[R-390] PSK-31 and Paint Removal

[Barry Hauser]

Barry --

Some followup on both ...

> Got some aircraft-grade stripper and when the weather cooperates and I can
get a good day outside, I plan to put it to the test.

The stuff I use is in a spray can and says aircraft something on it.  It's
of the caustic (non-flammable type).  I found that the panels have a variety
of paint/primer on them and it can take several applications of the stripper
to get it all off.  Best thing to use is one of those auto body
applicators -- basically plastic putty knives without handles. Apply the
stripper, allow the paint to bubble up a while, but don't wait too long or
it re-sets.  Scrape in full swipes across the panel right off the end to let
the glop slop off.  Before that, put the panel on a box that's somewhat
smaller so the edges are accessable.  If you want to preserve the back side
paint (and silk screening, if any), mask it off, but step the masking tape
in from the edges just a bit so you can strip full around and allow for the
prep, primer and new paint to wrap around the edges fully.

One problem concerns all those holes.  The stripper and resulting caustic
goop falls through and eeks it's way around the back under the masking --
unless you prep each hole with masking tape on the back side.  Then
puncture/trim out the masking tape patches so the slop can drip through.
That box I mentioned before should be open top -- i.e. fold the flaps down.

As for the "engravings", I found the best thing was a nylon auto detailing
brush -- like a giant toothbrush.  They sell these in a set of three --
nylon (or whatever), brass and steel bristles.  Don't be tempted to use the
brass one, it will scratch things up.  Be sure to be wearing glasses or
goggles because the stuff will splash or spit especially when you're using
the brush.  It takes some pressure and "vigor" to work the bristles down to
the bottom of the stampings.

You may well find that the stripper doesn't work on some of the old paint or
primer.  Before restoring to abrasives, try denatured alcohol or some other
solvent.  What was stubborn for the stripper may give way easily to
alternate chemistry.  I used denatured alcohol as the final scrub.  Don't
let different solvents mix -- might be hazardous.  Put down plenty of
newspaper or a tarp -- the fallout is messy and sticky and will take to
concrete quite well.

As you get most of the old paint off, check out the panel carefully.  If
there are a lot of small nicks, a pass with Scotch brite or a fine sanding
pad may work well. This may be a good idea simply to grain up the surface to
hold the primer.  Also, look closely at the "engravings".  The top edges
should be sharp and well defined so that it will be easy to fill them after
painting.  If not, do the Scotch brite thing.   Whatever you do, DO NOT USE
steel wool.  Tiny bits break off in the tiny pits in the panel or simply
fall out and stick no matter how you try to clean up.  That messes up the
paint job.  If there are some really deep gouges, fill with Plastic Aluminum
or that other stuff the guys swear by -- forgot the name of it -- from the
auto parts store.

After the surface is cleaned up and neutralized of all chemicals, ideally
you should immediately prime the panel, as oxidation starts right away.
Originally, I was going to Alum Prep, but couldn't find it readily.
Instead, I went with a self-etching primer.  Raw metal surfaces should be
etched before priming or else the surface may be too slick and result in
peeling later on.  The self etching primer I used was gray of a similar
shade to the finish coat.  This helps, because you've got to go very easy
with the finish or else you'll load up the etchings too much to fill
properly.  That means the finish coats may be a bit thin in places and
somewhat translucent.  So, I think you're better off with gray rather than
yellow zinc chromate or whatever.

If the finish coat goes wrong, don't fret.  There should be plenty of
stripper left in that can and the stuff you just put on will come off a lot
easier than what was there before.

>
> I had my first digital communications experience last night.  Had a cable
and connector appropriate to connect the laptop to the TS440S and was able
to copy PSK-31 sigs on 20M with ease.  I got a 1k:1k transformer and will
make a cable from the R390A to the laptop and see how it goes.  Something
different for a change and it was so easy!

Put the transformer in a small project box.  Run a coax out through a
grommet with a couple of crimpon terminals to the audio out terminals.
Mount a BNC jack in the box.  Find an existing screw or two that you can
safely remove from the back panel and drill holes in the bottom of the
project box to match up. (assembly instructions not exactly in order -- I'm
no Heathkit Inc.).  Fasten project box to rear panel with transformer,
grommetized captive cable and BNC jack.  Now here's the really important
part.  Make up a professional looking label that says something like:
"CV-2002XX  MULTI-MODE DATA INTERFACE" and maybe "Restricted".  Give
everything a coat of polyurethane so it looks MFP'ed.

Now, if someday you go and sell it, we just have to wait a couple of days
for the post, "Say, I just got this R-390A and it has this strange box on
the back of it which says ...."  That's the part where we all get our
jollies.

<sigh>  I'm getting very bad now, very bad ....

Barry (hoping you'll all blame this on the wrong one)