"38 Special" Mods
compiled by Jerry Parker
WA6OWO
Table of Contents
Mods to the 5 watt mod by Mike Gipe K1MG
NorCal 38 Special People:
Well, he said it was intended for experimentation! Ori, that is.
Right on
page 5 of the manual. I couldn't help myself. While the soldering
iron
was hot, I figured it wouldn't hurt to try one little thing.... It was the
badger made me do it... the badger voices...Nils! Take them bacckkkkkk!
When I started to assemble the 38 Special kit, I decided that the 5 watt
mod was so simple that I would just go ahead and put that in at the start.
After assembling everything, I put the board on the operating desk and
clip-leaded everything together. After playing with the receiver for a few
minutes, I noticed that the smoke was leaking out of the IRF510.
Oh ohh. What did I do wrong?
I clipped the FET out and checked it. It was shorted, but why was it
shorted? I started checking my wiring and solder slinging. I
double-checked with the schematic, and a light clicked on in my head (QRP
light, of course, most normal people wouldn't have seen it in the noise).
The MOSFET had a fixed voltage bias derived from the 12 volt power supply.
I was running the rig from a 13.8 volt 20 amp power supply. Hmmm. After
looking at the schematic some more, the noise in my head grew louder (It
must have been the badgers) and I started thinking about modifications.
First, however, I did some experimenting with varying the bias using the
circuit as shown in the manual. It behaves just as I expected it
would.
The bias circuit is set up to operate the MOSFET right on the knee of the
gate-drain control curve. As expected, the exact operating point with a
fixed DC bias will vary depending on the device temperature and
manufacturing variations. By adjusting the bias divider resistors, I set
up the amp for about 20 mA quiescent current with a supply of 12 volts.
This worked very nicely, and put out about 4 watts of nice clean power
(clean on a scope. I don't have a spectrum analyzer). After warming
up
the FET keydown until it was comfortably hot, the idle quiescent current
was up to about 60 mA, still acceptable, but indicative of the temperature
sensitivity of the knee. After cooling down, the current dropped back to
20 mA. The next experiment was very important. I increased the power
supply to 13.8 v and kept the same bias resistors. The quiescent current
jumped to 300 mA! This is because the bias voltage is set proportional to
the power supply voltage, and the knee of the gate is very abrupt at this
point. This, of course, is why my first FET self-destructed. It was
dissipating over four watts without heat sink during receive.
The simple bias circuit used is thus very sensitive to power supply
variations. The first improvement would be to move the bias divider over
to the regulated 8 volt line, and adjust the divider values so that the
quiescent current is between 10 and 50 mA.
But the badgers kept badgering. I thought, why not just DC couple the
gate, since we have an 8 volt swing from the 74HC240 driver. The circuit
is just a little simpler. The
biggest advantage is that the idle current during receive would be zero,
saving battery power, and reducing noise in the receiver.
The mod is:
1. Remove R101
2. Smile your almost done.
3. Replace C101 with a wire jumper.
4. Change C26 to 330pF. (Thanks to Paul Harden for this suggestion, which
helps stabilize things.)
5. Carefully adjust TC2 for clean sine wave output.
The amplifier now runs with zero current during receive. Its power output
varies smoothly from 4 watts with a 12 v supply to 5.3 watts with a 13.8
volt supply. The output is nice and clean and stable. This puts the
FET
into class D operation, and the impedance matching filter seems to be
exactly right. (Thanks to Paul Harden for confirming this.)
Your mileage may vary. That's why you should experiment and report your
findings to the list and QRPp. Ori built this radio so people could
experiment and learn.
Boy, this sure is a nice radio! And it sure is sweet with the matching
digital readout/24 hour clock accessory! I can't wait to make a QSO --
tonite, 0100 UTC. Catch me if you can!
Mike K1MG
Look before you leap by Roger Hightower N7KT
While checking out the manual, and gathering up the parts needed to add
the 5W, RIT and TiCK mods, I note that a number of components need to be
removed/replaced for these mods.
If you plan on doing it all at once like I do, then think about some
things. The RIT mode required that L1 be replaced with a 6.8uH molded
choke. Save some time and don't even wind L1; the core make a nice
addition to your junk box.
Similarly, the TiCK mod doesn't use R15, R16, R17, C31 and D8. Set
these aside in your junk box. _Then_, if you're planning on Mike, K1MG's
mods to the 5W mod, don't install R101 and C101. Install the wire
jumper at C101. Change C26 to 330 pF. As soon as we find out what
happened to Step 2 of Mikes post, check that out.
It'd be a shame to mess up such a fine board with a bunch of desoldering
attempts.
72/73
Roger N7KT
Return to Table of Contents
Errata #1 by Doug Hendricks KI6DS
Boy, the best laid plans seem
to go awry. Ok here are the errata that I
know of as of Sunday, Jan. 19, 1997.
1. The turns on L3 should be 8T. (Parts list on page 10 has it as
12T)
2. The turns on L4 should be 12T. (Parts list on page 10 has it as
15T)
3. "38 Special 5 Watt Power Amp Mod" schematic on page 11 has
C28 marked
C25, and C29 marked C26. The main schematic is correct.
4. Mike Gipe, K1MG has a much better 5W power amp mod. I strongly
suggest
that you do this one rather than the one in the manual, as it may cause
problems.
1. Remove R101 (If it has been
installed)
2. Replace C101 with a wire jumper.
(If it has been installed)
3. Change C26 to 330pF. (Thanks
to Paul Harden for this suggestion,
which helps stabilize things.)
4. Carefully adjust TC2 for clean
sine wave output.
