identifying carbon composition resistors - Page 1
Forum section: electronics / repair

author: mightyohm
author_rank: Regular Contributor
timestamp: September 28, 2023, 05:37:05 pm
content: 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?

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2023, 05:50:49 pm
content: 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.

author: Gyro
author_rank: Super Contributor
timestamp: September 28, 2023, 07:09:07 pm
content: 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

author: mightyohm
author_rank: Regular Contributor
timestamp: September 28, 2023, 07:49:01 pm
content: Quote from: Gyro on September 28, 2023, 07:09:07 pmMine has a different style resistor, see attached.

author: mightyohm
author_rank: Regular Contributor
timestamp: September 28, 2023, 07:50:31 pm
content: Quote from: TimFox on September 28, 2023, 05:50:49 pmExcellent - 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.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2023, 07:55:26 pm
content: Quote from: mightyohm on September 28, 2023, 07:50:31 pmOn 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.

author: Gyro
author_rank: Super Contributor
timestamp: September 28, 2023, 08:42:49 pm
content: Quote from: mightyohm on September 28, 2023, 07:49:01 pmThat 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.

author: mightyohm
author_rank: Regular Contributor
timestamp: September 28, 2023, 10:40:21 pm
content: Quote from: Gyro on September 28, 2023, 08:42:49 pmI'll leave it be. Thanks!

author: mightyohm
author_rank: Regular Contributor
timestamp: September 28, 2023, 10:41:58 pm
content: Quote from: TimFox on September 28, 2023, 07:55:26 pmI'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!