[AMRadio] ground sticks again


Alan Cohen acohen at texas.net
Mon Jun 5 21:03:44 EDT 2006


> Hey Dennis did you ever forget to remove a clip lead and hit the HV  
> button?
>

I'll plead guilty to that one.  The old RCA klystron-based UHF  
transmitters ran 18-25 KV, depending on the model with supplies that  
were capable of delivering a dozen or so amps all day, every day.   
They were huge rigs that you walked inside of to service.  As soon as  
you were inside, the first thing you did was use the shorting stick  
on all of the capacitors.  Next, you hung the stick on the HV bus  
coming off the rectifier stack.  You then used an additional shorting  
stick to ground out whatever part you were about to work on.

More than once I hit HV on with the stick still in place.  It was no  
big deal.  The breakers popped, you reset everything and tried  
again.  Not once did I ever see any real damage done to one of those  
rigs by forgetting to remove the shorting stick before beam on.

It's also interesting to point out that all of those transmitters was  
equipped with a special relay gizmo thingy that would short out high  
volts when the beam was turned off.  In all the 30 years off and on  
that I've worked on UHF television transmitters I never once  
experienced the big bang with the shorting stick.  The occasional arc  
and spark, but never wound up discharging the full load.  I know  
others who were not so fortunate, but lived to tell about it thanks  
to their shorting stick.  I should also point out that every UHF rig  
that I've ever seen has some sort of interlock that automatically  
grounds out the HV whenever any dangerous area is open.  In spite of  
such precautions, a shorting stick was always provided and always  
used before sticking one's hands inside.  Even so, I always felt a  
little squeamish whenever I first reach inside.

I can assure you that I've never, ever worried about popping a  
capacitor.  As far as I am concerned, using the shorting stick is a  
part of normal operation.  Any component that dies as a result was  
poorly designed in the first place.

Alan
WA2DZL




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