[Elecraft] VOX sensitivity K2/KSB2

[Dale Boresz]

g3pjt wrote:

>Hi
>
>Does anyone know if it is possible to increase the VOX sensitivity on the
>KSB2?
>
>But NOT by :
>    - increasing the audio gain/level input ( or compressor setting)
>
>
>Thanks
>
>Bob G3PJT
>
>  
>
Bob,

One of your countrymen, Stewart, G3RXQ submitted a very interesting 
posting to the list on 6/23/2003 with details concerning a number of 
SSB-related issues, among them increasing compression level (which I 
have done, and which works incredibly well with the Heil HC-5 element), 
and also a way to improve the VOX sensitivity. If you wish to locate the 
original posting, the subject line was: "SSB Mods (long)" Attached below 
is the snippet concerning the VOX sensitivity (which I have not tried 
since I don't use VOX):

##### BEGIN SNIPPET FROM Stewart - G3RXQ posting of 6/23/2003 #####
c) On my K2 the level of signal into comparator amplifier U4 pin 2 was marginal when compared to ALC THR from the DAC U2. This meant that the VOX operation was not consistent. 
The level from U3 is fixed 1:1) due to /COMP0 being at 0V during RX which means that no amount of reasonable mic gain will improve the VOX. Two options presented themselves for 
increasing the signal into U4. The first was to put a small amplification stage in circuit. This was discounted because of space constraints. The other which was adopted was to "wet"
the compressor by allowing a very small amount of compression to occur in the RX state. This is done by a small value resistor (4.7k) in series between /COMP0 and U3 pin 6.
I cut the track from U1/RP5 and U3 pin 6 where it runs along the left side of the KSB2. On the underside of the board I  connected the 4.7k resistor between RP5 pin 1 and U3 pin 6.
This value can be increased, but remember the K2 has no Anti-VOX. The VOX now works brilliantly.
##### END SNIPPET FROM Stewart #####

73, Dale / wa8sra