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Replicating old (dogbone) resistors

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Forum » Technique, Repair, Restoration, Home construction ** » Repair and restoration: Tips and Tricks » Replicating old (dogbone) resistors
           
Georg Richter
Georg Richter
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29.May.06 02:29
 
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Replicating old (dogbone) resistors

Author: Sylvain Vanier © 2002, All rights reserved.

Permission to be posted at RMorg by our honest member Sylvain, thank you very much!

Material list:

Putty epoxy

Dog bone resistor

Baby powder

Artist's acrylic paint OR Model paint (enamel)

Silicone (spray can) OR LPS oil spray can (Used in lieu of baby powder as a mold release)

Small paintbrush

Concept of reproducing type 1 dog bone resistor

These resistors, usually found in 1930's radios, originally consisted of a carbon composition encapsulated in ceramic or a similar substance. The wire leads are turned around the edges (see picture 1). These resistors came in various sizes, you can chose to create the mold from either a defective resistor or from a sample resistor from a radio. The process of creating a mold from the original is non destructive, however, in some cases the paint might be damaged or removed. Merely note the value of the resistor should the process remove the paint so you can repaint the original colors.

Some resistors built prior to RMA standards may bear different color codes. If you chose to mold replacement resistors in advance for stock, which may be used in a pre-RMA radios you could write the value of the resistor on the unit before they are painted.

Dog bone resistors and color code used:

MAKING THE MOLD

Although there are specialized products for making molds, like RTV
(2 parts fast setting silicones,) casting rubbers and silicones, I prefer to use the same product used for the dog bone resistor bodies, in this case putty epoxy.

The putty epoxy is fast curing, drying hard in less than 5 minutes. Other types of mold making products could be used but considerations must be made for heat resistance of a product subject to heat in a radio.

The old dog bone resistors came in a variety of shapes. Some were cylindrical or round others were hex shaped and a third style have metal end caps.

This article will only explain the replication of the hexagonal-shaped ones (the other varieties can be done as well.) Another option would be to use the case of a BIC pen or similar cut to the correct length and having two pieces of 18-20 gauge wire wrapped around the ends to create the form for the lead wires.



Cut and mix the 2 parts together until the color becomes uniform. Cover your hands with talcum powder to keep the epoxy from sticking to your fingers. You will feel heat buildup as you mix the putty. If the color is now uniform it's ready to mold the resistor.

Shape a nice oval blob of the epoxy and press it on a hard, flat surface. The epoxy may stick to a surface so I recommend using a waxed paper.

Check it by pressing a finger nail in the putty. Cover the resistor with talcum powder to provide a release agent then press it lengthwise half way into the putty. Wait about 5 to 10 minutes for the putty to harden.

SECOND HALF OF THE MOLD

Next, cover the bottom part of the mold and resistor with talcum powder then prepare another blob of putty about the same size as the first one.

Then place it on top of the lower half of the mold:



Mold, both halves showing:



REPLICATING RESISTORS

Choose the appropriate size wattage and value modern resistor to be molded in the dog bone form. If leads on the replacement resistor are too short, you can solder longer leads on before molding.

New style metal film resistor:



Metal film resistors are highly recommended but carbon film composition resistors can of course also be used. The metal film resistors come in all values and are usually smaller in shape for similar wattage compared to the carbon film ones. Benefits are; higher precision and stability (1%), are impervious to humidity and more stable (value wise) under heat. Drawback is they cost more than double the carbon type. But we are talking about a dime a piece here.

Next step is to cover the mold with a bit of talcum powder first before putting the putty in. Just blow the excess powder, press the putty in. If you forget the talcum powder, you're in trouble. You will never be able to part surfaces without damage !

Mix enough putty to fill in the mold. Press fit the new resistor in the first half of the mold after filling it with a bit of putty.

New resistor in first half of mold:

Fill in the other half of the mold with the remaining putty then firmly press both halves of the mold together.

Both halves ready to be pressed together:

Press firmly to get the excess out, trimming of the "flashing" is necessary once the resistor is removed from the mold.

Maintain pressure on the mold until the excess (flashing) becomes hard to the touch. This will take about 5 to 10 minutes depending upon room temperature.

Using a sharp bladed instrument separate both halves of the mold and carefully remove the resistor.

Removing the excess of flashing from the resistor is easy. Use an X-Acto knife or similar handcrafting knife to trim the "flashing" off the resistor. This is a one minute job and doesn't need to be perfect, the resistor will be painted after, so any tiny scratch or defect will add to originality. Look closely to the original resistor…NOT perfect, it's 1930's technology here, kinda sloppy, right ?

Unmolding the -new- resistor, can THIS be called NOS ?


PAINTING THE BODY

I recommend the use of artist's acrylic paint.

However, if a glossier finish is desired you could use plastic model enamel paint. I haven't tried using modeling paint so be sure it will tolerate the temperature it will encounter.

Finished product:



Can you spot where it is?





Remark by GR: http://www.oldradioz.com/old_index.htm is worth a visit
until http://www.oldradioz.com/index.htm is repaired.

This article was edited 29.May.06 16:44 by Georg Richter .

  
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