[Hallicrafters] Re: sx115 backlash

[Rocco Lardiere]

John,

This is NOT normal.  The normal "feel' of the main tuning should be light
and precise with no hint of backlash (the traditional term for this
condition) or sponginess.  Sounds like yours had an existing problem in this
area - hence the previous kludge, which should be removed.

First, make sure the dial lock is operating properly and not binding the
tuning train.

Generally, the cause of backlash  is due to something in the tuning gear
train and or capacitor not wanting to rotate freely.  I suspect the gear
train first and the capacitor second, but backlash points back towards the
capacitor.  One thing to suspect is the VFO variable capacitor bearings and
rotor contacts.  By loosening a semi-inconvenient set screw or two (use the
correct wrench - might be a Bristol Spline, in which case you may want to
invest in an Xcelite 99PS-60 or other fine set), you can free the capacitor
rotor or get close to it in the gear train and carefully rotate it with your
fingers (being ultra careful not to bend any plates on the cap or bend the
fragile red pointer!)  If it is tough to move, you have found the part of
the problem.  If it is smooth and free (find a good variable in your junk
box and get a feel for what is reasonable, if you don't know), then the
problem is in the gear train somewhere. I use DeOxit D5 on the spring rotor
contacts.  I use a teflon-based product made by Liquid Wrench called Super
Lubricant for the bearing points in the capacitor and gear train.  The trick
is to lubricate the bearings in the capacitor, but not the rotor contacts,
which should only see the DeOxit.  Others may have their favorite
lubrication products; I like teflon, because it tolerates heat very well.

Often the grease in the capacitor shaft bearing can dry out, and the rotor
needs to be free.  One warning - be very, very careful that any spray gets
sucked into the intended spot and cannot drip onto something visible and
vulnerable, such as a dial!

If you have found the problem to be the capacitor and all of this has not
loosened it, you may need to adjust the ball bearings of the cap (common on
SX 101's, BTW).  At the back of the cap you will see a red-painted shaft and
nut with the slotted shaft accessible from the back.  The idea is to
slightly loosen the nut, slightly back off the slotted shaft to loosen the
bearing races against the bearings, and then tighten the nut.  If you back
off too far, all those tiny bearing may fall into your receiver somewhere,
and good luck finding them!  However, if you just back off a bit, the shaft
should loosen, and you can lock it into just the right position (no play,
but free rotation) with the nut (which will try to retighten the slotted
shaft, of course).  Do all of this ONLY if you must.  It's easier than it
sounds.

Again - the capacitor itself should rotate smoothly and freely with no hint
of binding.  Usually you need to work the cap through its rotational range
several times to clear out the gunk after lubing/cleaning.

Gear train - you can loosen some set screws to find out what else might be
binding, understanding that you will need to carefully mechanically realign
the cap to the dial readout when you finish.  This gets into alignment, but
you are already familiar with that.  I lubricate every easily accessible
metal-metal bearing in my receivers' tuning train, and the difference is
usually dramatic.  The only exception is plastic or nylon gears (such as on
Drakes), that can absorb the lubricant and swell, causing more binding.  I
yield to the lube experts on what to use and where, but this method works
for me.

The SX 115 has an idler arm for the end stops that rolls on a shaft-mounted
disk to the right of the VFO - that might be causing friction if it is not
free to move and roll againt the disk.

SOMETHING in there is tight, and the backlash usually points to something at
the other end of the gear train (i.e., at the capacitor end).  Watch out for
mechanical alignment of the gears themselves, gunks of dried grease, etc.

Good luck - this problem can and should be fixed!  Just be very careful
around the fragile bits.  This is a VERY common problem with older
receivers.  And this is why they forced us EE majors to take mechanical
engineering courses (auto mechanics would have been a good idea, too).

Rocco N6KN

----- Original Message -----
From: "John Leer" <jleer at execpc.com>
To: <lardiere at ix.netcom.com>
Sent: Wednesday, December 18, 2002 3:48 PM
Subject: sx115


> Rocco, I have just completed repair and alignment of a very nice sx 115.
> I used the manual and your notes and the alignment seemed to turn out
> well. I have one problem you may have encountered. When tuning the vfo
> wants to spring back for want of a better term. I don't know if this is
> common to this receiver or peculiar to mine. The former owner has wedged
> a piece of sheet metal against the flywheel to act as a friction brake,
> but this seems like a hokey remedy to me. Do you have any suggestions?
> Thanks, 73 John K9XH
>
>