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Author Topic: AL-811 Blowing Fuses  (Read 1222 times)
AB9LS
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Posts: 2




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« on: January 05, 2007, 06:59:18 PM »

I have an AL-811 (3 tube)that has been sitting on my desk for about 6 months unused.  Now that 80m has opened up to SSB I decided that I needed to fire up the amp and burn in the tubes since it has sat so long.  When I fired up the amp it started up like normal and after about 5 seconds it blew a fuse.  I took the cover off and tried to find anything that is not right and couldn't see anything, so I replaced the fuse and tried again getting the same result.

The last time this amp was used it worked perfectly, the tubes are new and probably have 20 to 30 hours on them, and I upgraded the amp last year by installing a new paristic circuit board from Ameritron.

I would greatly appreciate if someone can point me in the right direction in finding the problem.

Thanks

Jon
AB9LS
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W1NK
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« Reply #1 on: January 05, 2007, 07:19:49 PM »

Jon--

I had a similar problem about a year and a half ago....

As it turned out, the plate of one of the 811s had developed a "soft spot" (for lack of a better term).  It looked like a shiny spot, as if I had put a small piece of tin foil on the plate.  Closer inspection of the other tubes revealed small, very faint areas of change in the normal color of the plates of the other tubes.

I could only assume that either I was being a little hard on the tubes (ie long periods of tuning trying to squeeze out the last possible watt) or the tubes Ameritron is using just aren't that robust. Or, a combination of the two.

My advice, check the tubes closely.  If you see any spots like I described, pull the tubes and replace 'em with a matched set of Taylor 811s from RF Parts ($53.85).

GL & 73
Frank, W1NK
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WB0MCO
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Posts: 81




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« Reply #2 on: January 05, 2007, 07:26:48 PM »

I would pull the tubes out and see if it blows the
fuse. Make sure the plate leads aren't touching
anything,support them with plastic tiewraps if
possible while the tubes are removed and power applied.
If the problem still exist then start with checking for a shorted component,diodes,caps,etc.
 Please beware of the high voltage,make sure unit is
unpluged from the wall outlet and the filter caps
are discharged.
Good Luck!
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AB9LS
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« Reply #3 on: January 05, 2007, 07:30:11 PM »

Thanks for the quick reply Frank, I will check the tubes as you instructed.  Like I said, they don't have much time on them, I put a new matched set of Taylor's in it when I got the amp.  

This does kinda make sense, because the amp has not been touched since I replaced the paristic suppression circuit last year, and it worked perfectly after that repair.
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VK2ACM
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« Reply #4 on: January 06, 2007, 03:45:44 AM »

Hi Jon,
The replies you have received already are correct. I would also check the grid resistors I think they are labelled R19-R22 they are 51 Ohm 5%. There should be Three of them make sure that none of them are blown.

I hope that this helps and that you get your amp back ASAP....73 de Clint VK2ACM
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