QRP Labs 10W HF Linear PA User manual

10W HF Linear PA kit assembly 1.00 1
10W HF Linear PA
10W HF Linear Power Amplifier kit assembly manual
A low-cost, high-performance HF Linear PA
covering 2-30MHz
Designed and produced by QRP Labs, 2018

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Contents
1. Introduction ..............................................................................................................................................................3
2. General Assembly guidelines ....................................................................................................................................3
3 ASSEMBLY .................................................................................................................................................................4
3.1 Parts list.............................................................................................................................................................4
3.2 Trace diagram and parts layout ........................................................................................................................7
3.3 Schematic..........................................................................................................................................................9
3.4 Wind and install inductor L201.......................................................................................................................10
3.5 Wind and install transformer T201.................................................................................................................11
3.6 Wind and install transformer T202.................................................................................................................13
3.7 Wind and install transformer T203.................................................................................................................14
3.8 install capacitor C213......................................................................................................................................16
3.9 Wind and install transformer T204.................................................................................................................16
3.10 Install capacitors C211 and C212....................................................................................................................18
3.11 Install remaining ceramic capacitors ..............................................................................................................18
3.12 Install inductors L202, L203, L204 and L205...................................................................................................19
3.13 Install resistors R203, R204, R209, R210.........................................................................................................19
3.14 Install resistors R201, R216 and R217.............................................................................................................20
3.15 Install resistors R207 and R208.......................................................................................................................20
3.16 Install resistors R213 and R218.......................................................................................................................21
3.17 Install resistors R212 and R214.......................................................................................................................21
3.18 Install resistor R202.........................................................................................................................................22
3.19 Install resistor R205.........................................................................................................................................22
3.20 Install resistor R206.........................................................................................................................................23
3.21 Install trimmer resistors R211 and R215.........................................................................................................23
3.22 Install capacitor C204......................................................................................................................................24
3.23 Install capacitor C214......................................................................................................................................24
3.24 Install diode D201 ...........................................................................................................................................25
3.25 Install transistor Q201.....................................................................................................................................25
3.26 Install transistors Q202, Q203 and Q204........................................................................................................26
3.27 Install transistors Q206 and Q209 ..................................................................................................................26
3.28 Install transistor Q205.....................................................................................................................................27
3.29 Install voltage regulator IC201........................................................................................................................27
3.30 Install transistors Q207 and Q208 ..................................................................................................................28
3.31 Install heatsink ................................................................................................................................................30
4 Applications information ........................................................................................................................................32
4.1 PA Precautions................................................................................................................................................32
4.2 Connections ....................................................................................................................................................32
4.3 Adjustment......................................................................................................................................................32
4.4 Tests undertaken ............................................................................................................................................33

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1. Introduction
This 10W HF Linear Power Amplifier has the following features:
•10W output from 2 to 30MHz, using 12V Supply
•Generously-sized heatsink, will not overheat even on continuous 100% duty-cycle modes
•2-stage amplifier provides 26dB of gain
•Push-pull driver and push-pull finals, for high linearity and low harmonic content
•+/- 1dB gain flatness from 2 to 30MHz
•4dB down at 4m (50MHz) and 8dB down on 4m (70MHz)
•Standard 50-ohm input and output
•Through-hole plated PCB, all through-hole components (no Surface Mount Devices)
•Standard inexpensive components throughout
•Tested for 1 hour at full-power 10W, 100% continuous duty-cycle with no forced air cooling
•Tested for 15 minutes at 20W, 100% continuous duty-cycle with no forced air cooling
•Tested at 20V supply
•Tested into open load, shorted load and various mismatches
We hope you enjoy building and operating this kit! Please read this assembly manual carefully,
and follow the instructions step by step in the recommended order.
The push-pull driver stage using two BS170’s is as used in the SoftRock transmitter stage.
I am very grateful to Allison KB1GMX for very valuable advice during the design and
development of this amplifier, for beta-testing prototypes, measurements and moral
support!
2. General Assembly guidelines
Assembly of this kit is quite straightforward. But there are quite a lot of components. So please
keep them methodically in trays or some convenient storage boxes. Be careful not to misplace
any. The usual kit-building recommendations apply: work in a well-lit area, with peace and quiet to
concentrate. Some of the semiconductors in the kit are sensitive to static discharge.
Therefore, observe Electrostatic discharge (ESD) precautions. And FOLLOW THE
INSTRUCTIONS!! Don’t try to be a hero and do it without instructions!
A jeweller’s loupe is really useful for inspecting small components and soldered joints. You’ll need
a fine-tipped soldering iron too. It is good to get into the habit of inspecting every joint with the
magnifying glass or jeweller’s loupe (like this one I use),
right after soldering. This way you can easily identify any
dry joints or solder bridges before they become a
problem later on when you are trying to test the project.
Triple check every component value and location
BEFORE soldering the component! It is easy to put
component leads into the wrong holes, so check, check
and check again! It is difficult to de-solder and replace
components, so it is much better to get them correctly
installed the first time. In the event of a mistake, it is
always best to detect and correct any errors as early as

