[TenTec] Model 963 questions (what can happen when you ground loop the Astron output)

[Stuart Rohre]

Don and the list.  I can give definitive report of the results of 
bonding the negative DC output post to the chassis of an Astron 35 Amp 
and its box.

A retired power engineer helping in our club station did this bonding, 
after some previous lightning events at our club station at Red Cross. 
He and I installed a wide flat bronze flashing ground bus around the 
shack, to which everything bonds, and this flashing goes to where there 
is a conduit in the wall that leads outside to an in ground metal water 
pipe to which the bus is bonded.  This faucet also has a braid to the 
nearby radio tower leg and a driven earth rod, and solid bus wire no. 6 
which is the telephone Demarc earth ground to the same rod at the tower 
leg and follows the bus flashing back thru the wall conduit.  And then 
runs up the inside wall to the telephone surge protector block.

As stated, the bonding of the 35 amp or any Astron negative of the 
linear supply to the chassis, makes that ground common to the AC green 
wire safety ground, which is chassis bonded in all Astrons we have been 
supplied since 1981.  (There are four at Red Cross).

The main one, always on the station's VHF and HF radio, is the 35 amp 
one.  Others are only plugged in on an as needed basis for extra 
stations in drills.

We had a lightning hit to a power pole destroying the top 1/3 of the 
pole, and the surge followed a guy wire of that pole to earth.  (pole 
ground wire was open circuit or burned in two).  This was 100 feet from 
the building where the shack and the 65 foot tower are located.

A branch strike either hit the tower, or came in power lines to the 
building.

Anyway, there was current brought down the coax shields, and onto the 
station common bus.  This current found a return through the ground loop 
of the Astron, and thru the ground foil of a dual band radio, vaporizing 
the DC negative trace to the radio connector, which was common with coax 
shield and case ground.  The current going into the output of the Astron 
negative, produced such a difference of voltage as to blow the MOVs on 
the input AC, and took out the LM 317 regulator chip and some of the 
output power pass transistors that are in parallel. Others were spared. 
  We might have had power transistors and linear regulator IC survive, 
had not the negative of the DC supply been tied to AC and chassis ground 
forming a ground loop.

I have worked in EMC for years. I believe that a disk 0.01 mf ceramic 
cap across the DC outputs would likely by pass the RF effect away from 
the linear regulator.  But, we only run 100 watts.  Adding caps to the 
MOV across the AC line would also help that path.  As I recall, I did 
put a disk on the DC output inside the supply.  I am still trying to 
convince the other ham that we should remove the ground loop.  BTW, our 
coaxes are run inside the BX tower tied to the legs of each section, but 
leave the tower at roof level as we have a roof penetration for all 
feedlines and rotor cable to enter the Radio Room.  Better would have 
been bringing the lines out of the tower at its base, which is still 2 
feet above earth on a concrete base.  Going into the shack right there 
was not possible due to telephone punch down blocks on the interior wall.

-Stuart
K5KVH