[R-390] capacitor analysis

[John Saeger]

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