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QRP Rig Modifications
UPDATE:
Larry East W1HUE recently posted his mods to various QRP rigs
here.
The following modifications were sent to me
by those individuals who originated and/or tested the mods.
I have done several (but not all) of these modifications to
my own rigs.
KD1JV AT Sprint III
Filter Response
(Mike KL7R)
I wanted to flatten the response curve a bit:
I poked around the coupling capacitors in the filter and tried
doubling the caps without much effect so I tried to remove one
to see if that made a difference. I picked the middle one for
no particular reason at all and removed its shunt cap (C8).
After removing C8, the 3db bandwidth is about
175 hz rather than the 50 hz it was before. Now, when I tune
in a signal, I have about 11 steps of the tune switch where
I can hear the signal and about 5 steps where it is relatively
loud.
I like it better this way, you may like the
tighter filter.
Photos:
Before
removing C8
After
removing C8
Filter Response
(Paul Stroud AA4XX)
I went ahead and performed this crytal
filter mod on my ATS3 over the weekend and really like the widened
response. It's still narrow enough for my taste while allowing
me to know who's above and below me--a real advantage for those
occasions when the calling station is more than 500 Hz off frequency.
I removed C8 and shorted across X2 to convert the original filter
to a 3-crytal Cohn filter.
The nice thing about the mod is that all the
solder points can be readily accessed without having to remove
the board from the Altoids tin.
Small Wonder Lab's DSW
Drive level adjustment
(Mike KK5F)
Relocate C116 from above the PCB to the same
holes, but below the PCB. Don’t let any of the the C116
lead or solder buildup project much above the top side of the
PCB for the hole that is closest to R25. Otherwise, it will
interfere with adjustment wheel of the trimmer pot to be installed.
Remove the 51 ohm resistor R25. Replace it
with a 100 ohm trimmer pot. The wiper and end connection of
a Bourns BP3352T 100 ohm will fit exactly into the existing
holes for R25. Make the adjustment dial side face the crystal
filter.
Adjusting this new drive control for 300 mA
total unit current consumption will usually result in the best
trade-off for power output vs current consumption. On my DSW-30,
going from 300 mA to 430 mA only changed power output from 2.0
to 2.3 watts. The increase in power output for high current
draw was even less for my DSW-20 and -40.
NOTE: If you do not wish to install a pot,
then just measure the current draw on your DSW before modification.
If it is less than 300 mA, try replacing R25 with a fixed value
resistor of LESS than 51 ohms. If it is more than 300 mA, try
values of R25 that are MORE than 51 ohms.
NOTE: A 100 ohm trim pot will allow adjustment
of power output from a little less than 1 watt to above 2 watts.
Changing the final transistor for more power
output
(Mike KK5F)
If you have a DSW with a 2SC799 as the final
transistor (Q7), you may be able to get more power output by
going to the other transistor that Dave at SWL now supplies
(2SC1947). This is a relatively expensive part, so Dave will
need about $5 each for one.
NOTE: Most DSWs originally included the new
transistor 2SC1947 in the kit.
Maximizing applied voltage to the DSW
(Mike KK5F)
The reverse polarity protection diode is in
series with the supply voltage. It is a Schottky diode with
only about 0.3 volts lost through forward voltage drop. If you
are battery powered, and wish to tweak every last bit of power
out of your DSW, then wire a short across the Schottky diode
D8. Install a shunt 1N4001 or similar diode from some convenient
positive supply voltage lead to ground, such that the diode
will conduct if power polarity is reversed. Install a fuse of
about 1 amp in the supply cable, so that it will blow if the
shunt diode conducts on reverse polarity.
NOTE: This method is less fool-proof than the
original design, and only results in an effective battery voltage
increase of 0.3 volts.
Final transistor heat sink
(Mike KK5F)
The final transistor (Q7) will get quite hot.
