[R-390] capacitor analysis

[Walter Wilson]

John,

I guess I don't understand what the big deal is.  You can replace every
paper capacitor in the R-390A plus the tantalum for about $30 or so.  And
when you replace them all, you're going to invest many hours doing the
replacements.  If you are going to all the trouble to replace them, why
worry about saving $15.

Below are a few items from others that have been posted to this list
previously.  I hope this helps you make your decision.  Just be careful not
to redesign the radio too much. [;-)

1) Ceramic capacitors tend to have hysteresis and other problems. What's
that mean?   It means their value changes slightly with applied voltage. So
their capacitance "wobbles" around applied audio and DC voltages. The
high-K, high value ones also have a lot of long-term capacitance drift, up
to 10% over years. None of this take away from the fact that ceramics are an
excellent choice for bypassing, and RF coupling use and the low-K and NPO
ceramics are excellent for use in RF tuned circuits. But for the reasons
I've stated, they're not the best choice for use in an audio path.

2)
Replace bypass caps with film (any type) or ceramic (any type).
Replace coupling with film (any type) or ceramic (any type).
Replace tone controlling with film of any type.
Replace power supply filtering electrolytic with voltage rating of 100% or
greater than original and 100-200% of capacitance.
Replace high stability with mica, *polystyrene* film, or stable ceramic
(NPO or COG).
Replace temperature compensating (only if ABSOLUTELY NECESSARY) with same
temperature characteristic ceramic.
Replace safety types with identical AC rating.

3) Not all poly capacitors are made equally.First there's that soft case of
some,and
the internal construction. Some have extended foil for extremely low
inductance
(Orange Drops), some don't. Some have different thickness of dielectric
giving
different reliability factors. The original capacitors were not extended
foil and
were not so effective as RF bypasses as IF bypasses.Besides`I've been using
Orange Drops for at least 40 years and still find them reliable. I don't
know about
the others that haven't been made that long.

4) I always test all replacement caps for leakage at full operating voltage
and for
value before I install them. I'm never had a new orange drop or CDE cap
fail.I
have with others.  As much trouble as it is to recap something like the IF
deck,there's not a shot in
hell that I'd try to save a couple of dollars and use a lesser grade of
capacitors.
You only talking about 18 or 19 axial leaded "paper"caps. I think that the
current
average price on the 400 and 600V OD's from Mouser right now to totally
recap
the IF deck is under $20.00. I'd spend the $20.00 on the OD's even if the
other
caps were totally free.

5) Sure you can replace SOME paper capacitors with disk ceramic but you have
to
face some consequences.0.1 at 400 volts is a value I've never seen in a disk
ceramic.Plus the very large values in disk ceramics have a very high
temperature coefficient of capacitance,negative.I've found them to lose as
much
as 85%of their room temperature value being close to the base of a tube.
Further they have low inductance which is generally good,except that
ordinary
paper capacitors can be close to series resonance at 455 KHz and actually
show a
lower impedance in the bypass than the same capacitance (neglecting the
effects
of heat) in a disk ceramic.

Walter Wilson - KK4DF
http://www.knology.net/~wewilson


----- Original Message -----
From: "John Saeger" <john@whimsey.com>
To: <R-390@mailman.qth.net>
Sent: Sunday, June 02, 2002 1:49 AM
Subject: Re: [R-390] capacitor analysis


> JAMES T BRANNIGAN wrote:
>
> > 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
>
> Yeah, if it ain't broke don't fix it.  I agree.

>>> SNIP <<<

> ;-)  But I have a hard time paying twice as much for an orange drop than a
> Cornell Dublier polyester, which is what I'm leaning toward right now.
You
> can get them in a 1KV rating.  But I'm still thinking about it...
>
> Thanks,  John
>
>
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