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The airworthiness directive
50-48-02 COLLINS RADIO: Applies to All Model 51R-1 and 51R-2 Receivers Incorporating Allite or Thordarson 90/150 Cycle Filters in Their Tone Localizer Indicator Circuits and to All Model 51R-1 Receivers Which Have Not Been Modified in Accordance With Collins 51R-1 Service Bulletin Number 7a. This Note Applies Only to Receivers Installed in Air Carrier Aircraft.
Compliance required by July 1, 1951.
A. To minimize the possibility of complete or partial in-service failure of the 90/150 cycle tone localizer system, replace all Allite and Thordarson 90- and 150-cycle filters with Collins Radio Co. approved replacement filters as follows:
REMOVE
REPLACE
ALLITE WITH FUNCTION COLLINS NO. THORDARSON NO. (SCHRADER) NO. COLLINS NO. 90 cycle 673 0160 00 T51677 C222220-1 673 0206 00 150 cycle 673 0161 00 T51678 C222220-2 673 0207 00
United Transformer Co. P/N F5024, 150 cycle filters, Collins P/N 673 0161 00, are satisfactory and should notbe replaced.
(Collins 51R-1 and 51R-2 Service Bulletin No. 14 dated September 21, 1950, covers this same subject.)
B. To minimize the possibility of course reversal due to the loss of automatic gain control action, modify the Model 51R-1 receiver in accordance with Collins 51R-1 Service Bulletin No. 7a dated August 29, 1950.