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 Post subject: Replacing a Polystyrene Capacitor
PostPosted: Mar Fri 06, 2020 5:00 pm 
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I am working on an Admiral 13in portable tv from 1968-69. It uses mostly Polystyrene clear capacitors. I am needing to replace one that looks bad and beat up. It is in the Horizontal AFC circuit. It comes off of pin 6 on the 8LT8.

It is capacitor C48 on the schematic below. The value is .0039 500v.

My question is can I use a film capacitor or do I need to use another Polystyrene or Mica?

Thanks!


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Admiral_1234FP.jpg
Admiral_1234FP.jpg [ 220.82 KiB | Viewed 1262 times ]

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 Post subject: Re: Replacing a Polystyrene Capacitor
PostPosted: Mar Fri 06, 2020 5:11 pm 
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Location: 18424 PA
I'm guessing any would work, 5% tolerance. If later you have problems you know where to look. I'd use a mica actually.


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 Post subject: Re: Replacing a Polystyrene Capacitor
PostPosted: Mar Fri 06, 2020 5:23 pm 
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Polystyrene caps are very stabile and extremely low in leakage. Some other charicteristics which may not apply here.
Quote:
Low dissipation factor and ESR.
High insulation resistance.
High stability of capacitance and DF vs temperature and frequency.
Operating Temperature: -40C to +85C.

.
I would go for a polystyrene replacement just to be sure.

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 Post subject: Re: Replacing a Polystyrene Capacitor
PostPosted: Mar Fri 06, 2020 5:36 pm 
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Use a capacitor with a documented dv/dt, the pulse shaping/timing are critical in horizontal circuits.

If that cap has not actually changed value or is leaking I would leave it alone. The wrong cap may effect how the horizontal oscillator locks to the signal. The set could be prone to horizontal tearing if the cap is not working properly. I used to use Sprague or Cornell dipped polys in these circuits, but not very often, seldom failed...

YMMV

Chas

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 Post subject: Re: Replacing a Polystyrene Capacitor
PostPosted: Mar Fri 06, 2020 6:24 pm 
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Yes, actually measure it. If it's on the mark, leave it alone. Someone probably got too close with a soldering iron.


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 Post subject: Re: Replacing a Polystyrene Capacitor
PostPosted: Mar Sat 07, 2020 9:58 am 
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Joined: Jan Thu 16, 2020 12:29 am
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In that circuit location in the AFC you can use any type of capacitor you want (of the same value and equal to or greater voltage rating), polystyrene, polyester,polypropylene, mica, ceramic, or oil filled. Just avoid vintage waxies as they leak and vintage mica's as they have the mica disease.

If you are wondering about the type of capacitor you are replacing, you need to look at its application: Is its ESR important (clearly not if it has a large resistor in series). Is its temperature properties important (very important for small value pF capacitors in things like turret tuners). Are its dielectric losses important. The latter two apply in oscillators where you want high Q and temperature stability.

Polystyrene caps have a lot of "favorable properties", so there is really no reason not to use one, especially if they were there in the first place and you can easily get a replacement. But, if they are hard to get, and the particular circuit configuration doesn't really need one, don't bother.


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 Post subject: Re: Replacing a Polystyrene Capacitor
PostPosted: Mar Mon 09, 2020 11:05 pm 
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Polystyrene caps are chosen where zero change with temperature is needed. You SHOULD NOT use any other plastic film capacitor because if you do the horizontal hold will DRIFT as the set warms up. This is bad.

It's rare for polystyrene caps to be bad, even after 50+ years. The only real failure mode is intermittent open circuit, where the cap will come good if a voltage suddenly appears across it. This is uncommon. But if you have a styro cap that's tuning a horizontal oscillator coil (yes) and the oscillator design is such that it won't start if the cap is O/C (yes), that's where you need to suspect it.

So, if there is an issue with the oscillator sometimes not starting, that cap IS your culprit in that circuit. I would replace it, and its mate, with dipped mica caps.

