.

Bob M.

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