Sprague TO-3 Tel-Ohmike calibration

Sprague Tel-Ohmike TO-3

Sprague Tel-Ohmike TO-3

This article discusses repair and calibration of the Sprague model TO-3 Tel-Ohmike Resistor-Condenser Analyzer.    Serious electrical voltages are present, repairs should only be attempted by a qualified technician. Copyrighted by Bob Putnak, all rights reserved.

Introduction to model TO-3

Model TO-3 Tel-Ohmike “Capacitor Tester” is based on the older Sprague design.
It is a balanced-bridge design with the following specifications:

  • Capacitance testing from 10 pf to 2000 uf, spread across 4 ranges
  • Electrolytic Capacitance and Power Factor measured with polarizing voltage is applied

TO-3 advertisement in Mansfield catalog, 1950

  • Resistance testing from 2.5 ohms to 25 meg, spread across 3 ranges
  • Capacitor Insulation Resistance test
  • Capacitor leakage test using a true load from 0 to approx 660 V (factory design). Note that a good argument can be made to drop the overall polarizing voltage down to 500 vdc (as discussed below).
  • Meter reads in DC Volts (up to 750), DC milliamps (up to 75 ma), or Megohms (up to 2.5 gigohms).

Repair and Calibration of Model TO-3

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Sprague Tel-Ohmike precision cap set – $43 free ship USA

NEED PRECISION REFERENCE CAPACITORS? — For those of you doing your own Tel-Ohmike repair, I sell a set of 5 precision reference capacitors for $43 free ship USA.   First, you get 2 precision reference caps (1 of 2.0 µf, 1 of 0.02 µf.)  Your original 200-pf cap is left installed.  Next, you get a complete set of 3 precision caps (1 of  2.0 µf, 1 of 0.02 µf, and 1 of 200-pf) for use in testing and calibrating your machine.  All 5 caps guaranteed accuracy 1% or better.

This set is applicable to any Tel-Ohmike model, not just TO-3.

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Sprague TO-3 remastered manual & updated schematic – $8 free ship USA

Need a remastered manual and updated schematic for Sprague TO-3 Telohmike?
$8 free ship USA.


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The original schematic for TO-3 has several mistakes and ambiguities.  Therefore, I have revised the schematic, corrected errors that I have found, and added notes to make the schematic more user-friendly.  The schematic is available as part of my TO-3 paperwork package. To elaborate: I corrected significant drawing mistakes at switch 2F, added labels to define the functionality of range-standard resistors, added the trimmer pots to the bridge pot, added clarification to the installed values of several resistors, added a ‘Notes’ section for better overall understanding, and digitally cleaned-up the schematic with photoshop for excellent readability.

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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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