Is this a Carbon or Metal Film resistor?

[gjpmho]

Hi All,

I'm repairing an amp and noticed some of the resistors that were installed in manufacturing don't appear to match the service manual. In this particular case, these resistors are noted as "critical" in the service manual. the part number is CFPS1/4
https://www.koaspeer.com/pdfs/CF.pdf

From the datasheet you can tell the service manual lists a Carbon Film Flame Proof resistor with a 1/4Watts power rating. 

What I'm seeing on the board is green/blue resistor which typically are Metal Film resistors, while orange/brown are typically Carbon Film resistors. I've removed one the resistors (which was burnt). Can anyone tell from these photos if this is indeed a Metal Film or Carbon Film resistor?

Thanks for the help.

[Gyro]

Welcome to the forum.

Yes, the photos do look rather like metal film resistors - but flameproof resistors always look different due to their cement coating rather than the normal lacquer. One way to solve the question would be to remove another resistor of the same type and check its temperature coefficient. With a metal film resistor you would normally expect around 50ppm/'C, a carbon film rather higher (according to the datasheet too), typically by about an order of magnitude.

As a rule of thumb, normally you can measure the resistance of a metal film resistor held in your fingers and measure a pretty stable resistance value (avoiding any leakage effects). With a carbon film resistor, you will see the value slowly drift with the heat of your fingers. If you have one of each type to hand, you can compare too. I can't get an idea of scale from your photos, but a carbon film resistor will typically be larger for the same power rating too, check the dimensions against the datasheet.

As you indicate these resistors are in 'critical' positions, intended to fail safely and prevent other parts of the circuit from catching fire, so you should probably replace with the same flavour of flameproof.

[strawberry]

carbon film resistors usually are not better than 5%

[magic]

I would say the dark resistive layer looks like carbon, MF resistors are grey inside.

Photograph on the first page of your CF resistor datasheet shows a few green ones.

I have seen green resistors in places where there would be no reason to use MF.

[gjpmho]

Carbon Film resistor, confirmed by KOA Speer. https://www.koaspeer.com/pdfs/Appendix_D.pdf

[gjpmho]

Thanks for the help.