Build this Radioman's R-C Bridge, April 1947 Radio News

While not many people are likely to build this R-C bridge circuit with vacuum tubes for use during design and troubleshooting, this 1947 Radio News magazine article has a good basic description of operation of any calibrated bridge circuit used to measure an unknown value. Interestingly, a "magic eye" or "cat's eye" tube is used in lieu of a meter movement to give a visual indication of an open, short, intermittence, poor power factor, and low "Q," as well as when the selected switch position correctly identifies the value of the resistor or capacitor under test. Note that in that era μfd = μF and μμfd = pF.

Fig. 1 - Front panel view of home-built test instrument. For correct identification of various controls refer to Fig. 3.

Complete construction details for a resistance capacitance bridge. This wide range instrument will find many applications in the service shop.

Here is a compact, wide-range resistance-capacitance bridge of marked usefulness, that can be built by any serviceman. Although it is sensitive, stable, and rugged, this instrument is inexpensive. Its attractive ranges will catch the eye of the amateur experimenter as well as the repairman. This small bridge is completely self-contained, as Fig. 1 shows, and it is fully a.c. operated. There are no tricks either in its operation or calibration, this instrument being entirely conventional and fool-proof and direct reading. We believe many radiomen will want to build it.

There are three capacitance ranges: 1 μμfd. to 0.1 μfd., 0.0001 μfd., to 10 μfd., and 0.01 μfd. to 1000 μfd. There like-wise are three resistance ranges: 1/10 ohm to 10,000 ohms, 100 ohms to 10 megohms, and 10,000 ohms to 1000 megohms.

The main dial has a single set of graduations from which both capacitance and resistance are read. The range switch, in its various capacitance positions multiplies the main dial readings by 10, 1000, and 100,000 and in "resistance positions multiplies the readings by 1, 1000, or 100,000. This arrangement provides the maximum simplicity and directness in operation of the bridge.

The exceptionally wide range of this bridge enables the operator to measure a variety of capacitances ranging all the way from small circuit values (down to 1 μμfd.) to the large capacitances of motor starter capacitors. Also, resistance measurements may be made from values considerably lower than 1 ohm to the high leakage resistances (up to 1000 megohms) encountered, say, in tubular capacitors.

Fig. 2 - Schematic diagram of RC bridge. Test instrument covers capacitance ranges from 1 μμfd. to 1000 μfd. and resistance ranges from 0.1 ohm to 1000 megohms.

The compact RC bridge has several special features worth mentioning. These contribute to its versatility and reliability.

Sensitivity. Very high sensitivity is provided by use of (1) a high-gain 6SJ7 bridge amplifier and (2) a high signal voltage. The signal voltage is approximately 50 volts at 60 cycles, supplied by the secondary of the signal transformer, T₁ (See Fig. 2).

The sensitivity is under full control, being variable from zero to maximum by means of the amplifier gain control, R₄. Sensitivity may be made so high that an almost imperceptible adjustment of the main dial will show up in the null detector.

Provision for External Signal Input. Transformer T₁ delivers a 60-cycle signal voltage to the bridge circuit. This will be satisfactory for all ordinary purposes. However, some operators may prefer to use a signal of some other frequency, such as 400 or 1000 cycles, for occasional experimental measurements. For this purpose, an external signal jack, J₂, is provided in the circuit. When a plug, connected to an external audio oscillator, is inserted into J₂ the jack contacts open and automatically disconnect the secondary of the 60-cycle transformer, T₁ from the bridge circuit.

Provision for External Null Indicator. The self-contained bridge null detector is a 6E5 magic eye tube which, driven by the 6SJ7 amplifier, makes a sensitive indicator. However, some operators may prefer to use an external null indicator (such as an oscilloscope, a.c. vacuum tube voltmeter, or headphones) in occasional experimental bridge measurements. Jack J₁ permits the quick connection of such an external indicator. Neither the 6SJ7 amplifier nor the 6E5 indicator need be placed out of operation when an external indicator is plugged in.

Power Supply. A regulated a.c. power supply is employed. Our experience with various small bridges convinces us that this feature is most desirable. A line operated (a.c.-d.c.) type of power supply would cause one of the measuring terminals to be "hot," especially on the resistance ranges, thereby creating a shock hazard. Amplifier and null detector operation tends to be unstable with an unregulated power supply.

Voltage regulation is obtained by means of the two gaseous regulator tubes connected in series, as shown in Fig. 2. The OC3/VR105 and OD3/VR150 together give a regulated d.c. voltage of 255 for the amplifier and indicator tubes. The regulator limiting resistor, R₁₀, with filter capacitors C₁₀ and C₁₁, provides sufficient filtration without having to use a filter choke.

