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.

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.

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...

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

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.

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

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

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

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.

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.

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...

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.

Thanks! Great tip!
Rob

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...

So after all the resistors have "broken-in" we change them.

Playing devil's advocate, why not re-use them as the higher value they read in other applications? Has an old resistor stopped aging?

What would prevent them from continuing to rise in value? Using the newer tech resistors that don't drift would seem to be a far better choice, and what, we're talking nickels per component.

I don't think he ever implied that it was a substitute for testing. He said first inspection. Like having a flat tire. If you are anal you will check them all with a gauge and insist on it. If you're schmart you will give a quick look and know that the one that is flat on the ground and the car is sitting low on is most likely out of air to some extent. Also I know as a tech in another field, often folks who replace everthing really can't diagnosis so it is their way of hiding that and it works although tis very hard on the customers pocket. For me with an old radio I might use a couple of time a year if it works decent I am happy and don't give a rats tail if two resistors have drifted in value. In fact they were probably never right anyway. It was a good tip. First look , quick check. Give the man a cigar. Tis helpful if one uses it.
donVe3LYX

The analogy to tires above, ^, isn't really a good comparison, since you can visually see an underinflated one, whereas determining a resistor's value can only be done with an instrument.

None of my various radios get used very often, but regardless, when I do a restoration, every cap beyond micas gets replaced, and every resistor gets measured. Those that are relatively within tolerance as per each resistors 4th band, or lack thereof, stay, and them that don't, go.
I just can't leave a known -to be considered marginal at best- component in the circuit, and consider that the set's been refurbished correctly.