[R-390] 390A IF cans/ caps

[Gary Geissinger]

Digi-Key has 200 PF mica caps 1% tolerance and an F tempo with is essentially no drift.  They are $4.03 each.


Sent from Yahoo Mail for iPhone


On Thursday, January 17, 2019, 7:21 PM, Larry H <larry41gm at gmail.com> wrote:

Here's some more info on the caps in T501/2.  The top cap is a ceramic
radial (dog bone) with the colors starting on the left end: brown, orange,
white, black, and red.  The chart that I found that I'm sure matches it has
the 1st color as the tempco, brown = -30.  And, the 5th color, red, is the
tolerance = +/- 2.  There is no voltage rating.  It's probably not
important in this low voltage circuit.

The bottom cap is the 200 pf red mica rectangle that has been identified
and documented earlier.  It has no tempco, so I assume it's close to 0
since it's mica

My IF is humming along steadily and happy.

My Kielbasa was awesome today.

Regards, Larry

On Thu, Jan 17, 2019 at 4:05 PM Charles Steinmetz <csteinmetz at yandex.com>
wrote:

> 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
>
>
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