I recommend some sort of TO-5 type heat sink. I had to scout
around to find one that was small enough to fit in the cramped
location of Q7. The clip on types like MFJ is supplying with
their MFJ Cubs would be ideal. I don’t know if MFJ will
sell you any or not. Use heat sink coumpound, and be careful
not to let the sink contact local components like D9.
Standard DSW enclosure mods
(Mike KK5F)
The blue anodization that coats most of the
surfaces of the standard DSW case is a good insulator. I used
a Dremel tool wire brush to remove some of the coating at the
mating edges of the case, the PCB standoff mounting holes, and
the control/connector mounting holes. This will create much
better shielding of the DSW internals, and prevent any spurious
signal pickup on the IF frequency (4.000, 5.182, or 5.068 MHz).
The early DSW-20 had an IF of 5.068 MHz, which Dave changed
to 5.182 due to signals from the SW broadcaster WWCR at 5.070
MHz. I managed to completely eliminate WWCR signals (I’m
only 90 miles from their transmitter) without changing to the
new IF frequency, by improving the shielding as described. I
figure that even if you don’t have a problem with WWCR,
it can’t hurt to keep out other potential unwanted signals.
The standard plastic PCB stand-offs can pop
back into the case from external pressure rubbing against the
outside protrusion. I replaced them with 4-40 x ½”
screws, using a lock washer and nut as a standoff bewteen the
PCB and the case bottom. This also more directly connects the
PCB to the case as a ground plane.
NOTE: 4-40 screws and the associated nut are
about the largest size that can be used. Otherwise there won’t
be enough clearance between the top nut and components on the
PCB.
DSW sideband change
The DSW-20 and -30 receive on the lower sideband.
The DSW-40 and -80 receive on the upper sideband.
The difference is due to whether the local oscillator injection
frequency is above (DSW-40, -80) or below (DSW-30, -30) the
received frequency. It is possible to get an intelligible signal
on voice sideband with the DSW. Unfortunately, the DSW-20, -40
and -80 receive the wrong sideband for voice on the associated
band. It is possible to reverse the sideband received by switching
the BFO injection frequency from ABOVE the IF frequency to BELOW
the IF frequency. Due to non-symmetrical crystal filter bandpass,
making this change can theoretically degrade the opposite sideband
rejection performance of the receiver. Thus, I have not made
such a mod on my DSWs. Bruce Prior, N7RR, has developed a detailed
modification for those who are interested in making this mod:
I stated mistakenly in my rave review (CQ Amateur Radio, December,
1999, p. 40) of the Small Wonder Labs DSW rigs, “The offset
of all DSW rigs is set for receiving on lower sideband.”
In fact, only the DSW-20 and the DSW-30 receive on the lower
sideband. The 80, 40-meter versions receive on upper sideband.
On the DSW-30, this feature makes no difference, since it is
not a phone band. However, the design sideband is the reverse
of customary sideband usage for the 80, 40 and 20 meter DSW
rigs.
CHANGING SIDEBANDS
After some experimentation and consultation with the DSW designer,
Dave Benson, NN1G, I offer the following very simple modification
which reverses the sideband on the DSW rigs. This could potentially
be very important to a DSW owner in a remote area who needs
to contact an SSB station in an emergency. As I noted in my
review, the rigs cover all of their respective amateur bands
and more, but if you can t understand the SSB stations on 80,
40 and 20 meters, the extra coverage is not very useful. What
is needed is to convert the DSW-80 and DSW-40 to receive LSB
and to convert the DSW-20 to receive USB. These conversions
can be accomplished inexpensively by adding a single fixed inductor.
Since the DSW-30 and DSW-40 both have an IF of 4 MHz, the modification
is the same. Similarly, the DSW-20 and DSW-80 both have a 5182-kHz
IF.
The identical added inductor reverses the sideband
of those rigs in opposite directions, however.
