GROUND RADIO BULLETIN NO. 77 - JANUARY 4, 1961 COMMS. DEPT. L.A.D.
98A VHF TRANSMITTER MAINTEMCE DATA
A recent failure of a Wilcox 98A VHF transmitter, and the subsequent repair job pointed up the fact that D.C. resistance data on the various transformers is not available from the Wilcox manual or any other source.
The following resistance readings taken from two separate units, currently in service, may be of help in trouble shooting. This type of information is especially valuable in cases where only a partial breakdown or failure has occured.
D.C. Resistance Data on Transformers - Wilcox 98A VHF Transmitter 1. Power Supply
Plate Transformer Plate to Fiate 54 ohms
C.T. to Plate 28 ohms
C.T. to Plate 26 ohms
Primary .6 ohms
Filter Chokes 49 ohms
Modulator
Interstage Transformer Plate to Plate 200 ohms
C.T. to Plate 95 ohms
C*T. to Plate IO5 ohms
Grid to Grid 190 ohms
C.T. to Grid 90 ohms
C.T. to Grid 100 ohms
Modulation Choke Plate to Plate 57.5 ohms
C.T. to Plate 25.5 ohms
C.T. to Plate 27.5 ohms
1750 Tap to Plate 17 ohms 1750 Tap to C.T. 47.5 ohms 1750 Tap to Plate 75 ohms 1750 Tap to 2250 Tap 5. ohms
Any deviation of more than 5% in readings should indicate a strong possibility of failure of the unit so checked. These components are fine quality and winding resistances vary only slightly from unit to unit.
Additional checks have revealed that the plate supply in this unit is overloaded on modulation peaks by as much as 150 ma. Consequently, a small downward modulation can be detected during loud audio input. An even, normal voice input will show little power fluctuation, either up or down, in a transmitter operating correctly.
FORM 9211—2107
