[QRP] NORCAL 40A QRO?

[Mike, af4lq]

k3hx at juno.com wrote:
> Looking for a mod to increase the power of my NorCal 40A.

Tim,

You might take a look at AE5X's page of mods for various rigs. He has an 
article there written by Gary Surrency regarding his work with the 40A:

<http://ae5x.com/mods.html>

The info on the 40A is two-thirds or so down the page. Additionally I 
have copied and pasted it into this message for you, below my signature.

At one time I was going mod my 40A and this is the best info I found in 
doing considerable searches on the subject. I never did the mod though 
as I'm getting 3 watts running 14v and the additional gain isn't enough 
to make it worthwhile, for me anyway.

Also, you might try the QRP-L archives. I've turned up a lot of 
interesting things on the 40A by running searches on "40A", "NC40A", 
etc. That site is at:

<http://www.kkn.net/archives/html/QRP-L/>

Good luck and I hope this helps. Take care.

73, Mike
af4lq
-------------------------paste-----------------------

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.