In the early days of television one of the common plastics used was Cellulose Acetate Butyrate commonly referred to as CAB.   Some formulations of CAB experience a high degree of decomposition and most experience at least some decomposition ranging from a chalky surface residue to complete crumbling of the part. 

TV collectors have experienced decomposition of CAB  in knobs and deflection yokes.  It is not uncommon to find an early color television with a completely disintegrated deflection yoke cover. The following tutorial will demonstrate how to make a reproduction deflection yoke cover to replace one which has disintegrated.  Many deflection yoke covers look as bad or worse than the photo of this cover which came from a CBS 205 Color set.  In many cases the yoke cover is crumbling and falling totally apart.

You will first need to obtain the following materials.  I buy my materials from Walthers Model Trains in Milwaukee.  They have an online catalog here   http://www.walthers.com/ and you can order the materials I use on their web site.

You will need Evergreen .080” Black Styrene sheet (Walthers #269-9117)  and Evergreen .040” Black Styrene Sheet (Walthers #269-9115)  and a bottle of Plaststruct plastic welding cement (Walthers #570-2)  These styrene sheets are 21” by 7” and are large enough to provide the material to make a yoke  cover for a  21” round tube color TV.  In this demonstration I will be making a cover for a CTC4 yoke, but the procedure is similar for other yoke covers, and can be adapted as needed.

Start by removing what is left of the original disintegrated yoke cover.  If the cover has any metal rings or other metal parts, save all of this because it will be re-attached later after the new cover is fabricated.  Measure the outside diameter of the body of the deflection yoke where the cover will be fitted to.  Take several measurements and find the diameter at the largest point. (deflection yoke bodies usually have some variance in the diameter)   Add .080” to that measurement.  This measurement will be the outside diameter of the styrene disc we are going to create in the next step.

Take a sheet of the .080” styrene sheet and cut off a square piece about ½” larger than the diameter of the cover disc you are going to make. 

 

In this photo I have divided the sheet into 3 equal pieces. Each one is 7” wide.  This sheet will make 3 discs.

 

The next step is to find the center of the piece by drawing a diagonal from corner to corner and scribing an “X” in the center of the sheet.

If you have a lathe follow these instructions:
Drill a 3/8” hole in the center of the styrene square. 

 

Then I mount the styrene in a special jig I created for use on my machine lathe. 

 

I use the lathe to create a perfectly round disc with the exact outside diameter I need. 

 

Then I use the cutter on the lathe to cut through the plastic disc and create an inside hole of the correct diameter.  The finished item is a styrene disc with a large hole in the center which the neck of the picture tube fits through.

If you don’t have a lathe follow these instructions:
With a good quality drafting compass which has needle points on both ends, set the span of your compass to ½ the outside diameter you calculated in the step above (yoke body diameter + .080”).  Using the compass scribe a circle onto the square piece of .080” black styrene sheet you cut earlier. Be sure to also mark the center point of the circle.  Now you need to also scribe a circle for the diameter of the center hole.  Using a tin snips do a rough cut of the circle leaving the disc about 1/8” larger than the scribed line.  At this point you are going to have to trim the last remaining 1/8” of material in a very precise manor.  If you have a grinder, or a belt or disc sander I would use one of those to precisely trim the disc to the scribed line.   You may also to possibly use a Dremel tool with a sanding drum.  In any case the object is to carefully and precisely trim the styrene disc down to the line you scribed with your compass.  You will also need to cut the center hole, and trim it to the line you scribed earlier.  This hole does not need to be quite so precise because it only allows for the neck of the crt to slip through it.  For a CTC4 yoke the hole is 2.5” in diameter.  You could buy a hole saw for your drill and cut the inner hole with it.

Set aside the disc we just made and lay a sheet of .040” thick black styrene on your work table.  We are going to make 2 strips of styrene which we will use to form the edge of the yoke cover.  One strip will be the exact width of the edge of the old cover and the second strip will be .080” narrower than the first strip.  In the case of the CTC4 yoke in this demonstration, the width of the edge of the old cover was .625”, so we will make one strip of .040” thick styrene .625” wide by 21” long, and one strip of .040” thick styrene .545” (.625”-.080”=.545”) wide.  The wide strip forms the outer edge and the narrow piece becomes the inner reinforcement piece.  As shown in the photo, I use a dial caliper set to the correct width to scribe a mark the correct dimension for scoring. 

Then using a utility knife and a steel straight edge, I score the styrene sheet.  Make at least 2 deep scores into the styrene using the utility knife. Once you have scored the styrene place the score over the edge of your work bench and bend at the score line.  The styrene sheet will “snap” at exactly the score line much like you would do to cut a piece of window glass.

Lay the disc we made previously on your work table and take the wider strip and wrap it around the OD of the disc.  We now need to trim the strip of styrene to the correct length so it is the exact length to go around the OD of the disc and so that both ends of the strip make a tight butt joint. 

After the strip is trimmed to the exact length needed, form the strip into a circular band and place a piece of plastic electrical tape on the outside face of the band so that the ends of the strip butt tightly against each other.  Now place the circular band around the outside edge of the your  disc.  The styrene band should fit snuggly around the disc without any sloppiness or gaps.  Adjust the edge of the band and the disc so that the edge of the band is flush with the face of the disc.

Now take the Plaststruct welding cement and coat the inside joint of the disc and band with the welding cement.  Wait a couple minutes and turn the assembly over and coat the front side of the joint with solvent.  Let the assembly sit overnight until the joint is fully set up.

The next day when the joint is fully set, take the narrower strip of styrene that we cut earlier, and place it against the inside surface of the outer edge band.  Cut this strip to the exact length needed to fit snugly inside of the outer band.  When you have a good fit install the inside band with it’s joint 180 degrees rotated from the joint on the outer band.  Now take your Plastruct welding cement and coat the joints liberally so that the inner and outer bands fuse together into one piece and also fuse to the disc.  Set aside for 24 hours to allow a full cure of the joints.

Now that the plastic cover is fabricated, you can take files and sand paper if you desire and file and sand the joints to make them clean and uniform.  I use a stationary disc/belt sander to clean up all the edges and then hand sand for a nice smooth finish.

For appearance sake you can use a semi gloss black spray paint to make the assembly look better.

 

This is what the finished yoke cover looks like prior to attaching the magnet shields.

 

In the case of the CTC4 yoke being demonstrated, there are some metal shields that we removed earlier that need to be attached to the new plastic yoke cover.  The parts are held together to the old rotten plastic cover, with brass rivets.  I use a side cutter to snip through the old rivets and discard them.   Using the metal disc I locate where to drill holes to re-attach the metal shield parts.   I use #4-40 solid brass screws and nuts to fasten the magnetic shield components to the plastic disc. I use non-ferrous fasteners because steel screws might interfere with the magnetic characteristics of the deflection yoke shield.

Now I place the cover onto the deflection yoke and mark where I need to make a notch for the wires.  I make the semi-circular notch using a Dremel tool and a miniature sanding drum.  And I also file off the barbs that are on the body of the yoke which held the old yoke cover from falling off.  If made properly this replacement cover will fit snugly around the yoke body and should not need any fasteners to hold it in place.  If the one you make is loose, I would use electrical tape to hold it in place, or a couple solid brass 4-40 by ¼” screws threaded through edge of the cover and into the body of the yoke.

Here is a completed photo of the CTC4 deflection yoke with its’ new cover.

 

Good Luck on your project!