[R-390] Newb question, slugs

[Roy Morgan]

On Feb 8, 2016, at 12:29 AM, Larry H <dinlarh at att.net> wrote:

> Hi Jason,  Welcome to the list and congratulations on the R-390A.
> The slugs are connected to the adjusting screw with a flexible coil tightly wound heavy wire.  It should only flex a little bit or it will bend and not easy to straighten up.  If it is bending at all, it is probably binding in the coil form.  Be very careful trying to clean the inside of the form - water and oil will damage it.  See doc online on this subject.  To see if it binding, remove the 2 small screw holding the adjustment plate on the rack and try to move it up and down by hand.

Jason,

The above suggestion will tell you if the slug is sticking in the coil form as it moves UP and DOWN.

WAs you are tightening the two screws removed from the slug plate, move the plate LEFT and RIGHT and FRONT to BACK to center the adjustment screw over the center of the slug.  Then the spring will not be bent and there will be no or minimum flexing of the spring as the slug moves up and down.

This “plate” we are talking about is a more or less triangular plate with rounded corners and the threaded adjustment shaft and  two screws that hold it to the slug rack.

Occasionally a spring will break at the slug. A small bead of epoxy will re secure it.  (NOT cyanoacrylate “super glue" - use epoxy such as JB Weld.) Clean the end of the slug and the spring.  Let no epoxy extend over the edge of the slug (or remove it if it does) - the slug must be perfectly smooth with no bumps or junk on the outside surface.

>  Source of sticking can be 1. foreign material in the form, 2. overheated coil form, 3. who knows.

If the slug is sticking, clean the coil form with Q tips, MAYbe with a bit of Windex or alcohol (just a tiny dampness on the Q tip - not a soaking).  Then if you want to go whole hog, apply a bit of car wax or butchers wax to the inside of the coil form and to the outside of the slug.  Work on the slug gently so as to not break the spring.


You asked:
> Is there a section of "piano wire" in between the slug and it's adjusting
> screw? Are they really supposed to deflect? Or is this thing just threaded
> brass, which is fatigued and about to snap?

It’s very much like piano wire (the thick kind that looks like a spring) but there is no central core of solid steel wire - it is a hollow spring, likely made of phosphor bronze or some similar material.  They are supposed to flex slightly if needed but only very slightly.  It is not threaded brass.  (In AM car radios of old that were slug tuned, and home radios of the same construction, the slugs were moved by relatively thin solid wire just like single strand piano wire.  In the great majority of slug tuned IF cans in radios, the slug is moved by a threaded rod, but the slug stays put once adjusted and there’s no need for a flexible connection to a separate adjusting screw.)
 
Carry on - you have good questions.

Roy

Roy Morgan
k1lky68 at gmail.com
K1LKY Since 1958