GROUND RADIO_______________bulletin no. 9 - July 27, 1949 COMMS. DEPT. L.A.D.
SPARE PARTS KITS
Effective immediately, the following changes should be made in the Spare Parts Kits. These changes, and the changes listed on Bulletin No. 7, should be noted in your Maintenance Manual on Dwg, GEN-25B-958-l.
Remove Condenser .000015MF 665-A-18207-150
Remove Coil 650-1060 KC 93U-8216
Change Transformer T54 8196-5706 to T-51770 8196-8086
Change Resistor 150,000 ohm 8191-7329 quantities from 0-0-2-2 to
3-3-3-3 on station classifications
12GLX-2A MODULATION
It is standard practice to detune the buffer coil of a 12GLX-2A transmitter so that the PA plate meter reads 20% less with the ICW and Oil keys down than it does with only the CW key down. Certain old buffer coils cannot be detuned enough to give the desired 2056 change in meter readings. In this case, new buffer coils have to be ordered. All coils supplied for the last few years and presently being supplied are of the new type.
With insufficient detuning, the difference in meter readings is greater than 20%. Vie have had a few cases where the excessive difference in meter readings was due to component failures. In one case bleeder resistor, item 33, was open destroying the voltage regulation to the modulator and buffer stages. In three other cases one or more of the buffer screen dropping resistors, item 20, had opened or changed value. Item 20 consists of three 150,000 ohm 2-watt resistors in parallel. Failure of any of these resistors reduces the buffer screen voltage and, consequently, the drive to the PA tubes. The PA grid current will fall below the required 55 Ilk and the ICW meter readings will be too low indicating overmodulation.
It must be pointed out that the system of modulation employed in these transmitió ters results in decreased antenna current under modulation. Excessive modulation changes in meter readings will only reduce the aerophare output and decrease the service area in which automatic bearings may be taken by aircraft.
