[Elecraft] sidetone mod now works

[Darrell Bellerive]

I am curious about doing this sidetone mod on a K2 rev B. Elecraft changed the 
sidetone circuit from serial number 3000 on, and wonder how effective the 
sidetone mod is on a rev B K2. My K2 is serial # 5093.

The mod is described at 
http://www.raunvis.hi.is/%7Ematti/TF3MA/sidetone_tf3ma.html

From the schematic diagram it looks like L1 82mH is removed and inserted 
between U10 pin 7 and C33. A new capacitor is then installed at the old L1 
position. From the charts on the mod page it looks like about 1 uF would be 
about right.

My L1 82 mH inductor has a DC resistance of 62 ohms. This is quite different 
from the 180 ohm version used in the mod. Would a series resistor be desired 
as Fred indicates in the message below? I have 100 ohm, 120 ohm, and 150 ohm 
resistors on hand. Where is this resistor installed between U10 pin 7 and the 
inductor or between the inductor and C33?

What type of capacitor would be best suited for this mod? My junk box only has 
1 uF 100 V 5% metallized polyester or 2.2 uF 16 V 10% dipped tantalum 
capacitors. The polyester caps are rather large. A couple of the 2.2s in 
series would be 1.1 uF. 

Comments and recommendations?

Darrell  VA7TO  K2#5093


On November 15, 2006 06:04 am, kt5x at cybermesa.com wrote:
> The sidetone mod by TF3MA was installed in my K2 right after Marti posted
> it.  I obtained a 47 mh inductor for this mod from Elecraft.  The mod
> resulted in an improved sidetone, but left room for further improvement,
> especially at lower tones.  Additionally there was an objectionable
> pin-prick sound at the make of the muting that was annoying to hear.
>
> Recently I decided to revisit this situation.
>
> I noticed in Marti's write-up that HIS 47 mh inductor had an internal
> resistance of 180 ohms.  With the K2 apart, I measured the inductor I had
> received from Elecraft and sure enough, it had an internal resistance of
> only 40 ohms.  This would entirely change the frequency response Marti's
> peaking high-pass filter.  Elecraft is apparently using an 82 mh inductor
> in its mod, I obtained that, and found it has an internal resistance of 60
> ohms which would seem to offer less than an ideal result.
>
> I put a 150 ohm resistor in series with the 47 mh / 40 ohm inductor
> originally used when I modified the radio some years ago.  Folks, the
> difference is DRAMATIC !!!  The sidetone is louder, clear, and is that way
> across its entire range from 400 to 800 cycles.  Not only that, the tone is
> so much stronger that the slight pin-prick click from the make of the
> muting line is all but completely inaudible in the strength of the sidetone
> and no longer objectionable.
>
> A big thank you to Marti and to W3FPR who both stayed with me through my
> frustration trying to make this thing right.  For a while they must have
> thought my hearing was the main problem because the radio seemed to have
> the right modifications in it to sound well.
>
> Maybe this will be helpful information for others.
>
> 73,  Fred KT5X


-- 
Darrell Bellerive
Amateur Radio Stations VA7TO and VE7CLA
Grand Forks, British Columbia, Canada