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.







Saturday, November 19, 2016

Some nice words about the SNA Jr.

I have been following Paul M0XPD "Shack Nasties"  Blog for quite some time.  This year at FDIM, Paul gave one of the seminar presentations.  I was pleasantly surprised when he mentioned my early version of the SNA Jr. in his presentation.  Later, I had a chance to speak with him about some of our common interests and projects.  I also gave him one of the extra SNA Jr II boards I had had made.  
A week or so ago I received an e-mail from him with a picture of the SNA Jr. that  he had just finished building.  Today I saw that he had updated his blog with a very nice write up about the SNA Jr II.  I want to thank him for his kind words.
http://m0xpd.blogspot.com/2016/11/sna-junior.html
 
If you have not read his blog before, it is one you should follow.  I know I always find something interesting and informative in each post.

Friday, November 11, 2016

Light Weight 3D Printed CW Paddle

I am a member of the North Georgia QRP Club ( NOGA). Many of the members are involved in portable operations such a SOTA.  One of the common discussions involves small light weight CW paddles for field use.  After getting the 3D printer and becoming familiar with its CAD software, I decided to see if I could come up with a small, very light weight paddle.  



The design I came up with is a combination of several different designs I found while searching the Net.  I also decided to customize it with the NOGA name built into the frame.



I printed up a few and gave them out at a recent meeting. The response seemed to be quite favorable. The suggestion was made that this was something we could sell at a upcoming Hamfest.  




I printed up a couple of sets in different colors for the hamfest, and we sold several in each color.  Now we are deciding if we want to make them available, and what would be a good price point including shipping.  I would like to get an idea of the interest in this.  So, if you think you would like to purchase one of these please drop me a e-mail at duwayne@kv4qb.us