A Simple Test

Radio Record, Volume II, Issue 37, 28 March 1929, Page 31

A Simple Test

‘VERY few receivers employing one ; RE valve followed by a detector reaction function perfectly. It is the reaction control that indicates in the first place whether the RF valve is completely neutralised. Here is a simple test to try. See first of all whether the receiver oscillates with the tickler coil, or. condenser in the case of throttle control, at zero. If on the lower wavelengths the receiver oscillates and is out of control, then the first valve is not balanced. Now tune to a distant station and see if by moving the reaction dial the valve goes in and out of oscillation at exactly the same reaction dial readings. If there is any lag and the dial has to be thrown further back than the point at where it went into oscillation, the first thing to look for is an nonneutralised RF valve. Now it often happens that the neutralising condenser is of too low a value, and this can be soon rectified by adding a little parallel capacity. More often than not it is interaction between the two main tuning coils that is the source of the trouble. Remedies for this complaint are obvious. Separate the coils, adjust their angles with respect to each other or, better still, screen one or both. Usually, to screen one only will be found sufficient, and if this method is reverted to always screen the coil upon which reaction is applied.

It not infrequently happens that areceiver employing some form of reaction suddenly refuses to oscillate, with a consequent loss of sensitivity. One or two ~* the most common causes of the trouble may interest some who have experienced this complaint. First of all check over the detector plate voltage. The battery may have run down, as the B negative to B + det. cells in the battery have more current drain than the remainder. ‘Try the effect of giving a higher voltage on the detector. Another point to watch is that the grid leak is connected via the coil in many cases to the filament positive wiring. A higher resistance grid leak makes the valve oscillate more readily. Try 3 to 7 megohms instead of the usual value of 2 meg~ ohms. By very slightly deneutralising the RF valve sometimes gives better control of reaction, although this is not always advised. A faulty grid condenser will cause the detector valve to become "off-colour." The only. remedy here is to replace with a new and tested one. A fixed condenser of .0005 to .002 mfd. capacity between terminal P on first audio transformer and B- will prevent choking due to the amount of wire on the primary of transformer. A detector valve that has lost its emission becomes dead, and a replacement of a faulty valve is obvious. A valve of low impedance usually oscillates smoother than a valve of. higher impedance. A few more turns on the tickler may improve.