[Hallicrafters] SX-100 (and other) gear train tips

[Roy Morgan]

On Feb 28, 2015, at 10:28 AM, james.liles at comcast.net wrote:

> Good morning Roy, 
> 
> I am always interested in tips, tricks, and experiences with these radios.
> Please share what shortcuts or make it easy information that you have with the SX-100 gear set or anything else.  May save a lot of time for us.
> Kindest regards Jim K9AXN

Jim,

Thanks for the invitation to share some tricks.  My notes file had my notes and also other ideas from Greg posted some years ago, so here are his ideas also.  These ideas follow from two phenomenon of radio nature:

- It’s a little known principle of the physics of field theory that a magical and troublesome field of attraction exists between small parts such as anti back lash springs, shaft circle clips, and tiny setscrews and an unseen force that hides in unknown and inaccessible parts of the work room.  Definite methods must be put in place to thwart the evil workings of this force.

- Common telephone cable wire, usually No. 22 plastic covered, has so many uses that one effort to catalog them all ended in the well-meaning worker being sent to the loony bin.  Only one use of such wire is mentioned here, partly in order to avoid any unpleasant influence on my mental health.



From: WA1KBQ at aol.com
Date: Sun, 23 Sep 2007 08:52:14 EDT
Subject: Re: [Hallicrafters] SX-88 Gear Train Rebuild
To: hallicrafters at mailman.qth.net

In a message dated 9/18/2007 3:40:52 P.M.  Atlantic Standard Time, 
roy.morgan at nist.gov writes:  (not the current email address)
Has anyone dis-assembled  an SX-88 gear train?

I have one to do and wonder what tricks there are to  put the anti backlash 
into the split gears and keep it there during  re-assembly.

I have done a couple. It's the only way to clean and properly  lubricate the 
bearings but they are a little tricky to get back together. When  you have it 
apart be sure to look at all the bearing balls with a magnifying  glass to see 
if any are flat spotted. The last one I did had this condition and  needed 
them replaced. I was able to locate the correct size replacement bearing  balls 
at McMaster Carr Industrial supply in Atlanta available in a box of 100.  

The main consideration for the SX-88 gear train is the reduction gears have to be  
timed so the end stop cams engage the followers at the precise time (position of gear train rotation). This will involve some trial and error (meaning disassembly and reassembly) as you try different gear tooth engagement positions to get them timed properly on both MT  and BS.  (Main tuning and Bandspread)

It was easier for me to leave the springs out for this step and 
put them  in after I had it right. Find a suitable pick to push them in place 
but be careful to have a means of containing them as they will quickly get 
away, fly  across the room and ruin your day trying to find the little wayward 
spring  again. These springs are not like any used in other common receivers so  
replacements would need to come from another SX-88. 

(Notes from K1LKY:  

1)Passing a length of dental floss through the spring, encompassing one or two of the spring turns and having its other ends tied together and fastened SECURELY to some solid thing will prevent the springs from sproing-ing off to unknown places.

2) Judicious use of No. 22 telephone cable solid conductor wire on the split gears will allow you to set the gear train parts into position without any spring loading - the wire is wound around the gear from one side to the other and passes through the teeth of the gear with the spring compressed one tooth of rotation.  Once the gear train is assembled, you can then cut the wire and remove it, allowing the springs to get into action.  More than one (or MAYbe two) teeth of preloading will cause excessive friction and rough feel to the gear train.  THOROUGH cleaning and greasing of the ball bearings and LIGHT oiling of the matching faces of the split gears and sleeve bearings will result in a system that has NO backlash and a very light touch in operation.)

Also, be careful of the preload on the shaft end bearings, apply just enough to accomplish zero end play. This adjustment *may* change when the receiver heats up - it would be a fine point to check this after a thorough warm up period.

When  the SX-88 reduction gear train is clean and adjusted properly and sparingly lubricated in the right places there are none finer when you give that silky  
smooth flywheel weighted knob a spin down the band, and then tune back and forth through a steady carrier in CW mode.

73, Greg

Further tips on the ball bearings and gears:  
 - If you discover flattened, rusted, or mis-shapen balls in the mechanism, it will be worthwhile to get new replacements.  Replacing all of the balls in one set is advised to avoid trouble from the new and old ones having slightly different diameters.  A terry cloth towel on the work bench will keep them from rolling away under the influence of that mysterious attractive force.
 - I used “One-Lube” grease on the radio I worked on.  This is synthetic grease available in grease-gun tubes at the auto store. (A modest investment gets you a ten-lifetime supply,) I suggest careful cleaning, and just barely enough grease in each race to hold the balls in place as you do the re-assembly.   
 - If I remember rightly, there’s a single bearing at the end of each shaft under the “preload” axial end play adjustment.    Very careful adjustment of this part pays off to reduce backlash. 
 - In the world of R-390’s, folks sometimes find enough tooth wear and burrs to need treatment: careful wiping of the gear face on fine crocus cloth against a very flat surface will remove the little burrs. (Follow with careful cleaning.  Some R-390A’s were in use 24/7 for years, tuned frequently to find “interesting signals”.)

Note related to the URM-25D signal generator:  Frequency shift when pushing and pulling the frequency set knob indicates looseness in the shaft end bearing.  Careful adjustment will cure the problem.  The F- model URM-25 has a different mechanism as I remember.

Roy

Roy Morgan
RoyMorgan at alum.mit.edu
K1LKY Since 1958