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

[Mike Hyder -N4NT-]

That is good information with regard to RF, I suppose.  If you examine the 
older Astron RS-35 diagrams, you will notice that the negative post is not 
connected to the chassis and that the negative post is connected directly to 
the voltage regulator chip.  In repeater service static discharges from 
nearby lightning can go directly into the regulator causing it to fail.  I 
do not know best practices but can tell you that my older Astron RS-35 would 
go out of regulation from RF entering the negative post and coupling 
directly to that chip.  The result on SSB was considerable distortion.

Stuart, I am curious as to whether you and your friends modify newer Astrons 
to remove the connection from the negative post to the chassis.

I never worked on any local repeaters here but understand they had regular 
failures of the Astrons which their trustees attributed to nearby lightning 
strokes.  They reported further that grounding the negative post to the 
chassis eliminated the problem.  They are reliable guys but I do not know of 
their experiences first-hand.

I am not recommending anything, just citing my experience with that supply 
and relating the advice Astron gave to me.  You can argue theories but you 
cannot argue with my personal experiences.

73, Mike N4NT

----- Original Message ----- 
From: "Stuart Rohre" <rohre at arlut.utexas.edu>
To: <tentec at contesting.com>
Sent: Friday, September 23, 2011 6:58 PM
Subject: Re: [TenTec] Model 963 questions (what can happen when you ground 
loop the Astron output)


> 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