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K2 PA Failures and Remote Antenna Switch

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Edward A. Dauer

K2 PA Failures and Remote Antenna Switch

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

>
>------------------------------
>
>Message: 17
>Date: Sat, 21 May 2016 13:59:50 -0700
>From: Josh Fiden <[hidden email]>
>To: [hidden email]
>Subject: Re: [Elecraft] K2 PA Failures and Remote Antenna Switch
> System
>Message-ID: <[hidden email]>
>Content-Type: text/plain; charset=utf-8; format=flowed
>
>Reviewing the schematic on Ameritron.com for RCS-4 there is a 220uF
>electrolytic across each relay coil. There are also a pair of diodes
>which will clamp the coil voltage, although it might not be immediately
>apparent since they're not directly across the coils. Unlikely that
>these parts are missing since they're not just clamps, but necessary for
>the selector to function.
>
>This does not look like any design flaw of the RCS-4. If I had to guess
>without seeing the hardware, it seems more likely that a failure
>occurred to cause hot switching of the antenna... if that was happening
>repeatedly, the finals wouldn't be happy!
>
>73,
>Josh W6XU
>
>> Inside the relay box I found no diodes and no capacitors across the
>>relays that might have been used in the design to avoid this kind of
>>problem.
>

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Josh Fiden

Re: K2 PA Failures and Remote Antenna Switch

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
>