Question with ID-5001 Weather Station

[Steve Harrison]

At 11:34 AM 11/27/98 EST, Robert S. Capon wrote:

>The chassis is painted black on the outside, and on the inside there is
>considerable paint "overspray".  There are a number of ground lugs
attached to
>the chassis, and I'm concerned that there won't be a good ground
connection to
>some of the lugs.  The manual doesn't call for sanding the chassis, so I'm
>wondering if all of the ground lugs will be connected later with wiring,
or if
>I should go ahead and sand the chassis.

Ground lugs in Heathkit gear were, in my opinion, one of the Archille's
heels of many kits. I do remember reading instructions to scrape away the
paint before installing such ground lugs on many kits; but my own concern
was how to do so cleanly? If there are any cosmetic problems inside some
Heathkits, it's likely to be where paint had to be scraped away with the
tip of screwdrivers or other sharp objects to make certain even a toothed
ground lug would really bite into the metal.

Even unpainted aluminum and steel chassis parts are not exempt from the
scraping requirement, however. I acquired a SB110A some years ago that
motorboated badly when the audio was turned up. Remembering a couple of
tips to try loosening and retightening all the mounting screws of the PC
boards, that was the first thing I tried but to no avail. I even recapped
all the bypass caps on the IF and audio board... with no noticeable effect
whatsoever. I wasted TWO MONTHS trying to track this problem down. I even
removed all the PC boards, carefully used steel wool on the copper ground
traces that were to contact the aluminum chassis when the boards were
bolted in place, and sandpaper on the edges of the aluminum chassis that
would contact the copper traces. Absolutely no effect whatsoever. Finally,
I sold the radio in frustration. I'm kicking myself in the back end even
now at that failure.

The new owner finally found and fixed the problem: it was the ground lug
located under the RF gain/audio gain pot on the front panel that grounded
the shield of the RG-174 cables carrying audio to and from the audio
control. He had to remove the pot, sand the bare aluminum under the lug and
the lug itself, and reassemble.

AND THAT WAS BARE METAL.

So: where you are told to mount a ground lug, YES: clean off the paint
underneath. After all, the ground lug isn't being placed there for
decoration: it's SUPPOSED to ground to the chassis. MAKE CERTAIN IT REALLY
DOES! Use your VOM on its lowest resistance scale to make absolute certain
that the resistance is as low as possible. Then WRIGGLE AND TWIST the
ground lug to make certain it REMAINS grounded to the metal.

I've noticed that some Heathkit aluminum chassis seem to have a coating of
aluminum oxide that protects them and prevents unsightly aluminum corrosion
over the years under ordinary household environmental conditions (not,
however, on the West Coast, such as the Los Angeles basin where there may
be heavy salt-air fog for thirty days of the year). If you very-lightly
place your VOM meter probes on such a chassis, you may find that the probe
tips are not punching through the outer layer of aluminum oxide
sufficiently to make metallic contact with the underlying conductive chassis.

As for the instructions on scraping away the paint: I bet that if you
thoroughly read the very basic instructions at the front of the manual on
how to assemble the kit, there *will* be a statement somewhere in there
that you *should*, indeed, scrape away any paint where a ground lug is to
be installed.

As for tieing all the ground lugs together with a single wire: DON'T!!! In
the case of sensitive, very-low-current or low-voltage-differential
circuits such as may be in your weather station, you could easily upset the
grounding point potential for a circuit, causing small differences in
ground currents from one circuit to flow through the ground points of other
circuits and thus upsetting precise offset currents or voltages. You just
might never be able to get the station to properly read or display one or
more sensors until you remove the wire tying all the ground lugs together
and properly sand the chassis.

If you are really concerned about cleanly scraping the paint away on the
chassis, and you have a good grinding wheel and some spare drill bits in
your shop, try grinding the tip of a bit to create a paint scraper. You
would leave a blunt pilot tip in the center that would fit inside the hole,
and which would have a shank that, when you rotate the modified drill bit,
would cleanly and evenly scrape away the paint around the hole to the bare
metal.

I've never seen such a tool marketed anywhere other than by Vector
Corporation; their device, available many years ago, was designed for use
on copperclad board. You would drill a component mounting hole in
unperforated copperclad board (actually, the tool was intended for use on
Vector's own copperclad perfboard for prototyping circuits), then stick the
tool's bit into the hole and carefully rotate the tool by hand. The shank
had two sharp cutter edges that would cut away the copper around the hole,
leaving a small insulated pad of copper around the hole itself to which you
could solder component leads. Unfortunately, the tool is way too small to
use with holes larger than those for about 0-80 screws, however! But I've
made my own using grinding wheels and old, dull drill bits.

73, Steve Ko0U/1

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