BITX20A

It has been a while since my last posting on this blog.  During that time several interesting things have happened, the most interesting to me being that Nigel G0EBQ worked up a modification of a basic BITX20A Kit PCB for operation on 17 meters.  After Dan performed spectrum tests on Nigel's conversion (it meets FCC spurious emission requirements), I wrote a BITX17A Assembly Manual for the new BITX17A Kit.  That manual, pictures of the BITX17A, and ordering information is now available at <http://www.qrpkits.com>.  Doug has even made a 20M to 17M conversion kit available for those who wany to convert their BITX20A units to be BITX17A transceivers.  I will be working up a short "conversion Instructions Manual" for that in the very near future.  After that...I plan on doing a total re-write of the existing BITX20A Assembly Manual so it will include things that we have learned in the past 8 months.  This new re-write will also include a troubleshooting section with schematic modules and DC voltage charts.  Until this new BITX20A Assembly Manual is available, you can download the BITX17A Assembly Manual if you need troubleshooting information for your BITX20A Kit.  The schematic sections and DC voltage charts are valid for both BITX17A and BITX20A.  To make the conversion of a BITX20A to a BITX17A, only the crystals and a few component values were changed to make it operate on 18.068 to 18.168 MHz..



Also during the past 10 months, Leonard KC0WOX has developed many excellent image files and videos to help with various BITX20A, BITX17A, and BITX Version-3 transceiver builds and related troubleshooting.  These are located at <http://golddredgervideo.com/kc0wox/>.

Over the past 14 months Sunil VU2SWX has continued to develop and improve his marketing and availability of the BITX20 Version-3 kit.  Information on this may be found at <http://www.cqbitx.blogspot.com/>

Information on the UK based 80 meter version of the original BITX transceiver is available at <http://www.radio-kits.co.uk/>

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February 4, 2008 Update:

Although most BITX20A kits have been stable, a few have experienced problems with oscillation in the RF PA section.  If you have that problem, then you might be interested in suggestions from  Martien PE1BWI

Re: BITX20A PA Oscillation?

My BITX-20 is not the famous kit but my own build unit. However I
liked the PA design so I build that part of the kit last week. My
unit also oscillated and I cured it by mounting a 220 Ohm resistor
between connection point C6/R13 and the collector of Q3. In this way
any parasitic effects from T3 (that could cause oscillation) are
attenuated by the resistor. Further I also mounted a 1000 Ohm resistor
between the drains of Q18 and Q19 for the same reason. The effect is
that the amplifier is now absolutely stable under all antenna
impedances (open/ short, various phases).

Since the impedances at these points are lower then the mounted
resistors no degradation of power could be measured. In fact mine
easily produces 15 Watt PEP without noticeable distortion.

You can try quickly and see if it helps in your situation.

Martien PE1BWI

7 October 2007 Schematic v-1.3c.gif
Click on the schematic to see larger image.
Click again for an even larger image.

You have three options for frequency coverage with the BITX20A kit.  Since the polyvaricon tuning capacitor is a two-section unit, you can use the A-section, the B-section, or both A and B sections in parallel.  There are trimmer capacitors built into the tuning capacitor.  These trimmer capacitor plates are visible through the plastic cover on the back of your main tuning cap.

Since the IF frequency is 11.0 mHz, your VFO will need to tune between 3.0 and 3.35 to cover the full SSB and CW sections of 20 meters.  However, it will probably not have that wide a tuning range with the components provided.  The fine tuning pot should only have 6 to 8 kHz (+/- 3 to 4 kHz) of tuning range.  Set it mid-scale when adjusting the main tuning calibration.  Close the main tuning capacitor by turning it fully counter-clockwise.  With a frequency counter or calibrated receiver connected to the top of R-27 (see image above) you can set the lower edge of your tuning range by adjusting the trimmer caps.  Now open the main tuning capacitor to find the top of your tuning range.

Click on this drawing to see larger size.

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You have three options for frequency coverage with the BITX20A kit.  Since the polyvaricon tuning capacitor is a two-section unit, you can use the A-section, the B-section, or both A and B sections in parallel.  There are trimmer capacitors built into the tuning capacitor.  These trimmer capacitors are visible through the plastic cover on the back of your main tuning cap.

Since the IF frequency is 11.0 mHz, your VFO will need to tune between 3.0 and 3.35 mHz to cover the full SSB and CW sections of 20 meters.  However, it will probably not have that wide a tuning range with the components provided.  The fine tuning pot should only have 6 to 8 kHz (+/- 3 to 4 kHz) of tuning range.  Set it mid-scale when adjusting the main tuning calibration.  Close the main tuning capacitor by turning it fully counter-clockwise.  With a frequency counter or calibrated receiver connected to the top of R-27 (see image above) you can set the lower edge of your tuning range by adjusting the trimmer caps.  Now open the main tuning capacitor to find the top of your tuning range.

