Re: A minor bug in the ubitx circuit

Ashhar Farhan

The volume with the 0.1uf cap in place of the R253 was too old. I placed a replaced a 10 K resistor in place of 1K at R250 and the volume is not reasonable.
You can solder the regular through hole resistors in place of the SMDs if your junk box is low on SMD components.

- f

On Wed, Dec 4, 2019 at 3:03 PM Ashhar Farhan <farhanbox@...> wrote:

Replace the R253 (a resistor) with a 0.1uf capacitor to get the sidetone going properly. 

The long story:
I was fooling around with an upgrade to Raduino, (more over the weekend) . I noticed that the sidetone was not proper. After a day's frustration, I put a scope probe on the sidetone line and saw that as the sidetone made a square wave of 0 to +5v and back, the average voltage rose to 2.5 v. This was directly coupled (well, me!) to the LM386 audio amp's volume control. Replacing the final resistor(R253) in the low pass filter with a 0.1uf capacitor fixes the problem. 
The problem was that the sidetone would 'vanish' after a start blip. That's because the LM386 was being pushed to the rail by the 2.5v DC on the input!
I am enclosing a picture of the mod. 

73, f

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