BE AM-6A RF Out Problem

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dd92251
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BE AM-6A RF Out Problem

Post by dd92251 » Sun Jul 07, 2019 10:37 am

Greetings... I have an AM-6A that has a RED Power Module warning lite that is ON all the time.

1) All of the PA Modules show good GREEN lites, no issues at all.

2) When I turn on the Power Level 1,2,3,4,or 5 buttons to get some RF output, I get a VSWR RED warning lite and more Reflected Power on the meter than I have Forward Power.

3) The Power Supply Lite stays GREEN no problem there.

4) This occurs regardless of whether the transmitter is feeding the antenna or Dummy load.

5) Even power cycling the AC breaker doesn't help.

6) The transmitter does have the BE Antenna Matching Unit installed in the rack.

7) Any ideas what the problem could be ?

Thanks
Dave Dybas

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kkiddkkidd
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Re: BE AM-6A RF Out Problem

Post by kkiddkkidd » Sun Jul 07, 2019 12:17 pm

I have never seen that in any of the 6's of 5e's that I have cared for but it sounds like something in the tuning network or lightning arc gap has gone bad. The directional coupler is between the combiner output/bandpass filter and input to the tuner.

IIRC, it is a pretty simple layout but a real pain to get to... I am not sure if I have ever torn into a 6 but monkeyed with an exasperating 5e for months before BE replaced it with a 6.

Good luck,
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Re: BE AM-6A RF Out Problem

Post by radio_guru » Sun Jul 07, 2019 12:40 pm

Dave....

When you adjust the matching network into the dummy load, does it change the reflected power at all? Do you see any current on the base current/common point meter when driving the antenna? If not, then the TX is clearly not putting out any power at the stud.

I agree with Kevin that it could be a problem in the sample or matching network. Particularly if the adjusting the match doesn't change the meter readings.

And I agree, this TX requires a degree in physical dexterity to reach some of the components.

Gil Housewright is my go-to guy at BE.

RG...

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Re: BE AM-6A RF Out Problem

Post by rfn » Sun Jul 07, 2019 7:45 pm

I have never tangled with anything BE but I did have a similar-sounding problem with a Nautel ND25. A couple of PA modules showing red indicators but all the the regular checks found no problems. Just as an experiment I swapped modules around and found the problem stayed with the slot rather than traveling with the modules.

Ultimately, and quite by accident, I got inside the combiner cabinet and found the braided wires connecting two of the coils to the circuit board had broken. I blame it on fan vibration over 20+ years of 24/7 operation. Since I found the problem while decommissioning (installing an NX25 replacement) I tacked one back on, fired it up one last time; the one associated PA module no longer had a red light. Didn't bother with the second one or check any of the others because (there being no market for it, even for parts), it was destined for the dump.

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RGORJANCE
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Re: BE AM-6A RF Out Problem

Post by RGORJANCE » Mon Jul 08, 2019 6:21 am

I service an AM-10A that had lots of difficulties in it's early days. I found numerous failures in the combiner boards and had to replace several.

Now, the other problems I had sounded similar to this one. Remove several modules associated with the bad indicating one in the same quad enclosure. Next, get a good flashlight and look carefully at the card edge connectors associated with the bad module. If there are any discolored connector pins in the socket, that is where you will find a problem. My experiences revealed that the printed circuit traces on the modules edge connector had also heated up. Removal of the quad enclosure is not one of the easiest jobs you will ever undertake, and is a real challenge for the lowest quad. I was able to locate the connectors at a very reasonable cost through either Mouser, or Digikey. I also found an easy way to replace only the affected pins by pulling the pins out of the new connectors.

I will do some digging thru my computer documents to find the typed procedure I developed used and PM it to you as soon as I locate it. I think I recorded the connector part number and source.

Also, in an AM-2.5, I had a tuning unit cap blitzed by lightning. It was a real PITA to get the tuning unit out of the xmttr, but that was partly caused by previous engineers designing the building only about 1/2 the size it should have been, but that's another war story.

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RGORJANCE
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Re: BE AM-6A RF Out Problem

Post by RGORJANCE » Mon Jul 08, 2019 7:11 am

Dave:

Sent you a PM with the details of quad removal. Unfortunately, I didn't have the connector make and model recorded. Will get that to you later.

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dd92251
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Re: BE AM-6A RF Out Problem

Post by dd92251 » Mon Jul 08, 2019 7:33 am

Bob Gorjance.... This issue is a bit weird, because all the PA Modules have Green Lites on them....there are no Red Fault Lites on the PA Modules themselves. The RED "RF Module" Fault lite is showing up on the Exciter and the Red VSWR Lite comes on when I try to turn on the RF. I'm going to look into the Antenna Matcher Unit today... Maybe the Cap in the "T" network is shorted ?

