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Old 12th May 2012, 05:07 PM   #1
cihtog is offline cihtog  United States
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Default Filter Cap Decade Box

Do any of you use any kind of decade setup for verifying defective filter caps?

I normally just replace them anyway, but I've come to point, at least on one repair, where dropping $75 to $100 on a couple of large value filter caps, with only a best guess to go on, doesn't really appeal to me.

So the thought occurred to me, "what about a handful of smaller value caps in parallel, one set for each rail?" ...say enough to cover up to about 20,000uF?

Just for swapping into the circuit long enough to rule out, or expose the currently installed caps as a problem.

Any thoughts as to why this may also be a bad idea, are welcome too!

Thanks!

(PS - I'm going to post a thread about the amp in question, in the appropriate forum as well.)
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Old 12th May 2012, 06:09 PM   #2
Enzo is offline Enzo  United States
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In repair, I rarely have to be precise with sub parts for troubleshooting.

If I suspect the 20,000uf cap in my amp is defective, chances are that a 10,000 I have around is good enough to sub in for test. If I then determine that indeed it made the difference, then I go get a 20,000uf part to complete the repair.

To me, a decade box is for situations where you need precise values or are trying to determine some exact value in a performance situation. Tuning in a circuit, so to speak. If you are trying to verify the need for a 20,000uf filter, I don't think the answer will be clouded by the difference between 20,000 and 19,100 say.

And imagine if your pair of caps cost $100, what must a whole decade box full of them cost? As much as a new car?
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Old 12th May 2012, 06:22 PM   #3
AndrewT is online now AndrewT  Scotland
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I built a decade box for resistance.
I have never considered that I need do similar for capacitance.
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Old 13th May 2012, 12:10 AM   #4
cihtog is offline cihtog  United States
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Ok,

Lets look at cost for a sec - the caps in the link below are Nichicon KW series 1000uF/100V Audio Grade caps. I've used these (KWs) as replacement filter caps before, and am quite happy with the results, or lack of issues - however you wish to look at it.

This particular value is $1.71 each @ mouser, if you buy 25 to 49 of them. That's about $85 shipped (aiming high on shipping $15), for two rails @ 20KuF/100v, and they stay on my bench for the life of the caps... I was thinking more along the lines of multi-part cans and fewer steps between, but this example works for discussion. Especially when the 3300/100 of the same cap times 4, that I just bought are $7.44 each, or the more expensive monsters mentioned in my first post are almost ten times that...

UKW2A102MHD Nichicon Aluminum Electrolytic Capacitors - Leaded

I guess "decade box" might have been too specific of a term - "close enough box" may have been more appropriate - I didn't necessary mean as a precision tool for selection. A handful of any value(s) could be pictured. I don't normally keep any large filter caps on hand. If I'm going to buy them, it's because they are going into something, when they get here. Waiting a couple of days on parts from Mouser is not a big deal for me - as I don't have a need or desire for a large variety of on hand parts. In fact, having a BOM neatly packaged, all in the same box, is the only way to fly for me.

The idea is - on the spot substitution of large, expensive, mostly screw terminal, caps - where "replace them anyway" ventures beyond the realm of cheap and routine, and I really just want to know if they are causing the problem - subbing in an approximate value does that.

So... how about a century, millennium, or a bicentennial box?

The more I think about it, splitting the decade above even further, for subbing in more than just two rails (stereo amps) sounds appealing.

Is there any reason that it would not work, or cause problems?

Is there any reason to question the longevity of the caps?

Is there any reason to suggest that, "this is a really bad idea, don't do it"?

The suspected bad caps in my original post, are 15000uF/90v and normally around $60 each from reputable suppliers. If making a re-usable/variable substitute is feasible, I'd like to do it - but the question remains, "if it really is a good idea, wouldn't everybody be doing it?"

Thanks again for any input ;-)
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Old 16th May 2012, 05:55 PM   #5
macboy is offline macboy  Canada
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If you used just 3 caps: 2200, 4700, and 10000 uF, you can cover the following 7 values:
2200
4700
6900
10000
12200
14700
16900

That should cover it, even for the smaller pre-amp side supplies.

If you add a forth smaller value of 1000uF, then you will cover 15 values from 1000 to ~18000 with no more than 1500 uF between steps. With only 4 caps.
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Old 16th May 2012, 06:03 PM   #6
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Donīt forget to add some high value resistor to discharge them when not in use, and to not have a bad surprise.
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Old 16th May 2012, 06:47 PM   #7
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I have decade boxes for small values, but not large. There's always something sitting around that's close enough to substitute. Better to build or buy a bridge or capacitance meter that will measure both value and loss, and be done with it. It only takes a few parts to build a bridge and it can even be clip-leaded together for emergencies. Here's an easy one specifically for electrolytics- Build a bridge for electrolytics.
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Old 17th May 2012, 01:42 AM   #8
cihtog is offline cihtog  United States
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All great suggestions - especially the last one! I do happen to have a filament TF laying around, as well as DVM, signal generator, and scope. I may just give the bridge a shot, as I would rather have tools, than spare parts any day. Thanks guys!
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