[Heathkit] Differntial earth

[Steve Beveridge]

Greetings folks,

This little snippet from Duane is an interesting/unfortunate observation
which has a very powerful message.

>due to lightning getting into the Shack via my ground rod system. I did
>not disconnect the ground during an  electrical storm. Unhooked all
>electrical connections from wall outlets,  disconnected all antennas >and
set switches to ground enable the Poly Phasers etc. You ground >them to
protect them and them must disconnect the ground to
> protect them! Sigh.

Everything seems to be right, but the problem was probably inside the
"boxes". In effect, poor earth connections between boards within the radio,
etc. can mean that there are small (probably very small) resistances between
the original earth and the "earth" which the board sees. At the sort of
potentials associated with lightning strikes, this means that there is a PD
between true earth and the earth as seen by the board - thus a 'differential
earth'. We know that PCB screws come loose - as may be seen by the myriad
'fixes' which simply involve re-tightening PCB screws.

I have been doing a total rebuild on a TS-850 which suffered a serious
lightning surge through the house wiring - not mine fortunately. This was a
'not-commercially-repairable' moment-of-weakness purchase (from the
insurance company) on my behalf.

There were quite substantial earth tracks 8 mm wide (or ~1/3") which had
simply been blown away, leaving gaps of 3 mm (1/8"). In one instance, there
were three blown sections in no more than 25 mm (1") of track. I am not
exactly sure how the rig was earthed, but suspect it had been earthed
through the PS, which was blown beyond repair. Most of the SMD transistors
on the IF board looked like little carbon blocks (and tested the same!).
Electros 150 mm (6") away had been blown up. A protection diode (on a
different board), nearly as big as my (average-sized) little finger, was in
full conducting mode.

It seems as though if everything goes "up" then "down" (with respect to
genuine earth) at the same time, there is no (little?) damage. However, even
a slight 'lag' in the return to true earth potential leads to damage. That
energy has to go somewhere.

A colleague (ex RF design and manufacturing engineer) always uses the outer
braid of RG58 to earth boards within his radios. Doesn't rely on the typical
push-on connectors. Sounds like a lot of hard work, but it seems to do the
trick.

The 850 is nearly there, but this has been a valuable learning foray into
something I had always taken as a fairly simple aspect of out hobby - the
earth!

Cheers
Steve
VK2XWL

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