Will keep you posted.

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RGORJANCE
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Re: BE AM-6A RF Out Problem

Post by RGORJANCE » Mon Jul 08, 2019 7:45 am

Ok, guess I miss-read the initial post. Could very well be in the tuner.

BTW-Gil at BE retired quite some time back, and at last word was hiding out like a hermit in the mountains of Montana, not wanting to look at another xmttr. Do I blame him? Not in the least! I'd much rather be leaning up against a tree with the line in the water, taking a summer siesta with the crock jug at my side! (no bait, no hook on the line)

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ncradioeng
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Re: BE AM-6A RF Out Problem

Post by ncradioeng » Mon Jul 08, 2019 8:05 am

If the PA power supply at the top of the rear cabinet has an issue, that can cause strange things to happen. You might try swapping it with one of the other supplies and see what happens.

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Re: BE AM-6A RF Out Problem

Post by RodeoJack » Mon Jul 08, 2019 10:07 am

Think I'd call BE and talk with them about that exciter fault light. If the board has gone and your shooting miscellaneous odd stuff through the transmitter, VSWR seems like a reasonable indication. Beyond that, given this is a strange one, I'd put a dummy load on the transmitter and bypass everything else until I'd validated the rig itself.

Regardless of anything else, those transmitters will run with a power supply board down, so I wonder if you might be dealing with more than one problem in there.

Since you haven't mentioned it, I assume you have solid indicators on the front panel, nothing's blinking that shouldn't and the modulation bar graph seems normal? The ECU supply has been a common source of trouble in the BE rigs I've taken care of, but it doesn't sound like that's what you're dealing with here.

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RGORJANCE
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Re: BE AM-6A RF Out Problem

Post by RGORJANCE » Mon Jul 08, 2019 10:31 am

Dave:

Don't know why I didn't think of this sooner. If you bypass the BE tuning unit, and fire the xmttr up at low power......say 500 watts, this might allow the xmttr to come up so you can either point the finger at the tuner, or something else.

I have a BE 1KW AM backup running direct into a phasor on a 4 twr array, and it works fine as all I had to do was to introduce a bit of reactance at the phasor input so it was happy at the 50 ohm resistance. The 2.5 kw rig has the tuner, so both are happy.

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grich
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Re: BE AM-6A RF Out Problem

Post by grich » Mon Jul 08, 2019 12:07 pm

I'm working on an AM-1A with a similar problem...symptom was high reflected power, even when running into a good dummy load. I was advised to first try the balance adjustment procedure on the Directional Coupler replacement manual (you can download it from the BE website). That got the reflected power reading to zero, but the forward power is reading low, and the modulation monitor sample level is too low. Gonna have to tear it apart and see what's what. Meanwhile, the transmitter is operating normally and makes full power...thankful for a good remote base current ammeter. ;)

But first, as already suggested above, bypass the tuning unit if possible and get the dummy load straight onto the transmitter output. The tuning units can get flaky, especially in a dirty or harsh environment.

ncradioeng
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Re: BE AM-6A RF Out Problem

Post by ncradioeng » Mon Jul 08, 2019 3:02 pm

If the top PA power supply isn't working, the transmitter will not come on. BTDT. I can't remember if one of the fuses was blown associated with that power supply or what, but there were no faults showing and I was going in circles until B-E made the comment concerning the situation with that supply.

dd92251
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Re: BE AM-6A RF Out Problem

Post by dd92251 » Tue Jul 09, 2019 1:42 pm

SOLVED !!! Inside the PA Harmonic Filter / Antenna Tuning Unit of the transmitter there was a burned up wire that became disconnected. On Coil L3 there is a 1/4-20 bolt that connects three wires together, the nut wasn't real tight on this bolt and the wire lugs connected with it and the bolt/nut itself overheated. The plastic on the coil where this bolt was located melted and some of the turns on L3 also lost their insulation to heat.

This is the second BE AM-6 transmitter I've come across that has had an issue in this part of the transmitter. For whatever reason the hardware gets loose inside this area. Probably a good idea to make a maintenance check of this area if you own one of these rigs.

Thanks to everyone for their suggestions ! :D

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Deep Thought
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Re: BE AM-6A RF Out Problem

Post by Deep Thought » Tue Jul 09, 2019 3:16 pm

I had a Nautel ND2.5 pull that trick...
toasty.jpg
Glad to hear you found it. :mrgreen:
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