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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 8th May 2021, 3:14 pm   #1
pentoad
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Default Polystyrene capacitors questions.

Are the old axial polystyrene capacitors (the clear ones with visable foil) generally reliable or do they deteriorate with age?
For example, should they be replaced if they are looking a bit wrinkled or yellow?
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Old 8th May 2021, 3:18 pm   #2
ms660
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Default Re: polystyrene capacitors questions

The "Suflex" types sometimes go O/C or high ESR, less common is them going S/C.

Lawrence.
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Old 8th May 2021, 4:31 pm   #3
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Default Re: polystyrene capacitors questions

I have never had one fail in situ, I have ruined one by keeping the iron on for too long though. I think Lawrence's failures may be because of mechanical stress, still a failure mode.
 
Old 8th May 2021, 5:03 pm   #4
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Default Re: Polystyrene capacitors questions.

THey're usually fairly good.

BUT don't let them go above 70C whether in equipment or storage. THey undergo a permanent shift in capacitance if you do.

David
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Old 8th May 2021, 5:11 pm   #5
ms660
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Default Re: Polystyrene capacitors questions.

In R&TV's, I've had them go O/C and also had them going S/C, in terms of reliability better than paper but not as good as mica.

Lawrence.
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Old 8th May 2021, 5:34 pm   #6
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Default Re: Polystyrene capacitors questions.

Suflex used to go s/c in various positions on a PetoScott 738 tv and if in a rush , just snip them out worked on that model.

I am back to the 1960,s here I add.
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Old 8th May 2021, 6:08 pm   #7
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Default Re: Polystyrene capacitors questions.

The Suflex sometimes went o/c completely and also intermittently I found. I think it was just the larger values though, say 1000pf + that I came across. One mobile radio had about a 2200pf or thereabouts in the noise squelch circuit and was one to look for. Some very high values used in specific af tone oscillators, sometimes many 10s of nF used to go o/c. Most of these larger type were GEC manufacture. Admittedly, the apparatus was not in the best of locations for temperature and humidity changes.
I never much worked on transistor portables, so the smaller ones I can't comment on.
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Old 9th May 2021, 6:49 pm   #8
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Default Re: Polystyrene capacitors questions.

I have had a number of O/C vintage polystyrenes of various makes.
Investigation revealed that the lead-in wires were a press-fit, bound into the foil windings. If the capacitor is not sealed, the connections can deteriorate.
I have also found some that change capacitance when squeezed, presumably because the windings were not tight enough.
Having said that, they are far more reliable than paper types.
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Old 9th May 2021, 7:39 pm   #9
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Default Re: Polystyrene capacitors questions.

Sounds like the usual problem of making reliable contact to aluminium.

David
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Old 9th May 2021, 7:55 pm   #10
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Default Re: Polystyrene capacitors questions.

I'll second the mention of problems with Suflex caps, they had a pretty bad reputation in the TV trade in various chassis as I recall.
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Old 9th May 2021, 11:38 pm   #11
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Default Re: Polystyrene capacitors questions.

Same here with the Suflex type, very unreliable in older TV sets. Never had any trouble with new ones in homebrew projects. Very stable, good for oscillators and filters.
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Old 10th May 2021, 12:54 am   #12
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Default Re: Polystyrene capacitors questions.

The smaller ones are normally 125/160 volts. but I have used some excellent 350V types.
Some large e.g. 100nF 250v+ were made by GEC and reliable. I have seen very few radial
lead types. There was a trend to clean PCBs using organic chemicals which could destroy
the caps, but isopropanol seems ok. Polystyrene capacitors were also made with an
outer aluminium tube casing, which survive well.
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Old 10th May 2021, 1:07 am   #13
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Default Re: Polystyrene capacitors questions.

I have been reading this thread with interest. In Australia polystyrene caps were very expensive, BUT also highly reliable. All I have seen made in Australia type were made by Ducon. Now if I see a waxy or electrolytic made by them, its replace on sight. If its polystyrene I very carefully remove it from old equipment and keep it. I have never had a failure yet!!. I bought many hundreds at the end of production and keep them for filters and oscillators as they are ultimately stable. I also have hundreds of Japanese smaller types from a few pF to maybe 1000pF and they are superb.I have never seen one that wasnt "rolled" tight enough, nor have I seen any that were not heat sealed into the plastic itself.

I guess different manufacturers in different countries used different methods.

Joe
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Old 10th May 2021, 8:25 am   #14
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Default Re: Polystyrene capacitors questions.

For precision filters, HP had polystyrene capacitors made to the exact values needed and 1% tolerance. Values could have been made up from E24 parts in parallel, but the filters needed to be small and strays needed to be low. I think it was the SEI arm of GEC which made them.

Incidentally, the tempco of polystyrene capacitors neatly cancels that of Siemens type N28 ferrite.

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Old 10th May 2021, 10:22 am   #15
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Default Re: Polystyrene capacitors questions.

These were quite a common failing on some of the excellent ITT/KB CVC5 hybrid chassis around the line oscillator stage. It use to result in loss of line sync. We would change out out all of them to clear the fault.
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Old 10th May 2021, 5:28 pm   #16
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Default Re: Polystyrene capacitors questions.

In my servicing days '58-73 had horrendous problems with the lower values in IFTs, where sensitivity would drop and touching any one of the pins on an IFT would bring it back to square one. Fitted either new polystyrenes or preferably S/micas - realign and off we go.

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