Capacitor Tester repair-calibration

(©2008 Bob Putnak, all rights reserved.)  This article discusses repair and calibration information for a number of vintage capacitor testers that all use the same fundamental test circuit.   The article currently covers the EICO 950-series, PACO C-20, Knight KG-670, Heathkit C-2 and C-3, etc.  (More models will be added on an ongoing basis as I find the time. ) All of these models are almost identical in functionality and circuitry.

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Set of 3 PRECISION REFERENCE CAPACITORS – for capacitor tester rebuild (Sprague Tel-Ohmike, Heathkit, Eico, Knight, Paco, etc…) For those of you doing your own cap tester repair, I sell a set of precision reference capacitors for $33 free ship USA. You get the entire set of 3 reference caps (1 of 200 pf, 1 of 0.02 µf, and 1 of 2.00 µf). All caps accuracy 1% or better and the 200 pf cap will be variable for maximum flexibility.

You NEED precision reference capacitors to achieve an accurate capacitor tester. If you want a 2nd set of precision capacitors to rebuild another unit, or a set to test the accuracy of your own tester, you can buy two sets for $53 free ship USA.

Other values of precision capacitors are available by request. Email for details.

1 set of 3 precision caps – $33 free ship

2 sets of 3 precision caps – $53 free ship


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These units are known as a Resistance-Capacitance Bridge, an R/C tester, but are most commonly used as a capacitor tester. Extreme caution must be followed with any vintage capacitor tester because very high voltages are present during repair and while operating. They should only be used by knowledgeable technicians.

EICO 950A

EICO 950A

PACO C-20 tester

PACO C-20 tester

Knight KG-670

Knight KG-670

Heathkit C-3

Heathkit C-3 (dark color scheme)

Heathkit C-3 (light color scheme)

Heathkit C-3 (light color scheme)

NRI Professional model 311

Eico 950B

Eico 950B

These models use a balanced bridge that measures capacitance from 10mmf up to 5000mf and resistance from 0.5 Ω up to 500-MΩ. During component value testing, the magic eye tube serves as the null-indicator. When the bridge is far from balanced, the target area of the magic eye tube glows completely closed green, and in fact overlaps. As the pointer dial approaches balance point, first the overlapping disappears and eventually the entire target area of the eye tube is completely open (dark). The bridge is balanced when the maximum dark area is indicated, and you can then read the value of your component on the faceplate scale.

Most models use the magic eye tube for both Leakage testing and component value testing.  With the Eico 950A, leakage of Paper/Mica caps is indicated by the #1629 magic eye tube, but electrolytic leakage is indicated by the Neon bulb.

Any technician who repairs vintage tube equipment (such as tube amps, antique radios, vintage jukeboxes) will find that a quality capacitor tester is a useful test instrument.

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