REPEATER DESIGN — NEWFOUNDLAND-NOVA SCOTIA LINKS 257 



design value (and for which the tubes are aged), one of two values of bias 

 resistance can be selected for VI and V2, and one of three values of bias 

 resistance for V3. 



All capacitors subject to the high- voltage supply voltage are of the oil- 

 filled paper type, and the others are of the silvered-mica type. Inductors 

 are air-cored spools which are multi-sectioned when used in high-imped- 

 ance circuits. All resistors are of the solid carbon-rod type, except for the 

 input-transformer termination, which consists of two high-stability 

 cracked-carbon resistors, and those in the feedback network, which are 

 wire wound. 



The input and output transformers employ 2-mil permalloy C lamina- 

 tions, and the latter core is gapped on account of the polarizing current. 

 A narrow Perspex spool fits the center limb, and conventional layer 

 windings are used; the screen is a sandwich made of copper foil with 

 adhesive polythene tape. 



3.3 Mechanical Design Details 



The arrangement can be seen from Fig. 1. At the A-end is a cast-brass 

 pot containing the resistors providing the amplifier high-voltage sup- 

 ply, and as this is bolted directly on to the housing bulkhead, the heat 

 generated is readily conducted away. The remainder of the units are in 

 cylindrical cans mounted in the insulating framework formed by four 

 Perspex bars. These are sprayed with copper on both faces to guarantee 

 the dc potential on these surfaces and eliminate the risk of ionization 

 at working voltages. The cans are not hermetically sealed but are dried 

 out with the repeater when it is finally sealed and filled with dry nitro- 

 gen. Perforated covers on the amplifier allow air circulation to reduce 

 the ambient temperature. 



Fig. 4 shows a typical can assembly, and Figs. 5 and 6 the construction 

 of the amplifier. It will be seen that the latter is a double-shelved struc- 

 ture with tubes alternating in direction, and an amplifier path is located 

 on each side of the chassis; the input and output transformers are at op- 

 posite ends, and the feedback network is contained in a hermetically 

 sealed can in the center of the unit. 



All cans are finished with a gold flash which is inert and gives a clean 

 appearance stimulating a high standard of workmanship. Tin plating 

 was formerly used, but it has been shown that tin tends to grow metallic 

 whiskers.^ Unfortunately certain capacitor cans had to be tin plated, 

 and extra precautions consisting of wide clearances or protective shields 

 have had to be taken. The risks from growth on soldered surfaces is not 