5. Page 9, left column, 2/3 way down the page, it says:
Solder in the following parts: R127,
R128, C133, C144 should be
replaced with solder in the following parts: C 202, R202. Note that C201
and R201 are not used. (Late change by the TiCK people.)
6. Page 9, left hand column 3/4 way down the page, VR2 is VR201, TR3 is
TR201.
Note: Pay particular attention to the Layout page for parts
location. R201
looks like R20 on the board, but is shown as R201 on the layout. Please do
not confuse it for R20 next to U5. Also, R301 is right below D3. It looks
like R30 on the board, but is R301.
I apologize profusely for the mistakes. If you find others, please email
me
so that I may add them to the list.
72,
Doug Hendricks KI6DS
#8 is Alive and Well by Grover K7TF
#8 is doing well at five watts, but do take a serious look at Micahel
Gipe's mod to the five watt mod that d-c couples the input to the IRF510.
You should do this mod!
For those who can't find the message, it consists of four simple steps
(quoting Michael):
1. Remove R101
2. Replace C101 with a wire jumper.
3. Change C26 to 330pF. (Thanks to Paul Harden for this suggestion, which
helps stabilize things.) [note: this cap is at pin 8 of U4.]
4. Carefully adjust TC2 for clean sine wave output.
Another note: my local radio shack told me that they no longer stocked the
1K pot that you need for the rf gain control. I used a 5k - works fb.
To Doug and Ori and Jim and everyone who played a part in this, a big thank
you. This is a terrific radio and a great project.
Cheers,
Grover K7TF
Return to Table of Contents
Audio Level by ORI AC6AN
There were some comments regarding the audio level of the
38S receiver. It can still hear S0 signals well, but the audio
is not ear piercing...
Here's what you could easily do: resistor R24 (in series with
the headphones) is responsible for some audio level loss
when low impedance headphones are used.
It is 47 ohm in the kit, since we found that it reduces the noise
level generated in the 5532A with certain low-impedance
headphones.
You may want to try lower values or short R24 with a jumper
wire. Your mileage may vary...
72
Using Sockets in the 38 Special by Paul Harden NA5N
I used IC sockets on everything when I
built the prototype as we wanted
to experiment with different manufacturers, HC240 vs. HCT240, etc. The
lab tests I posted on the 38S were from the socketized version, and I
saw no problems by using IC sockets.
72, Paul Harden NA5N
PS - Don't use an HCT240 for U4 - the HC240 only. The HCT240 is
spec'd at Vcc max=6.5v and the TTL output buffers do not give you the
parallel current advantage the HC240 does. In short, it don't work.
Return to Table of Contents
Mandatory Fix Revisited by Brad WB8YGG
Oh boy, this is almost funny!
I messed up the fix, now I think the fix to the fix is messed up.
Anyway, let me explain from the schematic perspective, and
skip the artwork discussion at least at first.
I hope this ends this :-).
Anyway, look at the schematic, and the lower right hand
corner in particular. See the A designator. comming from C32
220 pF. Ok, see where it connects on the left side of the
drawing at R19.
OK so far..I think I haven't made a mistake yet.. :-)..
Anyway, we want the A on the right side to connect to
the R19, C34 junction through a NEW 4.7K resistor.
Ok that said, right now, in artwork , C32 is connected to R19
directly, and that junction will have to be cut. Once Cut,
you need to connect C32 to the R19, C34 Junction via a NEW
4.7K resistor.
Now , looking at the front of the board, the side of
C32 that needs to be disconnected is the left side. And
the side of R19 that you want to connect C32 to through
the NEW 4.7K resistor is the bottom side of R19.
This Mod allows for the TiCK keyer telemetry tones
to be hear, where without it, they will not be heard,
especially when there are signals on the band.
Now, unless I mess this upagain..:-). Onward and upward.
73 everbody, and sorry for the confusion!
Brad WB8YGG & Gary
Embedded Research
On Sun, 19 Jan 1997 19:58:51 EST ori@juno.com (Ori K Mizrahi-Shalom)
writes:
>
>I saw a posting this morning regarding the keyer mod, suggesting
>to leave the 38S sidetone connected and not use pin 3 of the TiCK
>chip. This, of course, won't work. It's required to be able to change
>the programmable settings of the TiCK.
>When the PGM button is pressed, the TiCK enters the "menu" mode.
>In this mode it drives the audio without keying the transceiver.
>The address to all TiCK issues is Gary and Brad at Embedded
>Research, at e-mail address gmdsr@vivanet.com
>They can answer the questions regarding the TiCK in the 38S too,
>they have at least one working...
>
>72,
>
>ORI AC6AN
CORRECTION
by Bob Follett AB7ST
If you are having trouble reading Brad's instructions, its
probably because his paragraphs conflict. Looking at the
schematic, he is suggesting a new 4.7K resistor from C32
point A to the opposite (right) side, of R19. In his "how
to do it" paragraphs, he is connecting the new resistor to R17.
It probably should read:
"Cut the trace between C32 and R17, connect the NEW resister
between the side of C32 you just cut, and the C34 side of R19
(the end towards the bottom of the board, viewed from the top)"
72,
Bob Follett AB7ST
Tuning by Ori AC6AN
Rich N3SLR wrote:
...It is putting out 8.8 volts p-p which works out to ~194 milliwatts.
...I have terribly imperfect pitch ... a 415 hz tone coming out of the
speaker...
...The antenna here shows a 4.5:1 vswr on 10.116 .
The last sentence tells the whole story, Rich.
