[TenTec] Station power supply??

[Stuart Rohre]

Jim Brown has stated a very important point.  I think the folks at 
Astron are not thinking of some problems that bonding the negative of 
the DC supply to chassis can introduce, and in fact were introduced by a 
colleague who did bond the negative to AC and chassis ground on a club 
station Astron 20 amp supply.

What happened was a lightning strike to the Antenna tower and especially 
to a vertical Lo Band VHF ground plane sitting atop a beam array for HF, 
VHF, and UHF.  (Yes I know, a rotating ground plane, but that is another 
story, as the Low Band belongs to our host organization for the club 
station.)

The strike sent induced current along shields of the coax to the station 
where the coax was bonded to the building red iron roof girders, which 
was bonded to both earth ground, water pipe ground, and electrical 
conduits to 3 wire outlets. It appears that the surge that was induced 
in the conduits traveled to various radio gear and in this case through 
the AC ground of the Astron, to the primary of the power supply where it 
blew out the MOV across that line.  The current in the chassis also 
traveled through the negative lead from chassis and AC ground, and 
destroyed the 3 terminal voltage regulator.  In addition, there was 
enough chassis and negative lead current to enter a dual band radio and 
vaporize the negative power trace by arcing to the chassis,  while 
completing a circuit to the shield of the coax connected to the tower 
and its ground rod which was bonded by no 6 copper to the AC and water 
pipe ground point some 10 feet from the tower.

Current was spreading to and dissipating in many directions and paths.

The over voltage crow bar tripping risk here was handled in the 
rebuilding by putting a disk ceramic cap across the plus and minus DC 
output.

By the way, each radio chassis, and tuner chassis was bonded by braids 
to a copper/ bronze wide bus bar that ran behind the radio desk and 
around the room to the exit point for grounds.  It served as the 
grounding point for telephone switch protectors in that room, as well as 
the radio station coax arrestor panel.

By tying the negative of the Astron to the chassis, we ground looped
ourselves into possibly causing more damage than might have been 
experienced if the DC buses had been floating.  The VHF radio tied to 
the Astron was the only radio that suffered internal damage. And it was 
solely the vaporized negative trace.  The lightning may have hit the VHF 
beam coax connection, but the Low Band ground plane was the highest 
object on the tower.  That antenna has a double shielded coax feeding 
it, and after the event, no shorting was found in any of the beams nor 
the ground plane, or any coaxes.  With its folded unipole construction, 
the low band antenna probably shunted much current to the coax shield. 
Of course, this was a weekend storm event, and no one was at the club 
station to see what exactly transpired.

-Stuart Rohre
K5KVH