Harris FM-25K issue

FM does it with frequency!
Post Reply
Joe Friday
Posts: 70
Joined: Tue Dec 16, 2014 3:47 am

Harris FM-25K issue

Post by Joe Friday » Mon Nov 06, 2017 6:24 am

I take care of a FM-25K that I am pulling my hair out over. It is about 100 miles away and has a very intermittent condition I have had no luck in addressing.

The transmitter is configured to use an external BW Broadcast 600W box as the driver. It is properly interlocked and controlled by the 25K, just the same as the original internal exciter/IPA combination was, with the exception that following a fault reset, the BW has to go through the boot up process, which delays the return to air by about 30 seconds.

The problem I am having is intermittent IPA VSWR overloads. I recently swapped out the final for a fresh Econco rebuild. The transmitter had this issue prior to the tube replacement, but would not make full power, and the filament voltage had a great deal of control over the power out. Now I can make full power, but am still dealing with the intermittent IPA fault condition.

I have performed as good a visual inspection as I can in the grid cavity, and replaced the coax between the IPA reflectometer and the grid input on the side of the cavity. There is nothing obvious. The coax between the BW unit and the IPA reflectometer was new about two years ago when we did the driver retro-fit.

The IPA VSWR problem happens suddenly in some cases. I have seen it take place while monitoring the IPA reflected power indicator on the front of the transmitter, and seeing about .5w reflected. There are other times the IPA tuning changes drastically, but upon arrival I have always been able to tune the reflected power out and get the transmitter back on the air.

Previously, I have cleaned the surfaces that the shorting assembly in the IPA loading control makes contact with. When I last left the site the transmitter was running great. 24hrs later it was off the air again.

Any thoughts or suggestions welcome. Fortunately they have a reliable standby transmitter.

Thanks!

Kevin

TPT
Posts: 682
Joined: Mon Dec 03, 2007 3:18 pm
Location: St. Marys, WV

Re: Harris FM-25K issue

Post by TPT » Mon Nov 06, 2017 7:31 am

What is happening with the BW low power transmitter when an IPA fault occurs?

I have a BE 1 kw I use as a back-up transmitter, uses a 1/4 watt exciter card to drive the rest of the box. For no apparent reason this exciter will loose lock momentarily, then come back up, (a replacement card does the same thing). Luckily this unit can be driven by an external exciter, with a suitable pad on the output.

Best guess the culprit in the BE is a little reference oscillator in that exciter-- which looks like something you see on a computer motherboard. I would not be surprised if the BW uses a similar device for its synthesizer.

User avatar
RGORJANCE
Posts: 1388
Joined: Mon Dec 31, 2007 7:48 am
Location: RACINE, WI

Re: Harris FM-25K issue

Post by RGORJANCE » Mon Nov 06, 2017 7:52 am

This is a "for what it's worth" comment.

Recently, I have experienced issues with three rebuilt tubes. One, in a 10KW rig was gassy from the factory, and was arcing over causing several different overloads. After a couple of months it has stabilized. The other tube failed to work right out of the box.

The third replacement, last year, displayed grid VSWR but has dropped significantly after a couple thousand hours.
It also started out a bit goofy, but as it aged, it has settled down very nicely.

You might have a gassy tube.

Fossil

Steve Brown
Posts: 6
Joined: Tue Feb 21, 2017 4:23 pm

Re: Harris FM-25K issue

Post by Steve Brown » Mon Nov 06, 2017 9:09 am

You should check the components in the directional coupler box, the one that lives between the IPA and the cable that connects to the PA cavity. Minnesota Public Radio had a similar issue a few years back and thought like you did that the problem was a faulty cable connection. As I recall, Harris couldn't replace whole directional coupler assembly so the MPR engineers did an in house repair with some components and it then worked OK.

Steve Brown
Radio Rangers Minneapolis

Steve Brown
Posts: 6
Joined: Tue Feb 21, 2017 4:23 pm

Re: Harris FM-25K issue

Post by Steve Brown » Mon Nov 06, 2017 9:31 am

I found a Harris tech note that talks about failures of the module, which is a low pass filter and directional coupler combined. They say to look at variable caps C1 and C3 for clear glass, which means OK---dark glass means failure. Since the device is 50 ohms in and out, you can just terminate the output and drive the input with an exciter to see if the reflected power is OK or changes... Maybe do this with a big power load on the output and drive with your present IPA.

