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Hi bitxers,

Some have said they hear a click or perhaps a chirping on keying ptt, my bitx40 is no exception. When the ptt is pressed, the relay Rly1 is activated first, the +12tx line is enabling the microphone amplifier Q12 before rx audio Q16 has time to be disconnected by Rly2 (contacts M1 & M2) , therefore the microphone amplifier is active before the transmitter switching has completed hence a burst of feedback.
If you delay power to the microphone amplifier allowing Rly2 to settle
the "chirp" which is audio feedback does not occur.
As a test I disconnected resistor R127 which provides power to Q12 microphone amplifier and inserted a 2n7000 fet as a switch inline with r127 with a RC delay on the gate, chirp has now gone, it's not the most elegant hack so hope some one can came up with a better solution.

best 73 Alf vk2yac

Interestingly, I did the audio gain change (removing the cap that's between pin 1 and 8 on the lm386) and didn't like how the audio sounded on my speaker. When I put the cap back in, suddenly the chirp almost gone. It had been quite significant before.?
--
Ryan Flowers KC7RYS?

I have a loud "ptt-click" too in my BITX40.
I read some articles about the LM386 where it was mentioned that grounding pin 7 can be used to temporarily mute the audio.
Perhaps this feature could be used to cure the "ptt click", just by switching pin 7 to ground during transmit. Has anyone already tried this?

73, Allard

OK, I've done some experiments today:
1. connect a jumper wire between pin 7 and pin 4 (GND) - the audio was almost completely muted, but not 100%.
2. connect the jumper between pin 7 and pin 6 (+Vcc) - now the audio was muted completely!

So I'd guess this feature allows for a very elegant way to mute the audio during transmit - in fact much nicer than the way it's done in the current design (interrupt the input signal with relay K2).

Now I still need to find a way to somehow use the 12V at "TX" (from relay K1) for applying +Vcc to pin 7. Would it be safe to connect "TX" via 100 ohms to pin 7? In that case pin 7 would be at +Vcc during transmit, however at the same time pin 6 would then not be connected to the supply anymore (because it is connected to "RX"). I'm not sure if that would hurt the LM386.

Any ideas or other suggestions?

73, Allard

You are shorting out one of the input resistors and increasing Vcc on the input darlington pair.
This is not a good idea, since it increases the current drain and heat generated. Pin 2 is for
"Bypass" and is meant to go to ground through a capacitor to filter out line noise. Your method
upsets the voltage balance of the internal transistors. Interesting idea though...

The best solution is probably a FET mute such as that commonly found to mute CW
receive during key down. It should be at the input and might have to be somewhat heftier than a 2N7000 (which is commonly used)
but I think a BS170 should handle it.

john
AD5YE

Thanks Gian,
I will give it a try when I have time!
73, Allard

Digging up this old topic. I was testing out the mic on my in-progress bitx40 build and noticed a very loud chirp when activating PTT. Has anyone attempted and confirmed the fix proposed by Gian?

Thanks!
David