[R-390] capacitor analysis

[Helmut Usbeck]

If you change the caps to 1.0uf you might end up with some motorboating at
higher volume settings.  This happened to me when I did my audio mod.  Switched
back to .1 uf and everything was back to OK.  The real bottleneck is the
response of the output transformer. (among a couple of others)
Try www.zorkler.com for an audio mod in the local part of the deck.
regards,
Helm.  WB2ADT

John Saeger wrote:

> 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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