Re: New bitx40 build.


Yes the sidetone does leave a bit to be desired. No doubt you experienced a surging in the tone volume, where it starts out low and increases to a level
that is fairly loud.  A few of us have surmised that this is due to a rather large electrolytic cap on the output of the LM386 that is charging. 

I tried moving the sidetone injection to before the audio amp but found that after about a second I was losing sidetone. A closer inspection of the 
schematic of the BITX40 revealed that the supply voltage to the LM386 is getting removed during TX, so that explains the loss of sidetone.
Instead I built a small sidetone R/C filter according to the instructions in the manual for the K1EL keyer kits (manual online) and replaced one of the 
resistors with a trimmer pot so I could fine tune the level of the sidetone and this helps a lot.  One thing I did notice is that this "sidetone level surging"
seems to be much less noticeable with my Koss CS-100 headset (32 ohm impedance headphone) than with an 8 ohm speaker.  

One other mod that I highly recommend to improve the CW experience is anti-thump. I used the one documented in message 25871.
I still get some popping on the RX-> TX transition but it is reduced to the point where it is not uncomfortable to use headphones. 
I did some experimentation with the value of the resistor that is used to discharge the cap on PTT release with little success. I believe the 
the remaining noise I am hearing is a result of the reconnection of the supply voltage to the LM386 on the switch to RX. 


Michael VE3WMB / VA2NB 

P.S. I had a friend record my Bitx40 CW signal over the air and send me a snippet and I must admit that it sounds pretty good. No clicks, chirps or 
other artifacts. The keying sounds a little bit hard but it is not objectionable. IMHO it sounds a bit like an IC-703+. 

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