I have made the 49er mute mod to my 38s as suggested by Niel Kristjansson
Mar 22 1997 and it works terrific. It will take you about 15 minutes and
use about $2 worth of parts. I used the $.99 FET that Radio Shack sells,
2
X 4.7meg resistors in series, a small sig diode, and the .01uf cap. I
placed the JFET on the solder side of the board flat side towards the
board
across the trace cut. I had previously cut the trace bwtween c15 and the
input resistor to the audio amp, so I put the JFET (source and drain)
across that trace. I soldered the resistor to the left leg of the JFET as
viewed from the back of the board. Niel's original message follows.
Best of success,
Scott
> ------------------------------
> Date: Sat, 22 Mar 1997 17:41:47 -0700
> From: Niels Jensen Kristjansson <nkristja@cadvision.com>
> To: qrp-l@lehigh.edu
> Subject: [15428] 38S NO THUMPING
> Message-ID: <1.5.4.16.19970322053903.1f378040@cadvision.com>
> Mime-Version: 1.0
> Content-Type: text/plain; charset="iso-8859-1"
> Content-Transfer-Encoding: quoted-printable
>
> Just finished my 38S thump mod and it workes like a charm. A while back I
> built a 350mW audio amp on a little circuit board and installed it on to
> the volume on/off, control. The amp made the key thump worse to a degree
> of really bothersome. I remembered how the audio is muted during
transmitt
> on the 49-er, so I installed the same circuit in between U2 pin 11 and
C15
> where I had cut the trace. The circuit is only four components, 10Meg
> resistor, 10nF(0.01uF) cap, 1N4148 diode and a 2N4858 FET. The reason
for
> the choise of 2N4858 is that I had one in my junkbox. The components
> are mounted on the underside of the circuit board. The cathode of the
> diode goes to U4 pin 13, keying. Have a look at the 49-er schematic, Q1,
> R3, C8, D2 and it will all make sense, those who dont have one, look at
> the one on NorCals homepage. I now have a comletely thump free 38S with
> the 5W variable mod set at about 3.5W, this seems to be a happy medium
> between output power, battery consumption and the size of heatsink I use.
> Dont know if I will do the RIT mod as my offset seems to be just perfect
> also I plan on using a spare Curtis Chip keyer I have because the
sidetone
> osc in the 38S is such a good indicator of how the output RF signal is.
>
> 72 de Niels VE6NJK/TF3NJ
> ------------------------------
>
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