Download BITXxxA Assembly and Modification Tips

BITXxxA Assembly and Modification Tips

PCB Mounting: Use 1/4 inch standoffs and longer screws to fasten the PCB inside
the QRPKits.com chassis. This raises the PCB high enough to safely route all the offboard wire connections under the PCB instead of over the top. It makes things look a
lot cleaner and is easier to get at resistor and capacitor leads for troubleshooting and
alignment.

VFO Blocking Filter: L-10 and C-25 make up a band-pass filter for your transmit
frequency. This is there to block VFO harmonics from being transmitted. You can
try various values of L-10 and C-25 to insure that this BPF (Band Pass Filter) is
peaked at the optimum frequency for your normal operations. It is a minor
improvement, but makes one feel better to know that your BPF is exactly on
frequency.
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T-PAD: If you suspect problems with the C-16, C-17, C-19 impedance matching
pad, you can replace it temporarily with a 2.2 pf capacitor connected from the L-4 pad
for C-16 to the L-6 pad for C-19. This eliminates the 3-capacitor T-pad for coupling
the two tuned circuits together and makes tuning interaction a bit more touchy, but it
may be useful as a troubleshooting trick.
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Balanced Modulator: The original BITX20 BFO mixer had an RF bypass capacitor
at mid-point on the diode winding side of the transformer. If you are having difficulty
obtaining a good carrier balance on your BITxxA BFO mixer, you might want to try
adding a 0.001 mfd capacitor from center-tap of T-4 to ground on the diode side of
the transformer. It may help, and it may not, but is an interesting experiment to try.

Receive Antenna Coupling: C-50 in the antenna switch receive path is a 0.1 mfd
capacitor, which has a very low impedance at operating frequencies. In the interest of
adding some small amount of protection for the TR switch MOSFETs in case the
IRF510's go into oscillation, you may want to change C-50 to a value which has
between 10 and 50 ohms impedance at operating frequency. For 14.24 MHz this
would be 30 ohms = 390 pf, and for 17 meters 30 ohms would be 300 pf. Be sure to
use a capacitor that can handle up to 100 volts of RF. This should not reduce received
signal level by any noticeable amount. You can verify that by comparing the new
capacitor value with that when you bridge the old 0.1 mfd across the new capacitor.

Microphone PTT Wiring: When wiring the microphone jack, and the plug on your
microphone cable, put the PTT connection on the plug Tip. This makes it impossible
to short the full PTT relay voltage across your microphone element in case you
unplug or insert the microphone plug with power attached to the rig.

Optimizing the Crystal Filter:
For those purists who must have the best possible crystal filter performance, you can
adjust the values of C-56, C-55, and C-54 for the bandwidth you want (increased
capacitance decreases bandwidth, and decreased capacitance increases bandwidth).
C-56 and C-54 should be the same value, with C-55 approximately twice that value.
If there is a single dip or peak in the middle of the filter passband, try adjusting the
value of C-55 up or down to smooth out the top of your pass band. If you have
ripples (more than one peak or dip in the passband) then try adjusting input and output
impedance by changing C-62 and C-53 (transmit mode) and C-52 and C-89 (receive
mode) for minimum passband ripple. If the skirt shape is not what you want, try
swapping crystals around from one position to another until you have minimized all
skirt abnormalities. If you have a crystal which insists in causing a ripple in skirt
shape, you might want to swap that crystal with the BFO crystal to see if it is any
better.

TX PA RF Chokes: The purpose of L-3 and L-9 in the RF PA section is to act as RF
Chokes for feeding 12 volts to the drains of the IRF510s. Their individual impedance
should be approximately 2000 ohms at the operating frequency. This is important
information if you are modifying a BITXxxA for operation on a lower ham band. L3
and L9 would each be 45 uh (11 turns) for a BITX40A and 90 uh (16 turns) for a
BITX80A. On 160 meters you would want 176 uh or 23 turns.
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T1: T1 provides an impedance step-up from the drains of the IRF510s to the antenna
filter. Its secondary impedance is 50 ohms. The antenna filter impedance is 50 ohms
in and 50 ohms out.

