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Author Topic: AL-811H problem and solution  (Read 2564 times)

Posts: 244

« on: July 22, 2010, 11:24:20 AM »

I had a problem a while back with my Ameritron AL-811H, producing an oscillation, a fluttering sound in my transceiver's speaker, and the plate voltage needle vibrating.  A fast flutter sound, like a dragonfly on a screen door when I would key the mic with the amp on.  I could reduce power and make it stop.  How much depended on time of day, position of the moon, humidity, color of my shirt, and who knows.

I thought for a while it might be water on the antenna after heavy rain but that was not it, either.  I had pulled my antenna down and reworked the silicone grease and sealing tape (Scotch 130C, like Coax Seal but without the paper peeling).

Well, for some reason, it started again yesterday just before my favorite net.

I was able to reduce power to about 400 watts out and stop it.

I've finally solved it...  I noticed that different positions of the mic would affect it.  I had laid the mic on the notepad beside the transmitter ("digital logging program"... haha) and while checking things out, position of jumpers, power wiring, grounds, just reached out and pushed the PTT button on the mic, not holding the metal mic (Heil HM-10) in hand.  No fluttering on tx.

I picked up the mic, oscillation on tx.

Put it down, not touching the metal mic pushed the PTT button... oscillation on TX.  Then I noticed the metal body was touching not only the notepad, but the metal strip on the front edge of the workbench.  And so was the metal ground strap on the end of the bench, which goes down through a hole in the floor to the ground rods outside.  (My "shack" is in my metal workshop building in my back yard.  My ham gear on one end of a 16' workbench.)

Anyway, moving the mic around to different positions, I could make it start or stop.  And I got a little tingle on my hand with the mic in hand, back of my hand touching the metal strip along the edge of the workbench, while transmitting.

Duh... why didn't I think of RF in the shack?  Why should I be immune?

I started snapping ferrite beads on coax here and there (big baggie of 20 of them).  Tried some on the mic cable.  No luck.  Put them all back in the baggie.

Then I went outside, lowered my antenna (dipole hanger is hoisted up the mast via rope and pulley), and snapped 15 beads (no science, just seemed like a good number to try) on the coax right below the "ugly balun" (18 wraps of the LMR-240 coax on 4" dia PVC pipe).

Hoisted the antenna back up.  Back in the shack and try it...

Success!!!  Man I can't make it do that oscillation thing no matter what I do with the mic or wiggling cables around.

I could crank the amp up to full power, 65 watts in, 850 watts out, no problem.

Well, the oscillation/flutter is gone now.

Sheesh!!!  This is the kind of thing that will drive you nuts!

Paul - AE5JU

Posts: 21837

« Reply #1 on: July 22, 2010, 11:33:27 AM »

It still might have been the color of your shirt.

Did you change shirts when you tried this? Cheesy

Posts: 821

« Reply #2 on: July 22, 2010, 11:55:23 AM »

Very good...a common fix for common mode.

 I bet your favorite net was on 75 meters.  Those 18 turns @ 4" dia won't choke much at 3.9 MHz, but those extra 15 beads did the trick. 


Posts: 244

« Reply #3 on: July 22, 2010, 01:52:19 PM »

Yep, 75 meters, and the only place I use the amp.  I've done just fine with 100 w or less SSB on the higher bands (or 20-25 w PSK31).

And I thought it was an amp or tube problem.  In a way it was, but really it was a power and antenna problem.

Well, that's fixed.


Paul - AE5JU
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