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 Post subject: Recap bipolar cap?
PostPosted: Nov Mon 18, 2019 9:26 pm 
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Had to take a break from the tube stuff, fiddeling with an old Kenwood. Lots of components don't jive with manual vs what is in it. Anyway, second to last electrolytic of the complete recap came across this 3.3uF capacitor. On the schematic, it's polar, in the unit, it is bipolar and no (+) silkscreen like every other cap has. Don't happen to have a N.P. cap that size, from looking at the schematic and comparing it to the board, surely a polar cap here is ok? One lead definitely goes to ground, its part of the first stage where the AM signal comes in, just a decoupling cap. Am I missing something?

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 Post subject: Re: Recap bipolar cap?
PostPosted: Nov Mon 18, 2019 9:48 pm 
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I would use a 35V tantalum, 3.3uf or greater. Just make sure the tantalum polerity is orientated correctly according the schematic.

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 Post subject: Re: Recap bipolar cap?
PostPosted: Nov Mon 18, 2019 11:21 pm 
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It appears to be on an AVC circuit. Is there a possibility the line leading to it could get pulled negative by signal conditions? That would be a reason to use a non-polarized cap in that location.

If the line can go negative, a tantalum capacitor will not last long. Another non-polarized electrolytic would be a better bet.

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 Post subject: Re: Recap bipolar cap?
PostPosted: Nov Tue 19, 2019 3:01 am 
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Two polarized caps in series and with polarities reversed will work as a non-polarized cap, but the cap value is cut in half.
If I got that wrong, corrections will arrive soon. :lol:

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 Post subject: Re: Recap bipolar cap?
PostPosted: Nov Tue 19, 2019 9:57 am 
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why not just use a 3.3 film cap and be done with it?


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 Post subject: Re: Recap bipolar cap?
PostPosted: Nov Tue 19, 2019 3:22 pm 
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fonebone wrote:
why not just use a 3.3 film cap and be done with it?


I would if I had a 3.3 film lying around. :D

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 Post subject: Re: Recap bipolar cap?
PostPosted: Nov Tue 19, 2019 11:01 pm 
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What kind of 3.3 uF film cap can you shoehorn in on a crowded PCB like presumably that has? It would have to be mounted off-board. I for one do not recap the Japanese stuff unless the cap is bad. Not without good reason or suspicion.

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 Post subject: Re: Recap bipolar cap?
PostPosted: Nov Tue 19, 2019 11:32 pm 
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The schematic fragment posted does not indicate what different voltages might be encountered by this capacitor. Is there a complete schematic online that you might link?

Note that the voltages on the print do not exactly make sense, since the transistor is shown as having a positive bias current on the base. However, the only source of of this current, the 10k resistor, has a lower voltage at the other end. Herr Kirchhoff would not be amused.

Probably, a polarized electrolytic, a film cap, or a ceramic cap would all actually work.

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 Post subject: Re: Recap bipolar cap?
PostPosted: Nov Wed 20, 2019 1:33 am 
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I used a polar cap with no issues, have some non-polar coming Friday so will pop that in just for good measure. That cap tested 13.9uF with 6+ ESR just in case anyone wondered. Most of the electrolytics were very high over their proper values... :shock:

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 Post subject: Re: Recap bipolar cap?
PostPosted: Nov Wed 20, 2019 2:29 am 
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What is a polar cap used for in electronics? is it connected to the tranny??

DM


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 Post subject: Re: Recap bipolar cap?
PostPosted: Nov Wed 20, 2019 2:49 am 
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devilsmist wrote:
What is a polar cap used for in electronics? is it connected to the tranny??

DM


I believe he meant the polar ice cap. Seldom encountered in modern electronics.

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 Post subject: Re: Recap bipolar cap?
PostPosted: Nov Wed 20, 2019 7:12 am 
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wazz wrote:
What kind of 3.3 uF film cap can you shoehorn in on a crowded PCB like presumably that has? It would have to be mounted off-board. I for one do not recap the Japanese stuff unless the cap is bad. Not without good reason or suspicion.

well ive a 3.3 mfd 50v WIMA in my hand its .200 thick" .280" wide and is .450" tall. it has 5mm lead spacing im reasonably sure it would fit no problem. its a stacked film cap.


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