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 Confused on caps

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dynadude

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Location : Chapel Hill, NC

PostSubject: Confused on caps   Fri Oct 15, 2010 10:13 am

I'm not usually very dense, I admit I have my moments, but for the life of me I can't seem to get this clear in my head.

I know the difference in electrolytics and other types of caps, but when it comes to older sets I just kind of go blank and can't figure out which ones to use.

Say, the older sets (pre 1930s) with the power pack setups, or all the filter caps in a big square box filled with pitch.

Is there an easy way to determine what type caps to use for replacement? Is it a case by case kind of thing?

Many times they are low values, like 2 to 8 mf.
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smeezekitty

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PostSubject: Re: Confused on caps   Fri Oct 15, 2010 10:19 am

Usually its like this:
1pf - 0.001uf mica
1pf - 0.05uf ceramic
0.001uf - 2uf paper
2uf+ electrolytic
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processhead

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PostSubject: Re: Confused on caps   Fri Oct 15, 2010 10:26 am

The pre-1930s set often used paper caps for the original power supply filters.

However, a DC power supply does not care whether you replace filters caps with film or electrolytic style caps as long as the correct capacitance and working voltage is used.

It can be hard to find individual film caps in values larger than 2.0 mfd, but they can be connected in parallel to increase the total capacity. The electrolytic styles are readily available in larger capacities.

The good news is that no matter what style you use, they almost always fit better in the available space in the chassis because of their relative small physical size.
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pixellany



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PostSubject: Re: Confused on caps   Fri Oct 15, 2010 1:24 pm

This might be more than you wanted, but I think you'll find that the choice of technology and construction is very often dictated by resources. The resource we worry about the most is typically $, but size and mass is certainly important.

For example, there is nothing stopping you from building a ceramic disk capacitor for a power supply filter--let's say 40mf at 450 volts. Just one issue: It would be huge.

The other aspect is to be aware of the equivalent circuit for a particular real-world design. In some cases, you might be concerned with ESR (equivalent series resistance).

For RF work, consider a typical dipped mica cap--let's say 5 pf--with 1" leads. At low frequencies it is---a 5 pf capacitor.

At high frequencies, it's an inductor!!----above a certain frequency, the circuit would see it as simply a 2" piece of wire.
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pixellany



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PostSubject: Re: Confused on caps   Fri Oct 15, 2010 1:35 pm

In my RF example above, the crossover frequency (series resonance) is ~ 500MHz (assuming 22-gauge leads)

Thus--above 500MHz, that "capacitor" is now a ~50 nanohenry inductor

PS---the above is for 2pF, not 5
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werick



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PostSubject: Re: Confused on caps   Sat Oct 16, 2010 11:01 am

Hi:

I've been fooling around with pre-1930 AC TRF's for years. With the exception of a few, they all use paper capacitors in the power supply. I found old paper filter caps to be less troublesome than electrolytics. I have a few sets that still have good paper filter caps, I never changed them. I just put a fuse in the line.

When I do recap them, I usually recap main filters with 10 mfd 600V electrolytics (Watch the polarity on electrolytics). Some of these capacitors may be cross connected insde the can. So you need to pay attention to the schematic. I've used this procedure on a few sets with good results.

Sometimes I'll just cut them loose and mount the electolytics under the chassis. If it's a special set, I'll melt the tar or wax out of the filter can and put them in there. You have to be careful here. Sometimes there are filter chokes in those cans that might have to be reused.

Warren
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dynadude

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PostSubject: Re: Confused on caps   Sat Oct 16, 2010 1:47 pm

Thanks for the replies fellas. I think I've got the gist of it now. There may be some special circumstances where I might have to ask for some coaching again though. scratch
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