Re: RFI from uBitx TX after AGC and other mods installed
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First of all glad that you had good progress. I hadn't looked at the ¦Ìbitx 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 22¦ÌF 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. Regards
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:
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.
?
Rick?
KN4AIE?
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?
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