In circuit resistor testing?
 

Hey guys, whats the best way of testing resistors in circuit? I am using a digital multimeter. When I test a resistor, the rating of it is supposed to be 470K, i test it as 470M. I I pull it out of circuit, it reads correct..
Thanks

In general, you can't check a resistor in-circuit. Also, digital meters do very funny things when measuring resistances in-circuit. However you can verify if the reisitor is "definitely OPEN", or "MAY BE OK" with an old analog meter. You just can't measure "DEFINITELY OK". If you measure it at value or less than value, it "may be OK". If it measures higher than value, it probably is open or increased in value.

The other caution is that you may accidentally blow the meter by measuring a resistor which may be open, so your meter will get the charge from a capacitor somewhere upstream because the open resistor couldn't bleed that charge down. Always put a clip lead across any resistor for a second to short out any charge before you go try to test it in-circuit. That should protect you from a meter blowing Digital meters will still sometimes read funny because of harmonics in the circuit.

Charles

Oh heres the deal.. When I turn the color down all the way pure black and white picture, I can see some smearing to the right of text and images. This can olly mean that the color circuits must be ok. When I turn the color up, this color circuit is trying to process the smearing. So which circuit can cause the smearing with the raster display?

Have you checked the emission of the crt? A gun with low emission will make a smear of that color. Since you had other problems as well be sure all the power supply voltages are correct.

Bill R.

Well im still waiting on my crt tester. Some of these resistors measure off. Im suprised this set works! a 220k resistor measures 227k... Wow many of these resistors are slightly out of tolerance. What do you guys render a resistor as out of toleance, does this amount matter?

Resistors with the standard color code will have a last color band that is silver (10% tolerance) or gold (5% tolerance). I can't recall if no band means 20%. By the time I started in 1966, even places like Motorola consumer were using 10%, and better places like Zenith were regularly using 5% parts. So, 227K vs. 220K is just fine. Many circuits were built with 10% resistors unless it was found that 5% was really needed to prevent rejects in final test. Anything requiring closer tolerance than 5% usually meant you put an adjustable control in the circuit. On top of that, the carbon composition resistors varied in manufacturing such that 5% parts would be selected out of the batch, leaving 10% parts that were either high or low, but never nominal.

On carbon comp resistors, a gold last band is +/-5%, a sliver last band is +/-10%, and no last band is +/-20%

Generally, checking resistors in circuit is not possible. To some extent you can, if you follow the schematic while doing it. If there is no other DC current path across the resistor, it should measure normal. if it measures too high (out of original tolerance), that generally means it's bad, or at least lift one lead and try it again to make sure you really saw what you thought you did. If there IS another DC current path, you have to lift a lead. Theres no other way.

You can troubleshoot most problems by checking DC voltages. Start with the power supply. When you look at the schematic (if its a sams anyway) there will be DC voltages listed at several power supply taps, and on the elements of most tubes. You are looking for stuff that is way off.

Well tonight in my fury of resistor testing and replacing I came accross an interesting one, and im not sure if these are avaliable anymore, so if you know where I can get it, please let me know. It is a 27k 2w heat ceramic heat resistor, number R734. there are 3 of them, by my schematic, they feed the R G B into the neck of the crt. Or the prints call it AA Z and Y. This open resistor feeds Y, which is blue, but oddly I had a blue raster.. Hmmm

Well I don't have a schematic in front of me (and sadly its packed away deep), but IIRC those are the wires to the control grids. You would still have a blue raster I think(?).

Old color sets like the 16 have to produce a good b/w picture before the color signal is added back in. The (b/w) video is fed to the cathodes (it's called the y signal). The guns all have to track.

The chroma signals are added to the mix at the control grids. These are r-y, g-y, and b-y. As an example, b-y literally means "blue minus y", or blue minus the black and white component of the picture. As you turn the chroma control up, more of these three signals are added to the (already adjusted perfectly) black and white picture.

As for the resistors, if they're wirewound ceramics they aren't that hard to get. Otherwise, they might be metal film (I cant remember. pics would help). Those arent hard to get either. Anyway, look on mouser for "ohmite brown devils" if they're wirewound, and just "metal film" I suppose if they're those blue things marked with regular color bands. More wattage never hurts as long as the size is reasonable.

Well I got an update, remember how I was telling you guys about the smearing and the left side green raster, well I found an open power resistor that feeds y or blue amp. This resistor is a 27k 2w ceramic heat resistor with vent holes in the sides. Placement is r734. Assuming by the dark areas on the board these get darned hot! should probably replace r732 red amp and r736 green amp resistors as well. Do you have any idea what these resistors are called so I can order new ones?

Unless there's a controlled temperature tolerance, just use a regular white ceramic wirewound resistor of that value & wattage. At least for testing purposes. It probably does have a special characteristic to control the value over a wide temp range. The worst that will happen if you DO need a special resistor and can't get one will be that the gray scale will change slightly as the set warms up. Somebody with the schematic probably will ring in on this.

Charles

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I can get a pic of the resistor, but I took the old one and broke it open, sure enough, it's glass inside, solid, guess it is a ceramic comp resistor. this resistor is brown to light tan, and has color bars on it. It's about 8mm in diameter. I found them on mouser as ceramic composition resistors. Thanks for the help again guys!

Hey guys,
The resistors on my set are brown, im replacing the out of tolerance ones with radio shack 1/4 watt resistors, significantly smaller. Are these going to be okay? Im not sute of the wattage on the standard resistors in my set, I know the 1/4 inch resistors are 1 watt. But there are the smaller ones that are glossy and then there are these non glossy ones with thicker bands. Im replacing the borh of those types with the 1/4 watt resistors.

Most resistors in older TV's were at least 1/2 watt. The 1/4 watt would not be a good idea because they are under rated and will overheat and burn out.