Author Topic: Best replacements for polystyrene caps?  (Read 1054 times)

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Offline WyverntekGameRepairs

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Best replacements for polystyrene caps?
« on: December 24, 2019, 03:16:23 am »
Hi there! I'm working on my Atari 2600, and I have stumbled across 2 polystyrene capacitors. Those are not really made anymore, and are kinda expensive to get from most suppliers. I wish to replace them with better caps made of more reliable materials.
Polystyrene caps are indeed still made, but they are hard to come by these days. They have (for the most part) been dropped from production lines due to there being better alternatives. Size was a big factor in the death of these capacitors as well, as they were just too unreasonably big for any newer designs.
I am hoping there is an alternative capacitor that has almost the same, if not better, characteristics as a polystyrene cap.

The values of the capacitors are 820pF @ 50V.

Thanks!
-Sterling Ordes
Wyverntek Game Repairs
(Business License Pending)
 

Offline MasterT

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Re: Best replacements for polystyrene caps?
« Reply #1 on: December 24, 2019, 04:03:10 am »
Ceramic, COG type.
 

Offline james_s

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Re: Best replacements for polystyrene caps?
« Reply #2 on: December 24, 2019, 04:44:56 am »
Are they bad? I don't think I've ever had a polystyrene capacitor fail. If it works there is no reason to mess with it, they are typically excellent.
 

Offline bob91343

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Re: Best replacements for polystyrene caps?
« Reply #3 on: December 24, 2019, 04:56:30 am »
While polystyrene capacitors may be large, and don't offer high temperature capability, they were used due to their capacitance stability in some circuits.
 

Offline David Hess

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Re: Best replacements for polystyrene caps?
« Reply #4 on: December 24, 2019, 05:24:52 am »
Polypropylene is a good alternative if the application is actually relying on the performance of polystyrene.  Otherwise C0G, NP0, or any other plastic film capacitor is suitable.
 

Online TheMG

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Re: Best replacements for polystyrene caps?
« Reply #5 on: December 24, 2019, 07:05:46 am »
Unless you know for sure that they are bad, leave them alone. As others have said polystyrene film capacitors very rarely fail and they do not noticeably degrade over time.
 

Online Zero999

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Re: Best replacements for polystyrene caps?
« Reply #6 on: April 21, 2021, 02:33:28 pm »
I'm working on an old sample and hold circuit, using the NE5537 and 47nF polystyrene capacitors. I messed up by washing it in IPA, to remove the flux and it doesn't work properly any more. I know it's the capacitors, even though they measure good, on the bench, because I moved them from another junk board and it works. I believe some of the solvent leached into them, weakening the dielectric, causing the leackage current to drastically rise.

I've still got other boards to refurbish. Would polypropylene do?
 

Online Kleinstein

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Re: Best replacements for polystyrene caps?
« Reply #7 on: April 21, 2021, 02:52:16 pm »
For a sample and hold circuit the dielectric absorbtion is the critical parameter. Polypropylene is about as good as PS in this respect. It depends on the model / series which one is better. 47 nF is rather large to find a C0G versions, but still possible.  I have tested some (at 2.2 nF) and found TDK COG caps with DA quite a bit lower than the  PS and PP I tested. Other C0G where about on par with PS, PP.
 

Offline Gyro

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Re: Best replacements for polystyrene caps?
« Reply #8 on: April 21, 2021, 03:13:40 pm »
I'm working on an old sample and hold circuit, using the NE5537 and 47nF polystyrene capacitors. I messed up by washing it in IPA, to remove the flux and it doesn't work properly any more. I know it's the capacitors, even though they measure good, on the bench, because I moved them from another junk board and it works. I believe some of the solvent leached into them, weakening the dielectric, causing the leackage current to drastically rise.

I've still got other boards to refurbish. Would polypropylene do?

I accidentally did that on my Picoammeter project. The leakage shot up... but 24 hours later they were completely back to normal.
Chris

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Online Zero999

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Re: Best replacements for polystyrene caps?
« Reply #9 on: April 21, 2021, 03:30:43 pm »
For a sample and hold circuit the dielectric absorbtion is the critical parameter. Polypropylene is about as good as PS in this respect. It depends on the model / series which one is better. 47 nF is rather large to find a C0G versions, but still possible.  I have tested some (at 2.2 nF) and found TDK COG caps with DA quite a bit lower than the  PS and PP I tested. Other C0G where about on par with PS, PP.
47nF is also large for polystyrene. If they don't get better, after leaving them to dry out, I'll try replacing them with polypropylene.  C0G is SMT and this is a through hole board, so it could be done, but it will be messy.

I'm working on an old sample and hold circuit, using the NE5537 and 47nF polystyrene capacitors. I messed up by washing it in IPA, to remove the flux and it doesn't work properly any more. I know it's the capacitors, even though they measure good, on the bench, because I moved them from another junk board and it works. I believe some of the solvent leached into them, weakening the dielectric, causing the leackage current to drastically rise.

I've still got other boards to refurbish. Would polypropylene do?

I accidentally did that on my Picoammeter project. The leakage shot up... but 24 hours later they were completely back to normal.
I should have just left it alone. I'll hang on to the capacitors I removed, rather than discarding them, to see if they recover. It wasn't just IPA. I also used some hand sanitiser, which has been my go to for flux removal, since COVID. I like the way it hangs around on the board, rather than running off. I don't think much of it got on the capacitors. The IPA was just to wash any hand sanitiser off and capacitors got a fair soaking. I certainly won't do this again. :palm:
 

Online Kleinstein

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Re: Best replacements for polystyrene caps?
« Reply #10 on: April 21, 2021, 06:10:14 pm »
C0G caps are also available as THT - though internally they are likely a SMT part with connected wires and some epoxy dip.

Some extra heat (but not too much) could speed up the recovery process of the old PS caps quite a bit.
 

Offline schmitt trigger

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Re: Best replacements for polystyrene caps?
« Reply #11 on: April 21, 2021, 06:24:14 pm »
Have you tried cleaning the board thoroughly with warm distilled water?

That worked for me once.
 


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