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KPA500 Hum

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Roy Morris-6

KPA500 Hum

Referring to kit assembly instructions, Rev B, June 22, 2011:
The following is on page 33. When placing the transformer on the chassis
you are told to press it toward the Z-Bracket so it does not hit the rubber
bumpers installed earlier. Then
above figure 49 on the same page it says the bumpers press
against the side of the transformer near the bottom. Maybe some are
installing the transformer so it is resting against the Z-Bracket causing
vibrations (hum) and not firmly against the rubber bumpers as it should be.
Also some of the wires may be trapped (pressed) between the transformer and
the Z-Bracket causing the Z-Bracket to vibrate. I snugged the transformer
against the rubber bumpers, and I can see light between the transformer and
the Z-Bracket. I have not experienced any hum at all. Roy W4WFB

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Geoffrey Downs-3

Re: KPA500 Hum and Sticky Relay?

I did as you Roy but if I press my ear to the top of the KPA500 case I can
hear a faint hum. It is the same with my 12v linear power supply underneath
the bench. I do not believe it is a fault but fairly normal for linear power
supplies. Occasionally both the KPA500 and the power supply under the bench
will hum more loudly for a short time. I have always put this down to
variations in the mains supply waveform as referred to by Ian GM3SEK in his
earlier message.

As regards the "sticky relay" phenomenon - I have experienced this a couple
of times. Switching the KPA500 off and then back on again has cured it.

73 to all

Geoff
G3UCK

-----Original Message-----
From: Roy Morris
Sent: Tuesday, September 27, 2011 7:33 PM

I snugged the transformer
against the rubber bumpers, and I can see light between the transformer and
the Z-Bracket. I have not experienced any hum at all. Roy W4WFB

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Guy, K2AV

Re: KPA500 Hum

In reply to this post by Roy Morris-6

Another issue is what the amp is resting upon.

Back when I first got my Alpha 76PA, I had a noticeable hum when
transmitting. When I wised up a little, I tried it from different places
and noted that the volume had a very large variation depending on where
placed on the upper shelf or simply where elsewhere it was placed. I cut up
a little mouse pad that had become ugly into 1.5 inch circles and placed
them under the feet of the amp. Hum out in the room simply went away. You
can barely hear the note from a violin string that is strung between braces
on a workbench, but the same bow on the same string on a violin can be heard
thoughout the concert hall. The shelf was serving as the violin to the
Alpha's hum.

I still use the mousepad circles under the amp. If you put the the mouse
surface of the pad *on the bottom* then the amp will slide around if pushed
a little and is much easier to position or get into place.

73, Guy.

On Tue, Sep 27, 2011 at 2:33 PM, Roy Morris <[hidden email]> wrote:

> Referring to kit assembly instructions, Rev B, June 22, 2011:
> The following is on page 33. When placing the transformer on the chassis
> you are told to press it toward the Z-Bracket so it does not hit the rubber
> bumpers installed earlier. Then
> above figure 49 on the same page it says the bumpers press
> against the side of the transformer near the bottom. Maybe some are
> installing the transformer so it is resting against the Z-Bracket causing
> vibrations (hum) and not firmly against the rubber bumpers as it should be.
> Also some of the wires may be trapped (pressed) between the transformer and
> the Z-Bracket causing the Z-Bracket to vibrate. I snugged the transformer
> against the rubber bumpers, and I can see light between the transformer and
> the Z-Bracket. I have not experienced any hum at all. Roy W4WFB
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

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Don Wilhelm-4

Re: KPA500 Hum

Yes, the "resonating board" may explain the difference between various
perceptions of KPA500 owners about the hum level. To check for that, it
is a simple matter of lifting the KPA500 up with your hands - if the hum
level diminishes, then the surface it is sitting on is acting as an
acoustical amplifier.

There have been other items mentioned in this thread - the transformer
contacting the Z bracket is one valid problem caused by improper assembly.

The transformer itself has isolating foam washers that were to be placed
above and below the transformer core for acoustical isolation. I can
imagine if the bolt were tightened enough to compress those foam washers
completely, then hum might be transmitted to the KPA500 enclosure - try
loosening the bolt slightly to see if the hum goes away or diminishes
significantly.

73,
Don W3FPR

On 9/27/2011 5:17 PM, Guy Olinger K2AV wrote:

> Another issue is what the amp is resting upon.
>
> Back when I first got my Alpha 76PA, I had a noticeable hum when
> transmitting. When I wised up a little, I tried it from different places
> and noted that the volume had a very large variation depending on where
> placed on the upper shelf or simply where elsewhere it was placed. I cut up
> a little mouse pad that had become ugly into 1.5 inch circles and placed
> them under the feet of the amp. Hum out in the room simply went away. You
> can barely hear the note from a violin string that is strung between braces
> on a workbench, but the same bow on the same string on a violin can be heard
> thoughout the concert hall. The shelf was serving as the violin to the
> Alpha's hum.
>
> I still use the mousepad circles under the amp. If you put the the mouse
> surface of the pad *on the bottom* then the amp will slide around if pushed
> a little and is much easier to position or get into place.
>
>

Don Wilhelm-4

Re: KPA500 Hum

In reply to this post by Roy Morris-6

Dave,

Yes, you are correct, the foam may take some time to recover, but if
loosening the bolt makes a change, then that is the problem.

I am doing well, and in fact this weekend, my antenna farm will get
higher. I have a crew coming over to put the VHF/UHF antenna on the 48
foot tower - that project has been 3 years in the making, but it will
finally happen. My first time on VHF/UHF too, so this is a new
adventure for me.

We should have a good party on Saturday. 3 guys willing to climb the
tower (myself included), and at least 4 more for the ground crew. Most
everything is now ready to go up. Finally, my Yaesu FT-847 will find
some use, and I can check in with the locals on 2 meter FM.

73,
Don W3FPR

On 9/27/2011 8:51 PM, G3TJP wrote:

>
> Don, if they have crushed the foam washers, then loosening off may not
> help if the washers cannot recover. If they can recover, it might
> take 24 hours or so. If they are beneath the transformer and the
> cells have been creased, then they will never recover. It amazes me
> how folks can be surprised that transformers hum and that that it
> takes good practise to eliminate that which is caused by the stray
> magnetic field
>
> Hope you are keeping well, buddy? No floods, famine or limitations to
> the height of your antenna farm HI!
>
> 73 and stay well. Dave G3TJP
>