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possible (immediately after soldering the incorrect component). Again, a reminder: removing a
component and re-installing it later is often very difficult!
Components in the kit are installed vertically, to minimize PCB area and track-lengths. You will
need to bend the resistor wires as shown in the picture below. On the parts layout diagram, there
is a small circle around the hole where the body end of the resistor should be installed. This is not
critical (resistors don’t care which way around they are) but it is recommended, just to try to ensure
that the folded over wire-end of the
resistor doesn’t touch any other
wire or metal part.
Please refer to the layout diagrams
and PCB tracks diagrams and
follow the steps carefully.
The component colour coding of the layout diagram at every step of the assembly instructions is
as follows (kind of: components past, present and future):
•Components shaded grey have already been installed
•Components shaded red are the ones being installed in the current assembly step
•Components shaded white are the ones which have not yet been installed
VERY IMPORTANT SOLDERING ADVICE: when soldering a component pin that goes to ground,
you need to allow extra time with the soldering iron. Ground pads are separated from the
groundplane by what are called “thermals”, a gap between the pad and the groundplane which is
bridged in 4 places at 0, 90, 180 and 270-degree around the circle. This is standard practice.
However, you may STILL find that it is harder to solder ground connections, because the soldering
iron heat is dissipated away more quickly. If a component ground connection is broken it can
sometimes have catastrophic consequences. So please check and recheck, particularly with
ground connections.
3 ASSEMBLY
The PA board is mounted vertically on the rear PCB’s back edge. The PA transistor tabs and the
driver are bolted directly to the substantial heatsink provided in the kit.
Use a drill bit, twisted by hand, in each hole of the binocular cores, just to remove any
sharp edges, and prevent scratching the wire enamel.
NOTE: The two BN61-2402 binocular cores are supplied in a small labelled bag. This is
because they are visually identical to the BN43-2402 binocular core. Do not remove the
BN61-2402 from their small bag, or you would risk mixing them with the BN43-2402.
3.1 Parts list
This parts list shows the through-hole components to be inventoried and installed in the assembly.
Resistors
R201, 216, 217 220-ohm ¼W, red-red-black-black-brown (3pcs)
R203, 204, 209, 210 220-ohm ½W, red-red-black-black-brown (4pcs)
R202 22K, red-red-black-red-brown

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R205 2.2K, red-red-black-brown-brown
R213, 218 10K, brown-black-black-red-brown (2pcs)
R212, 214 47-ohm, yellow-purple-black-gold-brown (2pcs)
R207, 208 2.2-ohm, red-red-black-silver-brown (2pcs)
R206 33-ohm, orange-orange-black-gold-brown
R211, 215 4.7K trimmer potentiometer, label “472” (2pcs)
Capacitors
C201 - 203, 205 - 210, 215 0.1uF, code “104” (10pcs)
C204 10uF 25V electrolytic
C214 470uF 25V electrolytic
C211, C212 1uF, code “105” (2pcs)
C213 33pF, code “330”
Semiconductors
D201 1N4004
Q201 2N3904
Q203, 203, 204, 205 BS170 (4pcs)
Q206, 209 IRFU9024, substitute for BS250 (2pcs)
Q207, 208 IRF510 (2pcs)
IC201 78L05, 5V voltage regulator
Inductors
L201 BN43-2402 binocular core
L202, L203, L204, L205 1uH molded axial inductors
T201, 202 BN61-2402 binocular core (2pcs, separate bag)
T203 FT50-43 toroidal core
T204 BN43-202 binocular core
Miscellaneous
PCB PCB
Insulated wire 30cm of insulated wire
0.6mm wire 80cm of 0.6mm enamelled wire
0.4mm wire 15cm of 0.4mm enamelled wire
0.25mm wire 60cm of 0.25mm enamelled wire
Insulating silicone pad Insulating heatsink silicone pad (3pcs)
M6 nut M6 nut
Washer Large washer
Insulating washer 2pcs white plastic insulating washer
9mm M3 bolt 9mm M3 bolt
15mm M3 bolt 15mm M3 bolt
Rubber tube Short ~4mm length of rubber tube
Heatsink Custom-manufactured 130x28x25mm