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 Post subject: Re: Replacing a Polystyrene Capacitor
PostPosted: Mar Wed 11, 2020 12:28 pm 
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irob2345 wrote:
Polystyrene caps are chosen where zero change with temperature is needed. You SHOULD NOT use any other plastic film capacitor because if you do the horizontal hold will DRIFT as the set warms up. This is bad.



I doubt it will drift any noticeable amount.There are thousands of TV sets & video monitors that use polyester/polypropylene caps in the AFC loop (and H oscillators too) with no noticeable change in the phase (horizontal position) of the picture as they warm up. Though a polystyrene capacitor is agreeably better and probably, in a tube TV, the capacitors are exposed to a much wider temperature variation than in transistor sets, depending on where they are physically placed with respect to the hot parts.


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 Post subject: Re: Replacing a Polystyrene Capacitor
PostPosted: Mar Thu 12, 2020 12:18 am 
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Hugo, polyester caps in horizontal oscillator circuits DEFINITELY DO cause unacceptable drift with normal TV warmup temperature change.
Enough in many cases to cause loss of lock on channel change.

This from many years of TV servicing experience, including looking elsewhere for the fault!!

Kriesler, for one, used to use a special type of wound styro cap with "extended foil" to overcome the intermittent open-circuit-from-cold fault.
They looked like polyester caps and caught many techs service techs out as a result.

Before this type of cap was widely used, no-hor-osc-startup service calls were common with the onset of cold weather.
Replacing this cap was a stock fault operation whenever a stopped hor osc was suspected.

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 Post subject: Re: Replacing a Polystyrene Capacitor
PostPosted: Mar Thu 12, 2020 1:40 am 
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[quote="irob2345"

Kriesler, for one, used to use a special type of wound styro cap with "extended foil" to overcome the intermittent open-circuit-from-cold fault.
They looked like polyester caps and caught many techs service techs out as a result.

[/quote]

The worst capacitors in H oscillator circuits I ever came across were the black "bumble bee" type used extensively by RCA in their early 10" and 12" sets. They were actually waxed paper in there and how they thought they could get away with that was anyone's guess.

(PS: the cap C48, that the OP was referring to is in the AFC feedback loop and much more variability there is tolerated, not the polystyrene caps that tune the H osc coil which are more critical for temp stability)


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 Post subject: Re: Replacing a Polystyrene Capacitor
PostPosted: Mar Thu 12, 2020 2:21 am 
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The posted schematic is a little blurry and it took awhile for me to see which cap is the subject of this thread. It is at the lower left of the schematic next to the large black square with 39 in it.

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 Post subject: Re: Replacing a Polystyrene Capacitor
PostPosted: Mar Thu 12, 2020 2:46 am 
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Location: Urbana, Illinois
cparsons2391 wrote:
I am working on an Admiral 13in portable tv from 1968-69. It uses mostly Polystyrene clear capacitors. I am needing to replace one that looks bad and beat up. It is in the Horizontal AFC circuit. It comes off of pin 6 on the 8LT8.

It is capacitor C48 on the schematic below. The value is .0039 500v.

My question is can I use a film capacitor or do I need to use another Polystyrene or Mica?

Thanks!
Could you please post photos showing what these capacitors look like?

-EB

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 Post subject: Re: Replacing a Polystyrene Capacitor
PostPosted: Mar Thu 12, 2020 4:29 pm 
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Kriesler? Who?

Not the famous violist, no?

My service kit had a small assortment of horizontal circuit caps when I was on the road in the 60's-70's as a Massachusetts licensed Tv & Radio technician...

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 Post subject: Re: Replacing a Polystyrene Capacitor
PostPosted: Mar Fri 13, 2020 6:39 am 
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Location: Belrose, NSW, Australia
No, that was Fritz Kreisler. The e and the i reversed.

http://www.milesago.com/Radio/kriesler.htm

Re the Styro capacitors, here is a picture of some examples I was able to find.

Attachment:
Styroseal.png
Styroseal.png [ 275.98 KiB | Viewed 786 times ]


The "special" ones looked like most other wound foil caps.

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