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Author Topic: Ameritron 811H  (Read 4922 times)
KM5PS
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« on: March 13, 2011, 07:24:45 AM »

I have just recently bought a AL811H at a local hamfest.  This is my first amp.  When I first started using it I noticed that the amp was arcing  underneath the pcb board on top of the tubes.  I took it over to a local ham for him to look at.  We took the pcb off and the underside of the board was burned pretty bad.  We cleaned up the arcing traces and replaced the board with a washer on the underside of the board since half of the solder pad on the bottom of the board was gone.  Amp was working fine after that.  My then got a new board from another local ham who did not need the pcb board anymore.  This was a new board. Last night I installed the new board but I was  blowing the fuse on the back, everytime I turned on the amp. Looking from the back side of the board, it was the left side fuse.  Took off the new board and the board was burnt a little on the bottom again. Finally, put the old board back on with the washer and made sure the wire on top of the choke was not touching the board except for the metal ring .  Any suggestions why the board was shorting or why I was blowing fuses?  Also does it matter how the board is oriented right on top of the tubes? I do not have any instructions on how to the board goes on.  How tight should the screw be that holds the pcb board to the choke?  Any suggestions would be great..
Thanks
John  KM5PS
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KD8GKR
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« Reply #1 on: March 13, 2011, 07:57:45 AM »

Did you double check all solder connections? Look for arcing to the band switch shaft. What band do you use this on? I bought a AL-811(3 tube), and it had problems in the same area. Over driving amp on 10 meter was the only reason i could determine caused this on mine. I replaced the board and covered the burn mark on the band switch shaft and all was well again. Mine had a lock washer already installed under the parasitic board. Did you add a extra washer? Also if it has arced, does your meters track with each other? D16 diode was blown on mine as well.

The reason i concluded it was by 10 meter, this is were the arc lined up with the parasitic board.
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KD8GKR
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« Reply #2 on: March 13, 2011, 08:08:01 AM »

  Took off the new board and the board was burnt a little on the bottom again. Finally, put the old board back on with the washer and made sure the wire on top of the choke was not touching the board except for the metal ring . 
Thanks
John  KM5PS

After re-reading your post, i am a little confused about not touching anything but the metal ring. can you post a pic? its worth a 1000 words.
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KM5PS
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« Reply #3 on: March 13, 2011, 12:01:44 PM »

The metal ring is on the wire from the choke to the bottom of the board.  I was afraid after looking at the wire, that it might be touching one of the capacitors or resistors solder pads on the bottom side of the board.  Is the lock washer suppose to be mounted on top of the board or underneath?  Mine when I first took off the board the first time was on top underneath the screw head.  That is where it is now and the washer I put in is underneath the board and on top of the metal connector ring on the wire coming off of the choke.  The Grid Current meter seems to be reading higher than it did before.  Now it reads maxed out more easily when tuning than what it did before.  That is the only difference that I have seen so far.  I am also running about 55-60 watts into amp and getting 600 watts out on cw on 20 meters.  Only band I have used since worked on amp.  The link below shows the arcing on the new board when fuses were being blown.  The old board is a lot worse off.  Half of the pad on the bottom is gone.  Did not want to take old board back out yet, since amp is back to working.  http://km5ps.com/AL811H   for pictures.

John
KM5PS
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K2OWK
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« Reply #4 on: March 13, 2011, 03:32:20 PM »

I have a silly question. Why would you mess with the amplifier after repairing the old board, and getting the amplifier to work as it should. I am from the school of it it ain't broke don't try to fix it. Putting a new/used board in after fixing the old board makes no sense to me, unless you just like to play with the amp for learning experience.

Regards,
73s
K2OWK
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KM5PS
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« Reply #5 on: March 13, 2011, 05:40:40 PM »

K2OWK
The reason why I was trying replace the board was because of the bad burn around on the bottom of the board.  I was afraid that it might start arcing again.  It was also suggested to me by a couple of local hams who helped me, learning how to use the amp.  But of coarse after I had put the new board on and it started blowing fuses, I wished I had not done that of coarse.  It was also a learning experience for me also.  Now I get to replace Diode D16 which is bad, since it protected the meters.  Anyway thanks for the relpy..

73
km5ps
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KD8GKR
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« Reply #6 on: March 14, 2011, 08:11:52 AM »

I believe the lock washer is on the bottom without taking the cover off to verify. I also believe you are referring to little fine wire, i dont think it was connected to the ring where the post comes through the board, if im reading correctly and that is where you have it. It has also been awhile since i changed the parasitic board in my AL-811(non H). If i get time this week before work i will pull the cover off mine. When you bought this at the hamfest, did they say it minor issues? The one i bought, I was aware of the problems. Tuning input slug on Aux. Burned, Parasitic Board shorted, D16 blown, tubes bad, amp not loading on the low bands(80 and 160). There is also some recommended mods by W8JI for the "H" models. here a link http://www.w8ji.com/al811h_schematic.htm
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KM5PS
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« Reply #7 on: March 14, 2011, 07:55:50 PM »

New problem with 811h amp. Replaced the D16 diode tonight, and now when I tune up the amp, the amp is arcing on the Plate Tuning Capacitor.  The amp is also showing 500 watts out with only 25 watts of drive.  When it amp arc the power cuts out.  Anybody have any suggestions what might be wrong?  I have emailed W8JI to see if he could help me.  Odd that just replacing the D16 diode the amp is acting like this.  Any help wud be great. 

Thanks
John
km5ps
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W8JX
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« Reply #8 on: March 14, 2011, 08:05:35 PM »

500 watts out with 25 watts drive tends to suggest a high SWR in output circuit as this could cause high RF voltages and watt meter to read incorrectly too.
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KM5PS
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« Reply #9 on: March 14, 2011, 08:39:18 PM »

Did some more testing since last post and seems to only be arcing on 20 mtrs.  Tuned up on 15,40,80 mtrs and was okay.  Might be a bad spot on plate capacitor for the 20 mtrs.  Just guessing here. 

John
km5ps
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W8JX
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« Reply #10 on: March 14, 2011, 08:50:35 PM »

Check SWR on 20 and check band switch contacts. Amp might need to be Neutralized too as it can show up at 20 and above. Something is causing instability or improper loading which is shown as low drive and high output and RF arc over. 
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KM5PS
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« Reply #11 on: March 14, 2011, 10:01:46 PM »

W8JX-  How do you neutralize the al811h amp.  From what I have read about them you should not have too..  This is my first amp so I am new to all of this.

john
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W8JX
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« Reply #12 on: March 14, 2011, 10:48:56 PM »

As I recall the 4 tube 811H has ability to be neutralized while 3 tube does not. You would need to check manual here. 
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KM5PS
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« Reply #13 on: March 15, 2011, 05:41:01 AM »

Does anyone know how to neutralize an AL811h amp? I have read through the manual and can not see it anywhere.  Unless I have over looked it.  I have found where they talk about cleaning the air variable capacitors, but that is it.  Any help would be great..

John
km5ps
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W8JX
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« Reply #14 on: March 15, 2011, 07:49:19 AM »

W8JI has some general notes and comments about issues with 811H you might read for starters. He says amp is neutralized by design "if" everything is working properly.  The fact that it is a issue on one band points to something cause instability which is causing high RF voltages which are arcing over. A high SWR or band band-switch or capacitor in 20m circuit can cause this too.

http://www.w8ji.com/al811h_schematic.htm
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