GROUND RADIO BULLETIN NO. 3 - APRIL 27. 1949___________________COMMS. DEPT. L.A.D
12ACX-2 TRANSMITTER TUNING
"Old timers” with the Company are well familiar with the tuning characteristics.of a 12ACX-2 transmitter» For the benefit of new comers, we would like to point out that the 12ACX-2 transmitter will break into self-oscillation when it is timed with little or no antenna load. This oscillation does not appear until the key is released» The meters will then continue to show plate and grid current indicating self-oscillation. This oscillation may be stopped by turning off the plate voltage switch or by detuning the buffer plate circuit* If the antenna is coupled to the transmitter, the self-oscillation will cease and tuning will be normal.
4WTFA
Station Mechanic Selin at POS has called our attention to a recent annoyanoe which can happen at most any station* You will recall that the ¿jWTFA has an optional 200 ohm resistor (R-62) in the oscillator circuit» This resistor, when in the circuit, allows the crystal to oscillate even when the transmitter is not keyed. The weak signal from the continuously oscillating crystal can cause interference at frequencies which are harmonics of the crystal frequency in a receiver looated close to the transmitter» In the case reported, the key relay adjustment was such that vibration caused the relay armature to chatter against the back contact. This caused a tuneable hum or buzz to appear at harmonics of the crystal frequency. The obvious remedy was to correct the relay contact and spring adjustment.
IMPORTANT
Modification ”H” to the 12ACX-2 and 12GLX-2 transmitters was designed to replace the power relays with mercury-type relays» Most of these transmitters have been modified but a few unmodified transmitters still exist» The spare contacts and colls for the old-type relays will soon be declared obsolete and removed from stock» It is, therefore, necessary that the Senior Communications man at each station advise the DCS office immediately of any mercury relay conversion kits for 12ACX-2 and 12GLX-2 transmitters that are required» If no answer is received from a station, we will assume the transmitters at that station are already converted.
Stations having RFA-50 amplifiers will soon receive a mercury time delay relay mounted on a bracket» This assembly should be mounted in place of the R-2QQ relay in the RFA-50. This is known as modification ”C”» Stations as BRO and POS, where the. idea originated, should replace their present mercury time delay with the new unit for standardization purposes. The R-200 relay will then be removed from stock.
