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 Post subject: Cap incident and a few questions
PostPosted: Dec Tue 05, 2017 2:58 am 
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Location: Didsbury Alberta Canada
Hello all

I’m working on an RCA C-122, which I have not been able to locate a schematic, and thought I was done. It was recapped, except for the electrolytics as they looked to be newer, and was playing great for about 30 min. Then I heard the old unmistakable hissss which was followed by smoke. The cap in the attached link was the culprit, I’m assuming, as it’s split laterally. It is across pins 4 and 7 of the 35W4. Can someone assist with its value? The color code looks like yellow, purple orange then a dark color I cant make out, a space then yellow or gold. Secondarily, what could be the possible cause? Bad electrolytic perhaps? I’ll replace them regardless. Thanks for any help.

Rod

https://photos.app.goo.gl/dDBrj3aGcBsFSA8X2


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 Post subject: Re: Cap incident and a few questions
PostPosted: Dec Tue 05, 2017 3:15 am 
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That one should have been the first capacitor you replaced.........it's an infamous Sprague Black Beauty, and they are notorious for exploding quite loudly and even catching fire.

It's a .047 @ 400 volts, but I wouldn't replace it with the same type. You can use a modern "safety type" capacitor in it's place to avoid the same thing happening in the future should the new one ever fail.

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 Post subject: Re: Cap incident and a few questions
PostPosted: Dec Tue 05, 2017 3:17 am 
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Yellow = 4
Violet = 7
Orange = x1000

47000 pf = 0.047 mfd

http://www.electronics-tutorials.ws/cap ... cap_5.html

It's also known as "bumble bee" style paper capacitor. Known for high failure rate. Very likely died due to old age.

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 Post subject: Re: Cap incident and a few questions
PostPosted: Dec Tue 05, 2017 3:52 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 10972
Location: Mpls, Minnesota
Bumble Bee caps came in two variety's, paper in oil and Mylar film. If they have the solder blob on the end sealing the fill spout they are paper in oil, if there is no solder blob they are Mylar and probably still good.
Black Beauty's also came in the same variety's as the Bumble Bees but had no stripes. The ones with red lettering were Mylar and the yellow lettering were paper in oil and have a solder blob.

The paper in oil capacitors usually have very high leakage and should always be replaced. They will still work in low voltage applications like guitar tone controls and are in demand and fetch high prices for desired values.

Dave


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 Post subject: Re: Cap incident and a few questions
PostPosted: Dec Tue 05, 2017 5:04 am 
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Joined: Sep Sat 10, 2016 10:30 pm
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Location: Didsbury Alberta Canada
Always an education; thanks to all for the great info!

Rod


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 Post subject: Re: Cap incident and a few questions
PostPosted: Dec Sat 09, 2017 2:13 am 
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Joined: May Sat 28, 2016 8:23 pm
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If you're going to recap, recap, don't leave old caps behind because they "look newer". Newer could be 40 years old. Old caps especially bumblebees and similar plastic-cased types, regardless of how they work, are just time bombs. On old radios brought back in service after many decades, they will often work for a few minutes or hours, then decide to short or explode. I've seen radios where every one of these was cracked or split open. I wouldn't risk burning out a rare tube by not replacing a cap that literally costs pennies to replace.


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 Post subject: Re: Cap incident and a few questions
PostPosted: Dec Sat 09, 2017 4:51 pm 
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Location: Didsbury Alberta Canada
guyonearth wrote:
If you're going to recap, recap, don't leave old caps behind because they "look newer". Newer could be 40 years old. Old caps especially bumblebees and similar plastic-cased types, regardless of how they work, are just time bombs. On old radios brought back in service after many decades, they will often work for a few minutes or hours, then decide to short or explode. I've seen radios where every one of these was cracked or split open. I wouldn't risk burning out a rare tube by not replacing a cap that literally costs pennies to replace.




Sound advice and lesson learned. To be honest, I thought the bumblebee was a power resistor initially. After it lost its smoke, I tested it on a component tester which ID’d it as a cap of 30pfd. Won’t happen again!

Rod


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 Post subject: Re: Cap incident and a few questions
PostPosted: Dec Sat 09, 2017 9:21 pm 
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Hi Rod,

30pfd is WAY too low for a Black Beauty cap. Here's the color code explanation:
Image

The values are in pfd, but usually 10,000 or more.

These little gems are ALWAYS bad. They were oil-filled paper (except 200v had no oil). The seal fails and the oil leaks out, leaving an oilless paper cap with a voltage of maybe 100 volts.

- Leigh

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 Post subject: Re: Cap incident and a few questions
PostPosted: Dec Sat 09, 2017 9:36 pm 
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Joined: Sep Sat 10, 2016 10:30 pm
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Location: Didsbury Alberta Canada
Leigh wrote:
Hi Rod,

30pfd is WAY too low for a Black Beauty cap. Here's the color code explanation:
Image

The values are in pfd, but usually 10,000 or more.

These little gems are ALWAYS bad. They were oil-filled paper (except 200v had no oil). The seal fails and the oil leaks out, leaving an oilless paper cap with a voltage of maybe 100 volts.

- Leigh


Thanks for that Leigh


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 Post subject: Re: Cap incident and a few questions
PostPosted: Dec Mon 11, 2017 2:02 am 
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Joined: Nov Fri 14, 2014 12:22 am
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Location: Howell Michigan
Mr. Detrola wrote:
That one should have been the first capacitor you replaced.........it's an infamous Sprague Black Beauty, and they are notorious for exploding quite loudly and even catching fire.

It's a .047 @ 400 volts, but I wouldn't replace it with the same type. You can use a modern "safety type" capacitor in it's place to avoid the same thing happening in the future should the new one ever fail.


I had one that went off like a firecracker. Split right in half, a better cutaway view could not have been intentionally made.


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 Post subject: Re: Cap incident and a few questions
PostPosted: Dec Mon 11, 2017 2:09 am 
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Wrenchtwister wrote:
I thought the bumblebee was a power resistor initially. After it lost its smoke, I tested it on a component tester which ID’d it as a cap of 30pfd. Won’t happen again!

Rod

A component tester probably won't correctly identify a part after it has failed.

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 Post subject: Re: Cap incident and a few questions
PostPosted: Dec Mon 11, 2017 3:11 am 
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I have serviced radios where the bumblebee exploded, paper/foil confetti and Bakelite crumbs all over the chassis :shock:


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