EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

Some of the dogbone resistors have been removed from my allwave fifteen and replaced with a collection of rather dubious types, as it is my intention to tidy things up I want to replace these resistors with types similar to the original, realising tha new old stock may have drifted as far out as those in service left me with a problem,
After trawling through the Internet I found this
"http://www.radiomuseum.org/forum/replicating_old_dogbone_resistors.html"
The results are pretty spectacular, I used metal set epoxy for the mould and a type called milliput for the finished resistor, this takes longer to set than metal set and has a finer texture,
I used paste beesxax polish for mould release and 1watt metal film resistors, the finish is good enough to see the Color code paint from the original in the mounded resistor
Once painted it' s very hard to tell new from old. As in the photo.
Mike

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Mike;

That is a job well done! They look just like the originals and have the advantage of new parts inside with closer tolerances and correct values. I have also found that metal film resistors are quite reliable and stand overheating quite well as long as it is not for long periods of time.

Joe

Nice

Just fitted a couple of homemade dogbone resistors, Only problem is matching the colours.
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what the previous "repairer" fitted, I could not leave the chassis like this, there is an awful lot more to do.
Trying to work out why he has tried to be so precise with the values when the original resistors were 20% tolerance.
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Mike;

Yes, going for extreme close tolerance does not buy much as you commented. I do use 1% metal film types, not because the circuitry needs them but because in many cases they are less expensive than greater tolerance parts. In this situation, they are to be enclosed within your dogbone molded material, so no one would know. However, using two test selected resistors to get to a very accurate value is not helpful to the performance of the radio and makes for a rather unsightly appearance. There are a couple of aspects of metal film and wire-wound resistors that are beneficial: low intrinsic internal noise; positive resistance increase with advance of temperature (within specifications of the specific part). Carbon composition resistors have greater intrinsic internal noise in circuit. They also tend to go down in value under high heat and temperature conditions (when used in high wattage voltage dropping or load applications). This can lead to an avalanche decrease in resistance which can result in the resistor catching on fire - not good.

Of course in an AM type receiver, lower internal noise is not really an issue since the atmospheric noise is far greater. The noise produced by the various types of resistors is normally only an issue in applications such as preamp circuits which deal with very small signals.

This radio is going to be a beauty internally as well as externally. You do great work!

Joe

Thanks for your comments Joe, I have used a couple of resistors in each dogbone, firstly to spread the wattage, even though the original resistors were low power the replacements are potted in epoxy,

the second reason and I think the more important is the working voltage, the series I tend to use is around 300v per element, so doubling up is the way to go.

Mike

Joseph W Strickland said:

Mike;

Yes, going for extreme close tolerance does not buy much as you commented. I do use 1% metal film types, not because the circuitry needs them but because in many cases they are less expensive than greater tolerance parts. In this situation, they are to be enclosed within your dogbone molded material, so no one would know. However, using two test selected resistors to get to a very accurate value is not helpful to the performance of the radio and makes for a rather unsightly appearance. There are a couple of aspects of metal film and wire-wound resistors that are beneficial: low intrinsic internal noise; positive resistance increase with advance of temperature (within specifications of the specific part). Carbon composition resistors have greater intrinsic internal noise in circuit. They also tend to go down in value under high heat and temperature conditions (when used in high wattage voltage dropping or load applications). This can lead to an avalanche decrease in resistance which can result in the resistor catching on fire - not good.

Of course in an AM type receiver, lower internal noise is not really an issue since the atmospheric noise is far greater. The noise produced by the various types of resistors is normally only an issue in applications such as preamp circuits which deal with very small signals.

This radio is going to be a beauty internally as well as externally. You do great work!

Joe

Mike;

Yes, considering the voltage rating is just as important as the other specifications. People often forget to look at the voltage rating. It is not usually an issue with transistor and IC circuits, but in valve/vacuum tube applications it is. This is especially true of any variable resistors used in B+ circuitry. Sometimes a metal enclosed variable resistor that is required to serve in a B+ circuit has to be mounted to an insulating mounting material instead of having its case connected to chassis or circuit ground. I ran into that when dealing with an AC signal balance control for signals from a phase inverter to push-pull audio output tubes in a hi-fi amplifier. The resistance range was correct, but if mounted on the chassis it arced to chassis ground. After I mounted it on an insulating plastic surface the problem went away.

In the case of B+ dropping resistors or load resistors in B+ circuits, the DC voltage has to be considered plus any AC signal voltages that may ride on top of the DC. Sometimes it is enough when added together that it can cause an arc-over around the resistor, particularly with physically small 1/4 and 1/2 watt resistors, if they are in a high signal level situation. That is especially true in RF transmitter, modulator and oscillator circuits.

Joe

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