[Elecraft] K2 PA Failures and Remote Antenna Switch

[Dauer, Edward]

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 <josh at voodoolab.com>
>To: elecraft at mailman.qth.net
>Subject: Re: [Elecraft] K2 PA Failures and Remote Antenna Switch
>	System
>Message-ID: <5740CC46.3070300 at voodoolab.com>
>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.
>