[Elecraft] K2 PA Failures and Remote Antenna Switch

[Josh Fiden]

Hi Ted,

I see that you're right about no big caps on two of the relays. I was 
looking at the current version schematic, but there's an older one 
posted on W8JI's site with a completely different arrangement which must 
be what you had. That older circuit design is less than clever (IMHO).

When a relay is deenergized, the steering diodes & part of the bridge 
are all forward biased forming a clamp of sorts. Not as effective as if 
a diode were directly across the coil with short leads.

I did not mean to imply operator error as the cause of failure. I was 
guessing hot switching the antenna related to some intermittent hardware 
problem.

Glad you found the source of the problem. Sounds like you put that old 
RCS-4 right where it belongs!

73,
Josh W6XU

On 5/21/2016 9:05 PM, Dauer, Edward wrote:
> Josh —
>
> Thanks for looking further into it.  I looked pretty carefully before the
> RCS-4 went into the trash - no electrolytics in the relay box that I could
> see.  Maybe a manufacturing defect rather than a design defect?  But it’s
> in a landfill somewhere now so I can’t confirm that.  In my profession a
> skeptic might call that spoliation of the evidence.  But the traces on the
> ‘scope as I switched through the selector with no transmitter attached
> were unmistakable - very large very fast transient spikes.  Not a good
> thing, whatever the cause.
>
> I wonder if the diodes that are in the RCS-4 system are actually being
> used as flyback or suppressor diodes.  The RCS-4 power supply is low
> voltage AC, not DC.  The diodes in the control box separate the positive
> half cycle from the negative half cycle; as I read it the diodes that are
> in the relay box select for each relay the half cycle they like, telling
> the relays to do what the control switch says it asked them to do given
> that there is but one relay voltage conductor - the center lead of the RF
> coax.  Would they work double-duty as flybacks in that configuration?
> Pardon the anthropomorphisms - I have already confessed my intellectual
> limitations in the field.
>
> Hot switching is a theoretical possibility but if that means an
> operator-induced lack of synchrony between keying the TX and switching the
> antennas, I choose to believe it unlikely.  And failures three times and
> every time and only when on that antenna system but not on any other?
> Improbable, I would think.
>
> In any case, the system that used the RF coax for the antenna relay
> voltages is no longer here.  When it was here, I had repeated and rather
> expensive PA blowups.  As soon as it left everything worked just right.
> Reverting to my other profession’s argot, that’s a correlation that
> implies causation.  Lots of human behavior has been built on less.
>
> Anyway, life is good again.  My K2 is happy.  The people who sold me the
> new remote switch probably are too.
>
> Ted, KN1CBR
>