[Collins] 51J4 Crystal Filter

[Dr. Gerald N. Johnson]

75 to 100 pf would be expected values, though in 75S IF cans the tuning 
caps are as much as 500 pf. The classic reference of receiver design, 
Radiotron Tube Designer's Handbook suggests 100 pf is a nice value so 
the tuning doesn't change too much when changing tubes that contribute 3 
to 8 pf to the circuit capacitance and may vary a couple pf from one 
tube to the next (especially if different brands).

Depending on the construction, IF transformer capacitors have been known 
to go open. And coils too, often from the corrosion of an activated 
solder flux on wire smaller than hair. An ohmmeter can show the coil 
conductivity, and there is a simple test for an open capacitor. 
Temporarily wire in a capacitor, ideally a 120 pf compression mica 
trimmer, and tune it for a peak. If it peaks other than minimum C the 
original tuning cap is open. Then unhook it, check the value and solder 
in a fixed capacitor that value. Or for $1.14 from Mouser buy several 
120 pf trimmers and leave them in the circuit if they work. 
http://www.mouser.com/ProductDetail/Murata/TZ03R121N169B00/?qs=sGAEpiMZZMvkLkkMVBW%252bEMgRGSj6TTuq83rzMXqvshc%3d
81-TZ03R121N169B00

The switch, the variable capacitor (especially the shaft wiper), and the 
crystal are other possible places to lose signal in that circuit.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association


On 9/24/2011 8:18 AM, Jon L. Turner wrote:
> Hello All,
>
> I am going through T101 and T102 on a 51J4 that I purchased. I am wondering if anyone knows the value of the capacitors inside the cans. My tester shows 75 pf on the one in T101. I am not seeing a value on them in the schematic. The receiver seems to be low on sensitivity and it is looking like it is in this area of the receiver.
>
> Thank you,
>
> JT
>