Hi Lee:
At 03:36 AM 10/23/2005, you wrote:
I am still considering what my *grown up* rig will eventually be after
I've outlived my *first* rig. I always seem to go back to the K2 and that
is where my head and heart are. But the devil keeps popping up,
whispering in my ear that I'll never be able to strip and mount so many
toroid coils (and that I'll probably zap an IC).
My main worry is with the stripping process. The recommended method is the
melted solder thing where you tin and strip at the same time. But how do
you know you have not tinned the enamel, since the whole thing is covered
in solder?
Good question... easy answer... it's kinda like trying to 'wet down' an
oily plate... the water refuses to stick to the plate because of the oil.
Same for enameled wire... the enamel will not allow the solder to adhere to
the copper beneath it! The solder just rolls off.
Tinning a wire which is covered with HEAT-STRIPPABLE ENAMEL is not a
difficult task... though it does take a bit of time... heat... and possibly
a good technique.
There are (generally) two types of insulating enamel... heat-strippable,
and NON-heat-strippable (often referred to as FORMVAR-insulated wire).
Formvar is in insulation VARnish which is very(!) resistant to heat and
which you will generally have to scrape off the wire before it can be
tinned. In general, most Formvar insulations are dark (often reddish-)brown
in color.
On the other hand, heat-strippable insulations are generally much lighter
in color (golden or a clear red or green), and they will melt (and turn
black), offering up the bare copper beneath the insulation, once the right
amount of heat has been applied to them. From experience only, it appears
that the 'right amount of heat' must be in the range of 700 deg to 800 deg
F (370 deg to 425 deg C).
You can remove the insulation in any of a number of ways. I'll discuss a
few below:
1) SCRAPING it off with a (semi-)sharp blade. Hold the blade
against the wire at right angles and pull the blade to
the end of the wire, stripping off small strips of
insulation with each pull. Repeat all the way around the
wire, taking care to NOT NICK THE WIRE was you strip the
insulation.
The main problem I see with this method is that one can
impart small nicks to the wire which may eventually
result in weakening of the wire at the nick, especially
if the wire is in a location where it may be subject to
being flexed back and forth or in a connector which may
be installed and removed frequently.
The other possible problem with this method is that, if
you are not careful in blade-stripping of the enamel, you
will leave lengths of un-stripped enamel which can (if
not burned away during the subsequent tinning process)
result in a very poorly tinned wire.
2) SANDING it off with sandpaper, emery cloth, or some other
similar abrasive medium. This method can work pretty well
if you take the time to do it right. And it will not
cause damage to the wire as a result of nicking with a
blade. This method does often take some time to do
properly and, like blade stripping, can result in
'blotchy' tinning if you don't apply enough heat once you
tin the wire.
3) BURNING it off by use of a flame. This method seems to
work pretty well but once you have successfully melted
and burned the enamel, you still have to then clean away
the blackened residue which still adheres to the wire
itself. you CANNOT merely solder through the carbonized
insulation. This method takes some time to accomplish
properly, but it will generally result in a solderable
wire. You DO have to use some care when using certain
types of flame... the flame may be so hot, and
uncontrolled, that you not only succeed in burning off
the enamel but you also success in 'hardening' the copper
to the point that it becomes brittle and will break much
more easily once it is placed into use.
4) MELTING/BURNING it off by using the heat of molten
solder. I tend to prefer this method, though some find it
a bit more difficult to accomplish.
The 'tricks' (if there are some) to this method are using
a soldering iron which is hot enough, getting the melted
solder TO the COPPER where it will quickly spread the
heat, and being patient a bit patient.
If you have a temp-controlled soldering iron, you'll want
to set the tip temp up a bit higher than that which you
use for normal soldering. I tend to use a tip temp of
about 700 deg F (370 deg C) for soldering, but I bump
that up to about 750 deg F (400 deg C) when I'm melting
heat-strippable insulation. If you don't have a
temp-controlled iron, chances are that your tip temp is
already at or above the required temperature, so you
probably won't have to worry about tip temperature.
Additionally, a larger (width-wise) is recommended,
though a fine tip will work. You want to melt a blob of
solder into a clean iron tip, so it will be in good
thermal contact with the tip of the iron and so the iron
can continue to provide heat once the wire being stripped
has been inserted into the 'blob' of solder.
Heat transfers to the wire being stripped much more
easily if it can come into contact with a point of BARE
COPPER rather than if it is required to pass through the
insulating enamel in order to get to the copper. As a
result, once I have melted a small blob of solder onto
the tip of my iron, I'll start the stripping process by
inserting just the CUT end of the wire into the solder
blob, allowing it to come into direct contact with the
molten solder and to begin heating up the enamel.
Within a few seconds of the bare copper touching the
molten solder, I'll see some smoke and the enamel will
begin to bubble up. At this point, I'll slowly insert
more of the wire length into the solder blob, again
waiting for the enamel to melt and burn away. I continue
this until I have melted the enamel as far up on the
length of the wire as needed, and then I slowly remove
the wire from the solder blob by backing it out of the
blob just as I had inserted it. Te end result is a tinned
wire with a bit of 'dross' (burnt enamel) remaining at
the upper end of the tinning and possibly a bit of burnt
enamel on top of the tinner portion of the wire. I can
now use my thumbnail to remove any races of remaining
burnt enamel from the tinned wire.
Finally, if you happen to have a heated vacuum-type
desoldering tool, such as the Hakko 808, you can very
easily strip your wires by merely applying a bit of
solder to the nozzle of the stripper, then slowly
inserting the cut end of the wire into the nozzle. Almost
instantly, the enamel will start to melt and burn away,
tinning the wire as it is inserted further into the
nozzle. Then, once you have inserted the wire for enough
to fully tin it, press the vacuum switch while you remove
the wire from the nozzle. Almost too easy.
Finally, when tinning toroidal inductor leads, try to tin them up just a
bit PAST the edge of the toroidal core!!! Many builders stop tinning once
they reach the bottom edge of the core. This all too often results in some
unstripped wire being pulled down into the PC board hole once the toroidal
inductor has been installed and its wires have been pulled taut in
preparation for soldering. You do NOT want unstripped wire inside the
plated-thru holes of the PC board. Doing so can result in poorly soldered
joints which can (and will) fail at a crucial time. When I tin toroidal
inductor leads, I'll bend them out at right angles from the body of the
core, tin them almost all the way up to the body of the core, and then,
once they're cool and cleaned, bend them back down against the body of the
core. Of course, you don't want to risk having leads tinned in this manner
shorting out against each other, but the way in which Elecraft has you
winding your toroidal inductors, helps to ensure that the leads will remain
well separated one from the other, so shorting between individual tinned
leads should be a minor concern if they are installed into the right PC
board holes and if they are then pulled taut before they are soldered.
I see I've gone on much too long (again). So I'll stop here. But just
realize that tinning heat-strippable wires is NOT difficult, and certainly
not impossible, if you just use some care in the process.
73,
Tom Hammond N0SS
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