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3.2 Trace diagram and parts layout
Red = Top side; Blue = bottom side; Green = pads and vias.
There are only two layers (nothing is hidden in the middle). Not shown in these diagrams are the
extensive ground-planes, on both sides of the board. Practically everything on both layers that isn’t
a RED or BLUE track, is ground-plane! The two ground-planes are connected at frequent intervals
(not more than 0.1-inches) by vias.

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3.3 Schematic

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3.4 Wind and install inductor L201
Start with the inductors and transformers because they are difficult to install when the other
components are all around them.
The BN43-202 core is the one NOT in the small plastic zip-lock bag. First use a drill bit twisted
by hand, to gently remove any rough edges to the holes, that could cut into the wire.
L201 is a simple inductor made of 4 turns of the 0.4mm (Medium thickness) wire wound around
the BN43-2402 binocular core. “One turn” means the wire goes through both holes. The start and
end wires of the toroid wire therefore are both on the same side.
Install it on the PCB, cut the wires to 2mm and as usual, solder it for at least 10 seconds to ensure
the enamel burns off.
Test for DC continuity between the 12V pad of the PCB (indicated in the black circle) and the
positive pad of capacitor C204 (indicated in the red circle).

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3.5 Wind and install transformer T201
T201 is wound on a BN61-2402 binocular core (two of these
are in the small internal zipped bag). Again, use a drill bit,
twisted by hand in each end of each of the binocular core
holes, to remove any rough edges which could cut into the
wire enamel.
T201 has a primary consisting of 6 turns, and the secondaries
are made up of 3 bifilar turns. First use a drill bit twisted by
hand, to gently remove any rough edges to the holes, that
could cut into the wire.
Un-wind the thinnest wire. The best way to un-wind it, without
tangling it up, is to think of what the kit-packing person that
wound it up did. Then reverse his steps. So, first unwind the tightly wrapped part in the middle
where the end of the wire has been secured. Then, open out the spool of wire so that it is a circle.
Then unwind the spool, around your fingers, reversing the process of the person winding it in the
first place. Straighten the wire carefully and make sure there are no kinks.
Cut the thinnest wire into 4 equal pieces. Take ONE of those
pieces, bend it in the middle (do not cut it yet), and twist it into
bifilar-style wire. There are many ways to do this. You could clamp
one end and twist the other in a VERY LOW-speed drill or using an
electric screwdriver. Or just apply twists by hand, this is the method
I use.
The number of twists is not critical, and it will not be the end of the
world if it does not look perfectly neat.
Wind 3 turns of this bifilar wire on the core, remember that one “turn” goes through BOTH holes,
so in the end all the wires come out on the same side of the toroid.
Now take ONE more of the pieces of thinnest wire, and wind it through 6 turns. Remember, one
turn means the wire goes through BOTH holes of the toroid.
It is difficult to get the 6 turns through. It takes a great deal of patience. It can help to use a
wooden toothpick to poke in the holes, to move the existing wires to one side, to make space for
you to push through the new wire. Do not use anything metallic for this (such as a screwdriver)
which would scratch the enamel of the wire.
Be assured that it IS possible, I have wound many of these to prove it!
At the end of it, you have a transformer like the
picture, with 6 wires coming out of it. Two pairs
of wires will be the bifilar windings. Untwist
these ends but don’t remove the kinks from the
wire. This is how you can identify the primary
winding: since it was never twisted into bifilar
wire.
Now it is necessary to identify the wires of the
bifilar winding. So, tin the ends (by scraping
enamel off, or holding the wire ends in a blob
of molten solder for 10 seconds). Use a DVM

