[R-390] 390A IF cans/ caps

[Charles Steinmetz]

Jacques wrote:

> For your question, it needs a 2.2nF (2200pF + 220pF in series makes a 200pF) but there is not enough space within those IF transformers to take a 2.2nF there.
> However, a 200pF, 500V "dipped" mica fits (I tried it !).
> For the reason why it takes a 200pF mica is to maintain the original temperature compensation curve of the tuned circuits.
> This is the  reason why the Collins engineers designed those IF transformers using a 200pF, 2% mica cap in parallel with a 39pF, ceramic which obviously have a temperature coefficient to achieve a ~455kHz resonance frequency that holds over the operating temperature range.
> The mica caps have their own tempco, despite it is small, but the coils themselves have an even greater one.
> If it was not because of that, why they do not used a single 240pF, 5% mica there ??

Correct, as far as it goes.  BUT:

Different mica caps have different temperature coefficients of 
capacitance ("tempcos"), and different ceramic caps have different 
tempcos.  So, to achieve the same stability over temperature that the 
radio originally had, you need to choose a 200pF cap with THE SAME 
tempco that the original part had, and a 39pF cap with THE SAME tempco 
the original had.

Trouble is, (i) you may or may not have information regarding the 
original tempcos, and (ii) even if you do, it's not the good old days, 
when many small capacitors were available in a variety of tempcos -- 
series like "P100" and "N750" -- so it may well not be possible to 
source capacitors today that have the same tempcos as the originals.

"P100" indicates a positive tempco of 100ppm/K.  "N750" indicates a 
negative tempco of 750ppm/K.  Also, there is nothing magical about the 
construction of the caps -- ceramic, mica, glass, or polystyrene are all 
acceptable to replace either of the existing caps, as long as the tempco 
matches the original, the voltage rating is sufficient, and they 
physically fit.

Best regards,

Charles