Metal film resistor wattage
 

I want to replace a suspect 150 ohm, 1 watt carbon composition resistor. I have a junk Hitachi chassis with a metal film resistor of the correct value but I'm unsure about the wattage; seems physical sizes of these things are all over the place. If it helps at all the resistor I hope to salvage is R432 on a Hitachi NP83LX chassis.

Looks like at least 1W, possibly 2W. Should be good to go.

Awesome, thanks.

One small point to consider. Film resistors don't handle high surge currents as well as composition.

https://electronics.stackexchange.co...r-short-pulses

Fortunately if this is a poblem they should just fail open circuit without much drama.

Good t0 know. Maybe I should mine one of my non-op sets for one.

The suspect resistor is on a convergence board, got so hot from a previous user's tampering that it baked off every stripe but the green one.

Metal film should be OK in that position. The modern choice to replace composition resistors is metal oxide. Looking at a modern resistor it can be quite hard to tell the difference between oxide and film. if you can see the spiral under the glaze it's definitely film but you can't always see that.

The place where composition really worked well was surge limiters in, for example, the anodes of HT rectifiers. Now you'd use oxide or wirewound.

On a related point the surge limiters in series with rectifiers need to have a rather higher power rating than you might think. They're not only carrying the DC current but also the ripple current.

Ah, I had heard of that but didn't know the difference between oxide and film.

Metal film, metal oxide film, I get a lot of both on Google. Gah, it's crazy, no wonder I got confused.

The problem with metal film resistors is that the actual resistive element is a relatively thin spiral of material. This can burn out relatively easily from surge currents. Oxide and composition resistors are basically a mass of material which is less prone to burning out this way.

Good to hear it in lay terms, I still have a lot to learn. I guess it's no good to cherry-pick them from junk boards, I never really know what I'm getting.

There aren't that many carbon comps in this set, they number into the low 30s at most. Most are on the convergence board, geometry board, video output board and in the HV stage. The resistor complement of the main chassis boards is mostly carbon film. I've seen advice to replace 1/2 watt composition resistors with 1-watt metal oxide, but that's probably only good for tube devices. This is an early SS set; I was told not to exceed rated wattages in those because they can act as fuses to protect more critical components.

In just a few cases resistors are specified to act as fuses. Usually in the power supply area. Some are obvious - wirewounds with a soldered contact thatwill melt and spring apart. Others are not, just specially rated parts that will go open circuit peacefully. In modern kit these will be marked on the schematic along with warnings in the service data to replace only with the same type.

Convergence circuits are unlikely to need fusible resistors.

That must be what's meant by "critical safety resistor". There are a few in this set, mostly in the horizontal output stage, some of which look like ordinary carbon comps. They're marked on the schematic with an exclamation mark in a triangle.