Roller-Smith bridge ohmmeter

There is nothing digital about a vintage Roller-Smith bridge ohmmeter.  It can measure from 0.5 ohms to 50,000 ohms using 4 ranges.  This is a beautiful antique test instrument.  It has an attractive hardwood case, leather handle, and quality construction.  

The meter operates on 3 vdc, which can be supplied using two D-cell batteries or a portable external dc  power supply connected directly to the battery terminals.

Usage is very simple.  Connect the resistance to be tested to the terminal posts, select the appropriate range via plug socket connection, push the Test button and rotate dial until meter nulls at zero.  At this point, the bridge is balanced, and ohms is read by multiplying the dial scale reading by the plug-socket range.

Bridge-type equipment has always been very accurate, and this is no exception.

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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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©2009, All rights reserved.

The article will discuss a cheap and easy way to test panel meters. The meter in your tube tester is an example of a panel meter. Occasionally you see flaky meter operation and suspect that the panel meter may be defective.  Most techs know that it may not be safe to test a panel meter by placing an ohmmeter across its terminals — this attempt can ruin the panel meter (depending on the ohmmeter used) and will not reveal all of the panel meter specs anyway.

Most panel meters in vintage equipment (such as tube testers or capacitor testers) will be milliammeters or microammeters.  The good news is that these panel meters can be tested, although extreme caution must be exercised.  One mistake and you can easily ruin the meter, so “do not try this at home” until you have practiced with a large variety of junk meters and have sufficient experience and confidence in your skills. Even then, “triple check” everything before you proceed.

Simpson 2600 Calibrator

Many people think that you need to have expensive equipment to test panel meters.  It would be nice to own a Simpson 2600 Laboratory-grade Calibrator (which cost $1,620 in 1965 and weighs 132-lbs).  But you can make do with the following basic equipment: a variable power supply, a quality resistor decade box, a quality multimeter with mA and µA capability, and another quality digital multimeter that will accurately test millivolts.  Likewise, you need an understanding of what you are trying to accomplish.   Keep in mind that Ohm’s Law teaches that voltage, resistance, and current are all interrelated, therefore a panel meter that has its faceplate scale in “volts” CAN be tested as an ammeter.

In the following discussion, I will demonstrate testing a panel meter.  You will (1) determine the full-scale spec of the meter (using the printed scale as your reference for full scale), (2) evaluate whether the meter action is smooth and accurate across its entire range, and (3) determine the internal resistance of the meter.

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(©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.

PRECISION REFERENCE CAPACITORS — For those of you doing your own repair, I sell a set of precision reference capacitors for $29.99 + $3.50 postage USA/CAN.  Complete set of 3 reference caps (1 of 200 pf, 1 of 0.02 µf, and 1 of 2.00 µf).  All caps guaranteed 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 $49.99 + $4 postage USA/CAN.

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

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

PACO C-20 tester

Knight KG-670

Heathkit C-3 (dark color scheme)

Heathkit C-3 (light color scheme)

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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