The only reason I can imagine that a DSW-40 owner might want
to give this modification a miss is that the Canadian time-signal
station CHU on 7335 kHz transmits its French and English voice
signals on USB. WWV and WWVH on 10 and 15 MHz transmit on AM,
so the modification will not change your ability to copy its
signals on the DSW-30 and DSW-20. In addition, the modification
broadens the audio bandpass a bit, so CW reception is slightly
less selective. With the modification, SSB audio reception sounds
somewhat pinched, but it is quite understandable. In each case,
all you have to do is to place an inductor in parallel with
the trimmer capacitor C14.
INDUCTOR VALUES
All DSW-40 and DSW-30 kits include a grey 70-pF trimmer for
C14. Early model DSW-20 and DSW-80 kits have the same grey 70-pF
trimmer. In current model DSW-20 and DSW-80 kits, however, C14
is a black 90-pF C14 trimmer.
Check the color before choosing your inductor. Here are the
appropriate inductor values:
DSW-40: 15-uH inductor
DSW-30: 15-uH inductor
(Note: since 30-m is not a phone band, there is no point in
modifying the DSW-30 for USB reception.)
DSW-80 with a grey 70-pF trimmer capacitor C14: 8.2-uH inductor
DSW-80 with a black 90-pF trimmer capacitor C14: 6.8-uH inductor
DSW-20 with a grey 70-pF trimmer capacitor C14: 8.2-uH inductor
DSW-20 with a black 90-pF trimmer capacitor C14: 6.8-uH inductor
MODIFICATION PROCEDURE
To install the inductor in parallel with C14, solder it to the
two C14 pads on the bottom of the circuit board. The neatest
way is to use a surface-mount inductor. I had a tubular inductor
with axial leads available which I was able to bend around and
tack onto the circuit board. After soldering on the inductor,
re-align the offset pitch by adjusting C14 to match the sidetone.
You obtain the offset pitch through your earphones by applying
power while holding down the front-panel keyer button. Then
depress your key or keyer paddle to give you the comparison
sidetone pitch.
This modification will change the 80 and 40-m rigs to receive
LSB and the 30 and 20-m rigs to receive USB.
INDUCTOR SOURCES
For the surface-mount option, the 70-UMC1812 inductors for the
proper values from Mouser would do fine. Dave Benson recommends
the following Delevan surface-mount inductors, which are available
from Digi-Key:
15 uH: DN12153JCT
8.2 uH: DN12822JCT
6.8 uH: DN12682JCT
The tubular axial-lead 70-IM2 inductors available
from Mouser are bulkier and a bit trickier to solder beneath
the circuit-board. Now you can use your DSW rig for cross-mode
QSOs.
Off-frequency boot up
(Mike KK5F)
This is not a mod, just information.
The DSWs will sometimes boot up slightly off-frequency
for transmit (by about 0.5 kHz). The receive frequency is OK.
ANY turn of the frequency encoder, or even flipping on and off
the RIT will cause the transmit frequency to correct itself.
This effect is thus usually not noticed unless one begins transmitting
after power up without making ANY frequency adjustments. HOWEVER,
there is a consequence when in CALIBRATION mode (power up with
the keyer mode button depressed). When you are trying to adjust
the receiver offset, if the unit booted up with the slightly
incorrect transmit frequency, then your receiver offset adjustment
will be off by however much the transmit frequency is off. Powering
up, then momentarily pulling and re-inserting the power plug
to quickly cycle the power (all the while pushing the keyer
mode button) will get the unit to initialize the transmit frequency
correctly. The offset adjustment can then proceed.
Inability to adjust the offset to match
tones in CALIBRATION mode
(Mike KK5F)
First, refer to the info in item above.
If the offset still can NOT be adjusted to
match tones, then you probably have an early DSW-20 with a grey
70 pF offset adjustment capacitor (C14). Dave Benson will supply
you with a black 90 pF replacement.
Bandwidth
(Arne SM4INV)
 I
found the passband of the DSW-20 a little too wide when the
band is crowded so I decided to make a simple filter to narrow
it a bit. This is what I came up with:
The inductor is a Schaffner current compensated
choke on 2 x 39 mH but due to the mutual inductance and the
close coupling of the toroid core the inductance is almost quadrupled
if the two windings are properly connected in series. There
are two ways of connecting them in series - if you did it the
wrong way the inductance will be close to zero. You will notice
if you have done that!
