BITX40 Construction Notes

In the Hamfest India of 2014, the kit bag contained a PCB for 40 meters BITX called the BITX40. The BITX40 can be built very simply. Construction of the BITX should be a straightforward process with the help of these notes.

1. A mistake on the PCB

There is a mistake on the PCB as well as the circuit published in the souvenier. The T1A's common point has to be grounded. The correction is in Fig 1. 

Fig 1:

Here is a picture of our PCB with the ground jumper in place:

You will have to solder a small wire on the component side of the PCB. See Fig 2.

2. The Bandpass Filter

The bandpass filter in the original circuit had three inductors L1, L2, L3. However, we can get by with just two inductors. It makes the tuning up much easier. They are both wound on drinking straws.

- We only use L2 and L3. We skip L1, C2, C3. Solder a wire in place of C4. C1 now directly connects to L2.

- The L2 and L3 need 2.6uH inductancce. Wind 34 turns of 28 swg over 1/4 inch inner diameter.

- You can use 1/4 inch drinking straws for both the coils.

To wind the coils :

Use a pin to make a two holes across the one end of the straw.

Pass one end of the wire through them and wind them with light tension. Too much tension can collapse the straw, too less will result in very loose coil.

After 34 turns, hold the wire in place with your fingers, make another set of holes and pass the wire through it to secure the coil. Apply super-glue or Feviquick to secure the turns.

See the picture below:

The air wound coils are not self-shielding, unlike a toroid. Mount L2 and L3 at right angles to each other.

3. The VFO coil

The VFO needs a much larger inductor of 4.4uH. This needs 59 turns of 28 swg on 1/4 inch straw. See the picture below:

4. The RF transformers T1 and T2

The RF transformers are wound on TV baluns. First, prepare the trifilar winding for T1 and T2.

Take three 1 meter lengths of 28 swg. Twist them together so that there are 8-10 turns per inch.

Wind each of transformer with 8 turns. One pass through both holes counts as one turn. There will be three wires coming out from each end. Identify each wire's both ends with a VOM. The PCB is laid out as three pairs of pads. Each pair should take two ends of the same wire.

5. The T3 transformer

The T3 is made just like the T1 and T2 except that there are two wires twisted together instead of three.

6. The BFO RFC

The BFO RFC is just 10 turns on a TV balun.

7. The Simplified Diode Modulator

The diode modulator can be greatly simplified by eliminating the balancing circuitry. Skip the C52 (trimmer), C53 (the 10pf on the other side from C52) and the preset. Instead, just short all the three holes of the preset together.

Here is simplified modulator:

Without a manual balancing control, the diodes have to be matched. That is easy to do with a simple circuit (see below). Test all the diodes from your junkbox for the forward voltage drop. Choose the two with the closest match.

8. Alignment

There are four things to be aligned:

1. The VFO

With the tuning knob all the fully turned counter-clockwise, it should be oscillating at 11.915 Mhz. Use a general coverage receiver or a frequency counter to set the frequency. Tuning range should be from 11.915 Mhz to more than 12.020 Mhz

2. The Bandpass filter is easily tuned. Connect an antenna and turn on the BITX. Just peak the two trimmers for maximum noise.
3. The BFO setting is a little tricky.
   Attach an antenna and keep the trimmer C45 at maximum. Such that the moving plates are all inside the stationary plates.
   While listening to the noise, keep opening up the trimmer. The atmospheric noise will become bassy. Keep on opening it until it starts becoming shriller again and stop it right there. This is your coarse BFO setting.
   Now, tune around until you hear an SSB station. Tune it up for maximum sound. It may not sound clear at the moment.
   Lightly tweak C45 until the station is clear. The BFO is now set.
   
   

9. Conclusion

The BITX40 is the easiest BITX to align and assemble. It is also the most inexpensive one! If you have any more problem join the mailing list on http://groups.yahoo.com/groups/bitx20.