The W5VBO Variable power mod, with one or two small
corrections/additions [noted in brackets].
b
----------------------------------------------------------------
To: qrp-l@Lehigh.EDU
From: Brian Kassel <bkassel@dancris.com>
Subject: NC38s:(Long) ScQRPion Variable TX PWR Mod
Here is the way that I did a variable TX power modification the
my NC38S It only applies to those rigs which have undergone the
high power mod; that is, the IRF510/511 has been added.
What it does:
-------------
This mod allows the TX power to be completely variable from
milliwatts up to full output. The maximum power desired point
can also be set.
How it does it:
--------------
The mod consists of adding a pot in such a way that a variable
D.C. voltage, or bias, is added to the gate of the final FET.
By varying the pot, the D.C. voltage at the gate can be varied
from almost 0V (lowest power out) up to about 5.5 volts or so
(highest power out).
The parts required are:
-----------------------
1.) 1 ea. 50K pot
2.) 1 ea. 22K 1/4w resistor
[[ or
1.) 1 ea 100K linear pot
2.) 1 ea. 47k 1/4w resistor
IMPORTANT: The R should be 1/2 the max pot value. ]]
3.) A .1 UF capacitor at C-101, if you removed it per the Paul
Harden Mod.
4.) Oh yea, some wire too to hook up the pot.
Macro instructions for the knowledgeable/impatient:
---------------------------------------------------
The mod only requires the addition of a 25K to about 100K pot.
For this mod, the taper probably doesn't matter very much. I
used a 50K, linear taper unit. One end of the pot goes to
ground, one end goes to the cathode of D-7 via an added 22k 1/4
resistor. This resistor sets the highest available TX power out
when the pot is set for maximum. The center goes to the junction
of R-102 and C-102. This junction provides a +8 VDC when the key
is down (R-). This voltage is varied with the pot, and supplied
as the bias to the FET at the junction of R-102 and C-102. You
must remove R-103 also, which frees up the circuit for the mod.
Doing the mod, step by step :
------------------------------
1.) Replace C-101 (wire) with the capacitor that you may have removed
per the Paul Harden Mod. Or use a new one if you killed the
little beastie on the way out.
2.) Remove R-103, we will use the holes left by the removal of
the part to connect the new potentiometer.
3.) Solder one end of the 22K resistor to one end of the pot.
4.) Solder a wire to the other end of the resistor,
and run it to the cathode or banded side of D-7. CAREFULLY
solder the wire.
5.) Run a wire from the other end of the pot,
to TOP hole, where R-102 used to be. This hole should be ground.
Solder both ends of the wire. 6.) Run a wire from the center of
the pot to the BOTTOM hole, where R-102 used to be. This is the
connection to the junction of R-102 and C-102. Solder both ends.
[[ 6.) Optional: a 6v or even 5.2 volt zenor diode can be put across
the outer leads of the pot to make sure the voltage never exceeds
the Vgs limit of the FET. ]]
Smoke Trapping measures:
----------------------------
1.) Double check everything.
2.) See above
FINI:
-----
With the rig turned on and keyed, measure the DC. voltage at the
junction of C-102 and R-102. As you turn the pot, the voltage
should vary from about 0 to about 5.7 volts. If you get maximum
volts when the pot is CCW rather than CW, just reverse the
connections at both ends of the pot. If you find that at the
maximum setting of the pot you have too much output power, then
make the 22K resistor larger. If you find that at maximum
setting you don't get enough output then, oh heck, you know what
to do, right?
We already know that some of the final FET's are hotter than
others, so of course your mileage may vary, as they say in Texas.
Credits.
-------
The original idea for this mod came from the almost-famous
Dan Tayloe at our last ScQRPion get together.
I just kind'a worked out the physical details.
There are of course other methods out there I am sure on how
to accomplish the same end. This is simply the way that I
wound up doing mine.
********************************************************
* Brian D. Kassel W5VBO - QRP NERD 10 - ScQRPion # 0 *
* QRP-L #404 - ARCI #3623 *
********************************************************
. . . . . . - I own my own words - . . . . . .
Bill Acito
acito@asdg.enet.dec.com
|d|i|g|i|t|a|l| Digital Equipment Corporation Hudson, MA
W1PA qrp-ne qrp-l adv-rs arci norcal amsat-na arrl-life
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