You can of course use another inductor/capacitor/resistor
combination as long as the circuit istuned to about 800 Hz whitch
is the center of the DSW-20 passband. If the resistor value
is tohigh (or omitted) the filter will be very narrow and start
to ring. If you on the other handmake the resistor value too
small you will also lower the gain of the rx.
Try it, I think its a nice addition to a fine
rig! 72/73 de SM4INV, Arne.
Wilderness SST
Increased Tuning Range
Two varactors are included with the SST with
instructions to select the one that provides the tuning range
you are most interested in. To use them both:
Install a small SPDT switch (RS 275-635) on
the front panel.
Clipping the leads as short as possible, solder the hot side
of each varactor to the outer terminals of the switch (hot lead
of one varactor to one side of the switch, hot lead of the other
varactor to the other side).
Using a short wire, connect the center lead of the switch to
the hot side of the D4 mounting position on the circuit board.
With the D4-side of the board facing you, this will be the left-side
hole.
The two remaining leads of each varactor are now joined together
and connected to the other D4 mounting hole.
Tuning Linearity
Linearity can be a (minor) problem with varactor-tuned
rigs. To significantly reduce this in the SST, solder an 18k
ohm resistor across the tuning pot between the wiper lead and
the +8 volt lead. Experiment with this value as it varies from
band to band and from one type of varactor to another.
Howling
(Terry W0PFR)
My SST 40m version also howled when the audio
gain was at maximum. Looking at the application notes on the
LM386 audio chip that is in the SST, I noticed that there is
a comment that if the chip oscillates then an optional DC filter
can be installed to prevent the oscillation. The filter is only
a resistor and a by-pass capacitor placed in the lead supplying
power to the chip. I have installed the modification and it
seems to work. The changes can be done on the etched side of
the pc board.
1. cut the trace going to pin 6 of U3
2. solder a 100 ohm 1/4 watt resistor from pin 6 of U3 to the
contact next to pin 1 of U3 .
3. solder a .1mf disc cap from pin 6 of U3 to pin 4 of U3
Sidetone Level
(Wayne N6KR)
There are two ways to reduce the sidetone volume
in the SST - reduce the AF Gain or, as recommended by the rigs'
designer, Wayne N6KR:
Usually there is a good balance between the
incoming signals and the sidetone in the SST. But you're right--if
you have to turn it way up, the sidetone gets loud.
The only sure-fire way to mute the sidetone to a lower level
is to insert a low pinch-off JFET like a J201 into each leg
of the audio connection from the product detector to the audio
amp. This is how I do it in the NC40A. Actually you can probably
find other JFETs that will work; J310s are mostly low pinch-off,
too. MPF102s will work if you hand-select them for low pinch-off.
The source leads go to the '602 and the drains
to the original capacitors that go to the '386. Tie the gates
together and add a 1 to 10M pull-up resistor from the gates
to one of the source leads. Next, connect a diode from the gates
to the key input. Test the circuit thus far by keying the rig:
you should hear ZERO sidetone at this point, because pulling
the gates low cuts off the JFETS, making them look like an extremely
high resistance. If you hear a click on keydown, put a resistor
(start with 1K) in series with the gate diode. If you hear a
click on key-up, add a capacitor from gates to ground; start
with about 0.1uF and see if you can go smaller. (It will depend
on the pull-up resistor; 10M and .047 work well in most cases.)
Once this much is working, you can add a resistor
*across* one of the JFETs (source to drain) to allow some sidetone
to sneak through--as much or As little as you want. It will
take a large resistor, something like 1 to 15M in my experience.
20 Meter SST to 15 Meters
(Roy AB7CE)
Dozens of requests on moving 20mtr SST to 15mtrs, so here is
how I did it. If you make changes or improvements other than
these, please let me know so I can use them as well.