The power supply components are small in size and accordingly do not require a great deal of chassis space.

Ease of Calibration. The builder has to calibrate only one resistance range on the main dial. If the range switching resistors and capacitors are selected with care as to their values, the other ranges then automatically "fall in line."

The bridge calibration can be made easily with a number of hand-picked resistors, as will be explained in detail later in this article. This is a decided advantage, since most radiomen find it easier to obtain accurately measured resistors than a set of precision capacitors.

Fig. 3 - Front panel layout showing placement of the various operating controls.

Power Factor and Q Indication. Capacitor manufacturers have not given servicemen much definite, practical information in the way of limiting power factor and Q values. For this reason, repairmen are inclined to interpret power factor percentages in a variety of ways. The circuit of the bridge shown here has not been complicated, therefore, by the addition of a separate power factor control. But that does not mean that a capacitor with a bad power factor or Q cannot be spotted.

Normally, the 6E5 shadow pattern opens quickly and cleanly at null. However, when a capacitor under test has high power factor or low Q, the opening of the eye is not clean - the width of "fuzz" on each side of the shadow being proportional to the power factor. A very bad capacitor will produce a narrow eye opening with a great deal of fuzz on each side. There is no blurring at all, but a sharp line between the dark and bright portions of the eye pattern, when a good capacitor is connected to the bridge.

Indication of Shorts, Opens, and Intermittents. On any of the resistance or capacitance ranges of the bridge, the 6E5 indicator will open wide at the extreme left-hand (zero) end of the dial for shorts, and at the extreme right-hand (full-scale) end of the dial for opens. Intermittents are indicated by a flickering of the eye, when the dial is set for null, especially if the capacitor or resistor is rapped sharply.

The complete bridge schematic appears in Fig. 2, and an examination of this drawing will show the measuring circuit itself to be of simple, conventional design. Capacitors C₁, C₂ and C₃ are employed as standards for capacitance measurement. Resistors R₁, R₂ and R₃ are standards for the three resistance ranges. The bridge balance potentiometer, R_A, is a 3000-ohm wirewound volume control-type component. The rather high signal voltage does not cause bad heating in this potentiometer.

The 2-pole, 6-position range switch, S₁ automatically shifts the standards and measuring terminals from one bridge arm to the other when switching from resistance to capacitance functions. This enables the main dial to read in the same direction for both resistance and capacitance.

The 10 ohm, 10,000 ohm, and 1 megohm standard resistors, R₁, R₂ and R₃, must be hand picked for exact values. An accurately calibrated ohmmeter may be used for this purpose, if no other instrument is available. The author found that not many resistors had to be checked at the store in order to obtain three satisfactory standards. If the reader desires, precision instrument resistors may be employed as standards and the need for hand picking thereby eliminated.

The 0.0001, 0.01, and 1 μfd. standard capacitors, C₁, C₂, and C₃ likewise must be selected for exact values. It is best to order directly from the supplier or manufacturer capacitors having a tolerance of 1% or better. If an exact 1 μfd. value cannot be obtained for C₃, it is advisable to accept a somewhat lower capacitance and to connect enough mica capacitors in parallel with it to build its capacitance up to the required 1 μfd.

The accuracy of the bridge depends a great deal upon the accuracy of these resistance and capacitance standards, and the individual builder will do well to select them with the greatest possible care.

The signal transformer, T₁, is a 2-to-1 ratio interstage audio transformer with its secondary connected to the 115-volt line. The proper primary and secondary connections are indicated by standard lettering in Fig. 2. This transformer provides a bridge signal of a little more than 50 volts; and on long test runs, no large amount of heating was detected either in the signal transformer or in the bridge potentiometer. Any other type of transformer may be used, provided it will deliver 50 to 60 volts to the bridge circuit.

The remainder of the circuit is entirely straightforward, consisting of a conventional voltage-regulated power supply, previously described, and a standard 6SJ7 amplifier and 6E5 indicator.

Fig. 4 - Rear view of resistance-capacitance bridge. While the two OD3/VR150 tubes are visible, the 6SK7 and 5W4 tubes cannot be seen in this photograph. They are mounted directly behind the two voltage regulator tubes.

Fig. 5 - Photograph shows placement of various under-chassis components.

The bridge is built on a metal chassis, 12" long, 6" wide, and 2" high, and a metal front panel, 14" long and 7" high. It is enclosed in a metal cabinet. The external and internal appearance of the instrument and the arrangement of its parts may be seen in Figs. 1, 3, 4, and 5.

