PCS-Electronics

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?)

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?

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!

It must be stabilized than, good.

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

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.

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

Post a photo!

Marko