ZL2PD Accessories for the KN-Q7 QRP Transceiver
Accessories built so far include a linear
power supply, a switchmode power supply, a tuning aid, and a headset.
An antenna tuner and SWR meter will be described later.
KN-Q7 is a low power (QRP) transceiver kit designed in China by amateur
radio operator BA6BF.
In order to operate the new transceiver, I needed a power supply. I
began by building a power supply with a linear regulator, and later, I
modified a laptop power supply. The details are described below.
In addition, I have described how I connected a really cheap computer
microphone headset to the KN-Q7.
tuning aid to help adjusting a manual antenna tuner is also described
in detail here on my website.
to add some details of my new antenna tuner, complete with it's simple
SWR meter, soon.
A Linear 12VDC 2Amp AC Power Supply
might expect, I have several power supplies that have more than enough
capacity to power my new KN-Q7 transceiver. However, my largest power
supply is normally connected to my main 100W PEP SSB transceiver. It is
capable of 20A and more, but it’s not easy to connect other equipment
to it. I also have a smaller 8A power supply, but I use this to power
some other VHF and UHF transceivers.
In short, I wanted a simple power supply which I could dedicate to this
KN-Q7 40m QRP SSB transceiveris shown here with my new AC power supply
I've built and described below.
The design is simple and well-proven although it varies from the
traditional approach to allow the 2N3055 to be directly mounted on the
chassis or heatsink without the usual insulating washer
I had a really cheap 12V car battery charger in the workshop which I
thought could be easily modified to make a suitable power supply. I
quickly discovered that the charger’s transformer was not quite able to
supply the transceiver’s peak current load, even though the charger was
rated at 2.5A continuous operation! I also found that the cheap charger
transformer induced some low level hum into the transceiver when I sat
the transceiver on top of the supply.
Fortunately, I had a transformer in my box of bits which I had salvaged
a year or two back from an old stereo/CD audio system. These consumer
products seem to have a lifetime shorter than a mosquito! The
transformer looked big enough, and I measured its secondary voltage at
19VAC (no load). A bit high, but under load, it dropped to about 16VAC.
That would be OK. Another nice thing was that I could fit it quite
easily into the original charger’s plastic box.
The regulator section of the power supply is a slight variation on the
standard regulator design shown in the datasheets for the LM7812 12V 1A
regulator. To increase the current capability of the regulator, the
datasheet shows a circuit using a high power PNP transistor wrapping
around the 7812.
If the circuit is inverted, and a -12V regulator (LM7912)
is used instead, you gain two advantages. Firstly, I could use a common
cheap NPN power transistor such as the 2N3055 shown here. Secondly, the
transistor can be bolted directly to a metal chassis without the need
for insulating washers and other costly hardware normally required to
keep the collector/case of the high power device isolated from the
NOTE!! The connections for the LM7912 are NOT the same as those for the
I’ve used a 10,000uF smoothing capacitor in this power supply. The
usual design ‘rule of thumb’ is 2,500uF per amp, so 4700uF would be OK.
However, more is better, and besides, I had this larger capacitor on my
shelf along with all of the other parts. All told, this power supply
probably cost me less than $US2! And that was because I didn’t have the
LM7912 in my parts bin.
The audio system transformer has a copper band around the outside of
the windings, and that effectively prevents the induced hum problem
noted with the cheap charger transformer.
The LEDs were in the original charger, and
the charger’s PCB, complete with LEDs and front panel which matched the
LEDs, was reused. It was arranged in a very similar manner to that
shown in the schematic. I chose not to change this, although I think
there is some benefit from relocating the red LED to the actual output
of the power supply.
As it stands, with my transformer, the green LED acts as a current
indicator. Why? Because, on current peaks, the DC input to the
regulator falls below the voltage required to keep the combination of
the 12V zener diode and 2V green LED fully conducting. It dims nicely
at about 2A, and goes out at about 2.5A, giving me effectively a cheap
I also had to add a higher rated bridge rectifier to suit my new
transformer, using diodes also salvaged from older equipment, and I
added the slow blow fuse to the original charger PCB in place of the
original, unwanted, glass-encapulated thermal breaker. All quite easy
In service, the power supply worked really well. It’s easily capable of
powering the KN-Q7 QRP transceiver. The heatsink/metalwork is probably
too small for heavy duty ‘rag-chewing’, i.e. lengthy conversations with
the KN-Q7, but it seems fine for my typical two to four minutes of chat
followed by a similar period of listening to the other guy. If you plan
to talk longer than this, then use a metal box for the power supply and
bolt the 2N3055 (or similar) to the chassis.
A Switchmode 12V Power Supply
actually started out building a lightweight switched-mode power supply
Elsewhere on my website, you’ll see that I
have some experience at
modifying those sorts of power supplies. A SMPS like this is ideal to
holiday because they are so light.
However, they can generate a lot of
RF noise. And that proved to be a problem with the one I first tried to
use. After adding lots of extra filtering and
getting nowhere, I tossed it aside and went to find another one. I visited a
recycling centre in my town, and bought an old laptop switchmode power
supply. It was from a Toshiba computer, and cost me about $US3.
difficult task was to carefully break apart the plastic power supply
shell. This can be done by carefully easing the two halves of the shell
apart using a small screwdriver. Take care not to damage any internal
parts during the process.
The copper shielding was temporarily removed. The only modification
required was the addition of a tiny 1/8th watt resistor in parallel
with one of the voltage setting resistors around the TL431 in the
original power supply. The value was chosen after calculating the
approximate value I needed based on the TL431 specifications and the
values noted in the original power supply. You can see this in the
photo below. It's the tiny resistor in the upper centre of the picture.
testing, the copper shield was replaced and the plastic shell glued
together again. This modification was very successful, and the result
can be seen in the photos.
Using Computer Headsets with the KN-Q7
turned out to be one of the easiest modifications you can do to the
KN-Q7. A standard
PC headset used for Skype and similar services usually features a pair
of stereo headphones, typically 32 ohms impedance, and a boom
microphone. All of the ones I tested were electret types.
I had one in my workshop with only one
headphone (to allow you to still hear people talking to you without
having to remove the headset) and that was the one I used here. I think
it cost me perhaps $US2, and it's pictured to the right. These are
often very cheap now days.
Although the microphone connects using a stereo connector, in fact the
microphone is simply connected in common to both left and right hand
I removed the headphone and microphone stereo phono jacks from the
headset. The microphone wiring was connected directly to the microphone
pin and ground pin on the front panel microphone connector of the KN-Q7.
To allow me
to connect the headphone connections to the speaker output of the
KN-Q7, I needed to add a wire to connect the speaker output of the
radio to a spare pin on the same front panel connector. Easy! I added a
short brown wire from the top of the 10 ohm resistor (R12 on my
schematic of the radio, which you can download from here)
and that's all that was required. (It can easily be removed if you want
to restore your KN-Q7 to original)
The wiring is shown in the photo opposite.
The last step was to add a pushbutton for Push To Talk (PTT) It's the
white wire you can see in the photo, and the PTT switch can be seen in
the photo above at the end of a short section of grey cable.
More to Come!
antenna tuner - so I can use a variety of antennas when I take this
transceiver on my next holiday - Coming soon!
A simple SWR meter - This is built into the antenna tuner - Coming soon!
A tune-up aid - I use this to help me tune the antenna tuner - You can
see the details here.
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and select 'view image', you will see the picture or schematic at full
resolution - and the zoom feature will also allow you to see more
details if the picture or schematic is able to be enlarged.
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