38S 5watt mod


From: duane (duane@flinet.com)
Date: Tue Apr 15 1997 - 05:25:51 EDT

OK guys there has been a lot of mods explaining the 5watt
Mod to the 38S. Some say remove D-8 others say remove
C-101 and so on . Well I just completed the mod recommend
By Bill Acito acito@asdg.enet.dec.com and the mod worked
Great ! I was having a lot of trouble before until I did this one.
Also you can find a gif file on my ham page on the WWW at
http://www.flinet.com/~duane/ham/ham.html just follow the links to
my download page and look for 38S 5watt mod . This picture is the
schematic layout of the above recommend mod. I now have a FB
38s with an adjustable power output. My 38s has no other mod
at all and it works great.

Here is the org. post by Bill Acito

 << The W5VBO Variable power mod, with one or two small
      corrections/additions [noted in brackets].
      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-103 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-103 used to be. This is the
           connection to the junction of R-103 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
      With the rig turned on and keyed, measure the DC. voltage at the
      junction of C-102 and R-103. 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.
      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 *
      ******************************************************** >>

73's all I hope this helps
QRP-L# 710

    . . . . . . - I own my own words - . . . . . .
    Bill Acito
    |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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