Radio Broadcast (May 1928-Apr 1929)

SERVICING A RADIO RECEIVER •ervicing Home-Made Radio Receivers By B. IN THE repair of radio receivers the service man is confronted with two different types of problems; one manner of trouble is to be expected in the modern factory-built set, but it is impossible to anticipate the sort of defect which may be discovered in a home-assembled outfit. Fortunately, however, the man who "rolls his own" is not easily discouraged — he does not surrender to the extent of requesting the aid of a service man until he has exhausted his ideas as well as those of his friends. In the October article of this series there were listed, in the order of their importance, the causes of trouble experienced in the servicing of factory-made receivers. These disorders as well as others are encountered in home-made sets. However, very frequently when called upon to service sets of this type the poor results will be found to be caused by (i) the use of an incorrect, inefficient, or tricky circuit, (2) carelessness in wiring or assembling of parts, (3) the use of poorly selected apparatus, (4) improper arrangement of apparatus and wiring, and (5) all sorts of peculiar conditions which are the result of lack of knowledge on the part of the novice setbuilder. From the remarks given in the above paragraph it may be appreciated that in some cases, in order to make the set operate properly, it is only possible to salvage the apparatus and rebuild. However, the usual radio fan is not supercritical when regarding his own work; as a result a simple repair which will cause the set to operate— even if the performance is poor — is often considered entirely satisfactory. I might add that the inexpensive repair almost invariably is pre The third installment of a series of articles discussing the problems of a radio service man ferred, as the set-builder objects when it is necessary for him to pay for work he should be able to do himself. Home-made sets may be divided into three distinct classes, namely, the completely homemade set, the home-assembled kit set, and the home-modernized factory-built receiver. The first type usually presents the greatest problem to the service man, the kit sets often can be repaired without too much difficulty, and those which fall in the last class may or may not be reconditioned, depending upon their design, but frequently they should have been junked years ago. AN UNUSUAL SHORT CIRCUIT \ A / HILE on the subject of home-constructed ' " radio receivers, a few experiences with sets of this type may be of interest. A set which recently was given to us for repair typifies the unusual conditions which frequently are found. This particular set was made by a very careful workman; every connection was carefully soldered, the circuit was correctly wired, the best available parts were used throughout, and the construction was beyond criticism, except for one detail. An examination of the receiver showed that each by-pass condenser — they were 179 of the moulded-bakelite type — was mounted to the base panel with a machine screw passing through a hole drilled in the center of the condenser. Of course, drilling the holes through the condensers caused a short circuit in each case, thus making the receiver entirely inoperative. The builder of the receiver, admitting that his knowledge of radio was limited, explained that in constructing the set he tried to improve the mechanical design which was described in a newspaper radio supplement. Another interesting experience with a homeconstructed set proved very baffling for some time. The builder of this set had had considerable experience in building receivers for himself and his friends, and he had been very successful in most cases. However, the set in question proved to be his Waterloo. On the surface the construction of this set appeared to be perfect, but an electrical test showed many shorts throughout the circuit, The wiring was checked from beginning to end and it was found to be correct in every particular; nevertheless, shorts were existant in all parts of the circuit. Finally it was discovered that the front and base panels of the set were made of "self-shielding" material (these panels present the appearance of bakelite, but are imbedded with a wire mesh) which was as effective in short circuiting the various parts as an uninsulated metal chassis would have been. After insulating the apparatus and wiring from the wire mesh in the panels the set performed perfectly. A third experience concerns a home-constructed "Hi-Q 28" kit set which was wired for operation with a. c. tubes. The set was carefully