Resistors?

[MadMan]

Recapping my tv, and testing resistors where I'm able to get them out of circuit. Many have drifted beyond 10%. Looking for new ones, I just now noticed that many resistors for sale are rated to run at 200, 250, or 300 volts. Aren't there a lot of circuits that are running at 300-400 vdc? Should I be concerned about this?

[old_tv_nut]

You have to look at the voltage difference between the two ends of the resistor, not the total B+ voltage.

[Penthode]

A silver fourth band is 10% and a gold fourth band is 5%. No fourth band is 20%.

Drifting resistors should not pose much of a problem. In most cases 50% will work just fine.

I wouldn't get hung up because a resistor is 10% off.

[Electronic M]

Originally Posted by Penthode

A silver fourth band is 10% and a gold fourth band is 5%. No fourth band is 20%.

Drifting resistors should not pose much of a problem. In most cases 50% will work just fine.

I wouldn't get hung up because a resistor is 10% off.

In radio yes, but in TVs 3% outside manufacturer tolerance spec may be the difference between linear on frequency sweep and the opposite of that, or the difference between the factory alignment being good enough and not.

[MadMan]

Most are silver banded, and about 15 are right at the 10% mark and 7 are actually more like 15 or 20% out of whack. And I haven't even tested many yet.

I suppose I'll check voltage across them before replacing then? That's kind of what I'm getting here.

[old_tv_nut]

Originally Posted by MadMan

Most are silver banded, and about 15 are right at the 10% mark and 7 are actually more like 15 or 20% out of whack. And I haven't even tested many yet.

I suppose I'll check voltage across them before replacing then? That's kind of what I'm getting here.

Regarding resistance tolerance: Right at the 10% mark is common and normal for carbon composition resistors, as the production line produced resistors with a wide variance. The 5% resistors were selected out by final testing, leaving resistors that were all higher than +5% or lower than -5%. Then the 10% resistors were selected out. Then the 20% reistors were selected out, and the remainder, worse than 20%, were discarded. So, don't worry about resistors that are just at +/- 10% unless 5% is called for.

As far as the voltage goes, I feel like I would be a little concerned if the resistor was rated for less than 1/2 the B+ voltage, but I would check the voltage in the actual use to feel safe.

Maybe someone here has more intuition about this.

What type of resistors are you replacing the carbon composition resistors with? I think the concern with voltage may be that carbon film resistors may be made with a spiral element, which means some fraction of the total voltage appears between each turn of the spiral and the adjacent turns. More turns means closer spacing, but a lower percent of the total on each turn. Which effect wins out then determines a voltage rating.

[ppppenguin]

The parts that failed the 20% test were simply allocated to the next preferred value. The sequence of preferred values is designed so that can just about always be true.

Carbon composition resistors still have legitimate uses where they can survive higher voltages and surge currents. Also sometimes for low inductance in RF circuits though surface mount parts are usually better still for strays. Otherwise carbon film, metal oxide and metal film are better is all respects. For fixing vintage kit, 1W and 2W film resistors are often a good idea. They are often a similar size to old half watt resistors and will withstand a higher voltage than smaller modern parts.

For higher voltage capability the Vishay VR37 and VR68 parts are excellent. Readily available in UK/Europe, don't know about IS/Canada.

[old_tv_nut]

Originally Posted by ppppenguin

The parts that failed the 20% test were simply allocated to the next preferred value. The sequence of preferred values is designed so that can just about always be true...

Duh - of course! - Thanks for jogging my out-of-tolerance memory.

[MadMan]

Originally Posted by old_tv_nut

As far as the voltage goes, I feel like I would be a little concerned if the resistor was rated for less than 1/2 the B+ voltage.

That's why I asked.

[old_tv_nut]

Originally Posted by MadMan

That's why I asked.

See ppppenguin's note about 1 and 2 watt film resistors as subs for 1/2 watt carbon comp. Sounds reasonable to me.

[old_tv_nut]

In tube gear, a large part of tolerance budget has to be allotted to tube variation; then the stage should be designed with enough head room to absorb that plus a specified resistor tolerance. Stages with higher signal level will have less headroom for tube variation. In the case of video output stages, the tube is typically running wide open at max contrast setting, and therefore there is relatively great variation from tube to tube and less room for resistor tolerances while keeping signal excursions within limits.

Then there are circuits in which fixed resistors set a limit on adjustment range and touchiness of things like V hold, height, and linearity.

Based on that, without a sensitivity analysis of each stage, I would replace resistors that are out of the manufacturers spec, just because I don't know how close the designers cut it and it will help avoid problems when tubes are replaced.