[AMPS] Hot Switching

[measures]

>
>Hello, AMPers. Long time, no see.
>
>Someone asked me a question about hot switching today. I thought I
>understood this subject, but while I was thinking it through, I realized
>that I'm not entirely sure. The context was the unfortunate design flaw in
>the Yaesu FT-990 that causes it to apply RF a mere 3ms after keying the amp
>(it actually delays the RF envelope until 10ms after key closure, but
>inexplicably waits 7ms after key closure before keying the amp.) 

A potentially disastrous delay could be caused by the relay in the radio 
that keys the RL control line.  To avoid hotswitching this relay is 
removed and replaced with transistor switch.  

> The guy is looking at a QRO HF-2000, and got this off their website:
>
>"We have designed the transmit/receive (T/R) switching so that
>the relays are momentarily "hot-shotted" with a large voltage spike
>to speed switching. Also, the same circuit that controls the cathode
>bias switching also controls the T/R switching. This means that
>whenever the T/R relays are in transmit, the correct linear operation
>cathode bias will be applied. 

good news

>The T/R and cathode bias relays are
>sequenced so that no "hot switching" can occur. A fast open frame mechanical
>relay switches nearly as fast as some vacuum relays."
>
>Previously, I understood "hot switching" to mean that RF is applied to the
>tubes before the main relay has a chance to close. 

True enough.  One can also have hotswitching because the amp relays are 
not fully opened before the subsequent  RL signal arrives.  .  

>The tubes then transmit
>into thin air, causing arcing, pitting and burning of the T/R relay
>contacts. 

Not if the relays are quick enough.  

>The way I read it, QRO's sequencing circuit makes sure that the
>main T/R relay is closed before cutoff bias is removed from the tube, thus
>protecting the T/R relay contacts. True? However, since the 3ms provided by
>the FT-990 is not enough time for the T/R relay to close, won't the exciter
>RF be "hot switched" between the antenna and cathode input circuit? 

Yes.  

>Does
>this present a danger to the relay contacts that switch the exciter RF? If
>so, is this still called "hot switching?"

yes, yes.  
>
>BTW, the guy asked if he could use the amp in QSK. Seems to me that even
>though they claim to have speeded up the open frame relay to be "nearly as
>fast as some vacuum relays" (5-8ms?), it still won't be fast enough for QSK.
>True?
>
True, Dick  The puzzler is why QRO simply did not state how fast the 
relays switch in their amplifier.  

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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