[ARC5] replacing C3 and C8 of BC453A

[J Mcvey via ARC5]

If you ever have any doubt what the composition , value or tolerance of the caps might be, the original military manuals are quite specific.
I am presently working on a BC654 where I encountered several white hollow tube capacitors marked .008 . The manual said they were mica. I suspect that they were, since the couple random samples I took out to test were still  the correct value and had virtually no leakage.That saved me the  time and money to replace them. Others that had large rectangular plastic cases LOOKED like mica types but were listed as paper. Sure enough, these were very leaky.
BTW, as a military piece, I found the BC654 to be a disappointment on many levels...or maybe I just have a natural prejudice against floating ground series string sets...??? It's also very difficult work on or test in a disassembled state.Perhaps the army wanted a no-frills, easy to operate short distance radio, and that's exactly what they got! 

     On Friday, January 30, 2015 4:19 PM, Richard Knoppow <1oldlens1 at ix.netcom.com> wrote:
   

       The small tubular body (hollow) caps are ceramics.  The small 
white ones are usually temperature compensating caps and must be 
replaced with a cap of the same TC.  Also, its exact location may be 
important.  These caps rarely go bad but can.  There are larger 
"dog-bone" caps that are often brown, these are also ceramic caps. They 
show up in the Collins 51J/R-388 and likely many other places. These 
also rarely go bad but can so are not above suspicion.  Most other 
ceramics found  in boatanchor radios are disc ceramics. Again these are 
pretty reliable but many of them are Class-2 and do have an ageing 
curve.  One of the common defects is fracturing of the case where the 
leads enter due to mechanical stress.
      There are two types of mica caps: Silvered mica and stacked 
mica.  Silvered mica are very stable and were once considered to be 
extremely reliable.  The mica dielectric is electroplated with silver as 
the electrode.  These are the caps that suffer from silver-mica disease 
from oxidation  and migration of the silver. The stacked type use mica 
dielectric but have interleaved foil electrodes.  They are usually made 
with the stack under pressure and sealed.  This type was mainly used as 
a high-voltage capacitor. General Radio used this type of construction 
for its standard caps so they can be very stable. AFAIK, they do not 
suffer from silver-mica disease and do not have the symptoms of unstable 
capacitance.  RCA at one time made capacitors of this type under a 
patent they held. They can be found in the AR-88 receiver and in some 
other RCA products.  They are usually lozenge shaped and sometimes 
colored in an odd pinkish purple color but I've seen others that are 
brown.  These appear to be quite stable.  When mica was in short supply 
during WW-2 RCA used flat package paper caps in place of these. Most of 
them were made by either Micamold or Solar. In my AR-88 all were bad but 
the couple of RCA micas still used were good.
        Ceramic caps have low parasitic inductance and will work in many 
applications where mica caps were used but are not stable enough for 
timing circuits.  Modern plastic film caps seem to have very low 
parasitic inductance and will work in many circuits that used ceramic or 
mica.  They are very stable, have low leakage and ESR, and little 
dielectric absorption.  The latter is of no significance in most RF or 
audio circuits but can be in timing and some measurement circuits.  
They do not become degraded with time under voltage as do paper caps,


On 1/30/2015 12:18 PM, Roy Morgan wrote:
> On Jan 30, 2015, at 12:35 PM, Kenneth G. Gordon <kgordon2006 at frontier.com> wrote:
>
>> C-3, 100 pfd, is the white tubular capacitor on the bottom of the "socket" for
>> the antenna coil ...
>> And C-8, 200 pfd, is the white tubular job mounted on the bottom of the
>> mixer-tube's socket.
> Do I understand correctly that these small white-bodied tubular capacitors are ceramic?
>
> I found a shorted one in an SX-88 front end.  It had fried a plate dropping resistor and itself and was in a quite in accessible place.
>
> Roy
>
> Roy Morgan
> RoyMorgan at alum.mit.edu
> K1LKY Since 1958
>
>

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL

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