[R-390] capacitor analysis

[JAMES T BRANNIGAN]

More 2 cents..........
Except for electrolytics, black beauties and C-553, I see no reason to
replace good parts in a functioning radio.
That said, after all the work to identify and remove a component, it is a
false economy to replace it with a cheap substitute.
I have also learned, the hard way, not to re-engineer Collins radios.
Jim
----- Original Message -----
From: "John Saeger" <john@whimsey.com>
To: <R-390@mailman.qth.net>
Sent: Saturday, June 01, 2002 3:50 PM
Subject: [R-390] capacitor analysis


> Well, I'm still thinking about which capacitors I'm tempted to
pre-emptively
> change in my R390A so I've made a list along with their function.  Maybe
> this will help decide what kind of capacitors would be good to use as
> replacements.  I started out with the list on Nolan's page.
>
> My feeling is that if a capacitor is a bypass capacitor, it's not that
> critical.  As long as there is enough capacitance, a high enough voltage
> rating, and good enough quality and reliability, they could be just about
> anything.  Orange drops are great for the small ones.  Nothing wrong with
> them.  But I think other less expensive things will also work just as well
> in these positions.  Polyesters or even ceramics.  For the 2uf paper and
the
> 8uf tantalum, I'm likely to use big polyesters.  I'd make the
electrolytics
> polyesters too if it wasn't so expensive to do.  For now, I think I'm
stuck
> leaving them electrolytic.
>
> The capacitors that pass signals matter more.  The famous C553 IF signal
> capacitor is probably the most interesting case.  Maybe a low-dissipation
> factor polypropylene like an orange drop could make an actual difference
in
> the performance of the receiver.  Maybe.  My guess is that the receiver
has
> plenty of gain, and the signal to noise ratio as well as the gain of the
> vacuum tubes in the signal path make much more of a difference.  And since
> so much is at stake if this one goes bad, I think quality, and overkill in
> the voltage rating is the way to go.  I haven't decided for sure what to
do
> with this one, but I kinda like the idea of a 2KV ceramic here.  Maybe
even
> a 3KV.
>
> The audio stage is very interesting as well.  And I think that maybe here
is
> where the mandate to make it cheaper really took it's toll.  They started
> out on the right foot with C531, which is in the IF deck by making it a
> 0.1uf.  But then things went downhill.  Personally, with the possible
> exception of C601 which is the negative feedback capacitor, I'm tempted to
> make them all big.  At least 0.1uf.  But maybe even bigger.  Maybe 1.0uf
> polyesters if I can fit them.  I might leave C601 at 0.01uf since by
having
> poor bass response in the negative feedback, one might get better bass
> response by leaving it small.
>
> So there's my 2 cents.  But since I haven't actually done anything yet,
I'm
> still very interested in other opinions.  And if I've labeled any of the
> functions wrong, I'm interested in hearing about it too.
>
> Here's the list:
>
> Main Chassis
>
> C101
>    0.22uf paper        100V   bypass
> C103
>    50uf electrolytic   50V    bypass
> C104,C105,C106,C107
>    .068uf paper        ???    bypass
>
> RF Amp
>
> C227
>    0.047uf paper       100V   bypass
> C256,C309
>    0.1uf paper         200V   bypass
> C275
>    0.033uf paper       300V   bypass
>
> IF Amp
>
> C504,C505,C517,C521,C528,C536,C538,C543,C547,C548
>    0.1uf paper         200V   bypass
> C529,C533,C534,C541,C545
>    0.033uf paper       300V   bypass
> C531
>    0.1uf paper         200V   AF signal
> C549
>    0.01uf paper        300V   AF signal
> C551
>    2.0uf oily paper    500V   AGC time constant
> C553
>    0.01uf paper        300V   IF signal
>
> AF Amp
>
> C601
>    0.01uf paper        300V   AF signal (negative feedback)
> C602
>    0.033uf paper       300V   AF signal
> C603
>    3x30uf electrolytic 300V   bypass
> C604,C605,C607,C608
>    0.01uf paper        300V   AF signal
> C606
>    2x45uf electrolytic 300V   bypass
> C609
>    8uf tantalum        30V    bypass
>
> Thanks, John
>
>
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