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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 7th Aug 2020, 11:26 am   #1
G6ONEDave
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Default Capacitor voltage rating

Hi, I hope that someone out there can help with this. I have a quantity of 3000pf (0.003uf or 3nf) red tubular ceramic capacitors, possibly made by Erie. They do have the capacitance printed on them but no voltage rating. Probably 1960's vintage and were available from Radio Spares amongst others. I would like to use them in a restoration to replace some Hunts brown capacitors.
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Old 7th Aug 2020, 1:42 pm   #2
Terry_VK5TM
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Default Re: Capacitor voltage rating

At ~15 mm length and 1960's vintage, I would put them in the 100V class.

They are quite prevalent for sale on the auction sites - Radiospares Hi K ceramic capacitors - but their original packaging doesn't mention voltage.

I did come across a valve circuit in Radio Control Models and Electronics (September 1960) using them with a 90V HT.
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Old 7th Aug 2020, 1:49 pm   #3
paulsherwin
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Default Re: Capacitor voltage rating

I agree, I wouldn't use these for high voltage applications.
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Old 7th Aug 2020, 1:54 pm   #4
dazzlevision
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Default Re: Capacitor voltage rating

If these are "Radiospares" Hi-K types (or the OEM version), then they are probably designed for mains HT working, such as in TV and radios.

Photo from a Radiospares 1970 catalogue - but these were being sold long before then!
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Old 7th Aug 2020, 2:25 pm   #5
paulsherwin
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Default Re: Capacitor voltage rating

The ones in the ad appear to be 750V, but I would still be wary.
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Old 7th Aug 2020, 4:59 pm   #6
Alan_G3XAQ
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Default Re: Capacitor voltage rating

I have a hi-pot breakdown voltage tester I use for valves and vacuum capacitors. I wonder if a lash-up version would be viable for your capacitors, assuming you are prepared to sacrifice one?

If you don't have a variable HT supply available then maybe taking a sample of the highest voltage in your restoration project would do? Connect several high value resistors (usually 300V working) in series with the capacitor and put them across the HT [insert H&S disclaimer here]. Switch on. If the cap releases its magic smoke then you know it is unsuitable. If not you could be more scientific and add a microammeter to measure leakage current and gradually ramp up the HT while looking for any sudden jump in current.

Or maybe the mods and others are going to tell you not to even think of trying such an experiment?

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Old 7th Aug 2020, 5:28 pm   #7
G6ONEDave
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Default Re: Capacitor voltage rating

I must admit that I thought they would be good for 300vdc, not thought of trying a HT test though. Might have a go at that, as it only needs a tx, rectifier and elec cap, oh and a safety fuse or cut-out to protect the secondary parts against possible shorts. I am not worried about trying the test and it would be useful to know for future projects. Usually I would replace like for like but in the case of certain brands, it's not worth taking the chance imho.

The set that I am working on is the well known Ekco TMB272 and I was originally hoping to replace those pesky Hunts 3nf at 400vdc with these ceramics. As it happens I found some 3n3 630vdc polyesters, so am using those instead.

I would also like to add that I have been looking through the invaluable previous threads on the Ekco TMB272 and now realise just what a poor condition mine is in. However, I am determined not to let it get the better of me.
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Old 7th Aug 2020, 6:55 pm   #8
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Default Re: Capacitor voltage rating

The Erie 1965 cat lists their Hi-K Tubular Ceramicons as 500V d.c. working and Flash Test 1500 V d.c. but their stated sizes do not quite agree with your pictures i.e 0.54" x 0.255" (13.72 x 6.48mm) and 0.8" x 0.255" (20.32 x 6.48mm) - the RS ones seem closer.
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