[R-390] 390A IF cans/ caps

[Jacques Fortin]

Hi Roy,

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 ??
For the assumption that a 220pF mica can be fitted and the coil value compensated (adjusted) for, OK but what about the temp. behavior ??

73, Jacques, VE2JFE