Tx Audio Level: Audio input to the Microphone amplifier should be in the 30 to 45
millivolt range for full modulation levels. Excessive audio input to the microphone
amplifier will cause distortion and harmonic signal generation in the BFO mixer and
will appear as near-in spurs (2X and 3X the audio frequency) on your transmitted
signal. Do not overdrive the microphone input if you want a clean transmitter signal.
A clean 12 watts will get you more contacts than a dirty 16 watts.

Inserting External Audio: Since there is a 4700 ohm resistor (R-92) that provides
voltage for the electret microphone element, you need to either remove this resistor or
use a capacitor to block this DC from getting into any other type of external audio
source. But it is not as simple as adding a capacitor and leaving R-92 in place. This
would allow R-92 to pull the input to 12 volts and thus cause clipping or distortion of
your external audio input. If you do not want to go inside the BITXxxA and lift one
end of R-92, you can add an external resistor to ground which is the same value as R92 (probably 4700 ohms). This will set the input voltage at 1/2 Vcc or about 6 volts.
Then you still need to add an external coupling capacitor (10 mfd is okay) between
that 6V and your external audio source, and to observe proper polarity if this capacitor
is a polarized type.

Microphone Gain: Microphone gain in BITXxxA transceivers is controlled by the
emitter RCR network of the microphone amplifier (Q-15). DC gain is approximately
3, which is adequate for bias stabilization. Audio (AC) gain is set to approximately
10 by the amount of unbypassed emitter resistance versus the collector resistance
(1000/100 = 10). If you have a weak microphone you can change the amplifier gain
by changing the unbypassed feedback resistance (R-74) in this emitter circuit. But,
you should also change the value of R-93 to maintain the DC gain at approximately 3
for good bias control and to maintain enough voltage swing at the collector for quality
audio.
Those who understand this circuit have probably already guessed that you could use a
320 ohm potentiometer between emitter and ground and slide the capacitor up and
down this with the potentiometer wiper to make the microphone gain fully adjustable
between 3 and about 150 (X150 is way more audio than you would ever need).
A more sane way to do it would probably involve changing R-74 to a 200 ohm pot
with it's wiper connected to C-73 and changing R-93 to a 120 ohm fixed resistor.
That maintains the DC gain of 3 and supports variable AC gain adjustment between 5
and full gain of the transistor (probably around 150).

Reception Problem: Reports of no received signal, or weak received signal, are
almost always isolated to the bandpass filter made up of C-13, C-14, L-4 and C-20, C18, L6. The capacitors C-16, C-17, & C-19 are an impedance matching and isolation
pad between the two sections of this filter. You can replace this isolation pad with a
2.2 pf capacitor, but it will make tuning the LC networks a bit more interactive
between the two filter halves.
Begin troubleshooting weak received signal problems by using a 10 pf capacitor to
couple your antenna directly to the junction of C-24 and C-29. If strong received
signals are then available, you have a problem between this point and the regular
antenna connection...probably it will be that the L4 and L6 filters are not tuned
properly. These two tuned circuits should each have two points in the capacitor
adjustment where signals peak sharply. If you do not have these two peaking points,
then you may have to add or subtract one turn from the associated L-4 or L-6
inductor.
Another poor reception problem area is the VFO mixer transformers. If they are not
connected properly, or if they do not have continuity due to soldering over enamel
insulation, your received signal may be quite weak.