LOWPASS FILTER
These are standard values from any handbook:
L2, L3 = 11turns #26 on T37-6(yellow)
C34, C36 = 150pf, I used silver mica
C35 = 270pf " " "
XMIT MIXER
L1 = 18t #26 on T37-6(yellow)
C27 = 30pf, I used ceramic NPO VXO
X6 = 25mhz crystals. I used two, one on top, one under board.
RFC3 = 23t #26 on T37-2(red). This was a critical value for
proper oscillation and range. You may have to adjust a turn
plus or minus, or compress or spread turns to adjust to desired
tuning range.
RCV MIXER
RFC1 to 2.7uh(or close to it). Improves match to Filter and
recieve.
PA
2.2uh molded choke from base of Q2 to ground.
As above changes were all that was neccessary to put it on 21.050
to 21.070 with the MVAM108. Output was 1 watt, so I put in MRF237(ECG341)
for two watts out. Its also very easy to move to 17mtrs by using
same approach.
Addendum:
I put two crystals in both of my SST's to extend vxo range.
One is 20mtrs and the other is one I moved to 15mtrs. With the
MVAM108 varactors I was able to get about 20khz range. I was
experimenting with different varactors I had on hand. Was not
getting much difference. Then I tried a MV1404. WOW! The 20mtr
version is covering from 14.013 to 14.064. The 15mtr version
is covering from 21.039 to 21.073. Scope shows solid waveforms
across oscillator ranges. I don't know where to get them. They
don't show up in my cross reference, so I don't even know what
they are rated as. Mine were in a envelope of a dozen, that
was in one of Dans Small Parts
20lbs of parts for $10.00. They are not listed on his current
page. If any one knows what they are rated or where to get them,
they might inform the rest of the list.
Miscellaneous
(John AE5X)
The TiCK keyer in my SST is mounted on the
rear panel above the key jack. The new jack (required for paddles)
is mounted above the RF Gain control on the rear panel and the
pushbutton switch is mounted above the antenna (BNC) connector.
Using a Xerox machine, I reduced the the size of the TiCK's
program menu and laminated it to the top cover of my SST.
Also taped to the top panel is a tiny chart
showing my operating frequency at 9 O'clock, 12 O'clock and
3 O'clock positions for both positions of the SPDT varactor
switch.
To make tuning easier, a larger tuning knob
is helpful. I use a RS 274-402.
Wilderness NC40A
Multi-turn Tuning Potentiometer
 This
modification assumes you have the KC1 Frequency Annunciator
installed in your Norcal 40A. There are several Bournes multi-turn
potentiometers available from Mouser that will provide greater
tuning resolution. Pick your pot based on the resolution you
want, bearing in mind that the 10T version requires a lot of
spinning to go from one end of the Norcal’s band to the
other. If you mainly operate on 7040 kHz, this may be the best
choice for you. I like to hang out on the low end of the band
with frequent excursions to the QRP watering hole, so I chose
the 5T version and believe it to have been the right choice
for the way I operate. Here are the part numbers and prices
of the 5T and 3T potentiometers:
3 turn Bourns 10K ohm - 652-3543S-1-103 at
$22.62
5 turn Bourns 10K ohm - 652-3545S-1-103 at $23.30
Either of these two parts will install into
the Norcal with minimal fuss. For physical installation, the
hole in the front panel must be enlarged by 1/16th of an inch.
After doing this, secure the pot to the front panel with the
pot’s leads facing up. Due to the increased length of
the multi-turn pot, it is not a drop-in replacement. In other
words, the leads of the new pot won’t fit into the circuit
board like the original one does. I simply used a 3 conductor
piece of ribbon cable 3” long to connect the pot terminals
to their respective thru holes on the board (see photo).
Depending on which end of the band I’m
on, I get a tuning resolution of 12 kHz/revolution (bottom of
band) to 7 kHz/revolution (top end). This is a vast improvement
over the stock potentiometer’s 70 kHz/revolution and greatly
increases the operability of a fine rig.
