HINTS FOR BEGINNERS

Radio Record, Volume VII, Issue 8, 1 September 1933, Page 26

HINTS FOR BEGINNERS

A Selection of Useful Practical Hints for the Home Set‘Builder. © : "Warming Up" Howl: Many receivers usg transformer coupled audio stages badly while warming up. Changing valves does not affect a cure, nor does reversing the leads to the transformer. A simple remedy for this fault is to shunt the secondary of the first audio frequéncy transformer with a suitable resistor. Try a high value first, e.g., about 200,000 ohms, decreasing this until the howl is eliminated. Remember, however, that if too low a resistance is employed, the higher frequencies will suffer. Unwanted Capacity: In a shortwave set employing a metal chassis, it is sometimes advisable to lift some of the detector circuit components a little way above the metal base. This applies especially to the grid condenser if it is of the flat type, as quite a large

extra capacity from grid to earth may be produced if it is screwed down. flat on the chassis. Two small insulators of the reel type (once popular for indoor aerials) or two half-inch pieces of ebonite tubing, are, suitable for this purpose. As an alternative, one of the small condensers which can be soldered directly from the grid terminal of the dectector valve to the grid connection on the preceding coil might be employed. Using a Milliammeter: Every home constructor will find that a milliammeter is invaluable in keeping his set right ‘on form." It may be mounted on the front panel and permanently wired into the receiver, or used from time to time to make adjustments. When the set is new and working properly the current taken by each valve should be noted down by placing the milliammeter in the B+ lead to each valve, The total B current should also be taken by: placing the meter in the B- lead from the battery. The set should then be tested regularly about every month, and the readings obtained compared with. those taken formerly. If the plate current of the power valve suddenly begins to rise, you will know that in all probability the bias battery is out of order. Should the reading fall, the "A" or "B" bat-. tery is running down, or one or more

of the valves is losing its efficiency and may need replacing. If, when the meter is placed in the B- lead, the needle flickers substantially, it is an indication that the power valve is either being over-loaded or that the bias voltage applied to it is incorrect. To and fro flickers over three or four degrees mean: that overloading is occurring, while upward or downward kicks indicate too much or too little bias respectively. , .