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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