The main dial has a 3 3/4" diameter metal plate to which has been cemented a disc of thin white cardboard on which the special scale is hand drawn. After completing the calibration and drawing in the graduations with black India ink, this dial plate is covered with a matching disc of transparent celluloid or other plastic to prevent soiling.

The range switch plate (see Fig. 1) is also made of thin white cardboard which, after lettering, has been cemented to the front panel and covered with transparent celluloid. Mark this plate Rx1, Rx1000, Rx100,000, Cx10, Cx1000, and Cx100,000, corresponding to the switch positions shown in Fig. 2 and listed in detail in the range data in that drawing.

All of the standards, except the 1 μfd. capacitor, C₃, are connected directly between contacts of the range switch, S₁, and are supported by this switch. C₃ is mounted on the chassis where it may be seen directly behind the range switch in Fig. 4. Note the two parallel-connected mica capacitors on top of, and shunting C₃.

The two voltage regulator tubes may be seen in the rear of the chassis in Fig. 4. The 5W4 rectifier tube is directly behind the left hand regulator tube.

The 3000 ohm-bridge potentiometer is mounted in the center of the front panel and is supported back from the panel on a Mallory RB 249 metal bracket.

The 6E5 magic-eye indicator tube is mounted from the back of the front panel, where it may be seen directly above the power transformer in Fig. 4. An Amphenol 58-MEA6 magic eye assembly holds this tube in position. The 5 leads from the socket of this assembly pass through a grommet-lined hole in the chassis for connections to points underneath. The 1 megohm resistor, R₉ is enclosed in the tube socket shell of the magic eye assembly. The magic eye escutcheon, also supplied with the assembly, may be seen on the front panel in Figs. 1 and 3.

The voltage regulator limiting resistor, R₁₀, is mounted between the two voltage regulator tube sockets, under the chassis (see Fig. 5). The "On-Off" switch, S₂; sensitivity control R₄, pilot light bracket, and jacks J₁ and J₂ also are mounted under the chassis and may be seen directly back of the front panel in Fig. 5.

By employing two Johnson type 44 ceramic feed-through units as the "measuring terminals" (see Figs. 1 and 4), these terminals are kept out of contact with the metal front panel. Another one of these feed-through terminals passes through the chassis to connect the bridge circuit with coupling capacitor C₄, which is under the chassis near the 6SJ7 socket. This particular feed-through terminal is seen directly under the bridge potentiometer in Fig. 4.

Connections between the measuring terminals and the range switch, and between the range switch and potentiometer R_A are made with bare bus wire. These rigid connectors must be run as directly and over as short a path as possible, in order to minimize circuit capacitances.

It is advisable to shield the lead running between sensitivity control R₄ and the 6SJ7 control grid and to connect this shield to chassis at both ends. This is very important in order to eliminate extraneous pickup, especially if this lead runs for more than about 1 inch. Fig. 5 shows this procedure to have been followed in the author's bridge. Note that the lead from the sensitivity control has been enclosed in shield braid.

A single return point (ground) must be employed in the 6SJ7 stage; and J₁, R₄, R₅, C₅, and C₆ must be connected to this point. The most logical point is the No.1 contact of the 6SJ7 socket, since this contact also grounds the tube shell. The common return point is connected to chassis by means of a short, direct lead.

Signal input jack J, must be insulated from the chassis and panel. It may be seen supported by a bakelite washer in a large clearance hole in the panel, in Fig. 1.

After the wiring has been checked; remove the 6SJ7 and 6E5 tubes from their sockets, open the lead between filter capacitor C₁₁ and the OD3/VR150 tube at the point marked "X" in Fig. 2, and insert a 0-50 d.c. milliammeter in this line. The negative terminal of the meter must be connected to the regulator tube. Set the slider on R₁₀ at a trial point about 1/4 of the way from the C₁₁ end of this resistor, insert the line plug into a 115-volt a.c. outlet, and throw switch S₂ to its "On" position. Note the milliammeter reading. Again set the slider on R₁₀ carefully to give a milliammeter deflection of exactly 30 ma. and tighten the slider at this point. The voltage regulator now has been adjusted; and the milliammeter may be removed from the circuit, the connection "X" restored, and the 6SJ7 and 6E5 tubes returned to their sockets.

The bridge calibration is made with an assortment of accurately known resistors - with range switch S₁ in its No. 5 (Rx1000) position. For the calibration, obtain resistors having as many as possible of the following values: 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 4500, 5000, 5500, 6000, 6500, 7000, 7500, 8000, 8500, 9000, 9500, 10,000, 11,000, 12,000, 13,000, 14,000, 15,000, 16,000, 17,000, 18,000, 19,000, 20,000, 25,000, 30,000, 35,000, 40,000, 45,000, 50,000, 55,000, 60,000, 65,000, 70,000, 80,000, 90,000, 100,000, 150,000, 200,000, 250,000, 300,000, 400,000, 500,000, 1 megohm, and 10 megohms. The reader will see that a number of these values may be secured by connecting several of the lower values in series. If one or more decade resistance boxes are available, they may be used very satisfactorily to obtain the above values. Also, a series of volume control-type resistors might be set to the above calibration values, provided some means is available for measuring the control-settings.

