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Why did they do this????

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throbscottle

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I'm repairing my 'scope again. Found a couple of resistors gone high feeding the focus control. The control isn't affected so I'm leaving them in place for now, but I want to understand why they used the combination they did.
The design dates from late '60's so please bear this in mind.
The 2 resistors are in series and probably have about 1kV across them. One is carbon composition, the other I'm not sure what it's made of - carbon or metal film I think - it's unlike any other resistor in the 'scope, long and narrow. The composition one should be 330K and the possibly film one should be 470K, so 800K in total. The 330K is the only composition resistor in the 'scope. I think they're both original but I'm not sure. Most of the others are carbon film.
Any suggestions why these types of resistor would have been used? I'm thinking maybe the tempco's cancel out, but why bother here? It's working happily with over 1meg where it should be 800K.
 

MikeMl

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... The 2 resistors are in series and probably have about 1kV across them. ...
The max voltage rating of a standard axial-lead resistor is way less than a kV.
 

ronsimpson

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One is carbon composition
I had a "law" that any wire going to a CRT must have a Carbon Comp resistor in it. While the voltage might be low; there are times then the tube sparks inside and voltages get very high for a short time. Old tubes should be mostly over that but the new tubes would "snap crackle and pop" some times. The CC resistors can handle high watts for a short time while the film resistors opened up or changed value.
 

throbscottle

Well-Known Member
Mike - the CC one is pretty big and would have around 412V across it, the other one is extra long so it's probably a HV special. (Oho! I've just realised it gives me a way to measure the HV without a multiplier and without exceeding my meter's rating!)
Ron - That's interesting. I think I've come across that before. But I didn't notice any other CC resistors. I will have to take the back off again and have another look. Certainly a better explanation than anything I could think of.
 

schmitt trigger

Well-Known Member
From what I remember, CC resistors were very poor performers.

However they were non-inductive and had a superior surge withstanding.
 

throbscottle

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They also have the opposite t/c of film types, I believe, hence my original theory
 

MikeMl

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There are "fusible" resistors which if overheated fail by increasing their resistance at least 100:1 (i.e. they burn open). There are "standard" resistors, which can fail the other way, and go into thermal runaway because their resistance decreases with overheating.
 

throbscottle

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It's the oxide ones that are fusible isn't it?
I've never heard of a resistor going down in value!
 

Mosaic

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Metal film resistors also make good fuses. Too bad the manufacturers don't issue a spec for that. Discovered that when using a gaggle of them to make a 0.1 current sense.
 

MikeMl

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Mosaic

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Hmm:
R≤2.0Ω
Fusing time within 60 seconds at 36 times of rated power
R>2.0Ω
Fusing time within 60 seconds at 25 times of rated power
Fusing residual resistive value at least 100 times rated resistance

Sounds like a poor fuse solution there.
 

MikeMl

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Read the part about how they fail 'open' gracefully without first going into thermal runaway.
 

dr pepper

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I thought a gaggle was the same as a score - 20
 
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