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Old 11-21-2017, 10:35 AM
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OEM Electrolytics drifting high???

While working on the preamp section of a 1966 Packard Bell Console RPC-66, covered by SAMS 902, I have found some common Nichicon caps, the last few I need to replace.

The problem: C315 and C316 are labeled as 25 mf @ 3 volt and measured 124ufd (!) on a Fluke DMM.

C317, 318 and 319 labeled 50 uf @ 6V measured 221 ufd. I will double check on the EICO 950 but expect the same and checking for breakdown voltage, which the 950 does up to 500v, on a 6 volt cap is just impractical.

Note that my intended new replacement caps, measure 26 and 48 uf respectively. I'm used to original parts measuring lower, but not this.

Do these caps fail by increasing in capacitance? I expected a decrease in capacitance attendant with an increase of ESR.
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Old 11-21-2017, 10:41 AM
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I've seen paper caps and orange drops that were starting to go open test noticeably above their capacitance rating on my Heathkit C3...I may have seen it with lytics too, but it has not stuck in my mind.
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Old 11-21-2017, 11:23 AM
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I wondered if the caps were mis-labeled, with a digit missing but look as though the shrink-wrap label was applied carefully.

This order of magnitude is unbelievable. If I look at Sams, I will probably confirm what should be installed.

I mean, this receiver worked before but not well. I wonder how much improvement will be noted just by having correct values. Now I'm afraid of what the resistors may measure.

The build quality and ease of access on this chassis rivals that of a Zenith, not exaggerating either
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Last edited by DavGoodlin; 11-21-2017 at 11:27 AM.
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Old 11-21-2017, 11:32 AM
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Could this be not a real capacitance increase, but some weird effect of leakage (?) fooling the meter?
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Old 11-21-2017, 12:23 PM
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Quote:
Originally Posted by old_tv_nut View Post
Could this be not a real capacitance increase, but some weird effect of leakage (?) fooling the meter?
That is what I've always suspected... What is actually being measured in these meters, when on the "C" scale? RC time constant?, current transmitted at a given frequency?, phase angle at a given frequency?

jr
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Old 11-22-2017, 08:03 AM
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I measured these and the power supply caps on the older-type eye tube cap checker and everything agreed with the Fluke.

I'm not sure what the digital meters with readings in ufd and nfd see with electrolytics, just varying impedances at a fixed applied frequency, the 950 uses a wheatstone bridge circuit.
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Old 11-22-2017, 09:27 AM
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As an electrolytic ages the aluminum corrodes and swells making the internals tighter, thus the uF goes up before it fails.
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