Radio Broadcast (May 1928-Apr 1929)

A Compact and Inexpensive ^Trouble Shooter SINCE the beginning of the radio art, trouble-shooting has been one of the bugbears of the engineer, experimenter, and novice. Modern broadcast receiving sets often break down when least expected, and many times at points which the designing engineer had considered invulnerable to anything short of earthquakes, lightning, or dynamite. Most readers of this magazine have spent an evening attempting to locate the source of trouble in a balky or inoperative set. After prying out coils, transformers, condensers, and other apparatus; after testing them, and finally after several hours work, it may have been discovered that the trouble was due to a weak tube or poor connection. In itself the defect could probably have been remedied in a few minutes: but hours were spent in locating it. A tester to help in locating defective apparatus would be of great convenience. The instrument described here is dedicated to the purpose of saving time and patience. The "trouble shooter" is used to test each tube in the set while the set is in operation. Defective apparatus connected in the tube circuits will usually change the readings of the meters in the tester. Defective tubes or improper A, B, or C voltages can also be determined. The tester will not point an "accusing finger" at any certain piece of apparatus, but will aid in localizing any trouble in the set, thus saving much time. The tester makes use of a tube base which is inserted in the socket of that part of the set where trouble is suspected. A four-wire cable from the tube base is connected through meters to a socket in the tester. An ammeter is in series with the filament circuit, and another is connected in the plate circuit. The tube is inserted in the socket of the tester, and the set turned on. The filament and plate current drawn by the tube are indicated by the meters. The meter readings will usually give a clue as to where the trouble is located. A WHAT PARTS ARE NEEDED LIST of parts for the "trouble shooter" is as follows: Ai — i Milliammeter, o-io mA (Weston model 506 or Jewell model 135), or 1 milliammeter, 0-25 mA (Weston model 506, or Jewell model 135) A2 — 1 Milliammeter, 0-500 mA (Weston model 506 or Jewell model 135) R — 1 10,000-ohm resistor, current carrying capacity 25 mA. (for 0-25 milliammeter) or 1 20,000-ohm resistor, current carrying capacity 10 mA (for 0-10 milliammeter) 51 — 1 Small d.p.d.t. switch 52 — 1 Push-pull, or toggle switch 2 Binding posts 1 ux socket 1 ux tube base 1 Panel, 8" x 3I", or other convenient size 1 Small cabinet to fit panel The 0-10 or 0-25 milliammeter, Ai, is used to measure the plate current drawn by the tube. If it is expected to test sets using the 171 or 210 type of tubes, the 0-25 milliammeter should be used. For sets using a 1 12 type power tube or no power tubes the range of the 0-10 milliammeter CT'HE "trouble shooter" described in this article has the virtues of simplicity in construction and operation, and low cost. It does not have the universal range of the tester described by Mr. Messenger in the July number of Radio Broadcast; it is designed to measure the filament currents, plate currents and plate voltages of d.c. tubes while the set in which they are used is in operation. However, as the author points out, in most cases of receiver trouble the information given by these measurements will localise the defective parts of the circuit and make a blind search unnecessary. The compactness and portability of the "trouble shooter," together with its inexpensiveness and simplicity, should make it a very valuable instrument to the service man. — The Editor. will be sufficient. When the higher reading meter is used, a reading of one or two milliamperes will scarcely be visible on the scale; therefore if possible, use the 0-10 milliammeter. [It is a simple matter to use a low reading milliammeter, 0-10 mA, for example, and to place across it a shunt when higher currents are to be read. Such a shunt may be constructed of part of a rheostat and connected to a switch which is installed on the panel. The rheostat could be mounted inside the cabinet, of course. — The Editor ] The 10,000 or 20,000-ohm resistor, R, used with the plate current meter permits the meter to be used to measure B-battery voltage when the voltage is impressed across the two binding posts. If the 0-25 meter is used in the tester, the 10,000-ohm resistance should be used, and the reading in milliamperes multiplied by ten to give the reading in volts. The full scale voltage reading will be 250 volts. When the 0-10 milliammeter is used, the resistance should be 20,000 ohms; and the reading in mils multiplied by 20 to give volts. In this case the full scale reading will be 200 volts. Only an accurate high grade resistance should be used in order to insure correct voltage readings. Good resistors are usually guaranteed to O be within five per cent, of their marked value; therefore quite accurate voltage readings may be depended on. The principle by means of which an ammeter may be converted into a voltmeter as described in the preceding paragraph is simplicity itself. A known resistance when connected across a source of potential will allow a certain current to flow. If an ammeter is inserted in series with the resistor to measure the magnitude of the current, the voltage applied may be calculated by a simple application of Ohms law: E = IR. If R is made a constant value, E is directly proportional to 1 at all times. Therefore the voltage being measured is proportional to the deflection of the needle on the ammeter scale. Suppose a 0-10 milliammeter is to be used to measure voltages up to 200 volts. In order to determine the proper value of resistance to be used with the meter, Ohms law is again applied: R= E/I. Two hundred voltsdivided byo.oi ampere gives us 20,000 ohms resistance. The multiplication factor is gotten by dividing the maximum voltage to be measured by the maximum reading on the meter. In this case 200 volts divided by 10 gives a factor of 20. This means that the reading in milliamperes multiplied by 20 will give the voltage. construction of the tester npHE 1 anc tube. 'HE tube base is taken off a discarded id should be of the ux type. Break away as much of the glass as possible, and then scrape out the cement. When the base is thoroughly cleaned out, solder a flexible lead about two feet long to each of the contact pins inside the base. It is advisable to use leads of a different color for each of the four wires to prevent confusion when the unit is completed. The handle for the tube base may be made from a piece of broom stick. Saw a piece about three inches long from the top of the broom — making sure, of course, that your wife is not in the vicinity. Bore a hole lengthwise through the center large enough to accommodate the four flexible leads which you have soldered to the base. Now, whittle down the end of the handle until it fits snugly within the base. Pass the four leads through the hole; coat the end of the handle with glue, and force it into the base. The four leads should be tied or taped together in several places in order to make one compact cable. The free end of the cable connects to the UX-Socket UX-Plug FIG. 1 273