[R-390] Capacitor Heresy

[JamesMiller]

If the caps. were all pretty good, I wonder what it was that improved the
performance of the IF strip?

My experience has been likewise in the IF strip.  Here's a story from the RF
deck.  I had questionable performance on all bands, intermittant,
heat-related.  I had already replaced the .033 capacitor on the rear wafer
of the bandswitch.  I decided to go through the whole deck and replace ALL
of the ,005 disc capacitors.  I then checked the tightness of all chassis
screws holding ground lugs or tube sockets... yes some were a little loose,
...  Additionally, I sprayed the tube sockets top and bottom with "Big
Bath", which is an oil and moisture displacement spray.  Oil seeps down from
the gears to the chassis and eventualley leeches into the porus material
that the tube sockets are formed from... enough of an impedance change
results to affect performance a little.  After all this, Wow, what a
difference in performance now!  It is really "hot" on all bands.  Most of
the ,005 discs are used for screen and B+ bypass, a few are used to filterr
the filaments (I didn't replac all of the filament caps.).  I did not
measure the old ones.

Another thing I discovered in this radio... on one band (the upper AM
broadcast band) the carrier meter would change about 10-20 dB for no
apparent reason, usually a function of how long the radio had been on.  It
would play solid 60 dB on the local station for 30-45 minutes, then suddenly
drop 10-20 dB.  It was a local station and this was not due to propagation
changes.  After much head scratching and deduction, this was finally traced
to an intermittent failure in a capacitor in one of the band coils.  It was
in coil Z202-1, a 2400 pf cap (C-235-1)inside the coil can ... one end of
this cap "touches" the B+ line feeding the V201 RF Amp via the coil in Z202.
The other end of this cap. is in series with a 180 pf cap, both across the
coil.  My suspicion is that the radio was used a lot by the previous owner
on the broadcast band, hence this cap. was subjected to B+ longer than any
others, and developed a failure.  Alternately, the higher value combined
with aging resulted in a voltage breakdown of some kind.  The failure didn't
change the resonance of the coil greatly, but enough to reduce the signal
coupled to the next stage.  I bought a new can and it works well now.  I
suspect that any  questionable performance on other specific bands could be
traced to similarly failed capacitors in the coil cans for those bands
(C232-1, C234-1, C238-1, etc).   The moral is don't forget internal failures
in these cans when diagnosing problems. (PS I got the replacement can from
Fair Radio)...
Jim N4BE

----- 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.
> _______________________________________________
> R-390 mailing list
> R-390@mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/r-390