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Old 12-08-2016, 09:30 AM
madlabs madlabs is offline
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Sencore LC76, calibration and operation

I picked up a Sencore LC76 capacitor inductor analyzer for free. It didn't come with the power supply but it has a 12V input where the battery would go so I hooked it up to a 12V power supply. It lights up and fundamentally works but seems a little off in the pF range. I don't have a very good test setup, but here is what I have. I have a Fluke 117 and a cheap MK-328 tester. The 328 agrees with the Fluke quite well up in the nF range. The Fluke doesn't have pF range but testing some pF caps I have with the 328, including some 1% ones, the 328 seems to be right on the money. So I am measuring the Sencore against the 328. If I fiddle with the lead zero adjuster I can get it pretty close and it does fine on larger values. I think it is close enough for my purposes, it's not like I do anything exacting.

However, I am more interested in the accuracy of the leakage tester. That I really want to use. Since it is off in measuring capacitance values, does that effect the leakage test? Or are they separate circuits? Can one use a resistor to test the leakage? I tried this and it doesn't seem to match up. I took a 1m resistor, which measured out at 1,350,000 ohms. Then I tested it at 25, 50 and 100V. It should have read 96.6, 48.3 and 24.1uA respectively. But the Sencore showed 10, 80 and 140uA. Way off and far from linear. Is this a valid test of the leakage function? I should note that I measured the test voltages and they are fine.

I measure leakage in quite a few caps as well. Since I had the container of old parts from working on my Predicta handy, I tested a bunch of the old caps. They ranged from 0uA to 180ua to one that was over 10KuA and so out of range. Some of the values we 30, 19, 2,47 and 2uA. Then I tested a bunch of new 400V, .1uF film caps and they ranged from .3 to 1uA. Do these results make sense? While I see charts for aluminum electrolytics and tantalum caps, I don't see charts for paper, film, ceramic and mica. I take it that these one has to refer to the data sheet, but there must be some rules of thumb.

Then I starting playing with electrolytic caps. The Sencore agreed pretty well with the Fluke for value. Testing some slightly older but quality caps the leakage was well withing the charts and slowly ramped down. I was given a variety pack of cheap caps and most of them did fine. However, one whole bag of 100uF 50V caps kept climbing up and up until out of spec.. Quality control was out back having a smoke that day! Needless to say I tossed that bag and a good lesson that buying cheap caps (which I didn't) is stupid and to test caps before using them is a good plan.

So, any advice on using this gear or calibrating this gear? Any rules of thumb for different types of caps? Think I can use this gear for my crude purposes?

I haven't even started on the inductance and flyback and yoke tests but I will and I'm sure I'll have more questions!

Thanks!
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Old 12-08-2016, 09:57 AM
Findm-Keepm's Avatar
Findm-Keepm Findm-Keepm is offline
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Use a resistor to measure leakage - ohms law applies. I have the LC102 and the calibration uses several high-megohm values in calibrating the leakage at various voltage points.

I use a Sencore Field Calibrator originally sold with the LC53 and LC75 to verify calibration along with some Siemens 1% polystyrene capacitors for the lower ranges. For leakage/ohms, I simply use ohms law and some resistors - don't worry about the tolerance - my LC102's leakage accuracy is stated at 2% up to 500V, IIRC. After that, the leakage accuracy is quoted in percent plus an +/- least-significant-digit value. My inductance calibration checks are with some really nice mil-surplus coils with 2% tolerance. So far, spot on.

To be painfully honest, the LC102 leakage function is hardly used, as I can replace a cap quicker and cheaper than pulling one, testing it, and making a decision. Time=money...

Ringing tests on older flys and yokes is accurate, but the newer IHVTs sometimes trip up my Sencore - so I go the HR Diemen flyback tester (tests at real voltage values...) - this works with all but some Mitsubishi flybacks, which are like Hen's teeth anymore, so no worry..

All the LC-series from the LC75 and up are about the same schematically - just added features or ranges. Handy, but for real convenience, I use portables mostly anymore, and leave the LC102 to find real faults with stuff. The leakage/high resistance check is nice for measuring HV resistors and it'll even light up Nixie tube segments for testing.
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Last edited by Findm-Keepm; 12-08-2016 at 10:05 AM.
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Old 12-08-2016, 11:57 AM
madlabs madlabs is offline
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I am not planning to test caps in old gear with en eye to re-use except micas and ceramics to see if they are ok to keep or need replacing, as one would usually prefer to keep those. I'm sure I will check old paper caps for a while as they are pulled out just educate myself on how many are bad and just how bad they are. I'd also use it to check new caps before installation, not just that they are good but that I have the right value.

And I just figured out that it is much closer than I thought. I decided to re-check my leakage test with resistors and realized that with the leakage range switch set correctly for the DUT that it is pretty close. Good enough, I'm happy with that.

So, is there a rule of thumb for unmarked/unknown mica and ceramic caps for leakage? I'm trying to get an idea for when trouble shooting for those times when they do go bad. Same for paper caps, what would likely still work and what is just plain shot?

Pretty happy with it for a freebie. Only bummer is that it kinda shoves my Heathkit C3 with its lovely magic eye off the bench.
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Old 12-08-2016, 12:41 PM
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Findm-Keepm Findm-Keepm is offline
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Quote:
Originally Posted by madlabs View Post
I am not planning to test caps in old gear with en eye to re-use except micas and ceramics to see if they are ok to keep or need replacing, as one would usually prefer to keep those. I'm sure I will check old paper caps for a while as they are pulled out just educate myself on how many are bad and just how bad they are. I'd also use it to check new caps before installation, not just that they are good but that I have the right value.

And I just figured out that it is much closer than I thought. I decided to re-check my leakage test with resistors and realized that with the leakage range switch set correctly for the DUT that it is pretty close. Good enough, I'm happy with that.

So, is there a rule of thumb for unmarked/unknown mica and ceramic caps for leakage? I'm trying to get an idea for when trouble shooting for those times when they do go bad. Same for paper caps, what would likely still work and what is just plain shot?

Pretty happy with it for a freebie. Only bummer is that it kinda shoves my Heathkit C3 with its lovely magic eye off the bench.
99% of the ceramic and mica caps give an error when testing on my LC102 - no detectable leakage. In 38+ years of repairing electronics, I've seen exactly two mica caps that were leaky. I've seen one leaky ceramic - it burned, in fact, in a Motorola B/w set, a victim of high humidity and being in a smoker set...

Tubular ceramics? (White or off-white body, color dots along the cylinder) -Leave 'em alone - they are never bad, and are in some circumstances fragile, so let them be. Dipped micas, leave them alone. Chiclet micas/Dot code micas, give them a check for value - if good, leave them alone. Disc caps - usually they drift (as they should, by the tempco..) but rarely fail, and when they do, spectacularly. Paper/Wax/Bumblebees/Black Beauties/Elmenco ceramics? Gotta go...

On the Electrolytic side, everything older than the year 2K should be checked and replaced as needed - but some brands demand replacement - namely the
Callins caps - black with red ends, particularly found in Zenith TVs and radios.

A little experience under your belt with the LC76, and you'll soon be a wizard with caps....
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