[AMRadio] Shorting stick


Jim Wilhite w5jo at brightok.net
Fri Jun 2 19:22:54 EDT 2006


John I have a suspicion you do regular maintained on your rigs.  Each year I 
pull my high power rig out of the case and give it a visual, smell and 
resistance check looking for signs of failure.

I have a GK 500A that I checked last year and it had a stain on the power 
supply chassis that I had never noticed before.  The stain originated under 
a Low High Voltage transformer that was working fine.  I pulled the 
transformer and removed the end bells.  Sure enough the insulation paper was 
burned.  Although still working, it needed replacement.

I use a shorting stick made with a 5K ohm, 25 watt resistor to check the HV 
before even removing the connector from the power supply from the cabinet. 
I don't want a direct short for the cap as Don stated.

Much of what I do is learned from elmers of long ago who have passed now, 
but most of their wisdom lives on in others.  One had a 10 uf, 15 kv oil cap 
that he charged, then shorted with a screwdriver.  You probably have seen 
the fireworks that generated.  It will give one pause around HV.  He then 
waited about 20 minutes and shorted the terminals again.  Another bang but 
not as brilliant as the first.  The second was enough to seriously hurt you 
or make you hurt yourself.

All the commercial shorting sticks I have seen use phenolic sticks about 36 
inches long with a resistor in it, usually the higher the voltage, the 
higher the resistor.  They were supplied with broadcast transmitters.  This 
is something everyone who works with 600 volts or more of should be careful 
when servicing.

73  Jim
W5JO



> This is also true Jim.  This is why I use a VTVM with a 1 meg resistor in
> the probe.  I make up a special resistor probe divider network in addition
> to this to do measurements of HV with.  I occasionally check the 
> voltmeters
> on the front of the transmitter for calibration using this lash up. But it
> also makes a very good pre-checker for voltage before trouble shooting. 
> Of
> course if the transmitter is all working correctly and I just want to 
> check
> something or make a modification which I do on a regular basis, then I 
> watch
> the meters fall when I flip the plate switch off.  After they have reached
> Zero volts then I insert the shorting bar or stick to insure that the 
> plate
> voltage can't be activated again with out tripping the breaker HIHI.
>
> PS: the bleeder resistors had best be radiating heat after the plate 
> supply
> is off or I suspect trouble.
>
> John, WA5BXO
>
> -----Original Message-----
>
> True, but should something be wrong with the meter following a disaster, 
> it
> might not show a charge.  What an arc would be drawn if 3 KV remained
> somewhere and you gave it a direct short.
>
> I have always seen a high value resistor in these things.
>
> 73   Jim
> W5JO





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