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Author Topic: Filter cap replacement...  (Read 386 times)
WD0ERU
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Posts: 33




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« on: October 13, 2008, 07:16:04 AM »

I'm replacing a 60 uf filter cap in an old AM broadcast reciever. I have either a 100 uf or 47 uf at my disposal to replace it with. Which one would would you use? I could probably get an exact match if I ordered it, but for now, this is what I have laying around. Thanks in advance.

73's, David...
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AD4U
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Posts: 2186




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« Reply #1 on: October 13, 2008, 08:00:21 AM »

Use the 100, although the lower would probably be OK.  When replacing filter caps, typically you can go higher than the original capacitance assuming the voltage rating is the same or higher than the original.

Dick  AD4U
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K7KBN
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Posts: 2838




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« Reply #2 on: October 13, 2008, 08:02:29 AM »

Don't worry as much about the capacitance value as the DC working voltage rating.

If the new capacitors are rated at least the same as the one that's being replaced, then I'd probably go with the higher capacitance value.

Never replace a capacitor - any capacitor, whether it's a filter, bypass, feedthrough, whatever - with one rated at a lower DCWV than the original component.
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73
Pat K7KBN
CWO4 USNR Ret.
WD0ERU
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Posts: 33




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« Reply #3 on: October 13, 2008, 08:29:39 AM »

Thanks, that is what I thought. Voltage ratings are definitely higher than original as well.

73's David
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AC5UP
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« Reply #4 on: October 13, 2008, 09:27:46 AM »

A few years back I added an HP LCR meter to my work bench and it has been very instructive... Larger value electrolytics are not precision devices and typically run 50% or better above rated capacity. Meaning, a brand new 60 ufd cap might measure 80-90 ufd and be within manufacturer's spec. As time goes on the capacity will decrease and starting off "high" is one way of extending the service life of the part.

Point being that a healthy 47 ufd specimen may well be closer to 60 ufd than 47 ufd and the 100 ufd part offers a huge margin for aging. Either one should work FB.
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Never change a password on a Friday                
K4JSR
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Posts: 513




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« Reply #5 on: October 13, 2008, 09:39:52 AM »

Nelson, all this time I thought LC was a beast of Borden.

NEVERMIND!  That was Elsie!  :-O

Just remember that the mind is the second thing to go when you get older!

73,  Cal  K4JSR
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W5DWH
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Posts: 43




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« Reply #6 on: October 13, 2008, 12:49:40 PM »

Years ago they rated electrolytics as +80/-20% tolerance so you have plenty of leeway in value.
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W7ETA
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« Reply #7 on: October 13, 2008, 01:55:42 PM »

There just ain't as many value caps as there used to be.  Plus, the leads seem to be much smaller than in "days gone past".  So, you might not find the value and voltage you wanted.
73
Bob
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N4NYY
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Posts: 4820




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« Reply #8 on: October 13, 2008, 07:01:29 PM »

Go higher. You can probably find 68uf which will be a common value and an ideal replacement. Non-common value are expensive.
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