This info is a document that Harris released some time after the 25K was built, and has all sorts of valuable info about problems and solutions for these transmitters. If you don't have it I can send it to you in PDF form, or you may be able to get it from the Gates/Air site.

Steve B

User avatar
Dale H. Cook
Posts: 773
Joined: Thu Dec 20, 2007 9:08 am
Location: Roanoke/Lynchburg, VA
Contact:

Re: Harris FM-25K issue

Post by Dale H. Cook » Mon Nov 06, 2017 10:48 am

To follow up on Steve's response, you should insert a Bird Thruline wattmeter between the IPA and the PA. If the Bird shows VSWR higher than, say, 1.1:1 you have a problem in the PA causing a PA input impedance of other than 50+j0. If the Bird VSWR is very low the intermittent fault could be in the IPA or the PA - the purpose of the Bird measurement is to determine whether the PA input impedance is borderline and possibly causing an IPA VSWR overload under certain conditions. Do not trust the BW reflectometer - use a Bird. This is a problem that I have often seen with a solid-state IPA and a tube PA.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html

User avatar
Deep Thought
Posts: 3217
Joined: Thu Mar 20, 2008 9:23 am
Location: La Grange, IL
Contact:

Re: Harris FM-25K issue

Post by Deep Thought » Mon Nov 06, 2017 11:49 am

"Don't trust the BW reflectometer".

Indeed. I had one of their 600 watt transmitters practically burn itself up because the reflected power sensing did not work at all.

I did a similar conversion on a FM25K, feeding it with a Nautel VS300. One thing to make sure of is that you are only feeding enough power from the BW to make the TPO, because if you go much beyond 400 watts it'll smoke the coupler.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

Joe Friday
Posts: 70
Joined: Tue Dec 16, 2014 3:47 am

Re: Harris FM-25K issue

Post by Joe Friday » Tue Nov 07, 2017 6:59 am

Thank you to all who replied. To answer some of the questions posed in the responses:

I don't know what the BW transmitter is seeing when the IPA overload condition happens...It is about four racks away connected by a RG-8U jumper. When the 25K senses an overload, it interrupts power to the BW unit before I can get in front of it to see what is going on.

I have not had a Bird in circuit between the IPA and final grid...I will do so and see what is taking place. I believe that under normal conditions the 25K IPA reflectometer is accurately measuring the reflected power. When the final grid is tuned for minimum reflected power on the built in metering, it has always corresponded to low reflected power displayed on the BW unit.

I will disassemble the IPA reflectometer and have a look at it. The cable between it and the grid input on the side of the cavity was replaced because the dielectric had shrunk, leaving about 3/8" of the center conductor exposed at each connector. This allowed shield braid to come in close proximity to the center conductor at both ends of the jumper. If this situation did not contribute to my current problem, it was only a matter of time until it would have.

Thanks again for all who took time to respond. Good discussion has helped to get the wheels turning again...I now have some additional things to look at. I will post once I have the problem resolved.

Kevin

User avatar
Dale H. Cook
Posts: 773
Joined: Thu Dec 20, 2007 9:08 am
Location: Roanoke/Lynchburg, VA
Contact:

Re: Harris FM-25K issue

Post by Dale H. Cook » Tue Nov 07, 2017 10:04 am

Joe Friday wrote:
Tue Nov 07, 2017 6:59 am
...It is about four racks away connected by a RG-8U jumper.
It would be wise to make two measurements with the Bird - one at the PA input and one at the IPA output. That should reveal if the jumper is at fault.
Joe Friday wrote:
Tue Nov 07, 2017 6:59 am
I will disassemble the IPA reflectometer and have a look at it.
You should hold off on that until after making the Bird measurements. If they show that the PA or the jumper are at fault time spent disassembling the IPA reflectometer would probably be wasted.

If both Bird measurements show negligible reflected power during normal operation you will need to step up your game to catch an intermittent condition. That can be done with a spare computer (I use an old laptop for that purpose). The idea is to set up the computer to monitor a trigger condition (i.e., the IPA overload) while recording several seconds of one or more data sources (such as the Bird at the PA input) into buffers. When the overload occurs the buffers are written to the HD, and can be examined to show which data source coincides with the overload.