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to identify which pairs of wires have DC continuity.
Now if windings A and B refer to the identified as the bifilar windings, and C is the primary winding,
install the wires in the PCB as per the diagram. Note that the orientation of the core matches the
silkscreen print on the PCB.
Make sure that C is the primary winding –you can ensure this because it is only wire on each side
which is not tightly twisted bifilar wire. A and B are bifilar; C is the single wire.
Now test the DC continuity again between all three sets of wires (A, B, C) BEFORE soldering, to
make sure that there are no mistakes.
Also make sure that there is NO continuity between A, B and C wires, which would indicate a
short-circuit somewhere in the wiring, perhaps caused by a break in the enamel insulation.
When completely happy, cut the wires to 2mm below
the PCB and solder them, allowing 10 seconds of
heat to burn way the enamel. Inspect the joints with a
jeweller’s loupe or magnifying glass.
Finally do a continuity test, testing for continuity
between the “IN” and “GND” pads of the PA PCB,
holding the black and red probes as indicated by the
black and red circles respectively, in this diagram.
If there is no continuity, it indicates a problem with the
soldering of the primary winding which we called “C”.
Next test for continuity across the pads of R203, which is
winding “B”, and DC continuity across the pads of R204,
which is winding “A”. If either of these tests reveals no DC
continuity, then go back and check the soldering of the
appropriate T201 transformer winding.

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3.6 Wind and install transformer T202
T202 is wound on a BN61-2402 binocular core. It has a primary consisting of 3 bifilar turns, and a
secondary of a single 5-turn winding. First use a drill bit twisted by hand, to gently remove any
rough edges to the holes, that could cut into the wire.
Whilst the electrical definition of “primary” and “secondary” has swapped relative to T201 in the
previous section, the physical construction of the T202 transformer is very similar to T201. The
only difference is that the single-wire winding is 5 turns this time. Therefore, simply repeat the
procedure for winding the coil in the previous section, using 5 turns for the single-wire winding.
Similar to the previous section, let us identify the bifilar
windings as A and B, and the single-wire winding as C.
Again, untwist the bifilar winding ends and identify the
pairs of wires with the DC continuity test. Be sure to keep
the C winding (single-wire) separate and don’t confuse it
with the other two windings.
Installation on the PCB in the T202 position is very similar
to T201, but rotated through 90-degrees. Again, match the
shape of the binocular core to the drawing on the PCB
silkscreen.
Again, test the windings A, B and C for continuity of the
wire ends, through the right pairs of holes A, B and C,
BEFORE soldering. Only when you are sure that all 6
wires are in their correct holes, proceed with cutting them
to 2mm protrusion and soldering them. Again solder the
wires for 10 seconds or more until the enamel has burned
off and a good joint is made. With these thin wires the
burning method is quite easy.
Now undertake the usual continuity test.

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To test the “C” single-wire winding, connect the DVM probes to the top pad of R210 and the
bottom pad of R209. If you do not see DC continuity, go back and check the C winding soldering
joints.
To test the “B” bifilar winding, connect the DVM probes to the left side pad of C204 and the
indicated pad of the Q203 transistor. If you do not see DC continuity, go back and check the B
winding soldering joints.
To test the “A” bifilar winding, connect the DVM probes to the left side pad of C204 and the
indicated pad of the Q204 transistor. If you do not see DC continuity, go back and check the A
winding soldering joints.
Again, it is wise to check for any shorts between the C winding and the A/B windings. You should
not see a DC zero resistance (continuity) between any A/B to C test points. On the other hand, A
and B test points will show DC continuity between each other since the windings are connected in
the circuit.
3.7 Wind and install transformer
T203
T203 is wound on a FT50-43 toroidal core. It consists
of 10 turns of bifilar twisted 0.6mm wire.
10 turns of wire on a 0.5-inch size toroid will require
22cm of bifilar-wound wire. You should therefore start
by cutting say, 50cm of the 0.6mm wire. Prepare it by
twisting it tightly together the same as previous bifilar
wires. Wind 10 turns (10 times through the hole in the
toroid).
Untwist the ends, and tin the wire ends. This thicker
wire does not so easily burn off the enamel by the solder blob method. So, I find it helpful to
scrape the wire ends a little using a wire-cutter, THEN tin them with solder for 10 seconds.

