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 Post subject: Quick Visual Check of Old carbon Resistors
PostPosted: Dec Fri 16, 2016 12:11 am 
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Joined: Mar Tue 11, 2014 2:54 am
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I have noticed that a quick visual check if any resistors are thought to be opening up can save some time if you are not doing a complete replacement. Often , the color coded "paint" on the bad ones will start to look "bubbly" or "crusty". I do not know if they were a certain manufacture, but they often take on the same look when they are "open". I often start my trouble shooting with such a visual. Open S-G resistors will make a radio completely deaf.


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 Post subject: Re: Quick Visual Check of Old carbon Resistors
PostPosted: Dec Fri 16, 2016 9:15 am 
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It won't take but 15 minutes at most to spec all resistors in a garden variety AA5/6 circuit, which includes disconnecting one end of any resistor paralleling a lower resistance component, such as a coil.

Older carbon comp resistors tend to drift higher in value, and can often be found at 25% beyond their color band rating. There's no visual inspection that would reveal this.

While it's true that a bubbled resistor surface might indicate a problem, why not just do things correctly to begin with?

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 Post subject: Re: Quick Visual Check of Old carbon Resistors
PostPosted: Dec Fri 16, 2016 2:31 pm 
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fifties wrote:
It won't take but 15 minutes at most to spec all resistors in a garden variety AA5/6 circuit, which includes disconnecting one end of any resistor paralleling a lower resistance component, such as a coil.

Older carbon comp resistors tend to drift higher in value, and can often be found at 25% beyond their color band rating. There's no visual inspection that would reveal this.

While it's true that a bubbled resistor surface might indicate a problem, why not just do things correctly to begin with?


I guess that depends on the goal/technical experience of the person doing the repairs. I dont do this for a living, it is a hobby. If you really wanted to get technical, with the age of all the old parts an any of these radios, one should replace every capacitor and resistor under the chassis. Also, I dont think I have ever had a AA5 on my bench, it is always some variety of short wave receiver with a few more parts under the hood.


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 Post subject: Re: Quick Visual Check of Old carbon Resistors
PostPosted: Dec Fri 16, 2016 4:06 pm 
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Dogbone resistors and some brands of later axial carbons can test just fine and look O.K. However, it has been my recent practice to place a soldering iron on the lead, near but not next to the body while watching the ohmmeter. The resistance naturally rises. As soon as it does rise I remove the iron. If the resistor returns to its original value it stays. If it returns higher, even if within tolerance, it goes. The heat test is a check of long term stability. I'm sad to loose a colorful dogbone but returning to the project in the future for additional repairs that could have been avoided is not a better choice...

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 Post subject: Re: Quick Visual Check of Old carbon Resistors
PostPosted: Dec Fri 16, 2016 8:02 pm 
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Chas wrote:
Dogbone resistors and some brands of later axial carbons can test just fine and look O.K. However, it has been my recent practice to place a soldering iron on the lead, near but not next to the body while watching the ohmmeter. The resistance naturally rises. As soon as it does rise I remove the iron. If the resistor returns to its original value it stays. If it returns higher, even if within tolerance, it goes. The heat test is a check of long term stability.

I learn something new here quite often; thx for the tip!

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 Post subject: Re: Quick Visual Check of Old carbon Resistors
PostPosted: Dec Sat 17, 2016 5:42 am 
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Older carbon comp resistors tend to drift higher in value, and can often be found at 25% beyond their color band rating.

I"ve seen them at double their marked value and still looking beautiful. And yet, other similar resistors in the same set are within tolerance. I've also seen some resistors that don't look very good but test fine.

Visual inspection is worthwhile but don't expect it to substitute for actual testing.

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 Post subject: Re: Quick Visual Check of Old carbon Resistors
PostPosted: Dec Sat 17, 2016 5:52 am 
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Jim Mueller wrote:

Visual inspection is worthwhile but don't expect it to substitute for actual testing.

I don't think visual inspection is worthwhile beyond any indication of bubbling or burn marks.

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 Post subject: Re: Quick Visual Check of Old carbon Resistors
PostPosted: Jan Wed 11, 2017 11:30 pm 
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Chas, that is a technique that had never occurred to me. I have utilised cooling agents on suspected defective solid state components but never the reverse on older tube equipment.

