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Capacitor substitution question

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lokeycmos

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i have 3 identical capacitors in my old oscilloscope. the ratings are .1uF 1600VDC. they seem to be leaking what seems to be similar to oil. it looks like the 2 next to eachother may be smoothing the high voltage to the CRT, but i may be wrong. i dont have a schematic. my question is, i dont know what these caps are made of, could i safely replace them with these?


https://www.digikey.com/product-search/en?lang=en&site=us&KeyWords=338-1165-ND&x=13&y=15
 

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looking in wrong place?

Small caps (<.5uF) "usually" don't have a fluid to leak. It's electrolytic types... I'd check the area around the "piddle" marks. G.H. <<<)))
 
They likely are wax impregnated film capacitors.

Is the oscilloscope not working, and that's why you want to replace the caps?

The Digikey caps should work as a replacement.
 
Service grade (el cheapo) scopes tended to use voltage multipliers for the CRT voltages vs. a high voltage oscillator as Tektronix or Hewlett-Packard did on their scopes. It is oil leaking as it's probably oil-imprfegnated paper. Those caps are very unreliable. Your sub from Digi-Key appears to be OK as previously mentioned. If this scope has any paper caps in it, replace them with polyester (Mylar) types. Paper caps tend to leak and can often cause more-expensive secondary failures (e.g., open primaries of output transformers in old radios).
 
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I've replaced wax/paper caps in vintage radio's with poly film caps similar to the ones you reffered to, give them a try.
Ripple current and esr can be an issue in some circuits including voltage multipliers which these might well be (does the trace brightness affect the trace height?), that said 2012 poly films are gonna be a lot better than 1960's leaking wax papers.

Change them, run the scope for a while and see if they get warm, if not you sorted out the problem.
 
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I don't see ESR being any kind of a problem in a 1000v, low current circuit. ESR is primarily an electrolytic culprit anyway.
 
Nope maybe your right.
But if you put some tiny ceramic 100n's in they'd probably burn up.
 
Mmmmmm, simple ceramic caps, by their very construction method (a plate on either side of the dielectric with a wire soldered to each one), have some of the lowest ESRs anywhere. It's the cap types that consist of strips of plates and dielectrics that are rolled into cylinders that are very "ESRish-prone" because of both the chemistry in the dielectric and the lousy resistance in the plates and/or connecting conductors. A ceramic cap might arc over or puncture the dielectric because of excessive applied voltage, but I don't see them burning up because of high currents. Ceramic caps used for filtering and voltage multiplication in scopes tend to have values like 0.01 µF (10 nF) and voltage ratings in the 3KV to 5KV range when found in high-end oscilloscopes that use CRTs. In most of those, capacitor failures are very rare.
 
the mylar caps will have a stripe on the body indicating which end has the foil on the outside. this should be connected to the ground side of the circuit (except in a voltage multiplier where only 1 or 2 ground points exist and the rest of the caps are floating). the reason for this is to keep the cap from re-radiating noise, not for any "polarity" reasons, as mylar caps don't have anything resembling polarization. you may want to look through the scope for any more paper caps like that, and replace them all. i have an antique radio i rebuilt as a teenager, and i'd replace one paper cap, only to have another one fail soon afterwards (the first cap to fail protected the rest of them from failure by shorting first). i finally sat down with the schematic, figured out where all of the paper caps were, and replaced all of them with mylars. that's where i learned about the outside foil marking, as the schematic specifically said that the outside foil of the cap should be the ground end.
 
yes dean, punturing a whole in the dielectric might be a more correct definition of what I was trying to say, and probably why the odd time I've had one fail in a valve set I've been repairing.
The issue with valves is start up, a lot of the caps dont have much dc accross them during operation, but during start up when the rectifier heater warms up before the other heaters do there can be large voltages accross the caps.
 
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