As explained in the manual, the '240 will sound harsh if its input or
output is not tuned right. While this is not the most perfect way to tell
if your tuning is correct, it absolutely tells you when you're tuning is
incorrect.
A pitch change and a "harsh" signal are the characteristics of a
mismatch
at the antenna side or TC2.
Doug Hendricks posted earlier that you should check your radio with a
scope. That's a sure way to tune it, but good results can be achieved
if you tune into a known 50 ohm dummy load and adjust TC2 so it sounds
"clean". This is not enough, though. You should then use a receiver
that tunes the 30M band and listen to your signal. Tune a few KHz
around the carrier and listen if there is any excessive noise.
Then adjust TC2 once again and repeat the process.
This is a quick procedure (so is the scope) and you shouldn't feel
too bad bothering another local ham, in case you don't have a 30M
capable receiver.
The bonus is that you could converse on the air after your 38S
s aligned right...
72/73
ORI AC6AN
List of all external connections by Glen Leinweber VE3DNL
Perhaps a list of all the external connections, showing their silk-screened
numbers would serve as a good double-check for wiring all those pots,
switches and connectors. Something like this:
1 - Antenna HOT (center of coax cable) or coax connector center pin
2 - Antenna GND (shield of coax cable) or coax connector shell
3 - GROUND for D.C. power input
4 - POSITIVE D.C. +12 volt power input
5 - TicK keyer mod - paddle common (ground) & momentary switch keyer control
6 - TicK keyer mod - paddle DIT input
7 - TicK keyer mod - paddle DASH input
8 - TicK keyer mod - momentary switch for keyer control
9 - straight key input HOT
10- straight key input GND
11- R.F. gain variable resistor (1k) HOT
12- R.F. gain variable resistor (1k) WIPER
13- R.F. gain variable resistor (1K) GND
14- R.I.T. mod - to SPDT switch
15- R.I.T. mod - to SPDT switch pole
16- Main tuning variable resistor (100k) HOT
17- Main tuning variable resistor (100k) WIPER
18- Main tuning variable resistor (100k) LOW
19- Headphones output HOT
20- Headphones output GND
Super job, Doug. Going to enjoy using this rig. I love the KISS principle.
Glen VE3DNL
Simple Audio Level Mod by ORI AC6AN
More audio can be gained by changing the value of R24 or
shorting it. This seem to worsen the receiver noise level, but
should be a reasonable thing to try, if you really want more
audio with weak signals.
Another option is installing a 500 ohm pot for audio gain
instead of R24, but that might be an overkill for such a simple
radio.
72/73
ORI AC6AN
Receiver Mod by ORI AC6AN
The IF filter is not very sharp, and most selectivity is at the audio
stage. There are provisions for a half-lattice filter using two matched
crystals (or a 2-10 pF trimcap and the current crystal).
The tuning of the IF filter is very sensitive, so attempt this mod ony if
you have time, skill and a decent set of instruments.
This mod will buy you a better LSB rejection and a bit higher gain
in the receiver chain (who needs more anyway...)
My suggestion is that you build this kit and listen to the receiver
yourself. It might be just fine for your taste.
Sensitivity is fine, but there were some comments regarding the
audio level. I suspect this is an issue with some headphones
and a non-issue with other.
More audio can be gained by changing the value of R24 or shorting
it. This seem to worsen the receiver noise level, but should be
a reasonable thing to try, if you really want more audio with weak
signals.
Have Fun!
72/73
ORI AC6AN
Return to Table of Contents
C505 by Roy Gregson W6EMT
C505 ! .......I used a 330PF silver
mica cap, now have a beautiful looking
sine wave.
300PF to 390PF works Ok, but 330PF seems to give a better sine wave + a
smidgen more power out with proper adjustment of TC2. Also added a 1uH choke
in series with the 4.7uH at the xtal side. Now have coverage from 10.104 to
10.131. Was 10.113 to 10.133. 2.2uH choke would be better though
Roy W6EMT
Output Filter Mod
For The 5W IRF510 Version by Paul Harden NA5N
NorCal 38-Special - 5W IRF510 Version
OUTPUT FILTER MOD
The IRF510 5W mod was added to my prototype version and bench tested
carefully to address some of the reports of smoking IRF510's, chirpy
output, instability and spurs every few KHz around 10MHz. This is the
result of those lab tests and is PRELIMINARY at this point. I need to
get my 38S packed up, taken back home, and hope to get some good QSO's
on it this Sunday afternoon, to verify the results in the REAL WORLD.
For clarification, below is the output filter schematic. I am designating
the optional capacitor for the filter input as C501, as that is how it
is labeled on the 38S PCB (right next to L3). It is shown as C505 on
the IRF510 mod, 38S instruction book p. 11, but C501 on the PCB.
This actually consists of TWO different mods, where the second is just
an extension of the first.
MOD #1 - ADDING THE IRF510 5W AMPLIFIER
1. Make sure you cut the trace as explained in instruction #1 on page 8
in the 38S manual. Failure to do this will put +12v on the
HC240
outputs and surely cause destruction of the I.C.
2. Wind L101 per the instructions ... 27T on the T37-2 core.
(KD7S and NA5N believe in the "wind-em-tight"
method. My L101 with
27T was measured at 3.2uH with 12pF stray capacitance on a H-P
4332A
L-C-R meter).
3. This IRF510 5W amplifier is based on DIRECT coupling to the HC240
outputs.
DO NOT INSTALL 18K, R101
DO NOT INSTALL .1uF, C101; place a wire link in place of C101 for
direct input coupling.
