[Elecraft] [K3] Band data problem

[Don Wilhelm]

I would suggest if modification to the RCS-12 is in order, the 2N3904s 
be changed out for switching FETs such as the 2N7000.

The change Jim made will result in only 8 ma of drain current for the 
drivers in their off state.  I believe the band data drivers in the K3 
are similar to the KEYOUT-LP driver which is rated to sink up to 10 ma. 
according to the K3 ACC connector information.

73,
Don W3FPR
On 12/2/2012 12:24 PM, Joe Subich, W4TV wrote:
>
>> I didn't want yet another piece of clutter in my already box and
>> cable littered shack so I chose to mod the K3.
>
> In all probability an inverting buffer or possibly a pair of low drive,
> dual optoisolators could replace the 2N3904 transistors directly. I
> suspect the transistors are simply buffering and inverting the inputs
> to a PIC of some kind and are there to protect against 12V inputs.
>
> Alternatives are to add pull-ups (2K2 Ohm) in the RCS-12 (but that goes
> to the same issue of added dissipation on the K3 drivers) or replace
> the base resistors with 2K2 (the transistors should have sufficient
> gain to work with 1 mA instead of 1.5mA base drive.  Even adding 1K-1K5
> in each of the band data lines (connector at the RCS-12) might work as
> a "no modification/no external box" fix.  However, since I don't have
> an RCS-12 I have no way of trying it.
>