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Author Topic: Ameritron AL-82 Loud Bang  (Read 10825 times)
N1DYX
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Posts: 35




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« on: February 28, 2013, 09:35:14 AM »

I've had this AL-82 about a month that I bought used.  It shows no wear on anything and the tubes look crystal clear.  Every so often I hear a BANG from the thing which at first I thought was the sheet metal top banging from getting hot during use but it's not the case.  I actually can't tell where it's coming from but it scares the heck out of me when it happens.  I've looked at all the variable caps and see no signs of arching and looked over the tank circuit and see no damage, solder joints look fine in that area also.  I don't see any flash at all and I unfortunately have not been looking at the meter to see if the grid current pegs or anything, but listening stations do not detect any change in signal strength or mention they'v noticed anything.  I have read some posts about this occuring in other Ameritron amps but not with the 3-500z tube, I think it was 572's.

Does this sound famillair to anyone?  I have not contacted Ameritron yet about it but they have been very helpfulin the past with my questions about tuning this amp.

Thanks
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W8JX
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Posts: 5604




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« Reply #1 on: February 28, 2013, 10:04:47 AM »

Sounds like your may have a tube arcing over. Have you been running it hard to heat tubes up to keep them properly gettered? You want to get the plated red on those tubes and not baby them.
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K4RVN
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Posts: 760




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« Reply #2 on: February 28, 2013, 01:32:41 PM »

I would still guess that it is a cap even with no sign on the cap case. I personally had one in my AL 80 A do that and then it went dark with a hole in the cap case. You may also lose the 3-500 tube(s) but you may have glitch protection on that power supply. I did not, but I do now.

Frank
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W8JX
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Posts: 5604




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« Reply #3 on: February 28, 2013, 02:12:15 PM »

I would still guess that it is a cap even with no sign on the cap case. I personally had one in my AL 80 A do that and then it went dark with a hole in the cap case. You may also lose the 3-500 tube(s) but you may have glitch protection on that power supply. I did not, but I do now.

Frank

You could be right but the bang is being caused by a HV discharge honking the transformer and supply very hard for the brief discharge period.
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QRP4U2
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Posts: 109




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« Reply #4 on: February 28, 2013, 02:41:03 PM »

Is the sound that of a big thump or a snap?

If it is a big thump check the soft start relay/circuit and make sure the Soft Start relay contacts are not welded.

Phil - AC0OB
« Last Edit: February 28, 2013, 03:08:15 PM by QRP4U2 » Logged

AC0OB - Thermionic Emitters Rock!
N4ATS
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Posts: 807




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« Reply #5 on: February 28, 2013, 03:09:02 PM »

Typically , with the MFJ amps that come in for repair , it almost always turns out to be one of the HV caps , I just generally replace them all however replace the Chinese brand bleed resistors with a better brand , most likely one or more are open.

You can get better caps and resistors here for about the same price

www.Newark.com

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W1QJ
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« Reply #6 on: February 28, 2013, 03:12:31 PM »

We've been given very little information here.  All I know is I hear a bang.  A HV arc will manifest itself with a bang.  Tubes don't make noise when they arc.  Tubes have a vacuum and noise does not travel in a vacuum.  What you do hear when a tube arcs is the stress it puts on other parts that explode and make a bang noise.  Multiple bangs that occur with no apparent failures is usually a filter cap blowing off.  Ameritron put a piece of insulation under the caps near the chassis.  This gave a bit of insulation from a cap arcing to ground.  It might be missing.  If it is a newer unit with the shorter caps this is moot.  Unless the bypass cap on the plate transformer is arcing to ground then I would immediately remove the filter caps and check each one.  A filter cap that occasionally blows off will surely show itself physically upon a visual inspection.  The caps can easily be removed one by one by removing the screws and taking the caps out.  Take them all out, check them.  When they are out check each bleeder cap with an ohm meter and make sure none are open.  These amps are know for opening a bleeder resistor and that ruins the associated cap.  A doughnut says this is the problem.
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N1DYX
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Posts: 35




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« Reply #7 on: March 01, 2013, 09:28:17 AM »

Lots of good information here, thanks very much for all the replies.  Sorry about the sketchy technical details, I know "Loud Bang" is a little tough to diagnose. Smiley

I really havn't been pushing it hard, to be quite honest the way Ameritron tells you to tune this thing up exceeds the recommended current for the tube so I've been keeping the drive down to 30-40 watts.