4. Install remainder of IRF510 components per instructions, page 8.
5. The value of C26 should be increased from 220pF to around 330pF for
Xc=50 ohms at 10MHz. This is easiest done by soldering a
100pF cap
across C26 on the circuit side of the board.
6. INSTALL OPTIONAL CAPACITOR C501; the value should be from 330pF up
to 560pF, the mirror value of output capacitor C29. (Output
filter
caps C28 and C29 are incorrectly labeled C25, C26 on the IRF510
schematic).
7. If possible, monitor input current to the 38S during keydown to
ensure the IRF510 is not drawing >1A. If so, there's a
problem.
(Check the dc voltage on the gate; should be 0v on key up).
Normal operation, the current draw will be 400-600mA, depending
upon
the HC240 drive and final output power.
8. Readjust TC2 for a compromise between maximum output power and good
sounding tone in the earphones. It should not sound
raspy. Or,
monitor output purity on another receiver for a nice pure tone.
Adding C501 also makes finding the proper TC2 tuning easier and
more distinct.
For clarification of the mods and component designations, the output
filter is drawn below:
+12v
|
| -----> to receiver
L101 |
| |
From Q2
| C27 |
L3 L4
HC240 --- IRF510 -*-- .1 ---*--- T37-2 ---*--- T37-2 ---*----> ANTENNA
PA
|
| |
|
| |
C501
C28 C29
330-560pF
820pF 560pF
|
| |
|
| |
gnd gnd
gnd
This mod direct couples the IRF510 to the HC240 output for class C
operation; no current will flow until the HC240 output goes HI (>0v).
Reflected power in the output filter seems to easily transfer to the
HC240 input through the CMOS structure. Increasing C26 to 300pF
terminates the HC240 input to 50 ohms and attenuates many of the low
frequency spurs. Without C501, the 2nd harmonic was -24dBc; C501 thus
restores the 2nd harmonic to -40dBc in addition to removing the
spurious emissions around 10MHz and gives the amplifier stability.
I see no "chirping" or phase noise with this scheme in the lab, but
will verify on the air shortly.
The above MOD is MANDATORY in terms of removing R101 and C101. With
capacitor coupling, R101 puts about +4v on the gate, which on a
marginal IFR510, would put it perilously close to exceeding the maximum
drain current when RF is applied. C501 is probably mandatory to ensure
FCC compliance for the 2nd harmonic power. C501 and C26 mods together
ensure a cleaner output, better filter impedance and good stability.
The above output filter was analyzed using wide band noise source and
a RF Vector Impedance Analyzer (see 38S stock mod for discussion). The
above output filter yields:
Zin = 36 ohms -j50 (looking into C27 towards antenna)
Zout= 48 ohms -j85 (at antenna terminal looking in)
MOD #2
This mod is a continuation of the above and is preliminary at this
point until I verify its effects on the air. On the test bench, it
cleans up the output even further, gives the output filter some
additional voltage gain for higher output, and moves where the antenna
power to the receiver is pulled off to prevent overloading the NE602
receive mixer with the ten times increase in output power the IRF510
produces.
1. ADD an additional 200-270pF across the 820pF, C28 (for a net
capacitance of C28 now 1100-1200pF, or about 1/2 the 560pF input
and output capacitance of C501 and C29).
2. REMOVE C1 from the junction of C27, C501 and L3 and relocate to
the junction of L3, L4 and C25. C1 is the capacitor that
couples
the RF to the receiver input. I physically removed C1 from
the
board and soldered a new .1uF on the solder side from the original
C1-D1 hole to the L3-C28 junction.
3. Readjust TC2 for best tone and output, blah, blah, blah.
(REPLACE TC2 if you've adjusted it as much as I have in the
past 2 days -hi).
With C501 and the extra capacitance at C28, the junction of L3-L4 is
now at a lower voltage node than previously, which supplies less
p-p RF voltage to receiver for a nicer sounding sidetone. It drops
the MDS by about 2-3dB, but the resultant MDS on mine still measures
in at -125dBm, or still plenty of sensitivity to the receiver. BUT,
a little on the air will confirm this.
With MOD #2 the Zout is scarcely changed (46 ohms at -j80) and the
Zin of the filter is lowered even further to 16 ohms at -j80. The
16 ohms is closer to the ON resistance of the IRF510 than previous
filter Zin impedances, and the -80 degrees of (capacitive) phase shift
in the filter is the same on both ends, meaning it should be stable
and won't chirp. (One cause of chirp is when the input and output
impedance's of a filter are at different phase shifts, or capacitive
values, which causes the voltages to try and normalize when RF is
applied, causing the load on the PA amplifier to change and hence the
output filter to cause a momentary shift in frequency - or a "chirp").
The above mod is NOT MANDATORY, and again, preliminary at this point.
If you do try it, I would appreciate hearing from you. It is fairly
simply to perform, and DE-perform if you have to -hi.
I was doing these lab tests late Saturday afternoon and evening, and
watching my QRP-L mail ring with all the traffic about working AK.
And there I was, with MY 38S scattered in pieces all over the work
bench (and of course no antenna here). But I was having fun.
Hope this helps some of you experiencing various forms of instability
with the IRF510 mod, and always interested in the observations and
findings of others.
72,
Paul Harden NA5N
38 Mods QSO Update
by Paul Harden NA5N
Well, finally got the 38S kinda put back together, hauled it home,
put it on the aerial and had 3 QSO's while the superbowl was going on.
One QSO was with a QRO guy who lost patience with my "weak 449
signal."
Thanks for the nice report (hey, I'll take a 449). The other two were
more helpful, and provided no comments to suggest any chirping, etc.
