"Antonio Vernucci" wrote in message
. ..
Turns out to be a couple of misplaced decimal
points. First of all I mis-typed, the measured value is
0.048uf, not 4.8uf. Recalculating I get:

As a matter of fact a value of 4.8uF seemed real odd to
me.

Oh, Yikes! I did it again. The correct measured value
of the capacitor is 0.048 uf, D = 0.3

I calculate:

C parallel = 0.044 uf
R (AC) series = 995 ohms
R (AC) parallel = 12050 ohms
Xc, at 1000 hz = 3315 ohms

Someone please check this.

Your calculations seem correct to me (assuming that by Xc
you mean the reactance of Cs and not that of Cp, which is
3,617 ohm).

At this point, one would still have to explain how a
capacitor marked 0,02 uF can grow up to 0,044 uF, that is
more than twice its value.

Before formulating hypotheses (e.g. that the plates of
the capacitor are closer together than originally because
of loss of the wax impregnan) I would rather try to
reconfirm the measurement results.

Measuring the resistance of the capacitor by means of a
plain digital ammeter, do you obtain a value close enough
to 12 kohm? Repeating the measurement on a different
scale, do you obtain similar results? My experience with
lossy capacitors is that the apparent Rp varies quite a
lot with the scale.

Also it would be useful to repeat the test with the GR set
at a diffierent frequency (should this be possible).

73

Tony I0JX

There is no DC resistance, that is, its open circuit
for DC but I think there is an AC resistance component in
parallel with the capacitance (have to look this up). The
capacitance definitely measures high as do a couple of other
paper caps from the same RX. Measured on the TEK multimeter
the capacitance measures even higher. I checked the TEK
meter on a General Radio decade capacitor which is known to
be accurate and it measures correctly. There is definitely
something strange here. The hummer in the GR bridge is
definitely on frequency and it shows correct values on both
the GR decade box and on single precision caps. I really
think something has happened to the cap internally. Also, I
am not at all sure of the tolerance of these caps
originally, probably quite a lot on the high side. My Xc
calculation was made for the series cap value.
FWIW, I measured several other old paper caps including
a non-leaking Black Beauty. All were within reason of marked
value, all somewhat higher but nothing like the one in the
original thread. The dissipation factors were high compared
to new film capacitors but a couple of them were probably
still good caps. D ran from a minimum of around 0.03 to
around 0.5. C values were within about 20% of marked value,
all on the high side. Measured value of leaky (I mean the
oil has leaked out) BB caps shows them to usually be on the
low side.
Probably most of the paper caps in the S-40A did not
need to be replaced but I had the caps and it was not a
difficult job. Its difficult to know how much, if any, this
improves the performance. As mentioned two caps were
thoroughly gone, one a dead short and the other completely
open but most of the others were probably still servicable.
It would be interesting to know what the D of these caps was
when they were new.
It would be interesting to know what the new film caps
will be like in fifty years but I probably won't be around
then (but you never know what developments there will be in
medicine).
And, last but not least, thanks for checking my math.



--
Richard Knoppow
Los Angeles
WB6KBL