Radio Broadcast (May 1929-Apr 1930)

Practical Talks to Service Workers — /// ROUTINE PLATE CIRCUIT TESTING By JOHN S. DUNHAM QRV Radio Service, Inc. jrom the standpoint of the amount of service required, the plate circuits of a radio receiver are more important than the filament and grid circuits. That is so because more apparatus is associated with the plate circuits than with the other circuits. We have the primaries of a.f. and r.f. transformers, a.f. and r.f. chokes and by-pass condensers, interstage and output jacks, and a loud speaker. With battery-operated sets we have B batteries. In socket-powered receivers we have voltage dividers, variable resistors if the power supply is an external B-power unit, filter chokes, by-pass and filter condensers, rectifier tubes, buffer condensers if the rectifier is of the gaseous type, and the high-voltage secondary winding and primary winding of a power transformer. Plate circuits are also of greater importance to the serviceman because the voltages employed are of a higher order than those in the filament and grid circuits. The comparatively high potentials are the active cause of open audio transformers, loud speaker coils, and filter chokes. They are responsible for the breaking down of by-pass and filter condensers. They cause shorts and leakage paths through insulation. Because of all that, the serviceman must spend a great deal more time studying plate circuits and the functions and behavior of the included apparatus than it is necessary for him to spend studying the comparatively simple and abbreviated filament and grid circuits. Routine Tests We have mentioned before the value of routine in circuit testing, and the following of a routine when testing plate circuits is exactly as important as its application to other circuits or other work of any kind. Tests should start at the sockets, with very few exceptions. Continuity, as well as voltage tests which can be made from the sockets should be made imder the load of the tube which belongs in the particular socket, and with all the other tubes in their respective sockets. In socket-powered receivers, and in batteryoperated receivers when the B batteries are partially exhausted, the voltage across any tube rises as the remainder of the load is removed by taking other tubes out, an effect which is especially pronounced in modern socket-powered receivers using the series system of voltage division. What we desire to determine by a voltage test is not the no-load voltage, but the voltage under the normal working load of all the tubes. In past years, before the general advent of the high-resistance meter for service work, the meters used constituted a load greater than the normal load of any one tube and some times greater than the total tube load. The high-resistance meter, either as a single unit or incorporated in a test set, usually requires one milliampere for full-scale deflection, which constitutes a load that is negligible in comparison with the load of the tubes. Therefore, the meter has a negligible effect on the voltage across (he lubes or across any one tube. Then; are some (roubles in radio receiv The Arrow Electric Company, Jersey City, N. J., make a feature of their completely equipped service laboratory. This view of the showroom shows the neat test panel of the laboratory in the background. ers that do not show up under no-load conditions, but do show up plainly under the normal load. Intermittent opens sometimes fall into that class, and high-resistance joints, such as one where rosin has gotten under a large proportion of the solder at a soldered joint, are practically always in that class. In some cases highresistance shorts also act the same way. For example, assume that a by-pass condenser across a load has broken down in such manner that its d.c. resistance remains high. Suppose the load which that condenser constitutes is about equal to the load of a 171a tube across whose supply it is connected. In that case a voltage test at the tube socket with the tube removed would show normal voltage. With the tube in, however, the total load on the supply circuit would be twice that for which it was designed, and a voltage test across it at the socket would show a voltage considerably below normal. Whenever you can do so, make your voltage and continuity tests under normal load conditions. Many of the troubles in a receiver can be determined definitely without testing further than the sockets, by the employment of general knowledge of circuits and their behavior under given conditions, in conjunction with a clear analysis of the various items of evidence presented by the tests at the sockets. If, after each separate test is made, the serviceman will stop before going on to the next, and think out exactly what the result of that one test means in relation to the problem he is attacking, he will in most cases save himself a lot of unnecessary testing, and a lot of valuable lime. For example, if an Ep test at a detector socket shows no voltage, or fluctuating voltage, but the same test at the first a.f. socket shows normal steady voltage, the trouble is obviously not a general one having to do with the whole plate supply, but is confined solely to the detector supply circuit. If, however, an Ep test at the first a.f. socket does not show voltage, but a test at the detector socket does, the trouble is not thereby confined to the first a.f. local supply, because in most sets, regardless of type of power supply, the plate supply to the first a.f. and all the r.f. tubes is a common one. In that case it becomes necessary to test at one of the r.f. sockets to determine whether the trouble has to do with that common supply, or is confined to the branch of that common supply which goes to the first a.f. tube. If tests show lack of Ep at two sockets whose plate supply is common, and normal or slightly high voltage is found at other tubes whose supply is not common to those two, those facts are very definite evidence. If the set is a battery operated one, the trouble must exist between the battery and the point where the common lead branches to the several tubes supplied. If the set is socket powered, the trouble must exist between the voltage divider and the branching point. Open Circuits When Ep does not appear at the socket or sockets of one of the usual three branches of the total plale supply, and the cause is an open, part of the load 9 6 • •» JUNE 1 9 29 •