As reported by a couple of posts, the side tone does sound a bit chirpy,
or at least a rising in frequency after several seconds of CW action.
On the second QSO, I monitored this on another receiver, and could not
hear it. I *THINK* it may just be the sidetone oscillator warming up
on transmit. After all, the two drivers for the sidetone oscillator and
the ones for the PA amplifier are only a few angstroms apart on the
same chip. (Well, a few millimeters anyway).
Don't piddle with the C1 cap change. Not worth the effort on the air.
This was picking off a lower voltage node by moving C1 to the junction
of L3-L4. The couple of dB you loose to the receiver is two dB you
don't want to loose -hi. But the additional 270pF across the 820pF
does show a slight increase in output power, and it was slightly
noticeable by the fellow on the other end, who said it also sounded
"cleaner" (whatever that means). Not a pertinent change, but
obviously
cleans it up a bit, as shown in the lab.
My IRF510 was showing a scant more than 3W on my power meter. I forgot
to measure the total current drain, though. Is that what others are
getting? Or are you really getting a full 5W?
Nice little radio, though, isn't it?
72,
by Paul Harden NA5N
Output Filter Mod/Stock Ver by Paul Harden NA5N
NorCal 38-Special - stock version
OUTPUT FILTER MOD
This is the result of some careful bench testing of the 38S, mostly for
the IRF510 mod. The "stock" output filter for the 38S is a very
good
filter. The virtual square-wave from the HC240 is causing some
spurious power, most of it BELOW 10MHz. Most of it is more than 30dB
below the fundamental (-30dBc), but can cause a slightly elevated output
power reading on most power meters. This simple mod knocks down these
low frequency spurs considerably and boosts the output power of the
fundamental a bit (about 1.5dB or nearly 100mW). This mod is not
MANDATORY; the stock 38S is well within FCC compliance. This mod will
boost the output power a bit, attenuates the low-frequency spurs
(which produces the raspy sound when mistuned), and seems to make adjusting
TC2 a little more distinct.
For clarification, below is the output filter schematic. I am designating
the optional capacitor for the filter input as C501, as that is how it
is labeled on the 38S PCB (right next to L3). It is shown as C505 on
the IRF510 mod. C501 and C505 is the same capacitor. The space for
it
is already on the board, although no component should currently be there.
OUTPUT FILTER SCHEMATIC
-----> to receiver
|
|
From C27
|
L3 L4
HC240 ---- .1 ---*--- T37-2 ---*--- T37-2 ---*---->
ANTENNA
PA
|
| |
|
| |
C501
C28 C29
270pF
820pF 560pF
|
| |
|
| |
gnd gnd
gnd
MODIFICATION:
1. Add a 270pF capacitor at C501 (next to L3 on the 38S board).
Any value from 200-270pF does a good job; 270pF is best.
Above 300pF, output power is reduced while the overall drain
current increases on transmit and the low frequency spurs are
smoothed into a big power peak around 4MHz.
2. Change C26 from 220pF to 330pF. C26 is on the pin 10-11 end
of U4, the HC240. It is probably easier to leave C26 alone,
and
add a 100pF cap across C26 on the solder side for a total of
320pF. The Xc of 300pF is 50 ohms; this gives the reflected
power from the output filter and HC240 drivers a 50 ohm
termination,
which cleans up the output purity. C26 is the capacitor from
the
single HC240 driver, pin 8, to ground.
3. Readjust TC2 for a compromise between good output power and a
pure tone. This mod makes the area of the pure tone a bit
smaller
(20-30 degrees of TC2 rotation), but also much more distinct and
easier to set. (With the TicK keyer mod, you are not able to
hear the direct sidetone on keydown; you'll have to adjust TC2
for best sounding output one another receiver).
SOME DISCUSSION
The output filter was injected with a wide band noise source and
observed on a spectrum analyzer to "paint a picture" of the overall
response. Peak power occurred around 10MHz as one would expect, with
secondary power peaks occurring at 4, 8 and 25 MHz.
The output filter was then reconnected to the HC240 drivers at C27
and the input and output impedance was measured on an H-P 4815A RF
Vector Impedance Analyzer, which tells you the complex impedance values,
which was Zin=110 ohms at +80 degrees (+ phase shift means it appears
inductive). The Impedance Analyzer is then scanned in frequency to
find the points where there is zero phase shift ... this identifies
points of self resonance. This method also showed self resonance at
4,8 and 25 MHz. The above two mods were performed and the above tests
repeated, which showed the 4,8 and 25 MHz resonance points were
attenuated by 12-15dB, and the input/output impedance was lowered to
nearly 50 ohms with a -85 degree phase shift, meaning the output filter
now appears capacitive. With a 50 ohm dummy load added, Zout went to
48 ohms at -5 degrees, or very well nulled for 50 ohms at 10 MHz.
With or without this mod, the 2nd harmonic remains at -40dBc, or 10dB
better than the FCC requires.
72,
Paul Harden NA5N
38S Power Mod Report by Preston Douglas WJ2V
I have been faithfully following the mod reports, mostly because I am afraid
I will miss an essential one. So far, I have done the high power mod with
the excellent changes recommended by Mike Gripe KM1G (I am still amazed at
the number of technical wizards on this list--) plus the addition of C505
--
330pf SILVER MICA cap (don't use a ceramic in an output tank) as recommended
by Roy Gregson, WA6EMT. Incidentally, did you notice that Roy is the
designer of the excellent commercial line of EMTECH NWxx transceivers?