The amp was made in 2009 so I'm not sure if that insulation is there under the caps but I'll check it out for sure.

The sound is definately not a snap like a high voltage arc, it's a pretty solid sounding bang.

If this is a faulty high voltage cap, wouldn't there be smoke or smell coming out of this thing and then some kind of cascading failure?  I've blown electrolytics before and they stink like crazy. 

I'll pull the caps and check the bleeder resistors this weekend and see what I find, maybe the trouble is there if these are known faults.

Thanks for the Newark link for parts.

Thanks again, I'll let you know.
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N1DYX
Member

Posts: 35




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« Reply #8 on: March 01, 2013, 10:04:48 AM »

Typically , with the MFJ amps that come in for repair , it almost always turns out to be one of the HV caps , I just generally replace them all however replace the Chinese brand bleed resistors with a better brand , most likely one or more are open.

You can get better caps and resistors here for about the same price

www.Newark.com



Yikes $8-$9 apiece!  I ran across a thread somewhere else that recommended 220uf @500vdc as a better replacement for the 210uf @450vdc that comes in it.  Sound OK?
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W8JX
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Posts: 5604




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« Reply #9 on: March 01, 2013, 11:37:33 AM »

Yikes $8-$9 apiece!  I ran across a thread somewhere else that recommended 220uf @500vdc as a better replacement for the 210uf @450vdc that comes in it.  Sound OK?

Nothing wrong with that at all and actually a good idea as they are operating near the edge of their 450 volt rating anyway
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W1QJ
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Posts: 1444




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« Reply #10 on: March 01, 2013, 12:45:05 PM »

The real hot set up for the Al-82, 1200, and 1500 is to add 2 more caps to the string of 8 to make ot 10 caps.  That will give you 4500v total.  It is easy to do with a small piece of PC type board, make up 2 caps in series with bleeders and ft them to the left of the PC baord in between the middle wall and the transformer.  Pull the wire on the oribinal circuit baord and add in the 2 caps and then connect the other side back to the PC board.  Cut the trace on the PC board  going to the HV series resistors and run a wire over to the last cap.  Then use the green wire on the transformer to bump the HV up to 3900 to 4KV.  More power less grid current.  Serves 2 purposes, simple to do and adds headroom to the cap string.  Must admit, as much as I like Ameritron, those big box amps need 10 caps in the string.
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N1DYX
Member

Posts: 35




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« Reply #11 on: March 01, 2013, 06:14:13 PM »

The real hot set up for the Al-82, 1200, and 1500 is to add 2 more caps to the string of 8 to make ot 10 caps.  That will give you 4500v total.  It is easy to do with a small piece of PC type board, make up 2 caps in series with bleeders and ft them to the left of the PC baord in between the middle wall and the transformer.  Pull the wire on the oribinal circuit baord and add in the 2 caps and then connect the other side back to the PC board.  Cut the trace on the PC board  going to the HV series resistors and run a wire over to the last cap.  Then use the green wire on the transformer to bump the HV up to 3900 to 4KV.  More power less grid current.  Serves 2 purposes, simple to do and adds headroom to the cap string.  Must admit, as much as I like Ameritron, those big box amps need 10 caps in the string.

Thanks for the suggestion and the details, I'll look into it.!!  Anything to bring this grid current down.


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K8AXW
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Posts: 3725




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« Reply #12 on: March 02, 2013, 05:04:56 PM »

DYX:  With my limited experience in electronics (57 years) and 40 years in power generation, I've found one thing to be true.

A loud BANG leaves evidence.  Every time!  Finding that evidence might be time consuming, but if you look you'll find it.
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W1QJ
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Posts: 1444




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« Reply #13 on: March 02, 2013, 09:35:41 PM »

If the insulation is missing under the caps on the bottom a leaky cap will pulse to ground.  Those PS caps need to be checked.  Muliple bangs should show evidence for sure.
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W8JX
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Posts: 5604




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« Reply #14 on: March 03, 2013, 06:17:41 AM »

Then use the green wire on the transformer to bump the HV up to 3900 to 4KV. 

Tell me more about this mod Lou as this is first I have heard of it. 
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