identifying carbon composition resistors

[mightyohm]

Hey all,

I am restoring a Tektronix AM501. The service manual shows dozens of Allen Bradley carbon comp resistors in the BOM. However, I suspect that the unit in my hands is a later production item because the majority of resistors on the board appear, to my eyes, to be a film variety.
In the attached photos, it safe to assume that the more modern dogbone (?) shaped resistors are carbon/metal film, while the cylindrical resistors near the large blue filter caps are definitely carbon comp?

Any other suggestions for identifying possibly suspect carbon comp resistors in old equipment?

[TimFox]

I see a few of the old-style A-B CC resistors in your second photo: two 2.7 k/1 W (near the filter caps) and two smaller ones that might be 2.7 ohms (I can't read the colors well).
The others, as you suspect, appear to be film, but it's not clear if they be carbon-film or metal-film in conformally-coated packages.
Incidentally, the white objects in your third photo are NP0 ceramic capacitors in the true "dog-bone" package.

[Gyro]

The CC resistors in the second photo don't look to be using their high surge or low inductance properties (look like reservoir cap discharge resistors and ?). It's a fair bet that all of the 5% tan bodied resistors are 1/4W carbon film (that I've ever seen anyway) and the closer tolerance bluish greenish bodied, metal film.

It's worth looking out for any CC resistors in high frequency or output stage, but as far as I can see, it's only a 5MHz device.

Looking at the photo on TekWiki, there appears to be another one near the front panel, I'm not sure if it matches yours as yours has solder resist where the TekWiki one has gold plating...

https://w140.com/tekwiki/images/0/03/Tek-am501-side.jpg]https://w140.com/tekwiki/images/0/03/Tek-am501-side.jpg]https://w140.com/tekwiki/images/0/03/Tek-am501-side.jpg

[mightyohm]

Looking at the photo on TekWiki, there appears to be another one near the front panel, I'm not sure if it matches yours as yours has solder resist where the TekWiki one has gold plating...

https://w140.com/tekwiki/images/0/03/Tek-am501-side.jpg]https://w140.com/tekwiki/images/0/03/Tek-am501-side.jpg]https://w140.com/tekwiki/images/0/03/Tek-am501-side.jpg

Mine has a different style resistor, see attached.

[mightyohm]

I see a few of the old-style A-B CC resistors in your second photo: two 2.7 k/1 W (near the filter caps) and two smaller ones that might be 2.7 ohms (I can't read the colors well).
The others, as you suspect, appear to be film, but it's not clear if they be carbon-film or metal-film in conformally-coated packages.
Incidentally, the white objects in your third photo are NP0 ceramic capacitors in the true "dog-bone" package.

Excellent - those four resistors are the ones that I plan to test/replace.

Thanks for the tip on the ceramic capacitors. I'm not sure how to properly describe the shape of the package of the film resistors. They look like "normal" resistors to me. 

[TimFox]

I see a few of the old-style A-B CC resistors in your second photo: two 2.7 k/1 W (near the filter caps) and two smaller ones that might be 2.7 ohms (I can't read the colors well).
The others, as you suspect, appear to be film, but it's not clear if they be carbon-film or metal-film in conformally-coated packages.
Incidentally, the white objects in your third photo are NP0 ceramic capacitors in the true "dog-bone" package.

Excellent - those four resistors are the ones that I plan to test/replace.

Thanks for the tip on the ceramic capacitors. I'm not sure how to properly describe the shape of the package of the film resistors. They look like "normal" resistors to me. 

On the film resistors, I believe the normal term is "conformal coated", where the external layer follows the shape of what's underneath.
"Molded" film resistors used to be common:  the outside is a cylinder with molding flash, without the surface relief of the conformal coated units.
Allegedly, the end terminations in molded resistors were more reliable due to the support of the external molded plastic.

[Gyro]

Looking at the photo on TekWiki, there appears to be another one near the front panel, I'm not sure if it matches yours as yours has solder resist where the TekWiki one has gold plating...

https://w140.com/tekwiki/images/0/03/Tek-am501-side.jpg]https://w140.com/tekwiki/images/0/03/Tek-am501-side.jpg]https://w140.com/tekwiki/images/0/03/Tek-am501-side.jpg

Mine has a different style resistor, see attached.

That one is a metal oxide type. It looks as if it runs fairly warm too but they're tough and the PCB isn't discoloured.

[mightyohm]

That one is a metal oxide type. It looks as if it runs fairly warm too but they're tough and the PCB isn't discoloured.

I'll leave it be. Thanks!

[mightyohm]

On the film resistors, I believe the normal term is "conformal coated", where the external layer follows the shape of what's underneath.
"Molded" film resistors used to be common:  the outside is a cylinder with molding flash, without the surface relief of the conformal coated units.
Allegedly, the end terminations in molded resistors were more reliable due to the support of the external molded plastic.

I'll have to keep an eye out for these. I admit that I thought all PTH resistors looked pretty much the same. I guess not!

Thanks!