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Identify the pairs of windings. This could be confusing: now the windings CROSS when they are
inserted into the PCB holes.
Put another way: notice that the outline of the toroid
goes horizontally across the PCB. One pair of
twisted wires comes out on the upper side, the other
pair of twisted wires comes out on the lower side.
The upper two twisted wires are put into the upper
two holes; the lower two twisted wires are put into
the lower two holes. But there is an extra wire swap
such that the electrical connectivity is as shown by
the orange lines in the diagram.
Before cutting or soldering the wires, check again
very carefully, with a DVM touched to the wire ends,
that you have continuity between the two pairs of
points, as indicated by the orange lines. Only when you are totally sure the four wires are in their
correct holes, cut them to 2mm protrusion, scrape them a bit with the wire cutter, then solder them
(for at least 10 seconds).
For a continuity test to make sure that the four
wires have been properly soldered, simply
connect the DVM across the pads for C213.
If there is DC continuity this verifies that all four
wires are correctly soldered to their pads.
It does NOT verify that the 4 wires are in the
correct 4 holes –you will have had to correctly
identify the wires as described.
If there is no DC continuity then go back and
check the soldering of the 4 wires (and check
that the enamel was properly scraped/burnt off).

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3.8 install capacitor C213
Although we intended to install he transformers and inductors first, you can see that C213 is stuck
tightly between T203 and T204, and it will be easier to install it now rather than later. This is a
33pF capacitor with code “330”.
3.9 Wind and install transformer T204
T204 is wound on a BN43-202 binocular core, with a turns ratio of 2:3 using the remaining 0.6mm
(thick) wire. A 2-turn primary and 3-turn secondary. This transformer is relatively easy to wind.
First use a drill bit twisted by hand, to gently remove any rough edges to the holes, that could cut
into the wire.
Hold the binocular core orientated the same as the layout diagram, i.e. with the hole axes
horizontal.
Start the 2-turn winding on the RIGHT-hand side end of the binocular holes. Remember that each
“1 turn” means the wire passes through both holes of the binocular core. So, push the wire through
to the left end, and then back through the other hole to the right; then the same again for the
second turn. The ends of the 2-turn winding therefore both come out on the right-hand side.
Now do the same for the 3-turn winding, this time starting from the left-hand side (with the
binocular core being held in the orientation of the PCB layout diagram). The ends of the 3-turn
winding will come out on the left-hand side of the BN43-202 binocular core.
The only RISK is that you drop the core or somehow mix it up, so that you forget which end of the
binocular core has the 2-turn and which has the 3-turn.
Be very careful to keep a tight grip on this binocular core transformer and remember its
orientation. The amplifier won’t work well if the 2-turn and 3-turn windings are mixed up.
Insert the two right-hand winding ends (2-turns) in the corresponding two holes in the PCB which
are on either side of capacitor C213.

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Insert the two left-hand winding ends (3-turns) in their corresponding two holes also.
Pull the wires tight on the underside of the board. Cut the protrusions to 2mm, and scrape the wire
with wire-cutters. This thick wire doesn’t easily allow the soldering iron heat to burn off the enamel,
so scraping the enamel off first helps. The thick wire also absorbs heat easily so be sure to hold
the soldering iron on the joint for at least 10 seconds to be sure of a good connection.
The 3-turn “output” winding can be checked for DC continuity by looking for zero ohms resistance
between the “GND” and “OUT” hole pads at the bottom left corner of the PCB.
There is no way to check the continuity of the 2-turn “input” winding of T204 because there is
already DC continuity through the windings of T203. So careful soldering and an inspection with a
jeweller’s loupe or magnifying glass are the only options available.

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3.10 Install capacitors C211 and C212
These two capacitors are 1uF ceramic capacitors with label “105”.
3.11 Install remaining ceramic capacitors
The remaining 10 ceramic capacitors are all 0.1uF ceramic capacitors with label “104”.

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3.12 Install inductors L202,
L203, L204 and L205
These are 1uH axial molded inductors. One
lead is to be bent over so that they are
mounted vertically, like the resistors.
3.13 Install resistors R203, R204, R209, R210
These are 220-ohm 0.5W resistors. They have colour code red-red-black-black-brown, but you
can also easily identify them because they are larger than the other resistors.

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3.14 Install resistors R201, R216 and R217
These are 220-ohm resistors with colour code red-red-black-black-brown. Be careful with the
identification of resistors on this PA PCB because lots of them start with colour code red-red.
3.15 Install resistors R207 and R208
These are 2.2-ohm resistors with colour code red-red-black-silver-brown.