Hi gang,
Just read the message from Scott Thomas where he did my suggested mod,
here is a step by step how to for those less confident to sally forth.
I have done a mod to my 38S to eliminate the thump and it workes like a
charm. Earlier I built an audio amp that I mounted on a little board and
attached it to its volume control. The audio amp increased the thump to
such a level that I had to get rid of it. I have already sent this to a
couple of hams and am forwarding it to you now. The mod is quite simple
really, here is how it goes:
Parts needed, one of each.
: FET Transistor, 2N5484 or 2N4858( any equivalent will do).
: Diode 1N4148, or 1N914.
: Capacitor 0.01uF (10nF).
: Resistor 10M (1/8W for small size can be 1/4W).
1)
Look at the underside of the circuit board and orient it with the regulator
side towards you, I will make all references to this orientation.
Carefully cut the trace at the midway between U2-pin 11 and C15 on the
underside of the circuit board, now facing you, careful to the way you
count the pins on U2 as you are looking at the underside of the board.
2)
Identify the pins on your FET and bend the Gate pin away from the other two,
it should be the one furthest from the tap if you have a FET in a metal can.
Carefully scrape the protective paint of the trace you cut, about 1/32 (2mm)
on each side of the cut untill you see the bare copper foil, then pre tin
it, be careful not to over heat the traces as they are small and will lift
off the board if over heated.
3)
Trim the two leads on the Transistor, next to the tab and the middle one, to
bout 3/16 and lay the transistor on its side to the left of the cut in the
trace (I filed down a spot in the rim on the Transistor), carefully solder
the two trimmed leads one to each side of the cut leaving the Gate lead sticking
out in the air for now. The tab is towards the top of the board (north).
4)
Now take the 0.01uF Capacitor and lay it flat on the circuit boards ground about
1/2 inch up 45 deg to the right from the cut you made . The cap will be oriented
up down (or north south if you prefer) and solder the upper (north) end of the
cap to the hole in the ground trace that should be close to that end of it.
5)
Three ends will now be soldered to the other end of the cap. Solder the
third lead of the Transistor to the lower (south) end of the cap. Get the
diode and
solder the Anode (the lead that goes to the end that does NOT have the black
band) to the cap and the other end (the end WITH the black band) to U4-pin
13, this is due south, pin 13 on U4 is the keyline (T-). The third end is
the 10M
resistor from the caps south end to U2-pin 11. If you have a problem getting
all the ends to stay in one place when soldering them to the south end of the
cap, solder the other end of those components first and trim and bend the
leads to the cap and then solder them there.
Last thing to do is to put a small piece of tape or a drop of glue to make
sure the diode leads do not short out anything, check for solder bridges
with a magnifying glass, clean the solder flux first with isopropanol or
rubbing alcohol and a quetip. The thump is mostly on key up and the FET
will cut off the audio and not return it untill the thump is over. The
circuit is basically a muting circuit that is placed in the audio line and
does not turn the audio back on right away. The trick is to keep the thump
out of the audio
circuitry instead of trying to deal with it once it is there.
Do hope this helps and I have not confused you too much, if you have any
questions give me a shout. Good luck.
72 de Niels
VE6NJK/TF3NJ
: Argonaut 509, Epiphyte, 49-er on steroids and a thump free 38S
: And a distinctive swing ;-)
e-mail: nkristja@cadvision.com
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