With the components provided in the kit, my tuning ranges look like this:

Polyvaricon                                 Trimmer Open                              Trimmer Closed
Section-A                                            60 kHz                                              58 kHz
Section-B                                          124 kHz                                            120 kHz
Sections A+B                                   155 kHz                                            147 kHz


If you wish to change the tuning range(s) you will need to play with the values of C-37, L-7, and C-15.  Alternatively, you could use a switch and capacitor to make an additional tuning range by paralleling additional capacitance across the main tuning cap
.
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Schematic Version 1.3c.gif

Here are a front and rear view of a completed prototype BITX20A.


No labels for the knobs yet (I know where they are and what they do, thus no urgent need for labels).  The lower-left knob is for Fine Tuning,  The lower-right knob is Receiver Volume.  Large center knob is main tuning.  Those three LEDs in the upper-left may require some explanation.  The left-most is on when receiving.  The middle LED is on when transmitting, and the right LED shows Forward minus-reflected RF power on transmit.  These LEDs are my own addition and are not part of the BITX20A Kit.



Rear panel connections are Microphone, Speaker or Headphones, Antenna, and power.  A fan was added because my chassis is fully enclosed and it seemed like a good idea to remove some of the heat generated by those two RF-510s in the transmitter final amplifier.  This particular fan was once the CPU cooling fan in an old computer.  The fan is my own addition and not part of the BITX20A kit.

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Workspace Layout:





Receiver AF Amp Wired



Powering RX and TX Simultaneously for Testing Purposes



Trifilar Twister Tool


Toroid Lead Arrangement
     

30 pf Variable Capacitors (Grounding Detail)

These variable capacitors have a flat side and a rounded side.  The rounded side has a length of the metal tab exposed.  The rounded side, the one with the exposed metallic tab, should go to the ground side of your circuit.  This puts the screwdriver slot at ground potential and allows use of a metallic screwdriver to adjust the capacitance.  If you get it backwards, your screwdriver will detune the circuit when you try to adjust it.


PCB Topside Artwork (click to enlarge)


Latest Schematic (Click to enlarge)



AF Sections, IFs, Filter, Mixers, and VFO wired and tested.



Since my chassis is fully enclosed, I added a cooling fan on the rear panel.

Two parts bags for the BITX20A Version 1.3 arrived in today's mail.  Now all I need are the 11 MHz crystals that Jim is sending separately.  I will go ahead and start wiring this evening.



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Parts kit for the next BITX20A build was shipped yesterday (May 29, 2007).  It should be here by this weekend.  I have cleared off space on my workbench, cleared time on my work schedule, and cleaned the soldering iron tip. I'm ready!

This next round of building will be documented here in a format that I hope can serve as basis for the Assembly, Testing, and Operating manual.  Step-by-step assembly will be in a format that looks like this:

1.  C-24, 10 pf

    

2.  C-20, 47 pf

    

3.  C-19, 10 pf
   

4.  Solder and inspect connections

This layout provides check-boxes for marking each completed step during assembly.  Procedures will be provided for testing  each section as they are completed.

Dan says that he does not have time to assemble the parts kits for this next prototype build until after he returns from the Dayton Ham Radio Convention.  That means that we will not be starting the build until sometime after May 20, 2007.  I will post here again when the parts kits arrives.

       <http://www.hamvention.org/>

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The new PCB arrived today.  I looked it over using magnification and it looks very good.

Dan has added three new external connection pads to the layout.  These are TX-G-RX and are connected to the +TX voltage, Ground, and the +RX voltage.  This will provide a handy way to add modifications that require switching between transmit and receive modes.

Now, just waiting for parts to populate the board.

Arv   (May-5-2007)

The new prototype BITX20A PCBs have been ordered.  They should be distributed shortly after Doug, Dan, & Jim get back from attending Ozarkcon.  We will start this next round of construction & testing just as soon as PCBs and parts kits are available.

Keep watching this blog.  I will post here when construction starts.  The blog content format will be as close as I can come to an Assembly and Operating Manual layout.  This way my blog can be used to build the actual documentation that will accompany the kit.

Arv
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   GIF Image Schematic


Click on the diagram to see in in higher resolution.

CompuServ GIF Image:


JPEG Image:


PNG Image: