recapping PS - cap size discrepancies?

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aurelialuz
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recapping PS - cap size discrepancies?

Post by aurelialuz » Wed Jul 28, 2010 9:01 pm

i needed two 33uf 63v caps, and, simply because i live by the seat of my crazy pants, i didn't even check the size. so when i got the new ones, i couldn't help but notice there was a little bit of a difference. the big black one in the picture was the replacee, the two blues ones being held up the replacers. they're both 33uf 63v caps, but what makes the size so different?

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Post by The Scum » Wed Jul 28, 2010 9:46 pm

http://blog.makezine.com/archive/2007/1 ... itors.html










Of if the original is kinda old, newer caps are generally smaller. They've improved the materials science that goes into them, getting higher values in smaller volumes.

Is there a date code on the original?

Or there could be a significant difference in some other parameter...temperature grade being the first candidate.

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Post by RodC » Thu Jul 29, 2010 5:13 am

The Scum wrote:http://blog.makezine.com/archive/2007/1 ... itors.html










Of if the original is kinda old, newer caps are generally smaller. They've improved the materials science that goes into them, getting higher values in smaller volumes.

Is there a date code on the original?

Or there could be a significant difference in some other parameter...temperature grade being the first candidate.
+1

You should see the .1uf 500V cap in the PS of my 1937 hammond.
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Re: recapping PS - cap size discrepancies?

Post by Andy Peters » Thu Jul 29, 2010 9:11 pm

aurelialuz wrote:i needed two 33uf 63v caps, and, simply because i live by the seat of my crazy pants, i didn't even check the size. so when i got the new ones, i couldn't help but notice there was a little bit of a difference. the big black one in the picture was the replacee, the two blues ones being held up the replacers. they're both 33uf 63v caps, but what makes the size so different?

Image
Are the smaller blue caps tantalum?

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aurelialuz
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Post by aurelialuz » Thu Jul 29, 2010 11:01 pm

nope, they're not tantalums, i checked and rechecked and they're aluminum electrolytics.

so, i can use these? i'm not worried really about temperature. i'm not taking this board to the antartic.
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Post by RodC » Fri Jul 30, 2010 5:04 am

aurelialuz wrote:nope, they're not tantalums, i checked and rechecked and they're aluminum electrolytics.

so, i can use these? i'm not worried really about temperature. i'm not taking this board to the antartic.
They should be fine, they do look a bit small for 63V, but Its hard to get a good perspective.

If they say 63V then put em in and dont worry, its just the PS correct?
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Post by SeventhCircle » Fri Jul 30, 2010 9:48 am

Looks like the original is crimped on both ends and marked "BP" for bi-polar. Your polarized replacements won't be happy for long if they need to withstand sizable reverse voltage. Bipolar electrolytics are generally much bigger because they have double the insulating oxide layers. You can substitute two polarized electrolytics wired back-to-back for troubleshooting, but you should get a non-polarized part for long-term replacement.

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Post by RodC » Fri Jul 30, 2010 10:35 am

SeventhCircle wrote:Looks like the original is crimped on both ends and marked "BP" for bi-polar. Your polarized replacements won't be happy for long if they need to withstand sizable reverse voltage. Bipolar electrolytics are generally much bigger because they have double the insulating oxide layers. You can substitute two polarized electrolytics wired back-to-back for troubleshooting, but you should get a non-polarized part for long-term replacement.
Yup, good catch, I missed the BP and his hand was shading the one end, (and my eyes def are not what they use to be)
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Post by The Scum » Fri Jul 30, 2010 10:45 am

"BP" for bi-polar.
Good catch.

Is the original cap in question missing the line of arrows down the side, pointing to one terminal or the other (similar to the mark on the cap near the trimpot in the lower left of the photo)?
i'm not worried really about temperature. i'm not taking this board to the antartic.
Actually, your concern should be in the other direction. Electronics run hot. Heat degrades the electrolyte in 'lytic caps. This is why we have to replace them periodically.

Garden variety caps are rated to 85 deg C. Hi-temp caps are rated to 105. These values correlate to proper operation at the stated temp for a number of hours. Run them cooler, and they'll last longer, but they still degrade over time. You wouldn't be asking this question in the first place if this phenomena didn't occur.

There a local studio with an MCI 500 desk that the owner recently recapped...he had me there to do some tech work, and while we're talking in the control room, the desk emits a distinct "pffffft" noise. One of the coupling caps had failed, and vented electrolyte into the guts of the desk. Once we opened it up to investigate, we found that a number of other channels had done it, too. All the same cap...

Probably worth mentioning in context: MCI 500's run on the top side of warm...approaching hot. The amplifier stages use a totem-pole hybrid amp that hangs on +/-32 volt rails, and are probably good for 2-ish Watts each. Most of the modules have a giant PCB that fills the bucket, with a number of mezzanine boards on top of that...making a sandwich of boards that are each 1/2" apart, and 3 feet wide. No room for convection cooling.

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Post by aurelialuz » Mon Aug 02, 2010 8:17 am

SeventhCircle wrote:Looks like the original is crimped on both ends and marked "BP" for bi-polar. Your polarized replacements won't be happy for long if they need to withstand sizable reverse voltage. Bipolar electrolytics are generally much bigger because they have double the insulating oxide layers. You can substitute two polarized electrolytics wired back-to-back for troubleshooting, but you should get a non-polarized part for long-term replacement.
aha! interesting, interesting. back to mouser i guess. say, while i have you guys on the line, one cap on the console PS is labelled "250.0M 50.0." is that 250 uf? i assume the 50.0 is just 50v, it's about that size.
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