[AMRadio] Stock or modify? BC rig "value"

Donald Chester k4kyv at hotmail.com
Thu Jun 1 13:24:15 EDT 2006

>From: W4AWM at aol.com

> > The plate transformer in a
> > typical 1 kW tubed transmitter can deliver sufficient voltage and 
>current to
> > run an electric chair. YOU COULD GET KILLED!
>It isn't the voltage that kills, it's the current.  You could get fried on 
>12V battery if it gets you in the right place. But interlocks were put 
>for a purpose and defeating them is dumb, risky, foolish, idiotic and down
>right dangerous. For that matter, you can get fried wiorking on a receiver, 
>"Switch to safety!"

Very true, and I would never rewire around the interlock.  On my BC1-T, I do 
sometimes use a thin washer to temporarily defeat the interlock, since 
otherwise, with the front grille removed, the power is shut off to 
everything.  Even with the HV turned off, the exciter stage is killed as 
well.  That makes it difficult to take voltage measurements, or tweak up 
circuits as needed.  Of course, with the side or rear panels removed, the 
interior of the transmitter, HV and all, is accessible regardless of  the 

Working on these things carries the same hazards as a comparable homebrew 
transmitter, and requires that you be alert and know exactly what you are 
doing at all times.  Interlocks, designed to add a  layer of proctection 
against carelessness, do sometimes fail since switches have been known to 
mechanically stick and contacts to weld closed, so don't trust your life to 
a properly functioning interlock circuit - there is just too much stuff 
there that can malfunction.  Probably the safest interlocks are those that 
mechanically short out the HV using a spring loaded switch activated when 
opening the door or removing the panel, but even those are not 100.00% 

On my homebrew KW, I built in an interlock on the door that is opened for 
access to the plug-in coils, since these run with full modulated HV on them 
when the transmitter is operating.  I would never try to change coils with 
the rig in TX position.  The more redundancy the better.  When I change 
coils, the interlock is turned off, the rig is in standby position, the main 
HV toggle switch is in the "off" position, and I have made it a habit to 
observe the plate voltage meter to make sure it is at zero.  A shorting 
stick would'nt be a bad idea, either.

Don k4kyv


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