[Collins] More 75A-4 Recap

[Dr.Gerald Johnson]

Are you SURE you should be poking your soldering iron into this receiver?

Generally the electrolytic capacitors are dead from old age and the 
molded oiled paper capacitors have been leaky since they were wired 
into the radio, but mica tend to be fairly reliable.

There is a large size difference between 0.5 PF and 0.5 UF... The 200 volt 
rating hints at 0.5 UF paper, undoubtedly leaky. The 200 volt 0.5 UF 
paper would be between 3/4 and 1" diameter 2 to 2-1/2" long and the 
0.5 PF ceramic would be on the order of 1/8" diameter and 1/4" long. A 
huge size difference!

Allied Electronics shows a 4 pf dipped silver mica (molded mica haven't 
been made in nigh on 50 years) stock number 862-3103 Cornell Dubilier 
type CD6CD040C03 500 vdc rating, $1.67.

For the 0.5 UF 200 volt use an orange drop, Mouser catalog number 75-
225P200V0.47, Vishay part number 225P47492YD3, 0.47 UF 200 volts. $1.82

O.5 PF has been available from capacitor makers. It would be tiny. 
What is the function of V-22 pin 6? Is it audio or RF? Audio wouldn't use 
the 0.5 pf capacitor, only VHF RF would. Stray capacity to ground from 
wire and socket pin would be more than 0.5 pf, most tube input and 
output capacitances run more like 5 pf. 0.5 pf would be most useful as a 
coupling capacitor in a 144 MHz multiply tuned circuit filter and if all else 
fails two 1 pf capacitors in series make a workable solution. 0.5 pf makes 
a nice 50 ohm circuit coupling capacitor at 10 GHz, maybe a bypass for 
solid state circuits a 24 GHz. Part in a tube receiver, nah.

73, Jerry, K0CQ, Technical Advisor to the CRA

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.