Re: 7S14 Dual Trace Sampler Mercury Cell Replacement

Bruce Griffiths

On 09 April 2017 at 22:03 "Clark [TekScopes]" <> wrote:

The Photography guys also have a need for the mercury cells in some
single cell light meters. Their solution? An "N" alkaline cell with a
germanium diode in series. The discharge curves are identical in their
application. Just a thought.

On 04/09/2017 01:39 AM, 'Dennis Tillman' [TekScopes]

> >
Over the years there have been several solutions proposed for
replacing the
mercury cells in the front end of the 7S14 1GHz Dual Trace Dual Timebase
Sampling Plugin. This is an excellent plugin in part because, unlike the
finicky (I'm being polite) 7S11 and 7T11, it works the first time you
try to
use it without 30 to 60 minutes of fiddling with the controls before
you get
a useful display.

Unfortunately the 7S14 has an Achilles Heel. The sampling gates relied
on a
floating mercury cell for bias. Mercury cells were banned a long time ago
and there simply is no substitute. Mercury cells lasted nearly forever. In
fact it was not uncommon to find a 7S14 which was manufactured in 1970
working well into the 21st century. These mercury cell s were lasting over
30 years. Today, there is no battery technology that can come even
close to
that useful lifetime.

Several clever replacement solutions have been suggested over the years in
TekScopes. If I am not mistaken the general concept for most of them
involves a phototransistor and a photovoltaic cell. I have lost track
of the
various proposals that were made, but more importantly I am interested in
finding out which ones were implemented by our members and withstood the
test of time.

The reason I am asking is I realized it might be possible to make a small,
cheap, simple, printed circuit substitute for the mercury cells that
occupies the same form factor and requires only minor surgery on the
7S14 to
install. If this can be done then it will mean there is a way to bring all
those wonderful 7S14s back to life so they can continue to benefit the
of 7000 scopes.. A secondary goal is to prove that Tektronix was
capable of
designing a reliable, easy to setup, very functional sampling system that
didn't require the patience of a saint to get a stable display on the

I attempted to locate the threads / schematics / and user feedback on the
7S14 mercury cell substitutes in the archives but I had no success.
For some
reason the archive is excruciatingly slow to search through when I try
to do
it. If there is something I can do to speed my searches up I would love to
hear it.

If someone who has already downloaded the entire archive can search it and
provide me with the message numbers I will do the rest.

Thanks, in advance,

Dennis Tillman W7PF





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