Mixer Transformers:
Mixer transformers have an effect on receiver sensitivity. The trifilar winding wires
need to be tightly twisted in order to provide good impedance match and good
coupling. Should have approximately one complete twist in twice the wire diameter.
Twists should be evenly spaced. Space the turns equally around the core. Smaller
wire twists easier and makes better mixer transformers. Space the turns equally
around the core.
Wires on the left do not have tight enough twist, and the twists are not evenly spaced.
Wires on the right have adequate twist for good coupling and impedance match.
Properly built BFO Mixer transformers will provide better carrier rejection (deeper
null) than poorly constructed ones. Take your time with the mixer transformers. If
you are careful you can build broadband mixer transformers that as good as any of the
commercial equivalents.
When making your mixer transformers, leave at least 3 to 4 inches of extra wire on
each lead. This way you can thread individual wires through their PCB holes without
having them pull back out while you are trying to thread other wires through their
holes. You can even loosely tie a knot in the end of each wire after it has been
inserted through its hole to insure that it doesn't pull back through. Long leads also
allow you to re-test to insure that you have the proper winding connected to the
proper set of holes in the PCB. After soldering the wires in their respective holes, go
back and re-test with an ohmmeter to make sure that you have good solder
connections and no shorts between windings. Do this by measuring resistance
between pads that the wires are soldered to. This insures that you have adequate padto-wire continuity. If windings appear to be shorted together during your on-board
tests, take a look at the schematic. It may be that they are connected together on the
PCB instead of being shorted (this had caused several people to unnecessarily remove
and re-connect their mixer transformers).

Individual Section Current Measurements: As a troubleshooting technique, it is
possible to make some measurements and then calculations (educated guesses) as to
how much current each section is drawing. Most sections have decoupling resistors in
their voltage feed points. Use the voltage drop across these decoupling resistors to
tell you how much current is flowing through them. I = E / R ; where I is Amperes, E
is volts, and R is ohms.

LM386---measure the voltage drop across R82 and calculate the current from that
voltage drop and the 10 ohms of R82.

AF Pre-amp---measure the voltage drop across R79.

BFO---measure the voltage drop across R69.

VFO---measure the voltage drop across R35.

VFO Buffer Q6---measure the voltage drop across R26.

Receive 2nd IF amp---measure the voltage drop across R56.

Receive 1st IF amp---measure the voltage drop across R44.

Receive RF amp---measure the voltage drop across R25.

Microphone amp---measure the voltage drop across R76.

Transmit 1st IF amp---measure the voltage drop across R62.

Transmit 2nd IF amp---measure the voltage drop across R50.

Transmit 1st HF amp---measure the voltage drop across R19.

TX Driver Q20---measure the voltage drop across R90.

2nd TX Driver Q3---measure the voltage drop across R13.

TX PP Drivers Q18 & Q19---measure the voltage drop across R7.
NOTE: Any RF waveform may distort the DC voltage measurement.
Use a 4.7K resistor bypassed with 0.1 mfd to isolate your DC
measurement from its AC component.

Powering the KD1JV Digital Dial: Ground for the frequency counter can be
obtained at the ground end of C-30. Also connect the Ground lead for frequency
input to this same point. + 12 volt power for the frequency counter can be found at
the exposed lead of R-26. This keeps leads short and close to the front panel. Signal
input to the frequency counter is available at the exposed lead on R-27. Temporary
connection to BFO output for calibrating frequency offset can be obtained at the
exposed lead of R-67.

Wire: As much as possible, use the wire that was supplied with your kit. If you
substitute larger solid-wire in place of the smaller stranded wire it may be possible to
force it into the PCB holes, but if you ever need to replace one of these wires or want
to do any modifications, you may find that unsoldering heat has expanded this larger
and solid wire to the point that it cannot be removed from the PCB holes.

Cutting Lead Tails: When trimming component tails after components have been
soldered into your PCB, always use a sharp cutting instrument. Dull cutting pliers
may put a mashed wedge-shaped end on cut component leads which will not allow the
components to be unsoldered and removed from your PCB if you need to make
repairs or modifications.

