Re: RFI from uBitx TX after AGC and other mods installed
toggle quoted messageShow quoted text
First of all glad that you had good progress.I hadn't looked at the μbitx board but at the circuit diagram and there is no capacitor showing on pin 7 of the TDA2822.
My thinking was and is that the Q1 base - emitter junction is acting as a detector and feeding back the audio to the AF gain control through the input capacitor C1.
So you can try a higher value and see whether all the RFI will go away.
Regarding filtering at the voltage regulators, I always put the capacitors I suggested on both input and output as close as possible to the regulator, if possible across the leads on the pcb and also a 10 to 22μF capacitor on both input and output and never had a problem with RF on the regulators.
Furthermore, when a big voltage drop is required such as from 12V to 5V I usually put in a 7809 or 7808 before the 7805, splitting the heat between them.
They are cheap so better regulation (within their capacity) and less heat in each component and better component life.
As a note, we used to use different capacitor values when we used leaded components because due to the lead inductance they would not decouple efficiently at some frequencies.
We also used certain capacitor values with a particular leads length where the capacity with the leads length formed a series resonant circuit to efficiently decouple a particular frequency especially in screen grid circuits.
On Sun, Jul 1, 2018 at 9:02 PM, Rick Price <rickprice48@...> wrote: