[HBQRP] (no subject)

[Piatt, Darwin]

Just in case you have a BITX SSB kit and have not joined the Yahoo
group, here is a very informative e-mail, save it for reference when you
start building.
P.S. anyone post pictures of last meeting, yet?
73's Dar...

Re: General question on BITX30 construction 
Posted by: "Arv Evans" arvid.evans at gmail.com   k7hkl_arv 
Fri Sep 28, 2007 2:30 pm (PST) 
Rob, & Others

I'll try to answer some of the questions regarding complexity of the 
BITX20A kit from www.qrpkits.com.

1. Winding Trifilar transformers is not much different from winding
Bifilar ones. For Bifilar you twist two wires together and wind
then through the core material. For Trifilar you twist three
wires together and wind them through the core material. This web
site URL shows a neat tool for twisting wire to wind on your
toroid cores.

<http://arvid.evans.googlepages.com/toroids>

In this particular PCB layout, Dan arranged the pads so that you
do not have to twist any winding ends together to make up
center-tapped arrangements. Any interconnection of windings is
done on the PCB and not on the transformers.

2. There are a lot of parts involved in the BITX20A kit. I
approached this by sorting the parts and sticking one lead of each
part into a block Styrofoam packing material. This keeps parts
sorted and readily accessible while you build.

3. The PCB layout is quite compact. Resistors are stood up
vertically to save space and all parts are quite close to each
other. Dan marked the top side of this PCB to indicate which is
the folded down lead connection point for each resistor. This is
important because it provides test points for a number of signals
in case you have to troubleshoot the completed transceiver. When
bending the resistor leads, use a scrap of PCB material to bend
the folded down lead over. This insures clean and neat bends and
will help make your finished rig look quite professional.

I used a set of tweezers to arrange and install toroidal
transformer wires into the proper PCB pad. All other parts were
simply put in place with fingers. No other special tools were
required for parts placement. A thermally controlled soldering
iron would be an advantage because you will be using it for quite
long periods during the assembly process. My soldering iron is a
Weller W60P with iron coated tip. A small can containing coarse
steel wool is used to periodically clean the tip (fine steel wool
will pack and eventually become solder saturated).
<http://arvid.evans.googlepages.com/soldering_station_and_tip_cleaner>

4. Doug KI6DS provides the main tuning capacitor, fine tuning
potentiometer, and volume control potentiometer. Other off-board
components that you will have to provide include microphone jack,
speaker/earphones jack, power connector, power switch, and RF
connector. If your microphone does not include a PTT switch, you
will also have to provide a front-panel switch to perform this
function. Many builders of earlier BITX20 units have fabricated
their own chassis bysoldering together panels of PCB material.

5. I'm still working on documentation of the alignment process, but
here is a quick overview:
_Alignment:_
* Adjust the BFO/Carrier Oscillator frequency to be 200 to 400
Hz down the LSB side of the crystal filter passband. You
can do this in transmit mode with an RF probe and DC meter,
or in receive mode by using an off-the-air signal or signal
generator.

* Peak the receiver RF input coils for your favorite portion
of the 20 meter band by using either off-the-air signals of
a signal generator. These LC networks are also used in
transmit mode, but peaking them in receive mode is all that
is required and will provide optimal operation.

* Adjust the balanced modulator potentiometer and variable
capacitor for minimum carrier output (transmit mode) with
the microphone disconnected. Your RF probe and a DC meter
works fine for this.

* Preferably, use a frequency counter or calibrated 3.0 to 4.0
MHz capable receiver to set the VFO to frequency. The VFO
and 11.0 MHz IF operate together to generate a 20 meter band
signal. Thus, if you want to start receiving 20 meter
signals at 14.150 MHz, your VFO must start it's tuning range
at 14.150 - 11.0 = 3.150 MHz. 

Alternatively, you can set the VFO frequency by injecting a
signal at whatever frequency you want in the 20 meter band
and then adjust the VFO until you hear a beat note.

* Bias adjustment for the push-pull RF PA Driver is adjusted
to an idling current of 5 to 10 ma in transmit mode with the
RF drive (R-83) turned all the way down to zero. I'm still
working on a documented procedure for this but you can
measure the voltage across R-7 and calculate the idling
current in the two MOSFETS as you adjust R-8.

* Bias adjustment for the RF PA MOSFETs is also adjusted with
them idling under no drive (i.e. turn the RF drive R-83 all
the way down to zero). I think that Dan set his idling
current higher than I did, but mine are adjusted for 20 to
30 ma on each device. There is no convenient resistor to
monitor for current flow, so you have to monitor total
transmit mode power supply current for an increase of 20 to
30 ma as you bring R-2 and R-91 up from ground toward the
positive end of each pot.

The almost modular nature of the BITX20 design allows you to build this 
unit in sections. Then after you understand how each section works you 
can go on to the next part. In the Assembly Manual you will be 
encouraged to build your BITX20A kit by starting at the speaker and 
microphone end of things and working toward the antenna. This allows 
you to understand how each section works and to test each section as you

build it.

While this design does use bi-directional amplifier sections, they are 
just conventional single-direction amplifiers wired so that one side is 
passive while the other is active, depending on whether the circuit is 
providing "Receive DC voltage" or "Transmit DC voltage". Antenna 
switching is accomplished by using MOSFET devices as switches. A relay 
is used to switch the DC supply to Receive circuitry or Transmit
circuitry.

It is not advisable to ever transmit into an open or high impedance load

with this or any other solid state final type transmitter, so always 
connect an antenna & antenna tuner or dummy load before keying the rig.

For those that want to eventually make additions or modifications to 
this kit, I would suggest that you use a larger chassis than if you plan

on keeping the rig compact and do no later addition of functions or 
features. Because the PCB layout is tight, you will probably have to do 
any modifications as off-board constructs and thus will need a little 
extra chassis space.

Hope that helps,

Arv - K7HKL

Darwin (Dar) Piatt - W9HZC
Midwest HomeBrewer's and QRP Group
http://www.qsl.net/hbqrp 
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