[Elecraft] [K3] Band data problem

[Joe Subich, W4TV]

> 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.

73,

    ... Joe, W4TV


On 12/2/2012 11:28 AM, Jim Miller wrote:
> Hi Joe
>
> There's certainly more than one way to deal with this. External buffer
> chips are an inexpensive solution. I didn't want yet another piece of
> clutter in my already box and cable littered shack so I chose to mod the K3.
>
> The K3 driver chips are not an issue at all. Any additional external pullup
> would add to their load but would also provide the desired additional
> pullup drive and make it possible to increase the value from 820ohms to
> perhaps 1K. The KAT500/KPA500 add a fraction of a milliamp in total so they
> are not an issue. They are the only devices other than the RCS12 on the
> band outputs in my case.
>
> The only other consideration is that the increased internal pullup drive
> (lowered resistance) puts more load on the 5v regulator sourcing current to
> those pullups. Again not an issue.
>
> 73
>
> jim ab3cv
>
> On Sun, Dec 2, 2012 at 9:41 AM, Joe Subich, W4TV <lists at subich.com> wrote:
>
>> I would be careful when decreasing the value of the pull-up resistors
>> in the K3.  The "Band Data" outputs are open drain - decreasing the
>> pull-ups increases the current the outputs must sink at logic low on
>> top of any current sourced by the external devices.  It might be better
>> to buffer the RCS12 using something like the 74ACT244 (at less than
>> $0.25) to drive the 2N3904s.
>>
>