Obviously, he is not missing out on the fun of the 38S--and I betcha' a
number of successful 38S builders will graduate on to rigs like Roy's. So
in
short, the corrected mod I used (incorporating both the Gripe and Gregson
mods) is:
1. Remove (or do not use) R101
2. Replace C101 with a wire jumper
3. Change C26 to 330 pf from 220pf -- a regular disc cap is suitable here.
This is the cap immediately to the right of U4.
4. Add a 330pf silver mica cap at C505
I take no credit for the genius in the above. I am just repeating what
others have reported in a convenient fashion.
Anyway, I found that I can now tune the 38S 5 watter to a stable spot with
TC2. I still don't have a scope that will do anything above 1.6 meg, so I
use all the tricks I can to assure I am sending out a stable signal.
Lacking
a scope I found the best method is to use a freq counter (the counter will
at least show if you have an out of band spur that is so strong it captures
the counter. The 38S will do just that when mistuned. Once TC2 is
close,
the freq. counter quits showing a major spur at 9.8 MHz or so. Then, use a
general coverage receiver to find the correct signal and clean it up while
listening to the receiver. I can hear two tones on my Yaesu from my
38S
until it is just right, then there is only one clean tone. Actually, with
the power mod, I found the rig is *more* stable and easier to tune than
without it. Ironic, but there you are.
I have the IRF 510 sinked with back to back anodized aluminum "wing"
style
heat sinks which seem adequate at 12v, but run fairly hot at 13.8. It may
be
necessary to use insulated hardware and a mica sheet (Radio Shack sells this
as a mounting kit) to sink the IRF 510 to a metal case if you plan to run it
off 13.8v.
Made a contact too. Worked a fellow who is 78 years old in Tampa. He
reported a 559 for me. So it aint chirpin' !
OK, now the TICK and the RIT and the case.
Oh, and one more thing--Hey Doug and Ori and Jim--I think I got my money's
worth!
Thanks.
72,
Preston Douglas WJ2V
38S Problem Solved by Jim Bennett W6JHB
Fellow NC38S builders:
Yesterday I posted a note to the group asking for assistance in
resolving a problem I had with my NC38S after doing the 5 watt
power mod. I got quite a few replies, and I want to thank each
and every one of you, especially Ori, AC6AN and Paul, NA5N.
As I promised Paul, I'm posting the result so that others don't
do what I did to my rig.
The 5w mod calls for two mono caps @ .1mf (three caps if you
are not doing the suggested additional mod from Mike, K1MG)
plus some other parts. I did the mods as prescribed. After
I powered it up, it received for less than a minute before
blowing the fuse in my 13.8vdc supply. For a full description
see my prior posttitled: "38S Need Help (long)".
The bottom line was that Mr. know-it-all (me) was out of .1mf caps
and picked up some at the store - but did I get "mono" caps?
Nooooo. I thought tantalum caps would work just fine. I didn't
realize that they are electrolytic caps and have a plus side
and a minus side. Hmmmm...what WAS that black stripe doing
on one side of the cap?
Evidently I got one of them in the circuit with its positive
side where it wasn't supposed to be. It lasted about 40 seconds
and shorted internally, providing me with a nice path for my
8vdc to ground. I was lucky; no other components bit the dust
- just the fuse and the cap. Well, actually I did totally
mangle the IRF510 getting it off the board to test it!
So - I strongly suggest NOT using a tantalum capacitor where
a mono cap is called for. Unless you want to learn a lot
more about your circuit and how to spend lots of time
de-soldering components.
My 38S is now on the air, full power, no more fuse blowing.
All right, now I'll do the TiCK mod. What could possible
go wrong..........?
*----------------------------------------------------*
Jim Bennett W6JHB
|
|
Martinez,
CA
|
|
|
| NWQRP #431 QRP-l
#596 ARRL Life Member |
|
|
*----------------------------------------------------*
38S Tuning & 6.9uH Choke by Jim Hale KJ5TF
Thanks to Randy WJ4P for bringing up the chirp in my CW note here on
QRP-L. I again connected the 38 spec to my dummy load and monitored
its note on my QRP+ RCVR.
I heard the chirp that time and played with TC2 a little more, getting
what now sounds allot better.
Today on 30M I got my 1st 599 report :-) When I explained what
had
happened the day before I got a detailed and happy report of a clear
and solid note !
I looked carefully in my junk'e box and glad to say I found a 2.2uH choke
red red gold and put that in series with the existing 4.7uH for 6.9uH
which gives me coverage of 10.110 - 10.126mHz.
Thursday I made my first 38 S mobile QSO, after doing the 5w mod that
morning. I had 3 QSO's with a 30M Hamstick, = fun!
Thanks NorCal folks for a great little radio,
de Jim Hale KJ5TF AR QRP #2
IRF510 Specs by Paul Harden NA5N
DATA SHEET: IRF510 POWER MOSFET TRANSISTOR
Manufactured by Harris Semiconductors
Available at most Radio Shacks (although incorrectly labeled "IFR510")
Cost: $1.99 at Radio Shack (Cat. No. 276-2072)
DESCRIPTION.
The IRF510, IRF511, IRF512 and IRF513 are n-channel enhancement-mode
silicon-gate power field-effect transistors. These power MOSFET's are
designed for applications such as switching regulators, motor drivers,
relay drivers, and drivers for high-power bipolar switching transistors
requiring high speed and low gate-drive power. These types can be
driven directly from integrated circuits.
