[R-390] Capacitor Heresy

[Bill Smith]

I did need to replace several mica capacitors in R-390 RF cans which had
either opened or developed leagage, or both.  Many were of very small
capacitance values, in the range of 10-300pf.  Several resistors in the
power supply failed, and others in the IF strip have likely been stressed by
the power supply's failure, but values have not risen enough to warrant
replacement, yet.

The R-390 unit here was built with Vitamin Q capacitors and none of them
have failed.  That isn't the rule for capacitors, however, it is the
exception.  Although Vitamin-Q types (hermetically sealed, metal case
devices) have held up superbly, it is my experience that virtually all other
types fifty years and older have developed leakage.   For example, WWII
oil-filled bathtub types which have thought to be indestructable are now
commonly found with excessive leakage.   Square micas made of a pink moulded
composite case are notorious.    Many other old mica capacitors are open
and/or leaky also.

Exceptions can be found.  I have a Hallicrafers S-38B with original
capacitors which plays fine.  The electrolytics in those sets are a story in
and of themselves (they are still working fine).  But as a rule, all paper
caps and 10% of resistors can be assumed to be need of replacement in
virtually all equipment built before the 1960's.  Unfortunately, several
sets which hadn't been used for many years apparently worked well for the
first several hours but could be observed to "tighten up" and begin to
distort signals with use.

73 de Bill, AB6MT
billsmith@ispwest.com


----- Original Message -----
From: Scott Seickel
To: K2CBY@aol.com ; r-390@mailman.qth.net
Sent: Saturday, November 01, 2003 4:30 PM
Subject: Re: [R-390] Capacitor Heresy


Interesting.
I restored a Teledyne unit about 1-1/2 yrs ago.  Came to me in nice
condition
with all Teledyne modules and original meters.  Well kept.
I recapped the whole unit and tested all removed capacitors on a
Sencore LC75 analyzer.  I found about 2/3 of them to have had significant
leakage.  Several of the "Brown" caps also had cracks.  I would not second
guess recapping this set again.
My experience with the resistors in my set was similar to yours.

Scott

----- Original Message -----
From: <K2CBY@aol.com>
To: <r-390@mailman.qth.net>
Sent: Saturday, November 01, 2003 11:46 AM
Subject: [R-390] Capacitor Heresy


> It has long been an article of faith that you begin to rehab an
R-390/R-390A by yanking out all the paper capacitors -- particularly the
notorious "Brown Buggers" -- and replacing them with Orange Drops. I
recently went through this process with an R-390A Teledyne IF subchassis
(Contract No. 3785C-PC-63, Serial No. 6369), and the results were
surprising.
>
> After about four hours of squinting, sweating and cussing I managed to
excise all of the original capacitors and replace them with modern polyester
and polypropylene types of equivalent capacitance and voltage rating (more
on this later). Removing the BFO to access the capacitors connected to
points beneath the tuning shaft was a particular nightmare because some
clever techie (or maybe the factory) has Lock-tite-ed the set screws on the
bellows coupling. I persevered and managed to finish the job without singing
the wiring harness more than a couple of times (love that smell of scorched
plastic) and a minimum of other collateral damage. I then tested the IF
strip in the receiver and noted no real change in performance –in terms of
either gain or noise figure.
>
> This left me with a pile of 19 old capacitors on the bench. As I was about
to sweep them into the trash bucket I started to wonder "Just how bad are
these things?" so I started a little investigation.
>
> They all looked OK mechanically. There were no cracked cases, oozing gunk
or peculiar bulges. Nothing smelled out of the ordinary.
>
> Although I hadn't paid too much attention to it when I pulled the
capacitors, I noted that there were two distinct types. Teledyne (and
probably all the other contractors) used the brown Bakelite cased tubular
units only as bypass capacitors -- B plus line, cathode to ground, or screen
to ground. The plate to grid coupling capacitors were all metal cased with
plastic or epoxy seals – the type Sprague used to call "Vitamin Q," although
the ones in this unit were by Astron and General Instrument.
>
> I started out by measuring the capacitors on an ESI capacitance bridge at
a 1 kHz test frequency. Every last capacitor measured within 8% of rated
value. Since the spec on many of these was only 20%, I thought this was
pretty impressive. I then measured the dissipation factors. These ranged
from a worst case of .013 for the brown tubulars to less than 0.01 for all
the metal-cased units. For the 0.1 µF capacitors, the computed series
resistance was < 21 ohms in all cases; and for the 0.033 µF capacitors the
series resistance was less than 68 ohms. In all instances, the metal cased
units had less than half the series resistance of the brown tubulars.
>
> I then measured the insulation (shunt) resistance of the capacitors on a
ZM-11 bridge. For the 0.1 µF "Brown Buggers" the values ranged from 60 Meg
to 75 Meg; and for the brown 0.033 µF units, 800 Meg to 2,800 Meg. The
metal-cased capacitors ranged from 8,400 Meg to 10,000 Meg (the limit of
measurement). It should be noted that these measurements were taken with
applied voltages (up to 500) substantially higher than the rated working
voltages of the capacitors.
>
> By way of comparison, new out of the box Orange Drops had a measured D of
.004 and an insulation resistance in excess of 10,000 Meg (limit of
measurement) for both the 0.1 µF and the 0.033 µF units.
>
> I drew three conclusions from these tests. First, the Collins engineers
were no dopes. They confined the brown tubular capacitors to non-critical
applications and used premium-grade, metal-cased units where leakage
resistance and dissipation factor really made a difference. Second, the
original capacitors aged remarkably well. There wasn't one of them in my IF
strip that actually needed replacement. Third, unless you are a glutton for
punishment or just love to see the orange sparkle of fresh capacitors
glinting from inside the radio, it probably doesn't pay to re-cap unless the
receiver is showing symptoms of distress.
>
> The B plus and screen bypass capacitors are most likely to fail, and if
they start to leak the plate and screen voltages will be noticeably low. The
interstage coupling capacitors are pretty safe because they are higher
quality. If they commence to leak, the failure will be obvious because the
grid of the following stage will be driven into conduction, resulting in
zero or positive grid voltage and vastly excessive plate current. The
cathode bypass capacitors are least likely to fail since they operate at a
tiny fraction of their rated voltage.
>
> The only exception is the infamous C-552 (0.01 µF 300 dcwv) that couples
the plate of V501 to the mechanical filters. Because its failure will fry
the filters, it should ALWAYS be replaced with a top quality new part with a
voltage rating of at least 350. It's also easy to reach, and there is plenty
of room to fit a replacement.
>
> I have a couple of additional observations.
>
> Capacitors come in small packages these days, and by 21st Century
standards Orange Drops are pretty bulky. I instead used CDE Sub Miniature
Metallized Polyester  DME Types, which are about 1/4 the volume of an Orange
Drop. The types are DME2P1K 0.1 µF 250 volts (Mouser part no 5989-250V.1);
DME 4S33K 0.033 µF 400 volts (Mouser 5989-400V.033) and DME 6S1K 0.01 µF 600
v (Mouser 5989-600V.01) for C552.
>
> Although the capacitors looked pretty good in my IF subchassis, the
resistors were another story. I measured each of them, and almost half were
more than 15% out of tolerance. Invariably, the resistance was higher than
it was supposed to be – in a couple of cases, about 25% high. I also found a
couple that were charred – but amazingly enough one of these still on value.
I left the grid resistors and AGC bus resistors pretty much alone since
those values are non-critical but changed all the others that were more than
10% off.
>
> In conclusion, maybe it pays to keep a closer eye on resistors than to
routinely replace all capacitors.
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