A spare computer configured to record conditions preceeding an overload is a powerful tool for debugging intermittent faults.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html

Lee_Wheeler
Posts: 83
Joined: Sun Jul 24, 2016 1:53 pm
Location: Kansas City

Re: Harris FM-25K issue

Post by Lee_Wheeler » Fri Nov 10, 2017 6:24 pm

The suggestions on the low pass filter / directional coupler are very good. One other place to look is at the cable from the directional coupler to the PA. I recently had to replace all of them on the 4 25Ks that I take care of. One burned up completely and on the rest of them the cable was very brittle and they would crack both the outer jacket and the inner dielectric if they were flexed.

...Lee

ChuckG
Posts: 927
Joined: Tue Mar 11, 2008 11:12 pm
Location: Moo
Contact:

Re: Harris FM-25K issue

Post by ChuckG » Fri Nov 10, 2017 7:04 pm

Joe Friday wrote:
Mon Nov 06, 2017 6:24 am
There are other times the IPA tuning changes drastically, but upon arrival I have always been able to tune the reflected power out and get the transmitter back on the air.
That points towards something after that tuning control, or the control itself. I haven't been inside a 25K for a long time, but if there are caps in that circuit, they're suspect, as is any coax or connectors... I had a 10K do something similar to this, found some of the grid bypass caps had worked loose.
<><><><><><><><><>
Chuck Gennaro
Central Wisconsin

The Wattcher
Posts: 9
Joined: Thu Sep 30, 2010 9:12 pm

Re: Harris FM-25K issue

Post by The Wattcher » Mon Nov 20, 2017 3:34 pm

Drop the bottom cover on the cavity and inspect/replace the short coax (about 6" to 8" long)from the PA BNC input connector to
the Input tuning cap. Early 25K's used RG-58 for the jumper. It will not handle high IPA output power for a long period.
It was later replaced with RG-400. Which works well.

Joe Friday
Posts: 70
Joined: Tue Dec 16, 2014 3:47 am

Re: Harris FM-25K issue

Post by Joe Friday » Sun Dec 03, 2017 7:32 pm

The coax on my unit is the upgraded RG400, and is in good condition, but it is one thing in the circuit that has not been changed. Still having issues. Most recently, the transmitter was brought up having been off air for four days, and ran perfectly without any re tuning necessary for about 18 hours...then the IPA VSWR fault showed up again. A Bird 43 agrees with the metering in the IPA reflectometer. I have not been in the building when the transmitter has failed, so I have not been able to make the Bird measurement when the problem occurs.

I have changed out the IPA directional coupler/second harmonic trap, and did find an issue with the old one. The input connector had a broken solder connection to the circuit board. I don't know how this was not causing a problem, but apparently it was not.

I am beginning to wonder about the BW TX600V2 that I am using as the driver. I have never had an issue with the frequency randomly changing on a BW, and I have a number of them in translator service. If it was randomly changing frequency it could be causing the issue I am having.

To this point I have:

Inspected and verified the integrity of the RG-8 coax between the external BW driver and the IPA directional coupler.

Replace the IPA directional coupler/second harmonic trap (with an upgraded unit from an HT series transmitter).

Replaced the RG-213 coax between the IPA directional coupler/PA grid input.

Cleaned the IPA grid tuning sliding shorting bar assembly.

Changed the final with a rebuilt Econco.

I have another experienced engineer coming Tuesday to look over what I have done thus far...I would like to get this classic back on the air, and it is running with good efficiency and AM noise until the IPA VSWR problem crops up. Fortunately they have a reliable HT10 as a lower power backup.

Thanks!

User avatar
kkiddkkidd
Posts: 560
Joined: Mon Dec 03, 2007 11:13 am
Location: Lawrenceburg, TN

Re: Harris FM-25K issue

Post by kkiddkkidd » Mon Dec 04, 2017 9:56 am

Any chance the power is blinking off for a few seconds or there is a power quality issue? I think that you said that sometimes it required physically retuning the IPA input to clear the fault but was just wondering if the BW might lock on wrong freq briefly at startup or burp off during a power quality problem.

You might want to contact your power provider to see if they have a power quality monitor.

Good luck,
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com

Post Reply