Increasing RF Output
Gary Surrency AB7MY:
I have spent some considerable time increasing
the RF output of the NC40A. The following modifications have
worked well for more than two years, and do not require any
surgery to the PCB or anything drastic like that.
You will need to get some MRF237's or 2N3924's for the PA stage.
Either one will achieve 5 watts output, with some additional
mods to be described. My sources for these have recently dried
up. The 2N3924 is a direct drop-in, whereas the MRF237's C and
E leads are reversed from the PCB layout. There are two ways
to accomodate that. You can put the '237 on the bottom of the
PCB, and press its case against a small puddle of thermal grease
on the inside of the bottom cover as you solder the leads from
the top. This adequately heatsinks the PA at 5 watts.The '237
case is grounded, so there's no need for insulation.
Or, you can bend the Base lead between the
E and C leads, and rotate the '237 180 degrees so that the leads
now fit the PCB pinout and install it from the top. That allows
the stock heat sink to be used, but avoid long keydown operation
or the PA will overheat at 5 watts quickly. Ordinary intermittent
CW duty cycle does not cause the PA to overheat, but a larger
heat sink is preferable anyway. I've been using mine with the
stock heatsink on a 2N3924 during several QRP contests, and
it has not failed.
Both devices are designed for 12-13 volt operation,
instead of 28 volts like the 2N3553, etc. Better efficiency
at 12-13 volts is the result, but they are rather fragile electrically
and subject to SWR damage if your antenna is not well matched.
This is due to low breakdown voltage ratings resulting from
the original application these devices were designed for. But
they work very well if you remember to be careful.
You also have to modify the low-pass filter
(LPF), so that it presents the proper, lower impedance load
to the PA for 5 watt operation. I found that the following changes
were necessary:
Component Old Values New Values
C45 330pf 470pf
L7 18t on T37-2 16t on T37-2
C46 820pf 820pf (no change)
L8 18t on T37-2 18 or 19t on T37-2
C47 330pf 470pf
Good ceramic discs are OK, as that is what I used. But you could
use silver mica or polystyrene caps if so inclined, as they
have better Q and less loss.
By using an antenna analyzer, such as the MFJ-259B,
you can look into the antenna jack and determine if the PA collector
load is correct. Use a test resistor of 15-19 ohms tacked across
the PA transistor collector to ground. A single 15 ohm, 18 ohm,
or pair of 33 ohm or 39 ohm 1/4 watt resistors in parallel is
fine.Leave the power off, and the PA device installed. Tune
the analyzer to 7.040 MHz, and read the SWR and reactance.
With the LPF values from above, you should
be able to make slight adjustments to the toroid windings to
get the analyzer to "see" 50 ohms resistive, with
no X value or reactance. Now the PA is set up to operate at
5 watts under ~13.8 v so it can work "hard" enough
to develop 5 watts. Using the analyzer is a "real-world"
way to verify the theory, and the although the measurements
seem to be pretty "touchy" as you adjust the LPF components,
the actual PA operation will not be so sensitive to small errors.
If you get close - it is fine.
Also, the reduced collector impedance allows
the stock 36v zener at D12 to be retained, since the peak collector
voltage is held down by the lower collector Z. That demonstrates
the LPF is matching the 50 ohm antenna load correctly to the
15-19 ohm collector load, as we just measured with the analyzer.
If these steps aren't taken, the stock zener will overheat at
higher operating voltages. Also, the complexity of a tapped
collector choke is avoided, and the associated wiring nightmare
that would be required with the original PCB layout.
I have used the antenna analyzer / PA collector
test resistor method with every rig I own, to verify and if
necessary, make the required LPF changes to correct any PA to
antenna load matching problems. I posted this procedure on the
QRP-L some time back, with the original idea coming from "Solid
State Design" by Wes Hayward and Doug DeMaw. I just use
a different approach with the availability of modern test gear.
