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 Post subject: Capacitor bands?
PostPosted: Jan Mon 01, 2018 12:33 am 
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Location: Wayne, NJ
Hi,
so, I didn't know capacitors also came with band markings like resistors and I have to replace these large black plastic caps....the schematic tells me what their value is, but what kind of cap are they?


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Last edited by addyboy on Jan Mon 01, 2018 10:58 pm, edited 1 time in total.
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 Post subject: Re: Capacitor bands?
PostPosted: Jan Mon 01, 2018 12:37 am 
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They are paper caps, in molded cases. At the time they were typically advertised as being better quality than the wax coated types, but that has proven to be untrue over the years. Depending on where they are connected in the circuit, they have a tendency to short and explode, sometimes catching fire in the process.

In low level audio applications where there is no appreciable DC voltage applied to them, they may still be usable even though somewhat leaky. Don't throw the old ones away, they will sell on eBay for several times what you pay for the replacements to musicians and audio folks who want that type of capacitor. Certain values (unfortunately yours aren't) can go for significant money!

Some recent sales: https://www.ebay.com/sch/i.html?_from=R ... 5573.m1684

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 Post subject: Re: Capacitor bands?
PostPosted: Jan Mon 01, 2018 12:56 am 
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Those are Sprague Black Beauty capacitors, and they're bad, as in BAD.

Here's the Sprague color code chart for them (note there are one or two voltage bands):
Image
That's sort of hard to read. The bands (left-to-right) are 1st digit, 2nd digit, multiplier, tolerance. Then there's a space, followed by one or two bands for the voltage.

The problem is they're not just simple paper, they're oil-filled paper.
That's how Sprague got such a high voltage rating in such a small package.

Over time, the plastic case develops leaks and the oil drains out, leaving behind dry paper.
That had a working voltage less than 200 volts when new. Now it's much less.
You often seen these caps with visible oil on the surface.

- Leigh

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 Post subject: Re: Capacitor bands?
PostPosted: Jan Mon 01, 2018 3:20 am 
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Eee Gads! $15 for a leaky old NOS Bumble Bee cap? Dang, some folks will buy anything if they're convinced "original" is the same as "best."

I have a coffee can somewhere out in my shop with probably 20 or 30 NOS Bumble Bees. I guess I should dig 'em out and put 'em up on eBay before the word gets out that they're really pretty sad old caps.

- Scott


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 Post subject: Re: Capacitor bands?
PostPosted: Jan Mon 01, 2018 5:46 am 
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Those are the famous Black Beauty capacitors. The color bands give the capacity, voltage rating, and tolerance per the chart posted above. They were excellent, premium caps in their day--HP and Tektronix used them by the ton, as did many other manufacturers like Zenith in this case. But at this point in time the failure rate is very high; a good estimate that 95% or more of the caps will not pass a leakage test at their rated voltages.

Problem is inherent in what happens when paper is sealed in a very tight enclosure that cannot "breathe." The paper used in capacitors is nearly pure cellulose. Molecular breakdown is inevitable and results in carbon and water molecules being released. The water cannot escape so it saturates the paper and the result is low leakage resistance. If the circuit has enough energy behind it the water turns to steam and splits the case of the capacitor--sometimes with a loud bang!

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 Post subject: Re: Capacitor bands?
PostPosted: Jan Mon 01, 2018 9:55 pm 
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Location: Wayne, NJ
Thanks guys.....awesome stuff!

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