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
• 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

PRECISION REFERENCE CAPACITORS — For those of you doing your own Tel-Ohmike repair, I sell a set of 5 precision reference capacitors for $43 + $4 postage USA/CAN.   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.

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

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 can be downloaded as a PDF [to3 schematic].  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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