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 Post subject: Why are capacitor testers like heathkit and eico not accurat
PostPosted: Mar Thu 01, 2018 5:20 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3195
Location: Waukegan, IL
Why are Heathkit and Eico capacitor checkers like the Heath C3 or Eico 950b not accurate on the lower third of either end of the dial when performing resistance and capacitance value checks? but they are accurate toward the middle. and is there a way to fix this?

and yes, mine are recapped if anyone asks.

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Tony Pignaloni


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 Post subject: Re: Why are capacitor testers like heathkit and eico not acc
PostPosted: Mar Thu 01, 2018 7:37 pm 
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Joined: Jun Sun 23, 2013 9:03 pm
Posts: 407
Location: Canandaigua, NY
Don't know much about 'em, but the C3 is a bridge and should be accurate if the reference caps are accurate and have low losses. If old paper caps, then not so much. Also, check to see if it measures series or parallel and remember that the loss dial is power factor, not dissipation factor as is common today. Over some of the range they're probably near equal anyway. What kind of caps are you measuring? The only other thing I can think of is how good the pot is. Remember that GR and others used very large and very accurate wirewound pots, giving excellent dial accuracy. A cheap and physically small carbon pot won't do nearly as well.


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 Post subject: Re: Why are capacitor testers like heathkit and eico not acc
PostPosted: Mar Thu 01, 2018 10:04 pm 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 3462
Location: Florida
I have cap testers from EICO, Heath, Knight, PACO, and NRI. They all used decent wire-wound pots and other components in the bridge circuits and work well but they are not precise laboratory instruments and were not mean to be. They were intended to identify bad caps and do a good job of that.

RRM


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 Post subject: Re: Why are capacitor testers like heathkit and eico not acc
PostPosted: Mar Thu 01, 2018 10:53 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 17937
Location: Albuquerque, NM 87123
I would say the "accuracy" is in the 3% range for Heath and EICO. This was considered good enough for parts that were rated at 10 or 20%.

Has your equipment been refurbished? For the Heath C3 I use 2 watt 2% resistors in the divider network and I match a couple of mylar 1uf caps to make the 2uf capacitor; I also use closely matched .01uf caps to make the .02uf capacitor. This will improve the accuracy, but it certainly does not make the C3 laboratory grade.

Note: the 10k resistor in the divider network needs to be changed to 5.6k to make the 25 volt range accurate.


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 Post subject: Re: Why are capacitor testers like heathkit and eico not acc
PostPosted: Mar Thu 01, 2018 11:32 pm 
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Joined: May Wed 18, 2011 2:40 am
Posts: 3620
Location: Littleton, MA
Capacitance bridges work by using a potentiometer to compare the ratio between the standard capacitor and the capacitor under test. If the capacitor under test is near the value of the standard capacitor, when the bridge is balanced the potentiometer will be near the half-way point. If the capacitor under test is far away from the value of the standard capacitor, when the bridge is balanced the potentiometer will be near the one of the ends.

Now the ratio between the two sides of the potentiometer (wiper to one end, wiper to the other end) doesn't perfectly follow the angle of the rotation of the potentiometer. The amount of error is the "linearity error" of the potentiometer. A 10-turn pot might spec a 1% linearity error.

Now let us apply this to your capacitor testers. Let us assume the cheap pot in the heathkit or eico has a 2% linearity error as an example, and has a value of 100K ohms. When the capacitor under test is the same value as the standard capacitor for the range, the pot is centered --- 50K ohms on one side, and 50K ohms on the other, ideally. Now look at the effect of the linearity error of the pot. With a 2% linearity error, when the pot is centered the resistance on one side might be 49K ohms and the other side 51K ohms. The ratio of the two sides is not affected much by the linearity error, it is still nearly 1:1.

But if the capacitor under test is one-tenth the value of the standard capacitor for the range, or ten times the value, the bridge will null with the pot roughly at 9K ohms on one side and 91K ohms on the other. Now a 2% error in the low-resistance side of the pot might be 8K ohms on one side and 92K ohms on the other, a ratio of 12.5 rather than 10, or a 25% error. You can see how the error is multiplied by the ratio between the two sides of the pot.

So, when the pot is near the center, the bridge reading error is roughly equal to the linearity error of the pot. But when the pot is 9% away from one of the ends, the bridge reading error is roughly equal to ten times the linearity error of the potentiometer. Thus the accuracy of the bridge degrades when you measure values near either end of the range.

A laboratory quality bridge will have a range every decade, instead of every two decades like the Heathkit and the Eico, to avoid this problem.

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Steve Byan http://www.byan-roper.org/steve/steve-at-play/


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 Post subject: Re: Why are capacitor testers like heathkit and eico not acc
PostPosted: Mar Sat 03, 2018 7:40 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3195
Location: Waukegan, IL
ok. that makes sense.

i have two eico 950bs. one pot measures 10.8k. the other ones measures 9.5k.

the weird part is, instrument a is accurate on the left side, while instrument b is accurate on the right side.

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Tony Pignaloni


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 Post subject: Re: Why are capacitor testers like heathkit and eico not acc
PostPosted: Mar Sun 04, 2018 12:23 am 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 3462
Location: Florida
What did you use for the bridge caps (200pf, 0.02uf, 2uf)?

RRM


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