[R-390] Seeking Advice: First Contract Number IF Deck

[Larry H]

Hi Jacques,  I don't have any of the older IF decks, but I've thought about
this situation a few times.  Since the filter inductance or cap values may
have changed with age, I think there will probably be some benefit to
having the right cap values installed (I don't think the inductance would
change much over the years).  But, that would depend on the current level
of signals getting through them.  I think if they are all about the same,
then the cap values are probably OK.  If it needs more gain, then you might
try adding a temp trimmer across each one, one at a time, noting the gain
or loss as you connect it and tune it.  If the gain goes down as you
connect a trimmer, then you will need to reduce the fixed cap value,
first.  The other factor to consider is the flatness of the signal in the
band pass.  Is it higher on one end or the other?  Are the filters
otherwise OK?  Any audio distortion at low or high signal levels?

Sounds like a fun experiment.

Regards, Larry

On Tue, Feb 2, 2021 at 11:16 AM Jacques Fortin <jacques.f at videotron.ca>
wrote:

> Hi to all,
>
> I do not know how much "First Contract Number" Collins R-390A (14214-PH-51)
> are still in existence, but they came with an IF deck having no tuning
> adjustments for the input / output of the mechanical filters.
> Selected fixed capacitors values were used instead.
> Does it worth to modify this deck to use trimmers, like in the following
> versions ?
> How can I be sure that the original fixed capacitor values originally
> selected are still the right ones ?
>
> 73, Jacques, VE2JFE in Montreal
>
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