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 Post subject: Re: Capacitor blowup
PostPosted: May Thu 16, 2019 1:18 am 
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Got a call one day to look at an RCA radio. Seller decided to warm up the radio while I was driving over. This is what I saw when I got there. I ended up buying it for far less than he originally asked for.



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 Post subject: Re: Capacitor blowup
PostPosted: May Thu 16, 2019 2:08 am 
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:shock:

I hope the power trans was ok! I'd leave the boric acid stain--it adds to it's heritage :D

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 Post subject: Re: Capacitor blowup
PostPosted: May Thu 16, 2019 3:39 am 
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A radio with a chimney .... just when you think you've seen it all ...

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 Post subject: Re: Capacitor blowup
PostPosted: May Thu 16, 2019 11:41 am 
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ktewell wrote:
My first and hopefully last cap blast. I almost had to change my underwear. That’s all the paper under it. It was on for awhile then bang.


So was it in backward? I see those are the Miec brand. I always wondered if those were reliable?


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 Post subject: Re: Capacitor blowup
PostPosted: May Thu 16, 2019 12:19 pm 
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Is this correct? if so, I had it backwards.


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 Post subject: Re: Capacitor blowup
PostPosted: May Thu 16, 2019 1:06 pm 
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Location: St. Louis, MO, USA
I have a Lafyette LSG-10 signal generator that I have used for many years with no problems. Last year it quit cold. When I opened it, there were 2 modern filter caps that had blown up. It seems the previous owner had mistakenly put 40 uF 10 WV ones in instead of 10uF 400 WV. How it worked that long is a mystery.

Dennis


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 Post subject: Re: Capacitor blowup
PostPosted: May Thu 16, 2019 4:20 pm 
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ktewell wrote:
Is this correct? if so, I had it backwards.


Actually, the way you have it marked IS backwards.

The center tap of the HV winding is negative, the filament of the 80 is positive.

Even if you had it correct, sometimes there is more than 450 volts right at the first filter cap between the CT and rectifier.
That will indeed push the cap past its limit.

-Steve

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 Post subject: Re: Capacitor blowup
PostPosted: May Thu 16, 2019 4:28 pm 
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Then I had it right and it popped anyway. Because, I had it opposite of the drawing above when it popped. Should I get a 600 volt cap?

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 Post subject: Re: Capacitor blowup
PostPosted: May Thu 16, 2019 4:31 pm 
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ktewell wrote:
Then I had it right and it popped anyway. Because, I had it opposite of the drawing above when it popped. Should I get a 600 volt cap?
Hi Kevin,

The WVDC rating of a capacitor is the DC voltage PLUS the peak ripple voltage. For the cap at the rectifier cathode, the ripple voltage may be quite significant. You can measure it with a peak-reading AC voltmeter.

- Leigh

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 Post subject: Re: Capacitor blowup
PostPosted: May Thu 16, 2019 4:40 pm 
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ktewell wrote:
Then I had it right and it popped anyway. Because, I had it opposite of the drawing above when it popped. Should I get a 600 volt cap?


Yes, i would reccomend a 600 volter there. 4 uf is small enough you could easily find a mylar or poly cap.
https://www.tubesandmore.com/products/c ... ylene-fast
The 4.7 uf is $5.10.

Or put two 2.2 uf 630V yellow ones in parallel with each other. Ends up being about the same price.

http://www.radiodaze.com/axial-film-2-2mf-630v/

-Steve

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 Post subject: Re: Capacitor blowup
PostPosted: May Fri 17, 2019 3:32 am 
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i use sprague atom capacitors they have never gave me a problem.

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 Post subject: Re: Capacitor blowup
PostPosted: Jun Thu 06, 2019 12:49 am 
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I got new capacitors is their a negative side to these caps?


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 Post subject: Re: Capacitor blowup
PostPosted: Jun Thu 06, 2019 1:02 am 
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perhaps are they not electrolytics and they are film caps ?

to me, they look like film caps, but then again, i could be wrong.

look where the leads come out of the top and bottom of the cylinder. is one surface a different color than the other ?

if so, do tell.

steve

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 Post subject: Re: Capacitor blowup
PostPosted: Jun Thu 06, 2019 1:05 am 
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Same color on both ends. I bought what Steve told me to buy :D

https://www.tubesandmore.com/products/c ... ylene-fast

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 Post subject: Re: Capacitor blowup
PostPosted: Jun Thu 06, 2019 1:19 am 
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Nope, it doesn't matter which end is used as positive or negative.

-Steve

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 Post subject: Re: Capacitor blowup
PostPosted: Jun Thu 06, 2019 1:21 am 
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Thanks Steve just wanted to make sure

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 Post subject: Re: Capacitor blowup
PostPosted: Jun Thu 06, 2019 2:47 pm 
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A type 80 rectifier has a directly heated cathode and will have electron flow long before the other tubes in that radio begin to draw current and load the supply voltage (20-30 seconds.) A C-L-C input filter requires a minimum load, or the voltage will be quite higher than the operating voltage. Thus, after a few seconds after turn on, that capacitor may be seeing a higher than expected voltage until the rest of the set "warms up." Besides ripple rating, many of those older caps had a surge voltage rating just for that purpose. A 600 volt cap would be a good investment. Or you could use two 450 volt caps in series, with twice the capacitance value and equalizing resistors, to be even more conservative. It may be cheaper as well.

Pete

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 Post subject: Re: Capacitor blowup
PostPosted: Jun Thu 06, 2019 8:41 pm 
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-or- install a CL-90 on the mains in.

i had to do that with two of my restored RCA-86K7 radios, plus i used 500 volt capacitors for the electros as i do on any transformered set.

both fully restored sets would hit over 500 volts at turn on with the directly heated 5W4. i also tend to believe that the input resistance/impedance was high as well to create such a surge before the rest of the radio started to conduct.

using a 5Z4 (i think) instead of the 5W4 solved the turn on surge problem since it was indrectly heated. the B+ slowly, safely, and climbed up nicely, but one of the offsets was a higher B+ which was not a big issue. that could have been lived with.

the second offset was that the 5Z4 (i think) that i installed drew 500 more mA on the filaments than a 5W4. i did not want to put an extra 500 mA on an eighty year old transformer.

between the 500 volt capacitors and the CL-90, the startup is very nice.

steve

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 Post subject: Re: Capacitor blowup
PostPosted: Jun Thu 06, 2019 8:55 pm 
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What is a CL-90 :D

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 Post subject: Re: Capacitor blowup
PostPosted: Jun Thu 06, 2019 9:07 pm 
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https://www.ametherm.com/inrush-current/

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