Crackling Audio

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Crackling Audio

Postby Gilly » Wed Mar 02, 2011 8:06 am

Hi Everyone

Thought best to create another post from the last D-limit C-limit posting....

My new 80M AM station is now going 100% :D ..... great audio .... high efficiency and modulates positively to 200% without a problem....

This TX owes me a lot as it had a number of annoying problems....the main one being what I would describe as crackling audio mainly when modulating over 50% .....but was there on carrier as well.
After going through what Steve suggested I just could not see any parasitic oscillations at all ... everything looked normal....
The problem was in the PWM output filter area.....
In my other PWM modulators the cases I built had a thick sheet of Aluminum on the base on which I use a small sheet of PCB about 2" X 3" to solder the capacitors to....and bolt it to the case using 4 screws / nuts in each of the corners.....this gave a good earth to the case..etc:
But in this box I ran out of thick sheet and only had thin sheets....so to make the case strong I used wood with the thin sheet on top.....
I still attached the PCB using 4 screws /nuts BUT the screw head was against the wood and the only contact to the case ( ground ) was the shaft of the bolt just touching the thin sheet of Aluminum as it went through the wood / thin sheet of Aluminum and the PCB :(
So the capacitors had a very poor connection to ground although when I tested the Modulator it all went OK when using a 5 ohm dummy load......on the CRO anyway..... :roll:

I only found this after becoming desperate and going through everything as if someone else had built it....when I saw the filter area it hit me straight away :mrgreen:

But it was all worth it and makes a good story for people who are interested that is !


Wayne

VK3ALK
Gilly
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Re: Crackling Audio

Postby steve_qix » Thu Mar 03, 2011 7:27 am

I'm glad you solved the mystery! That would certainly cause problems :D

Regards,

Steve
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Re: Crackling Audio

Postby Gilly » Tue Mar 08, 2011 6:53 pm

I should not have said the TX was going 100%...

It has one last annoying issue...

Keying the TX gives an identifying quick buzz....
Looking at the CRO there are small pulses at the carrier level...similar in time to the Buzz...
Replacing the TX with a Dummy load shows what would be best described as negative ringing....
The digital drive on the gates of the IRFP260's show quite normal operation.....

The HV voltage 130V is fine....

The sequencer keys the HV last etc:

Apart from this all goes great...

Wayne
Gilly
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Re: Crackling Audio

Postby steve_qix » Wed Mar 09, 2011 7:44 pm

Hmmmmmm.. Is it possible to post the oscilloscope pictures? Something we see may give us a clue to what might be happening.

Regards,

Steve
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Re: Crackling Audio

Postby Gilly » Thu Mar 10, 2011 2:01 am

http://waytec.lasielle.net/main.php?g2_itemId=386

Image

For some reason the IMG link is not working....have changed my File Server and things are different :?

The CRO is set for 1ms per division.....
The image is poor as there appears as though 3 waveforms are there when its only one.....
Was difficult keeping camera steady and keying the TX.....

The CRO lead is attached to the output of the Modulator and monitoring the Output RF signal the waveform is appox the same as well.....

Also when the voltage is in the Tuneup position of 65Volts the effect is greatly reduced......


Wayne
Gilly
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Re: Crackling Audio

Postby Gilly » Thu Mar 10, 2011 5:24 am

Hi Steve

Just leave it with me for the moment.....

I'm getting an analog waveform in my PWM output board along with the digital square wave.....thats in sync with the buzz.....etc:
I did not notice it until I took a closer look......the good thing about analog CRO is that you can see a trail etc:

For some reason this TX system has had too many annoying faults and I had better have a closer look at things...
Maybe there is some oscillation going on in there somewhere.....

Will let you know how it goes...


Wayne
Gilly
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Re: Crackling Audio

Postby Gilly » Fri Mar 11, 2011 6:29 am

Well its all going......

I should be embarrassed :oops:

Never did believe the 12 Volt supply to the TX drivers was good enough ..... Have a motorbike battery I use at times as a power supply .... so substituted it and all symptoms disappeared.....



Wayne
Gilly
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Re: Crackling Audio

Postby steve_qix » Fri Mar 11, 2011 11:35 am

Excellent !!!! :P

Regards,

Steve
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