Don't forget to remove the 15-18 ohm collector-to-ground
test resistor before connecting the DC power again, and *don't*
transmit into your antenna analyzer by forgetting to remove
it and re-attaching your antenna or dummy load and wattmeter.
You have been warned!
A little more drive is also required, so the
following steps are needed:
I changed Q5 from a J309A to a J310. Select
the best J310 if you have several to try.
The 100 ohm resistor at R14 is not changed,
due to the low impedance input from T1 that is OK, and we don't
want to reduce the already marginal drive that we have for a
larger PA stage.
A MPS2222a from Radio Shack, etc., was used
for Q6 instead of the original PN2222a. Again, hand selecting
a "hot" device is recommended while observing the
wattmeter indication.
If 5 watts is still not possible (and it should
be now) you can do this:
Some VFO's or NE602's used at U4 may be a little
down on output. If this seems to be the case, you can replace
the 5 pf cap used at C31 with another one, or use a slightly
larger cap, of no more than 8-10 pf. Some of the 5 pf caps I
have tested were low in value, preventing sufficient VFO signal
into U4. But an NE602 is very easily overdriven into spurious
output, especially with the simple one-pole bandpass filter
following U4 in the NC40. I found that 5.6 pf to 6 pf at C31
would almost always help a low VFO signal into U4, and still
prevent spurious mixer output. Radio Shack carries a package
of small pf value caps if you need some.
If you have a small pot of 200 ohms, it is
a better choice for the drive trimmer, R13.
If you have leading edge audio clicks in the
headphones with a KC-1 installed, a 100 ohm resistor in series
with the Mute lead from the KC-1 to the anode of D2 on the NC40A
PCB will cure it.
After these mods, reduce the drive for about
1 watt of output and then adjust the TX bandpass filter trimmer
C39 for maximum indication on the wattmeter. Keep the output
at 1 watt or less to avoid overheating the PA as you tweak the
trimmer for maximum drive. Allow the PA to cool a bit, then
quickly set the drive pot to any output level up to 5 watts.
Make sure your antenna is a good 50 ohm resistive load before
operating, since the higher power capability will quickly fry
the PA, even with protective zener D12 in place. That is the
only caution when using these TO-39 PA devices, since they aren't
very tough. I always use my MFJ-259B to pre-match the antenna
before transmitting.
You could install a larger PA transistor in
place of the MRF237 or 2N3924, as they are getting hard to find.
But suitable devices like the 2SC2078, 2SC1969, or MRF476 are
all in TO-220 power tab packages, and require the leads to be
bent oddly to fit the PCB layout. The extra drive and LPF changes
described above are adequate to achieve 5 watts or more with
these larger RF transistors (I know - I tried it), but they
don't allow the same simplicity of the original NC40A design.
The original 2N3553 will almost achieve 5 watts with all of
the other mods, so you might try it that way with your stock
transistor. If yours has the 2SC799, it simply won't handle
the extra power.
On my NC40A with all of these mods, I actually
have to reduce the drive pot to avoid exceeding 5 watts of output
at 13.8 v to 14.0 v. Even with a gel cell during portable operation
- it is still possible to get 5 watts output by increasing the
drive pot setting.
With the stock collector choke in the rig,
5 watts is getting pretty close to its current rating. But it
only gets slightly warm during normal operation at or below
5 watts. You could probably gain a bit of output by replacing
the small, stock 18uH choke with one made from a ferrite core,
such as a T37-43 or T37-61 wound with 8-10 turns of #24 wire.
That was the case with the SW-40+. The series resistance and
losses would be less, allowing more current for the PA and more
RF into the antenna. Mine still has the itty bitty stock choke,
since I haven't gotten 'round to it, yet. ;-)
Oh yes, I've also installed a 10-turn
tuning pot and it is a very nice improvement over the stock
pot. You'll need the KC1 installed as I have, or some other
method to determine the frequency. Works very FB.
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