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Old 6th Oct 2017, 2:49 pm   #1
Eichberger
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Default Should I replace these capacitors?

I have been testing the caps in my Tandberg 12 with an ESR meter but am not sure if they need changing. I know there are different schools of thought about this but I was planning on only changing them if there is an indication of a problem.

So there are quite a few small 2.5uf 64v electrolytics. I measured two and got ESR of 7ohms and 5.4 ohms. Capacitance was fine - over in fact 2.9uf.

There is a big (in size) capacitor for the motor which is 1.2uf 400v. ESR is 0.38 ohms.

There are also two big electrolytics in the psu 2500uf and 1250uf. ESR for those is 0.04 and 0.05.

Thanks for any advice or thoughts on these.
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Old 6th Oct 2017, 6:01 pm   #2
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Default Re: Should I replace these capacitors.

The 2.5uf are possibly leaky, that's why the value is reading high. Their ESR is too high.
The rest seem to be OK.
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Old 6th Oct 2017, 10:12 pm   #3
Phil G4SPZ
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Default Re: Should I replace these capacitors.

In my experience, if a capacitor reads significantly higher than its marked value, it's likely to be faulty.
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Old 6th Oct 2017, 10:40 pm   #4
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Default Re: Should I replace these capacitors.

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Originally Posted by Boater Sam View Post
The 2.5uf are possibly leaky, that's why the value is reading high. Their ESR is too high.
The rest seem to be OK.
Thanks for the advice. I've done a fair bit of diy with modern stuff but this is my first go at restoration so it's a steep learning curve!


Quote:
Originally Posted by Phil G4SPZ View Post
In my experience, if a capacitor reads significantly higher than its marked value, it's likely to be faulty.
Right, I wasn't away of that. The two big electrolytics came out quite high too. The 2500uf measured as 3427uf and the 1250uf measured as 1470uf. They both also have a corroded/degraded spot on the ends - see attachment. Should I replace. There is a fair bit of hum on the deck's output and I'm fairly sure there are no ground loop problems.
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Old 6th Oct 2017, 10:44 pm   #5
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Default Re: Should I replace these capacitors.

If in doubt, whip it out.
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Old 6th Oct 2017, 10:53 pm   #6
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Default Re: Should I replace these capacitors.

The bulge indicates some leakage - it's a deliberate weak spot, to allow controlled leakage and prevent pressure build-up that could cause an explosion. Replace on sight!
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Old 8th Oct 2017, 5:24 pm   #7
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Default Re: Should I replace these capacitors.

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Originally Posted by Phil G4SPZ View Post
The bulge indicates some leakage - it's a deliberate weak spot, to allow controlled leakage and prevent pressure build-up that could cause an explosion. Replace on sight!
Great, thanks. That makes sense.
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Old 8th Oct 2017, 5:41 pm   #8
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Default Re: Should I replace these capacitors.

Quote:
Originally Posted by Phil G4SPZ View Post
In my experience, if a capacitor reads significantly higher than its marked value, it's likely to be faulty.
This is a very good rule of thumb.
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Old 9th Oct 2017, 3:02 pm   #9
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Default Re: Should I replace these capacitors?

On the other hand, aren't electrolytics often speced as for example -20%/+50% so that statistically a high value would almost be expected.

I agree with the 'leakage causes high readings' sentiment for other types of caps though.
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Old 9th Oct 2017, 3:44 pm   #10
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Default Re: Should I replace these capacitors?

The 2500u could be replaced with a 2200u or probably 3300u and the 1250 with a 1200u. even if they are part of an RC filter the change in value won't hurt much. I wonder why those odd value's were chosen in the first place? A look at the schematic might explain things.

Low value electrolytic's tend to need replacing more than higher value ones IME. Again 2.2u might do as replacemments. Are they orange coloured caps by any chance? Also it may be possible to replace these with non electrolytics as low value electrolytic's are getting harder to find as they're no longer made.

Andy.
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Old 9th Oct 2017, 6:01 pm   #11
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Default Re: Should I replace these capacitors?

I was just beginning to think about this. The 2.5u caps (blue, not orange) are specified as -10/100%. I am assuming this means +100% -10%. So I'm thinking a 3.3u 20% cap should be fine?

There are a number of higher value Frako branded caps in metal cases. I've been reading that these all need replacing due to the possibility of shorting when they fail. Most of them are reading high, 25% or more over. ESR for 100u of 1.2 ohms.

Would Nichicon KZ or FG range be good replacements?
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Old 9th Oct 2017, 6:25 pm   #12
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Default Re: Should I replace these capacitors?

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On the other hand, aren't electrolytics often speced as for example -20%/+50% so that statistically a high value would almost be expected...
True, electrolytics are stated as having quite wide tolerances, but in my experience good ones usually measure very close to the nominal value. As is often the case, the actual capacitance value isn't that critical to circuit operation; it's high ESR or excessive leakage that upsets things. In the presence of such defects, separately or combined, many simple capacitance testers seem to get confused and register a high capacitance. Anything above about +10% should be cause for concern.
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Old 10th Oct 2017, 8:07 am   #13
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Default Re: Should I replace these capacitors?

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Originally Posted by Diabolical Artificer View Post
The 2500u could be replaced with a 2200u or probably 3300u and the 1250 with a 1200u. even if they are part of an RC filter the change in value won't hurt much. I wonder why those odd value's were chosen in the first place? A look at the schematic might explain things.
They were the standard values of the day, before the E3 series (1000, 2200, 4700) and E6 series (1000, 1500, 2200, 3300, 4700, 6800) became the norm. Resistors went with those series much earlier than capacitors for some reason. Could be because especially electrolytics had such wide tolerances that there was little point in switching from the 1000, 1250, 2500, 5000 etc series to the standard E3/E6.
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Old 10th Oct 2017, 9:14 am   #14
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Default Re: Should I replace these capacitors?

It is generally accepted that an electrolytic's capacitance falls with age. Therefore if an old one shows a rise in value much above its stated original value, despite the wide tolerance, it will be leakage that is being measured not capacitance.
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Old 10th Oct 2017, 1:33 pm   #15
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Default Re: Should I replace these capacitors?

"So I'm thinking a 3.3u 20% cap should be fine? " Yep.

"Would Nichicon KZ or FG range be good replacements?" Yes again, or Vishay/BC do good caps too. They do a good axial range. Get 105 deg C if you can.

F&T do nice caps for use on PSU's in the style of your old caps. Available from Hifi collective or Watford Valves amongst other places. Ebay prices tend to be a lot higher than Farnell/RS and there's a possibility you'll get fakes or iffy/old caps.

Thanks for the heads up Ricard.

Andy.
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Old 11th Oct 2017, 12:33 pm   #16
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Default Re: Should I replace these capacitors?

Thanks for all of your help with this. Got all the electrolytics ordered. Should I also look at replacing the little metallised paper film caps? I desoldered a 3300pf one and it measured 3500pf.
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