Setting the BFO frequency: Your crystal filter has a fixed frequency passband.
Yours will be different from the picture below...this is just a representative drawing to
show how and why.
Looking at this drawing you see a filter with a -6 db bandwidth of 2.8 KHz, centered
at 9.996500 MHz. For best audio the BFO frequency should be placed about -12db
down the lower skirt of this filter, or in this case at 9.994500 MHz.
NOTE: For the BITXxxA models your IF and BFO frequencies will be different from
the 10.0 MHz shown on this drawing. The drawing was made for an original
Manhattan-construction BITX-20 that used 10.0 MHz crystals for the BFO and IF
filter.
Placing the BFO frequency this way will provide an additional 12 db of carrier
suppression, and also will attenuate the lower 500 Hz of the operators voice so less
RF power will be wasted in transmitting unnecessary lower frequency information.
So... what frequency to set the BFO for......it depends! It depends on the filter shape
factor, the skirt frequencies, and how far down the skirt you want to place your BFO
for best performance.
Next question may be "how does one do this?". If we assume that the builder has
relatively little test equipment, but has purchased the BITX kit with a frequency
counter, it becomes fairly easy to accomplish.
o
Use a diode detector and digital or analog DC voltmeter to measure the carrier
output of your 2nd transmit IF amplifier (using the amplifier stage to isolate
the crystal filter from any loading by the metering components).
You could also use an oscilloscope to observe the RF voltages.
o
Connect the frequency counter to the BFO output just before the balanced
modulator.
o
Turn the Tx drive control all the way down to zero to protect the RF PA
and set transmit bias controls for no current flow in the RF PA devices.
o
Unbalance the balanced modulator by turning both resistor and capacitor
to the extreme right or left of their range.
Try both directions to see which gives you the highest voltage reading.
o
Key the transmitter and observe the RF voltage at the output of the Tx
2nd IF amplifier.
o
Slowly tune the BFO through the filter passband, observing and writing
down frequency at the -6db points and the lower skirt edge where voltage
has decreased by another 6db (the -12db point). You can even graph
the shape of your filter by observing voltages at various frequency settings.
Halving of the voltage represents -6db point. Halving it again will be the
-12db point.
NOTE: Such a graph will look rough because it is a voltage representation
of the filter, and not a true db view of the shape. Convert your voltages
to DB to see the more traditional smoothed filter shape in db.
Now you have all the information you need to properly set the BFO frequency relative to
your particular IF filter characteristics.

MONO-BAND BITX20 DIPPER and ATU:
This diagram represents a composite of several products shown on the
QRPKITS.COM web page. The idea is to build an inexpensive single-box solution
for antenna tuning and operating that will fit well with field day use of a wire antenna
and counterpoise or ground. The idea for a crystal controlled Antenna Dipper came
from the web page of Steve KD1JV. This BITX version incorporates a Dummy
Load, Antenna Dipper, and Antenna Tuner into an integrated package. Having the
dummy load as part of this provides a couple of benefits (1) it keeps you from keying
your BITX transceiver into an open circuit when the ATU is switched to the dipper,
and (2) it provides a 50 ohm load for testing the dipper when the transceiver is
connected to the ATU. Activating the dipper when the transceiver is switched to the
ATU allows use of the 14.318 crystal as a reference frequency (handy if you are not
using a digital dial for your BITX20). Much of what you need to test and align a new
BITX build is incorporated into this composite instrument.
There are several possibilities for modification of this circuit, including (1) adding a
rectifier and voltmeter to the dummy load so you can measure actual power output of
the BITX transceiver, and (2) replacing the crystal with a tuned circuit to make the
dipper part tunable over the whole 20M band. And, of course this design could be
modified for use on any other ham band. A simple current transformer arrangement
on the transmitter output lead could be used to provide indication of RF current going
to the dummy load or to the antenna. If you add switching of the oscillator crystals
this circuit can be used for multiple bands...particularly interesting if you have built a
multi-band version of the BITX transceiver.
If T3 secondary had a 12-position switch with taps every 2 turns, the ATU part could
probably be used to match low impedance antennas (coax fed ones) as well as the
randon-length end-fed wire.
C5 does not have to be 360 pf. Probably a 100 pf unit would be adequate, and easier
to adjust. 28 turns on a T50-6 core with 100 pf will resonate at just over 8 MHz. 14.3
MHz should be at the 32 pf point.
This design can be modified for single band use on any band where you have a crystal
in the center of the range where you intend to operate. I chose 14.318 crystal because
I have a number of them in the junk box and because they are readily available for
quite resonable cost.
NOTE: I am in the process of building this, so the design has not been tested. Please
let me know if you do build this and if it works as planned. I am also interested in
what changes you might have to make to optimumize the design. No, there is no PCB
or PCB layout yet.
