[Elecraft] SSB Mods (long)

[Stewart Baker]

Hi Guys,

Rig K2 #3527 - KNB2,KSB2,K160RX,KPA100,KAT100.

Now the build and alignment is over I have been making some comparisons =
with the other station rig a FT1000MP.
In my opinion on receive the K2 leaves the 1000 standing, however I felt =
that the K2 SSB TX performance needed some tinkering to get it right.

The areas on my K2 that needed attention were:-=20
a)  Low mic gain when using the Yaesu MD-100 dynamic microphone.
b) Speech compression maximum (4:1) too low. I normally run about 8dB =
compression on the FT1000MP. Viewing the RF on a scope showed the =
difference.
c) VOX.  Needing to be very close to the microphone to get satisfactory =
operation. This more a function of the level out of the SSM2165 IC than =
mic gain.

I made that following mods to the KSB2 module and am now very satisfied =
with the performance both for rag chewing and contesting.

a) I built a 1 transistor common emitter amplifier on a piece of perf =
board which was superglued to the side a computer header socket mating =
with P1 on the Front panel board.
The complete assembly fits snugly between the Front Panel board and the =
Control Board. I am able to adjust the gain by changing a fixed resistor =
in the emitter decoupling.
For the MD-100 about 22dB seems right.

b) Looking at the spec sheet for the SSM2165 IC it is possible to get a =
15:1 compression ratio. This seemed to me to be rather high, but I =
wanted more than 4:1.
I carefully removed RP5 res pack and replaced it with a 6 pin male 0.1" =
connector. This lets me make changes to resistor values with minimum =
fuss.
I decided that the new compression ratios for  existing menu entries =
would be as follows (new value in brackets) 4:1 (10:1), 3:1(8:1), =
2:1(5:1), 1:1(1:1).
Using Pin 1 of the connector as common for all the resistors (1/8w 5%) =
their  values and pins are :-  pin 6 (150k), pin 5 (470k), pin 4 (220k). =


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.=20
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=20
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.

I now have a K2 that performs as well on SSB TX as all the other modes. =
Anyone for an FT1000MP ?

73

Stewart G3RXQ


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