Re: RFI from uBitx TX after AGC and other mods installed
John, Don Lawrence, others
Interesting Outcome! Shielding was no help, replacing pcb with perfboard agc helped a little, but back to pcb. bypass's on 78L05 were .68 on input and .1 on output still no help and removed 78Ll05. Did not try 1nf,10nf,100nf in parallel on 78L05 because I had already removed it. Because pin 7 of U1 (TDA2822) is already bypassed with C78 did not do. Don suggested to filter the 5v supply. I get the 5 volts from the radino pin 3 (green wire) and use it for both AGC and mike preamp. Used a 470uf and .01 on the 5V no improvement. Don suggested not to try the emitter-base capacitor because of forming a blocking oscillator. It looked to me that because the emitter was already grounded that it probably just bypass any unwanted signal to ground and not oscillate, connecting to the collector might have been another story.. What the heck nothing ventured nothing gained. Tried the .01 and turned on expecting possible oscillations of some sort. AGC off transmitting nothing as before, AGC on transmitting at full volume only occasionally get a very small occasional hint of audio coming through the speaker no oscillations of any kind. Before as soon as audio was present from the mike it started coming through and sometimes went into oscillations. Hardly ever run at full volume so it looks like your theory and suggestion Lawrence may work. ARC still appears to work as before but will need a strong signal to verify. Still have to experiment with a couple of different values to see what's best. Hope this spells relief with the agc on for the few of this it affects. Thanks to all for the input and suggestions.
From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Lawrence Galea
Sent: Sunday, July 01, 2018 12:28 PM
Subject: Re: [BITX20] RFI from uBitx TX after AGC and other mods installed
Also try putting a 100n directly across Q1 base-emitter on the AGC board.
It will cut off some of the higher AF as it would be in series with C1 to earth across the Af gain control.
However, this would not be of any serious consequences.
If you don't want as much AF HF cut off you can try a smaller capacitor, say 10n.
I am thinking that Q1 base-emitter junction is acting as a detector and the AF is being coupled back to the AF gain control since the level is varying with the AF Gain control.
One could also add say a 10n capacitor directly on pin 7 of the TDA2822 or across the AF Gain control as an rf decoupling.
Can you try and report?
On Sun, Jul 1, 2018 at 5:33 PM, Lawrence Galea <9h1avlaw@...> wrote: