Wednesday, November 30, 2016

Building the BITX 40 v 3, receiver portion

12/2/16   Added link to Eagle files for making AGC Board



A couple of things came up and I was delayed in building Farhan's SMD BITX 40.  With the availability and price point on the board, I decided I will stop work on the version I was designing and building.   I will instead spend my time on trying some modifications to enhance the board from Farhan.  
For those who are building with this board, there is a website devoted to different mods for it. 

http://bitxhacks.blogspot.com

Since I haven't gotten around to build a si5351 VFO yet,  I installed a 10 turn tuning pot.  This made it much easier to tune, but I found there was quite a bit of drift during warm up.  Looking over the bitxhacks blog, one of the first mods I made was to change some of the resistor values in the bidirectional amplifiers.  Changing them from 100 to 220 ohms reduces current in those stages,  should reduce heating of that part of the circuit board.  I also coated the VFO coil with several coats of clear nail polish to prevent  coil winding movement.


Another change I made was to cut the short trace going from  switched 12 volt to the  U2 (VFO  9 volt regulator) input pin. Then I ran a wire from the input pin of U2 to un-switched 12 volts.  I also added a .1 uF capacitor from the regulator input pin to ground, and a 47uF and .1uF  from the output to ground.  This keeps the VFO circuit on at all times, and hopefully further reduces warm up drift. 
After all of this it is much better, but I still have some drift, up and down in frequency even after warm up.  Ordering some NPO capacitors to use in the VFO and will see if this helps.

With much of the drift problem solved, I next looked at the audio.  Since the BITX does not have AGC, having to adjust audio level when going from station to station can be aggravating.  Farhan posted a simple AGC circuit on the bitxhacks blog, and I decided to add it to mine.  


I took his hand drawn schematic, and input it
to Eagle and laid out a simple board that can be built as mostly SMD or Muppet style. 




 
After etching a couple boards, I built up one of each to give a try.  For the SMD version I also added a solder mask to make assembly easier.  Adding the AGC board to the BITX took care of leveling the audio, but the added circuitry reduced the audio level.  Replacing the 1 uF. capacitor from pins 1 - 8 of the LM386 with a 10uF. brought the gain back up to where it was.


Link to Eagle files and a .pdf with mirrored top layer image for making toner transfer boards.

https://www.dropbox.com/sh/pg3otf9cwtekkoo/AADGvAfmP_E96njuNP5UX_Y9a?dl=0

Next to add the frequency counter.  With a little change to the gain on the counter pre-amp I had it reading correctly.  Using the setup function on the counter, I measured the BFO frequency, and set the counter mode to subtract the VFO for proper frequency display.  There was some noise introduced on the audio output.  A 220uF capacitor across the counter power supply leads, and a 10mH. choke in series with the positive  supply lead took care of that.

Except for a small amount of frequency drift the receiver portion of the BITX is working.  Now to wire up the microphone and test the  transmitter.  It is starting to look like a transceiver.







6 comments:

  1. Hello DuWayne,

    Greetings from EA5, Spain. I am a user of EAGLE and would like to have access to the layout files. I would be very obliged to you. TIA. Und auf wiedersehen!

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  2. Added link to AGC layout files

    https://www.dropbox.com/sh/pg3otf9cwtekkoo/AADGvAfmP_E96njuNP5UX_Y9a?dl=0

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  3. This comment has been removed by the author.

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  5. Hello DuWayne,

    Would you be willing to upload and share the .brd files to OSH Park https://oshpark.com/ allowing the boards to be available and purchased buy other Bitx owners? I have found other Bitx Pcb's at OSH Park to be useful, https://oshpark.com/shared_projects/6mnfQQ6e

    73's
    Neil KM4PHK

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  6. is not the VFO circuit on all the time anyway? isn't it used both on receive and transmit? thanks
    jer aa1of

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