Last edited · 2 revisions  

 



From post  

After much poking and probing with instruments I found the reason for the power pop when you press the PTT.

When PTT is pressed the TX voltage floods the mic preamp. Capacitor C122 charges up and the charging current flows into t4 and then
D15 - thereby for a fraction of a second the carrier leaks through and if you are listing with another RX you will hear a "tong" kind of sound.

Solution is to stop this flow through the diode then R106 - R220 to ground.

In my analog VFO version I desoldered the R106's center pin and put in a 0.1 capacitor in series. This stops the capacitor charge current from routing though the mixer diode.

For better measure add a 1K resistor across C107. I found a constant 30mV there which was unbalancing my diodes and the carrier balance pot R106 was more to clockwise than needed!

For the digital VFO board or boards without R106 trimmer, cut the track between D20 and R38 and solder a smd 0.1 cap there.

Result is a significantly reduced pop which is not bothersome anymore.

Farhan please incorporate this hack.

Cheers for a pop free Sunday!

--
Raj, vu2zap
Bengaluru, South India.


PS: Try this:

1. Power the audio stages directly by connecting 12V line to the junction of R111, R113 and D18.

Now you will hear your own voice during TX and we need to mute it.

2. Take a 2N7000 or similar and connect SOURCE to ground, DRAIN to the top of the volume control (M1/M2).
feed the TX 12V through a 10K or nearby value to the gate.

This should mute the amp during TX.

If this works for you then remove the diode D18!

Caveat: I have not tried it as I am at the coffee farm this week. Itching to get back to my board but it will be Monday next!
While in TX mode some RF might get into the amp stage and be just as annoying.
Then we have to work more on that! A 0.01 across the volume control might fix this if it happens.

Cheers
Raj