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 Post subject: insulation on capacitor shells
PostPosted: Jan Wed 11, 2017 12:42 am 
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Joined: Nov Thu 22, 2007 11:31 pm
Posts: 819
Location: Johnston, Iowa
All, I just smoked the buffer capacitor on the vibrator circuit on a Detrola 8030 car radio. After I replaced all the resistors, film and electrolytic caps I put in a new .008uf 1.6KV ceramic buffer cap and the radio played great. I then put on the metal shield that covers the vibrator pins, the buffer capacitor and rectifier pins. When I did this the tip of the buffer capacitor shell touched the metal shield and a hole immediately burned into the cap where it hit the shield. It’s amazing how much smoke can come out of such a small capacitor. I always assumed that capacitor shells were well insulated and I never worried about the shell shorting out. Was this due to a faulty cap or should I have been more careful to make sure that the capacitor body was isolated from other components?
Thanks in advance,
Keith


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 Post subject: Re: insulation on capacitor shells
PostPosted: Jan Wed 11, 2017 1:17 am 
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Joined: Feb Sat 06, 2016 1:47 am
Posts: 2059
Location: Santee Calif. 92071
Ceramic does not conduct. Not the capacitors fault. Something else got in contact with chassis ground when you placed the metal cover back on. Inspect parts and lead dress that may contact the shield before that happens again. Don't damage the transformer.


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 Post subject: Re: insulation on capacitor shells
PostPosted: Jan Wed 11, 2017 2:01 am 
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Joined: Nov Thu 22, 2007 11:31 pm
Posts: 819
Location: Johnston, Iowa
Interesting, so it's just a coincidence that I toasted just the buffer capacitor? What kind of short would take out the buffer cap? I put a new one in and it plays again.
Keith


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 Post subject: Re: insulation on capacitor shells
PostPosted: Jan Wed 11, 2017 2:30 am 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 6855
Location: Long Island
When you say "ceramic capacitor," that's a lot of different types of caps. If you were talking about one of the ubiquitous ceramic disc caps, what you describe is entirely possible. The ceramic is on the inside. The outside is covered with epoxy on many of the modern ones, and no, it won't necessarily hold up if you put high voltage on it and press it against a chassis.

At one time, ceramic tube capacitors were available which were impervious to this kind of thing, but I don't think they've been made in years.

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 Post subject: Re: insulation on capacitor shells
PostPosted: Jan Wed 11, 2017 2:56 am 
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Joined: Nov Thu 22, 2007 11:31 pm
Posts: 819
Location: Johnston, Iowa
Well, I learn something new on this forum every day. I thought all the ceramics were the same. This I believe was one of the modern epoxy covered caps.
Here's a pic.


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buffer cap.jpg
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 Post subject: Re: insulation on capacitor shells
PostPosted: Jan Wed 11, 2017 3:04 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 30418
Location: Maryland 20709, USA
keith49vj3 wrote:
Here's a pic.
Hi Keith,

The voltage rating of a capacitor is between the two plates.

The withstanding voltage across the package insulation may or may not be specified.
If it is, you'll only find it on the manufacturer's datasheet for the part.

The instantaneous peak voltages in a vibrator power supply can be very high.

- Leigh

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 Post subject: Re: insulation on capacitor shells
PostPosted: Jan Wed 11, 2017 3:07 am 
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Joined: Feb Sat 06, 2016 1:47 am
Posts: 2059
Location: Santee Calif. 92071
Yes that's an epoxy covered capacitor. Agree with above. High Voltage to close to ground. Arced through the epoxy. If you used the same type of capacitor in the same location again would be better to use like fish paper insulation to avoid that again.


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 Post subject: Re: insulation on capacitor shells
PostPosted: Jan Wed 11, 2017 3:15 am 
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Joined: Nov Thu 22, 2007 11:31 pm
Posts: 819
Location: Johnston, Iowa
Thanks guys, another lesson learned. I'm thinking it was because the cap was pressed again the chassis ground. Could anything else have caused the buffer cap to short? It's playing with a new cap installed but I haven't put the cover shield back on yet.
Keith


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 Post subject: Re: insulation on capacitor shells
PostPosted: Jan Wed 11, 2017 5:45 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2250
Location: Lakewood, California
Keith,
Learned my lesson with this type of capacitor back in 1979 on a 54 Ford deluxe 4SF-756 radio. Capacitor looked the same as yours, a ceramic epoxy coated disc cap rated .008mfd at 2kv and marked “BUFFER”. Radio came back with an arced through hole punched in the coating and dead shorted. Replaced it with a Sprague “Orange Drop” 1.6kv and no more trouble. That was the only disc type buffer I ever used and although it may have been a fluke, no more of them for me.

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 Post subject: Re: insulation on capacitor shells
PostPosted: Jan Thu 12, 2017 1:13 am 
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Joined: Dec Sat 24, 2011 9:17 pm
Posts: 1083
Location: Vancouver Island, Canada
Thanks, Keith. I'm sure you have saved the life of a few of those caps by posting this. 8)

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 Post subject: Re: insulation on capacitor shells
PostPosted: Jan Thu 12, 2017 2:10 am 
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Joined: Nov Thu 22, 2007 11:31 pm
Posts: 819
Location: Johnston, Iowa
I just installed another cap with a 4KV voltage rating that I picked up cheap on ebay. It's a Sprague 8200M Y5S ceramic. This time I enclosed the cap in heat shrink tubing. It's playing well with the shield on. I have no idea what the Y5S code means. Hopefully this is suitable for my needs.
Keith


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