MAXIMUM
RATINGS
IRF510 IRF511 IRF512
--------------------------------------------------------------------
Vds Drain-source voltage
100v 80v 100v
Vdgr Drain-gate voltage
100v 80v 100v Rgs=20K
Vgs Gate-source voltage
+/-20v +/-20v +/-20v
Id Continuous drain current
5.6A 5.6A 4.9A
ELECTRICAL CHARACTERISTICS (All types unless
otherwise stated)
--------------------------------------------------------------------
Igss Gate-source leakage
500nA (forward) -500nA (reverse)
Idss Drain current, Vg=0v 250uA
Id-on On state drain current 5.6A IFR510,
IFR511
4.9A IFR512, IFR513
Rds-on Drain-source "on" Res. 0.4-0.54
ohms (device ON resistance)
Cis Input
capacitance
135pF (at Vds=12v, Cis=180pF)
Cos Output
capacitance
80pF (at Vds=12v, Cos=130pF)
Td-on Turn-on delay
time 8-11nS ) These
parameters define
Tr Rise
time
25-36nS ) how fast the MOSFET turns
Td-off Turn-off delay time 15-21nS )
on and off when gate is
Tf Fall
time
12-21nS ) driven with a square wave
Vsd Diode forward voltage
2.5v (dropped across the source-drain
due to the internal diode)
SOME DATA FROM THE PERFORMANCE CURVES
Output drain current (Id) vs. gate-source voltage (Vgs) at Vd=+12v
Vgs=4v Id= 0A
Vgs=5v Id= 1A
Vgs=6v Id= 2.8A
Vgs=7v Id= 4.8A
Vgs=8v Id= 6.8A
NOTE: Therefore, for a 5W QRP power amplifier, the gate-source voltage
should not exceed 5-6v; otherwise excessive
current will attempt
to flow. A continuous applied Vgs >7.5v
will cause Id to
exceed the maximum drain current rating of 5.6A
(IRF510). This
will cause "catastrophic substrate
failure" (commonly known
as smoke!).
What is the maximum frequency? Max. frequency is not specified, but
since Tr= 36nS (rise time) and Tf = 21nS (fall time), a total device
delay of 57nS occurs, worse case. f=1/t = 1/57nS = 17.5 MHz. Total
"typical" device delay is 25+12ns= 37nS for f= 27 MHz. This does
not
take into account L/C loading of the output filter, etc., which will
lower the maximum frequency which the MOSFET will toggle on and off.
PIN-OUT
_______
|
O |
| | <--- Metal Flange
(Drain) ISOLATE FROM GROUND!!!
-------
| IRF |
| 510 | <--- Plastic TO-220 case
| |
|_____|
I I I
I I I <--- Leads (Max. temp = 300C
for 10 seconds, max)
I I I
G D S
-----------------------------------------------------------------
GL de Paul Harden NA5N (1-97)
No Scope Tuneup Trick by Bill Jones KD7S
I didn't install the original sidetone circuit in my NC38S because I went
straight for the TiCK sidetone. As a consequence, I couldn't tune TC2 for
*best sounding sidetone* as suggested in the manual. I figured my antique
EICO scope would be good enough to do the job. Wrong! It has about a
4 mHz
bandwidth. So I found another way to do the job. Here's what I did.
I took my main station rig (Icom IC-728) and fed a 10.110 mHz signal through
my MFJ 948 transmatch to my Butternut HF6V-X roof-mounted vertical. I
adjusted the transmatch VERY carefully so there was absolutely no sign of
any reflected power whatsoever. Then I switched over to my 38 Special and
adjusted TC2 for zero reflected power while simultaneously listening to the
signal on my Icom. The cleanest sounding signal corresponded exactly with
zero reflected power. In other words, most (if not all) of the spurious
output is gone. About five degrees of TC2 rotation, one way or the other,
will cause the reflected power on the transmatch to begin showing up again.
Another five degrees and the signal starts to sound cruddy again in the
Icom. Your mileage may vary but it sure works for me.
Now, on to another problem. While I have no sign of chirp, even at five
watts output, I have noticed that when I begin sending from a cold start
there is slow downward frequency shift of about a 150 Hz. After about
twenty seconds of transmission, it stabilizes. However, if I stop sending
for more than about five minutes or so, it starts all over again.
Something is heating up but I can't find the culprit. Does anybody else
have this problem?
==============================
Bill Jones - KD7S <><
Sanger, California
==============================
38S and Voltage Regulators by ORI AC6AN
For portable operation I would also consider the following
environment-friendly
solutions:
(1) a 7.2V NiCad pack (with reduced power) - just bypass the regulator!
(2) 7-8 cells of 1.2V NiCads with a low-drop diode in series,
again bypass the regulator
(3) 9.6V NiCad pack with a low-drop regulator
(4) 9V from 6 rechargeable alkalines with a low-drop regulator
The capacity of standard AA Nicad cells is about 600 mAh,
but that's at low discharge of 60mA. Maybe someone can give
a realistic value for 100mA. To be conservative at 70%
capacity this should be good for 6 hours with 40% key down.
Not bad, but don't count on more than 200mW by the end of the
day.
The rechargeable alkalines are rated at 1300 mAh (probably
mid-life, after 10 charges). The current consumption is
not too high to consider them good for their rating,
so they will provide for a long day in the field at full
output power (a whoping 300mW!)
The 7.2V NiCad (in a 9V-style shell) is rated at 150mAh.
I would carry a few "spare" ones in the pocket...
Of course, all those are NOT compatible with the IRF-510 amp.
For that one you should use a 12V/4Ah battery for a few
hours of contest-style operation (40% key down at 0.9A,
60% at 40mA < 400mAh/hour). As a side benefit, the extra
weight in the backpack might convince you to drop all
quasi-QRO ideas and go back to the basic radio configuration...
hi hi It's obvious that the 5W boost increases your
signal strength and gets you more replies, but it's
interesting to hear whether it really extends the
range...