The following calibration procedure must be followed: (1) Connect bridge to 115-volt a.c. supply and throw switch S₂ to "On." (2) After tubes have heated and 6E5 indicator glows brightly, throw range switch to position 5 (Rx1000). (3) Connect 100-ohm resistor to measuring terminals. (4) Set sensitivity control so that 6E5 eye just closes, and adjust main dial (R_A) for null, as indicated by sharp, complete opening of 6E5 shadow. This null should be obtained at left-hand end of main dial. If it occurs at right-hand end of dial, reverse the two outside connections to potentiometer R_A. (5) Adjust sensitivity control R₄ for sharpest and clearest opening of eye pattern. (6) Mark this dial setting 0.1. (7) Repeat process with 200 ohm resistor, marking corresponding null point 0.2 on main dial. (8) Repeat with the various resistors listed earlier, marking dial points according to the following system: 100 ohms = 0.1, 200 ohms = 0.2, 1000 ohms = 1, 1500 ohms = 1.5, 10,000 = 10, 1 megohm = 1000, 10 megohms = 10,000, etc. The reading should be marked in hundreds up to and including 900, then from 1000 through 9500 divisions should be marked for each 500 ohms, from 10,000 to 20,000 should be marked in steps of 1000. From 25,000 through 65,000 markings should be at intervals of 5000: From 70,000 the readings should be marked at: 80,000; 90,000; 100,000; 150,000; 200,000; 250,000; 300,000; 400,000; 500,000; 1 megohm and 10 megohms.

It is not possible to calibrate a few points and then to divide the dial scale by hand to obtain the rest of the graduations since response of potentiometer R_A is not linear. It becomes necessary therefore to calibrate as many individual points as practicable. If the reader has the required resistors, he is advised to calibrate even more points between 500,000 ohms and 10 megohms than we have suggested.

After the calibration is completed, the dial scale may be drawn-in permanently with black India ink and covered with transparent celluloid or other plastic, to prevent soiling and marring.

Use of the bridge is straightforward and rapid. (1) Switch-on bridge power. (2) Connect capacitor or resistor to be tested to the "Measuring Terminals." (3) Set range switch S₁ to trial resistance or capacitance range. (4) Set sensitivity control R₄ so that 6E5 eye just closes, and adjust main dial for null, as indicated by wide, clear opening of 6E5 eye at some point along dial range. (5) Adjust sensitivity control R₄ for clearest and least "jumpy" operation of eye. (6) Read capacitance or resistance value on main dial and multiply dial reading by multiplier indicated by range switch setting. (7) If null is not found, switch to next R or C range, as case may be, and repeat adjustments. (8) If null occurs in upper fifth of main dial, where divisions are crowded and comparatively difficult to read, switch to next highest range to obtain null in wide open (more accurate to read) portion of dial.

A capacitor with high power factor or low Q may be discovered by a fuzzy appearance of the edges of the 6E5 shadow pattern at null. If no null point is found (either with a resistor or capacitor under test) and the eye opens sharply at the extreme right-hand (full-scale) setting of the dial, an open circuit is indicated. If no null point is found and the eye opens sharply at the extreme left-hand (zero) set-ting of the dial, a short circuit is indicated. An intermittent is shown by a flickering of the eye at null, especially if the capacitor or resistor under test is rapped sharply.

The operator will observe that by advancing sensitivity control R, the bridge may be made so sensitive that the eye indicator responds to individual wire turns of potentiometer RA as the main dial is adjusted!

If it is desired to employ some other bridge signal frequency than 60 cycles, obtain the desired frequency from an audio oscillator or combination of oscillator and audio amplifier, plugging the output of the external signal source into jack J2. When the plug is inserted into this jack, the internal 60-cycle signal source will be removed from the bridge automatically.

If it is desired to use some other null detector than the magic eye tube, plug the external detector into jack J. This connection will not disturb the bridge operation and no changes need be made. The bridge sensitivity con-trol will have no effect upon the external detector; however, this will be no disadvantage, since, in most cases, external null detectors have gain controls or input voltage adjustments of their own.

Build This Radioman's R-C BridgeApril 1947 Radio News

Build This Radioman's R-C Bridge
April 1947 Radio News