Capacitor failure report
Engineer
I don't spend much of my time on boatanchors but this may be of
interest.
While restoring a Bendix RA 10DB receiver as part of a CASM, Toronto,
Lancaster bomber restoration project, see:
http://casmuseum.org/avro_683_lancaster_x.shtml
one of the decoupling caps failed spectacularly. It was C54. First
there was a smell as plate decoupling resistor R15 (1000 ohms)
overheated, then smoke (I think from from R15 paint), then C54 got
very hot and oozed a white bubbling substance... B+ had dropped from
221 VDC to 183 VDC... hit the off-switch fast! Fortunately, no
dynamotor damage. This unissued, NOS Bendix radio had earlier been
powered for about a hour or so in several check-out sessions without
any trouble.
The cap is one of several marked "Solar TYPE [blank] MADE IN USA .1
MFD 400 VOLTS DC GRD (at one end)" It has a rectangular brown
plastic case (bakelite?), about 1 3/8 x 5/8 x 5/16 inches in size.
There are several of these in the set, some but not all are under B+.
I'll be watching them. I presume this is a mica cap but others may
know better.
Just one small event in a myriad of restoration experiences, but
factual data... not just an anecdote!
Cheers,
Roger
Richard Knoppow
"Engineer" <junk...@rogers.com> wrote in message
news:00f50a7a-7931-4a8c...@m35g2000vbi.googlegroups.com...
The value is somewhat high for a mica cap but its
possible and the shape is right. If it is mica its probably
a stacked cap rather than silvered mica. I suspect that any
modern plastic cap would replace it. GRD probably indicates
the outside foil. Grounding that gives some shielding to the
cap. It was common for tubular paper caps to indicate the
outside foil by a line at one end and this may be the same
sort of thing.
I don't know the mechanism of its shorting but that is
evidently what happened.
--
Richard Knoppow
Los Angeles
WB6KBL
dick...@ix.netcom.com
Roger D Johnson
IIRC many of these mica lookalikes are actually paper. On the smaller
postage stamp sizes the first color dot indicates type.
73, Roger
--
Remember the USS Liberty (AGTR-5)
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Remove tilde (~) in address to answer.
Ed Engelken
save yourself lots of trouble if you replace them all now before
something is damaged the next time one of them shorts. --Ed
Scott Dorsey
>The cap is one of several marked "Solar TYPE [blank] MADE IN USA .1
>MFD 400 VOLTS DC GRD (at one end)" It has a rectangular brown
>plastic case (bakelite?), about 1 3/8 x 5/8 x 5/16 inches in size.
>There are several of these in the set, some but not all are under B+.
>I'll be watching them. I presume this is a mica cap but others may
>know better.
It's paper.
If you have ANY of these in any location where they can take out an IF
can when they fail, PLEASE replace them now. The Xicon film caps are
cheap and seem reliable.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Engineer
Thanks for all replies. Yes, it's paper! The dielectric is super
thin, brown, translucent (likely impregnated), skin-like paper with
very low tensile strength - I did not test for burst and tear but will
certainly be low!
I plan to replace all of these that are under any B+ stress before
further restoration. I'll be using modern 0.12 uF, 630 volt, black
plastic, rectangular caps (Korean made.)
Rare original IF's saved, Scott... and lessons learned!
Cheers,
Roger
Bill Baka
> <snip>
> Thanks for all replies. Yes, it's paper! The dielectric is super
> thin, brown, translucent (likely impregnated), skin-like paper with
> very low tensile strength - I did not test for burst and tear but will
> certainly be low!
> I plan to replace all of these that are under any B+ stress before
> further restoration. I'll be using modern 0.12 uF, 630 volt, black
> plastic, rectangular caps (Korean made.)
Everything I buy from Korea is checked very carefully.
If I were you I would replace all those tubular paper capacitors with
the wax on them. I found some that the leads had come loose and you
could twirl the capacitor while the leads were still soldered into the
radio.
> Rare original IF's saved, Scott... and lessons learned!
> Cheers,
> Roger
Best of luck.
Bill Baka
Engineer
Thanks, Bill. I've used these rectagular black plastic 0.12 (and
0.18) caps for a while now as coupling caps in various amplifiers and
radios with no problems - virtually zero mV positive voltages measured
on any g1's. I have a bag of each, acquired from a retired EE and
radio hobbyist. Also used them for RF decoupling in radios, again no
problems.
Cheers,
Roger
Bill Baka
>
> Thanks, Bill. I've used these rectagular black plastic 0.12 (and
> 0.18) caps for a while now as coupling caps in various amplifiers and
> radios with no problems - virtually zero mV positive voltages measured
> on any g1's. I have a bag of each, acquired from a retired EE and
> radio hobbyist. Also used them for RF decoupling in radios, again no
> problems.
> Cheers,
> Roger
Roger,
I have an old Echophone on my computer desk right now and the bottom is
missing as well as the back. It has normal round resistors, octal tubes,
and 5 wax covered paper caps. There are other caps that look near the
same but no wax on them. Could be the wax oozes out after 60 years or
so. There is a foil .25uF @ 200V and 2 electrolytics at 80 uF @ 150VDC
and a 30uF @ 150VDC.
I have a Heathkit cap checker and a Fluke 77 for checking the resistors
so next comes the fire up and check for noise. It isn't, as I always
say, rocket science, but a fair amount of work.
Of course a schematic would have been nice but I think I can trace it
out with not too much problem. There is a cap dangling but I think I
know where it goes.
My new, small project.
Thanks for listening.
Bill Baka
Engineer
Two comments...
Bill, good luck with the radio project. I have an old cap checker
with eye tube (in virtually new condition) but find it of little use,
rather it's just a curiosity. A DVM (use your Fluke 77) and a
variable B+ voltage source is much more useful. A Q&D check on
coupling caps is to measure the voltage on the following g1 - anything
over +2 mV to ground is suspect. But don't bother to check the wax
caps, just replace them all - electrolytics, too.
An earlier post suggested I replace the risky brown 0.1 uF caps in the
Bendix RA 10DB receiver if their failure could take out an original
IFT. As far as I can see, no shorted decoupling cap can take out any
IFT since they are all above the IFT's. But, if they fail, they will
take out the 1Kohm decoupling resistor and stress the dynamotor so, as
I said, all the ones with B+ on them will be replaced.
Cheers,
Roger
Richard Knoppow
"Bill Baka" <bb...@comcast.net> wrote in message
news:hctkhk$7qu$1...@news.eternal-september.org...
I am pretty sure that there are some Echophone
schematics on BAMA or Nostalgia Air. Echophone was bought by
Hallicrafters at some point but they continued to make
Echophone products for a time. The famous Hallicrafters S-38
series appears to be based on the Echophone.
--