An excellent suggestion. Thank you for sharing it.

Tony


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 Post subject: Re: Quick Visual Check of Old carbon Resistors
PostPosted: Jan Thu 12, 2017 4:18 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
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Location: Santa Rosa, CA
A visual inspection is always worth doing.
While its common to see higher value carbon resistors go
up in value I've also seen some go down in value. These
are usually lower value resistors that have been run too hot.
In one recent restoration (Racal RA-17) a 470 ohm resistor
was reading 280 ohms! This is the worst I have seen so far.
Steve


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 Post subject: Re: Quick Visual Check of Old carbon Resistors
PostPosted: Jan Thu 12, 2017 4:35 pm 
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A visual inspection of the entire apparatus should always be the first step. Looking for signs of overheating, evidence of past repairs gone wrong**, bad solder joints, etc. etc.

But, with the obvious exceptions, you cannot find bad parts by their appearance.

With the older resistors, I will test a few. If half of them are bad, I stop testing and just replace them all.



**If something has had the benefit of significant previous repair efforts, it gets a line by line check against the schematic.

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 Post subject: Re: Quick Visual Check of Old carbon Resistors
PostPosted: Jan Thu 19, 2017 5:13 pm 
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The physics of carbon tells us that it has a negative temperature/resistance coefficient. It is very common to encounter a carbon composition resistor that has decreased in value. It is usually a resistor whose coded value is below 50K or below. I personally have encountered this many times over the years.


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 Post subject: Re: Quick Visual Check of Old carbon Resistors
PostPosted: Jan Fri 20, 2017 3:10 pm 
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eeprof wrote:
The physics of carbon tells us that it has a negative temperature/resistance coefficient. It is very common to encounter a carbon composition resistor that has decreased in value. It is usually a resistor whose coded value is below 50K or below. I personally have encountered this many times over the years.

The temperature coefficient of carbon is not a concern, if it was then the radio would have issues on the first day it was turned on. The problem is aging of the resistors.

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 Post subject: Re: Quick Visual Check of Old carbon Resistors
PostPosted: Jan Fri 20, 2017 4:13 pm 
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Location: The Old Dominion
eeprof wrote:
The physics of carbon tells us that it has a negative temperature/resistance coefficient. It is very common to encounter a carbon composition resistor that has decreased in value. It is usually a resistor whose coded value is below 50K or below. I personally have encountered this many times over the years.


Then how does one explain all of the carbon composition resistors (used and new) that drift up by 20% or more?

I've never seen one drift low, out of tolerance. Never. YMMV, but I have a full shoebox of carbon composition resistors that all are out of tolerance, having Increased in value. Multiple brands - Ohmite, IRC, Stackpole, and Allen Bradley...

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When fuses go to work, they quit forever.


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 Post subject: Re: Quick Visual Check of Old carbon Resistors
PostPosted: Jan Sat 21, 2017 2:24 am 
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Joined: Mar Sat 28, 2009 8:27 pm
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Location: Georgia
The resistors will not "drift" lower but will indeed go down in value when in actual operation.. Just ask an old time serviceman who has been servicing for a number of years. The reasons for drifting high is the corrosion of the metal embedded ferrules within the carbon slug ends that the lead wires are connected to.


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 Post subject: Re: Quick Visual Check of Old carbon Resistors
PostPosted: Jan Sat 21, 2017 3:05 am 
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Chas wrote:
Dogbone resistors and some brands of later axial carbons can test just fine and look O.K. However, it has been my recent practice to place a soldering iron on the lead, near but not next to the body while watching the ohmmeter. The resistance naturally rises. As soon as it does rise I remove the iron. If the resistor returns to its original value it stays. If it returns higher, even if within tolerance, it goes. The heat test is a check of long term stability. I'm sad to loose a colorful dogbone but returning to the project in the future for additional repairs that could have been avoided is not a better choice...

Thanks! Great tip!
Rob


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 Post subject: Re: Quick Visual Check of Old carbon Resistors
PostPosted: Feb Sun 05, 2017 6:35 am 
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An overall visual inspection is always a good idea. On my last refurb job, I found a partially cracked carbon resistor. Interestingly, it still measured within tolerance but it was obvious that it had gotten very, very hot. It was on its last leg for sure...

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