Tips and Jottings

Radio Record, Volume II, Issue 10, 21 September 1928, Page 30

Tips and Jottings

Use Good Resisters. ,, Too much care cannot be taken: in the choice of resistances for use in any part of a receiver, for there is no more prolific source of artificial’ "static" than faulty resistances. © . R.F. Instability. ‘THE failure to stabilise an R.I. citcuit may be due to a number of causes, the most common being shortcircuited turns in either the primary or neutralising windings on the R.F. transformer, high-resistance contact between the transformer pins and sockets on the base, or faulty neutralising condenser. Should high-frequen-cy oscillations find their way into the audio portion of the receiver, this trouble will arise, but incorporating an R.F. choke in the plate circuit of the detector valve should preclude this possibility. The components concerned should be carefully examined for uny of the faults mentioned above, special attention being given to the neutralising condenser. Certain types have a rather high minimum capacity, but in most cases this can be reduced by stripping off a few of the plates. High-resistance contact between pins and sockets can usually be traced to either dirty pins or sockets; alternatively, if the split type of pin is used, this may require "splaying out" before a good contact will result. A careful examination of the R.F. transformer will enable any short-circuit-ed turns to be located. Grid Leak to Filament. N some cireuits it will be noticed. that the grid leak is placed between grid and fdlament instead of across the grid condenser. It will be found in most cases that very little difference is noticeable whether the grid leak is connected divectly across the grid condenser or from the grid to the filament. In certain receivers, ' however, it has been found that slight--ly better results are obtained by using the grid-to-filament connection. In this ‘ase the end of the grid leak farther from the grid of the tube should go to A positive in order to give the grid the positive bias’ necessary for gridleak detection with special tubes. If the detector is the first tube of the set and the grid return is connected to the positive side. of the filament circuit. the leak may be used directly across the grid condenser, since the grid will obtain its positive bias through the tuning coil. The same applies when the detector tube follows a stuge of radio-frequency amplification, if the grid return of the R.¥. transformer goes to A positive. — Since this is the usual connection of the grid return in most receivers, the placing of the grid leak across the grid © condenser will probably prove most sitisfactory. -. Sd