International projectionist (Oct 1931-Sept 1933)

INTERNATIONAL PROJECTIONIST VOLUME II NUMBER 4 JUNE, 1932 TESTING FOR 'SHORTED' FILTER CONDENSERS A. C. Schroeder MEMBER OF LOCAL UNION 130. LOS AJVCELES. CALIFORNIA FILTER condensers are used to filter or "smooth out" the current before it reaches the plates of the amplifier tubes. They remove the hum which would be present if the supply were to be used just as it comes from the rectifiers. They also act as a reservoir to store current and keep it in readiness so that it can be used for peak loads, which occur when the reproduction is very loud, especially on the lower frequencies. When one of the filter condensers punctures, the show stops; or should we say when the show stops, how do we know if the trouble is a filter condenser? If the condenser shorts completely, the sound will stop at once. The reproduction will be good until the last. It will not first become distorted, nor will the volume drop gradually. In this instance there is no indication that the trouble is a condenser or something else. If the set has a plate current meter it will read zero on the faulty amplifier. Circuit Similarity Glancing quickly at the tubes we wiU see that they are lighted, showing that the trouble is not a blown fuse. At the same time we will notice that the plates of the rectifiers are red hot. An overheated plate means that there is an abnormal current flowing in the plate circuit, and a shorted filter condenser would cause such a condition. Looking at Figure 1 we see part of the wiring diagram of the 42-A amplifier. This portion of the 43-A is practically the same as in other makes ; in fact there are many amplifiers of different makes that have practically the same or very nearly the same wiring. Any amplifier and current supply can be divided as shqwn in Figure 2. There is one exception, and that is when batteries are used. Even then it is sometimes required that filters be used, but usually for a different purpose. The dotted lines in Figure 1 divide the apparatus of the 42-A amplifier into three divisions. In the amplifier there is actually no division of parts, nor are they partitioned off. The dotted lines serve only to show the similarity of each section to corresponding sections in other amplifiers, as is shown in Figure 4, which is part of the wiring of one type of RCA amplifier. Tube Current Check While the components in any or all three of the divisions may vary in different amplifiers, they perform the saine functions and are subject to the same trouble which can be diagnosed in the same manner. The filter may have one, two, or three banks of condensers ; it may have one or more chokes. In some cases there is also a voltage divider, which really forms a fourth division, but this unit need not concern us here. [7] When no meter is present in the plate circuit we must ascertain if current is getting to the tubes by some other method, such as a voltmeter hooked from the plate to the filament terminals of the socket. If the plates of the amplifier tubes are red hot, it is sufficient indication that there is a flow of current and that the condensers are not shorted, for it they were, the current could not get as far as the amplifier tubes. This can be seen in Figure 1 and in Figure 4. When one of the filter condensers is shorted, the current flows from the rectifier filament through the plus wire on the left, up to whichever condenser is shorted, then back to the negative wire, to the high voltage transformer, and so back to the plates and the filaments of the rectifier tubes. It cannot get up to the amplifier tubes. When the condenser becomes only "partially shorted" the action is different There is not a steady flow of current through the punctured spot, but instead the current flows intermittently. This can be seen by observing the plate current meter, which will be jumping back and forth quite rapidly. The sound will have dropped to a low level and will be very distorted, sounding fuzzy and whiskery, somewhat like a person with a very hoarse throat. When the meter is seen to act as de