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
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.
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.
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.