Harris HT25-FM

FM does it with frequency!
Post Reply
ClassicRadioGuy
Posts: 3
Joined: Thu May 31, 2018 7:34 am

Harris HT25-FM

Post by ClassicRadioGuy » Thu May 31, 2018 8:12 am

I have a Harris HT25FM that has a high pitch whining noise over the carrier, you can really only hear it when the DJ stops talking. All of this came about after a storm that decided to pop up one evening, causing the Transmitter to fluctuate from high to low power. I could never get to the transmitter fast enough to catch this happening. When I opened up the Transmitter I noticed that a BNC cable going to the IPA RFDriver had been bend at a complete right angle, and fell apart in my hands when I touched it. This was squeezed in-between the Preamp and the driver, causing it to be stressed beyond belief. So, I replaced that BNC cable, bypassed the flex patch running it right back to the Exciter. Before, the Exciter ran 10.5 watts, now it has to run 24Watts to make 100% power on the Transmitter. Could this be a simple IPA issue starting to show signs of failing ? Gates Air/Harris, has no idea and have never heard of this type of issue before.

This transmitter has a FAX-50 Exciter off to the side mounted in a rack, has a new cables running into the Transmitter. Bypassed the PreAmp completely and running the exciter straight into the RFdriver. to accomplish this, the Flex patch was bypassed allowing the Exciter to plug its BNC cable right in to the RFdriver.

I pulled the FAX-50 Exciter and swapped it for my back up Exciter thinking that would solve this whining issue. That however, increased the winning noise level to be greater than what it was on the FAX-50 Exciter. I have placed a Mod Monitor to check all audio levels, and a spectrum analyzer to view and read what all is going on. Audio was clear and clean for the most part of the Spectrum Analyzer, however you could see the whining noise a little. I even pulled the audio from the Exciter that runs in to the Transmitter, Noise is still there and heard.
Ive contacted Harris, they are even clueless as to what this could be, mentioning just buying a new Transmitter lol.

grich
Posts: 393
Joined: Fri Sep 02, 2011 11:19 am
Location: MP89.5, Mason City Subdivision

Re: Harris HT25-FM

Post by grich » Thu May 31, 2018 11:50 am

Rotting RF cables between exciter and IPA, and exicter and PA deck are common for this age of transmitter. Check them all.

Are there other stations transmitting from this site? I've seen issues with co-located AM transmitters getting into the FM transmitter and creating spurs on the FM carrier +/- the AM carrier frequency. Make sure your exciter rack and transmitter are properly grounded/bonded.

User avatar
Deep Thought
Posts: 3383
Joined: Thu Mar 20, 2008 9:23 am
Location: La Grange, IL
Contact:

Re: Harris HT25-FM

Post by Deep Thought » Thu May 31, 2018 12:09 pm

Check the AM noise...I'll bet it is very high and the IPA power supply has issues especially if the power was going bouncy bouncy during the storm. There is a 36,000 uF/100VDC filter capacitor on the IPA supply (C4) which you might want to take a look at. Are the IPA voltage and current both "normal"?
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

fm-engineer
Posts: 23
Joined: Mon May 02, 2016 6:22 pm

Re: Harris HT25-FM

Post by fm-engineer » Thu May 31, 2018 6:51 pm

What is the stereo generator? 8000 and 8100 Optimod used to develop a whine or whistle, but I can't exactly remember why. Otherwise, I agree with the others. Check the cables. Change the soup can caps. You may want to open up the IPA and look at the combining circuit. Check the IPA current and see if its normal. If its running low, check the Wilkenson combiner solder joints. I've had two of those lose power over the years (blue version) where re-soldering the combiner fixed the problem.

ClassicRadioGuy
Posts: 3
Joined: Thu May 31, 2018 7:34 am

Re: Harris HT25-FM

Post by ClassicRadioGuy » Fri Jun 01, 2018 6:45 am

Ok All of the soup Can caps and a few small ones have been replaced soon after this event. seemed to help it a little but not by much. The RF cable and audio cable from the Exciter are all new, I will look over the ones in side the TX and replace the ones that I can. The Exciter is mounted on a rack off to the side of the TX and is well grounded. I am wondering if the IPA is just starting to make it way out the door.

ClassicRadioGuy
Posts: 3
Joined: Thu May 31, 2018 7:34 am

Re: Harris HT25-FM

Post by ClassicRadioGuy » Fri Jun 01, 2018 6:47 am

Oh,,,, and I did pull the audio from the Exciter, that eliminates the Optimod and everything else going back to the Station. Just leaving the Transmitter running with no audio from the Studio, but that noise it still there.

Kelly
Posts: 500
Joined: Thu Aug 21, 2008 5:32 pm
Location: Washington D.C. Area

Re: Harris HT25-FM

Post by Kelly » Sun Jun 03, 2018 5:21 pm

A total long shot, but check the MOV's in the HV power supply cabinet. Mine is an HT30, but I had something similar happen where after a storm the MOV just inside the right side panel was making a screaming noise. With the blowers (and in my case a rotary phase converter) running, it's hard to hear, but it was driving my AM noise detected on the Belar mod monitor, through the roof. If failed, usually the MOV is visually damaged, if not blown to pieces.
Skype:kellyalford Twitter: @KellyAlford

Post Reply