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1372 HANDBOOK OF PROJECTION FOR
of the same transformer. The filament circuits are simple and rarely give trouble. When they do it is easily located and remedied.
The contacts between the tube prongs and the springs in the sockets must be kept perfectly clean, as must also the grid and plate contacts, which are, of course, not a part of the filament circuit, but set up a lot of crackling noises when they are dirty, just as do the filament contacts.
When testing the filament circuit only, turn the switch to the filament position. This leaves the plate supply dead and there will be no high voltage in the set.
On the front of the panel are four tubes and a meter. These and the nature of the sound reproduced, will usually give us a fair idea of what has happened. Fuzzy notes of music, or as it is termed in the lingo of projection, "whiskery" music, usually is caused by a tube that has gone bad. Usually this is accompanied by loss of volume.
An overloaded tube will cause a rattling sound at the horns, but this is not likely to happen in the 42 amplifier if the tubes are in good condition, as there is hardly sufficient amplification ahead of it to cause overload, save perhaps when using a film or disc recorded at a very high level.
A leaky condenser sometimes will set up crackling noises in the horns, though this will depend largely upon the location of the condenser in the circuit, the voltage across it, etcetera.
A broken down condenser usually will cause a crackling noise which can be heard coming from the spark itself; also it gives off a peculiar burning odor which,