Have Fun!
ORI AC6AN
Keyer Mod Suggested by Mike Gipe K1MG
I just finished putting the Tick keyer mod into my NC38S. I summed the
Tick sidetone into the audio op amp input as suggested by Brad Mitchell,
and it works just fine.
One more thing that you MUST do, though, is replace D8 with a wire jumper.
Although the input of the 74HC240 gate which you remove from the circuit
when switching to the Tick sidetone does have unbalanced leakage such that
it will settle to a logic 0, the current provided is so small that it may
take several minutes to settle, drawing excessive current, and it will
forever be sensitive to noise.
Mike Gipe K1MG
DIAGNOSTIC VOLTAGE MEASUREMENTS FOR THE "38 SPECIAL" by Paul Harden NA5N
Voltage measurements are VDC unless otherwise indicated, using
a DVM or voltmeter in both receive and transmit modes. Due to
variations in voltmeters and Vcc, voltages below are +/- 0.2v.
38-Special measured has 5W amplifier, but NO RIT or keyer mod.
NOTE: U1-3 = U3, pin 3.
On XMIT voltages, ensure you are connected to a
dummy load.
U1 NE602 RCV Mixer U3 NE602 TX
Mixer/ U5 NE5532 Audio
Prod. Detector Filter/Amp
RCV XMIT
RCV XMIT
RCV XMIT
-----------------
----------------- -----------------
U1-1 1.4V 1.7V U3-1
1.4V 1.4V U5-1 3.9V 3.9V
U1-2 1.4 2.2
U3-2 1.4 1.4
U5-2 3.9 3.9
U1-3 0
0 U3-3
0 0 U5-3
3.9 3.9
U1-4 6.7 4.7
U3-4 6.7 6.7
U5-4 0 0
U1-5 6.7 4.7
U3-5 6.7 6.7
U5-5 3.9 3.9
U1-6 7.4 7.4
U3-6 7.7 7.7
U5-6 3.9 3.9
U1-7 6.6 6.5
U3-7 7.0 7.0
U5-7 3.9 3.9
U1-8 7.8 7.8
U3-8 7.8 7.8
U5-8 7.8 7.8
-----------------
----------------- -----------------
U2 CD4066 Switch U4 HC240 Drivers, PA and
sidetone osc.
RCV XMIT
RCV XMIT
RCV XMIT
-----------------
----------------------------------------
U2-1 7.8V 7.8V U4-1
5.7V 0V U4-11 7.8V
3.5V
U2-2 7.8
0 U4-2 0
4.2 U4-12 0
3.6
U2-3 7.8
0 U4-3 0
4.0 U4-13 5.7
0
U2-4 0
0 U4-4 0
4.3 U4-14 0
3.6
U2-5 0
7.6 U4-5 7.8
3.8 U4-15 0.6 3.8
U2-6 0
7.6 U4-6
0 4.4 U4-16
0 3.6
U2-7 0
0 U4-7 0
7.6 U4-17 7.8 3.8
U2-8 0
6.6 U4-8
0 4.4 U4-18
0 3.6
U2-9 6.6 6.6
U4-9 0 4.4
U4-19 0 0
U2-10 6.6 6.6
U4-10 0
0 U4-20 7.8 7.8
U2-11 6.6 6.6
----------------------------------------
U2-12 5.6 0
U2-13 5.6 0
U2-14 7.8 7.8
-----------------
Q1 2N3904 RF Driver & bi-phase franistan
RCV XMIT
-----------------
Q1-E 0V 0.9V
Q1-B 0.6 1.5
Q1-C 7.8 3.5
------------------
DIODES (-A= Anode/band end -K =cathode)
-----------------------------------------------
D1-A D1-K D2-A D2-K D4-A D4-K D5-A
D5-K
-----------------------------------------------
4.8V 5.4V 4.8V 5.4V 5.7V 5.7V 5.7V
5.7 <--- RCV
7.6 7.8 7.6 7.8
0 0 0
0 <--- XMIT -300mW
8.1 7.4 8.1 7.3
0 0 0
0 <--- XMIT - 5Watts
-----------------------------------------------
D6-A D6-K D7-A D7-K D8-A D8-K
-----------------------------------
5.7 6.1 0.1 0.6 0.1
0.6 <--- RCV
0 0.5 7.6 1.5
7.6 3.9 <--- XMIT, either power
-----------------------------------
D3-A 0-8V, depending on setting of the TUNE pot
D3-K 0V (ground)
AC VOLTAGES - Most of the *AC* voltages in the 38-S are too small
to register on a DVM/voltmeter, except as follows:
HC240 RF output at U4-12,14,16 = 0.14Vac on XMIT (approx.)
NE5532 Audio output at U5-1 registers about 0.05Vac on XMIT and
about 0.03Vac on RCV with a moderately
strong signal.
If you're having problems with your 38-Special ...
Check the voltages listed above in both receive and transmit.
Since most of the voltages are switched on and off fairly hard
between RCV and XMIT, a problem should be evident. A measured
voltage that is close to the above is probably OK. Voltages
on the NE5539 are primarily the 1/2Vcc biasing (virtual ground),
developed by R25, R26 and C38.
GL Paul Harden NA5N
Material
and concepts presented on The American QRP Club (TM) website is Copyright 2003 by The American
QRP Club, Inc.
These pages are designed and maintained by George Heron, N2APB
Page last updated: April
15, 2004
