Thought best to create another post from the last D-limit C-limit posting....
My new 80M AM station is now going 100%
..... 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.....
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
But it was all worth it and makes a good story for people who are interested that is !
Wayne
VK3ALK
