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Author Topic: AL-811 High Input VSWR Assistance  (Read 2410 times)
NT4I
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Posts: 17




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« on: November 04, 2012, 09:41:35 AM »

Hello all, I recently acquired an Ameritron AL-811, (not the H) manufactured in 2006 as per the QC stickers on the inside of the amplifier. I had to perform some minor repairs and the amplifier is working as per the specifications except the input VSWR is about 2 to 1 on all bands. The seller stated that he had just put in new tubes prior to selling, however, I have no way of validating or dismissing his claim.
I realize that there are input tuning slugs on the rear to minimize the input VSWR, but my question is, it normal for the input slugs to require re-tuning when new tubes are installed?
In performing re-tuning of the input VSWR slugs, any recommendations, tips or hints would be appreciated. It seems pretty straight forward, but I'm sure someone out there has perfected the process.

Thanks in advance for any responses. 73'
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N4ATS
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Posts: 811




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« Reply #1 on: November 04, 2012, 11:00:50 AM »

As a general rule , I always reset the tank circuits on all the 811's I get in for repair as they are usually not set very well from the beginning. I usually set them all while looking into a dummy load for best overall performance...

Good job on the fixes you did as well....What fixes did you have to do so others can see?

However , did you try it into a dummy load at 50 Ohms?
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WX7G
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« Reply #2 on: November 04, 2012, 11:03:54 AM »

The tuning slugs do not need retuning when the tubes are replaced.

The fact that all bands are affected leads me to think the cause of the 2:1 VSWR is something common to all bands and not that someone adjusted every slug.

1) How you tried another SWR meter?

2) What plate and grid current are you measuring?

3) What minor repairs did you perform?
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NT4I
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Posts: 17




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« Reply #3 on: November 04, 2012, 11:56:02 AM »

The main repair was the replacement of the load capacitor, parasitic board and general re-touching of suspect solder joints.
The amp was loaded into a know good dummy load the the VSWR meter between the radio and amp is a Bird 43. I have found over the years that sometimes the jumper length between the radio and amp will affect the VSWR (even though it should have no affect as long as its 50 ohms). Several lengths were tried but the results remained the same.
The amp fully loaded is about 550 ma of Ip and grid is 150ma. No load voltage 1700 and fully loaded about 1500. Output keyed down on 40 meters is about 500 watts with PeP around 600 watts. One thing I did notice is that when I advance the load control (clockwise) the grid current does not go down much (10 ma or so), but not knowing what to expect it's subjective at this point. The manual only states that advancing the load control will reduce the grid current.
For giggles and grins I'm off to RS for a hex adjustment kit and I'll see if I can adjust the slugs for a better match.....   
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WX7G
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« Reply #4 on: November 04, 2012, 11:29:54 PM »

I suspect one of the tubes has low emission. This can be checked by applying RF drive until the plates show a dull red color. If one tube does not show color that's the bad one.
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KG4FET
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Posts: 26




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« Reply #5 on: November 09, 2012, 04:12:48 AM »

I had the same issue with my AL811H.  I would check the relay FIRST, before you start messing with tuning slugs. Output may not be making good contact.  I checked mine by using a PLASTIC rod to push the relay output contact (top contact) in WITH THE AMP KEYED. My 3:1 + swr dropped to 1:5 or less on all bands.  Turned out my 1998 made 811H relay contacts had been pushed back and were dirty. With a quick cleaning and pulling the output contact forward the amp is working 100%.

Tuning slugs and the blue tuned input wafer switch were also my first concerns.  If the repairs you made required you removing the bandswitch rod it (blue wafer switch) could be out of alignment as well. The breaks in it should line up with the rivets just above (right side) and just below (left side looking from the front of the amp) the mounting "humps" with the bandswitch in the 160M position. This  information came right from the 811 Tech at MFJ.
GL ES 73
DE KG4FET Sandor   
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