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Humm!
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seven
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Post Humm! Reply with quote
Ok i am using one of the 150w amps (BW do them aswell i think) and just a standard PLL driver (500mw-1w)

I have boxed it all up and all is working fine but i get a hummmmm when i turn the radio up full when on the dead carrier.
I have used a 300w toroidal transformer and two 10000uF smoothing caps.

I know the placement of the boards in relation to the transformer is important and i took that into account when boxing it up but i still get the hum/buzzing.
I have tryied so many things now but no success although i twisted the transformer round a little and the humm seems to be a little quieter (why is that?)

Could it just be my cheap dummy load? or anything other than the transmitter.

As i turn the power up though the humm gets worse/louder.


This is so frustrating, i have spent hours trying to fix it.

I just hope someone can suggest something. Ive got a feeling it will be something simple (it must be right?)
Tue Jan 08, 2008 4:28 am View user's profile Send private message
pcs
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Post Reply with quote
Is your power supply stabilized? If not, you are adding a bit of AM 100Hz signal to your carrier, you can hear it
as humm.

Why not use a switching power supply?

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Marko - PCS Electronics
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Tue Jan 08, 2008 10:09 am View user's profile Send private message Visit poster's website
seven
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Post Reply with quote
I think ive done it!

A ground loop i belive (you may be able to verify)


Took me a while to find it but i fixed it by just connecting the negative of the rectifier straight to ground instead of having a ground wire going from the smoothing cap to the amp ground plane. So no ground wire to the smoothing caps now

Brilliant!!

I knew it would be something like that... one wire!


cheers again for your help and time Marko!
Tue Jan 08, 2008 5:39 pm View user's profile Send private message
pcs
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Post Reply with quote
It must be stabilized than, good.

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Marko - PCS Electronics
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Turn your PC into a FM radio station!
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fax +386 4 2316 128
Tue Jan 08, 2008 6:59 pm View user's profile Send private message Visit poster's website
seven
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Post Reply with quote
Just tested it properly. There is still some hum there but not as much.
However, when i power the exciter with my bench PSU and run the rest as normal there is no humm atall so the problem lies in the supply to the exciter i imagine?

I am just using a 7812 regulator decoupled with a 0.1uf and 47uf on the input and output of the regulator. Any other tips ?

cheers
Tue Jan 08, 2008 8:12 pm View user's profile Send private message
pcs
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One thing, it is best to place amp and exciter in separate boxes. This is why all major
big-power transmitters are always separated above, say 200-500W of power.

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Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Tue Jan 08, 2008 8:45 pm View user's profile Send private message Visit poster's website
seven
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Post Reply with quote
pcs wrote:
One thing, it is best to place amp and exciter in separate boxes. This is why all major
big-power transmitters are always separated above, say 200-500W of power.

Yeh i would do if it was that kind of power.

I know it cant be audio getting into the RF stage on the exciter. Its the power to the exciter even though i have it on a decoupled regulator.

strange
Thu Jan 10, 2008 5:03 am View user's profile Send private message
pcs
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Post a photo!

Marko

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Marko - PCS Electronics
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Thu Jan 10, 2008 11:18 am View user's profile Send private message Visit poster's website
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