"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

Richard Knoppow wrote:

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).

I suspect they'll be hanging in there. Some, actually many, of the film
caps from the 1950s are still reliable and I can only guess that modern
ones will be better still...with the exceptions of some that may turn
out to have come from crummy manufacturers.

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).

Well, in that case I believe that the calculations we did cannot be strictly
valid. Anyway, it is an interesting issue.

By the way I also have an HT-40 and found no need for changing capacitors, as
the leaky capacitors are placed where they make no harm (e.g. screen grid
bypass).

73

Tony I0JX
Rome, Italy

Antonio Vernucci wrote:


By the way I also have an HT-40 and found no need for changing
capacitors, as the leaky capacitors are placed where they make no harm
(e.g. screen grid bypass).

73

Tony I0JX
Rome, Italy

I always consider 'leaky' to mean that they are somewhere along the road
to 'short' and just haven't arrived yet. Leakage, particularly on HT
circuits, will ultimately lead to heating and then is just a matter of time.


I figure we all have a certain mental tolerance of how much we can live
with in a certain part of the circuit but the other factor is 'time'.
Same applies to drifted carbon resistors.

-Bill

I always consider 'leaky' to mean that they are somewhere along the road to
'short' and just haven't arrived yet. Leakage, particularly on HT circuits,
will ultimately lead to heating and then is just a matter of time.

You are certainly right in saying that changing the leaky capacitors would be
the correct way to go from the technical viewpoint.

However I adopt a different phylosophy. I use the HT-40 (or other boatanchors
radios) for maybe a few hours per year in total, just for fun. For normal
operations, I use recent equipment.

So, the risk of failure caused by the HT-40 leaky capacitors is moderate. On the
other hand, if I would replace the leaky capacitors, I would have the certainty
(and not just the risk) of spoiling the originality of the radio.

So, in the end I prefer to leave the radio as it is, as long as it works
acceptably.

73

Tony I0JX

So, the risk of failure caused by the HT-40 leaky capacitors is moderate.
On the
other hand, if I would replace the leaky capacitors, I would have the
certainty
(and not just the risk) of spoiling the originality of the radio.

I am of the opinion the originality of the radio has been compromised when
the caps (and other parts) go bad and do not perform up to original
engineering intent. Like a Fender Stratocaster or Les Paul with an old leaky
bubble bee cap that sounds nice, mellow, fuzzy...(insert description). It
did not sound that way from the factory either. This old bubble bee is a
whole new sound. Pleasing and desirable, perhaps, but not original.

And there is the engineering part of me that says if it is not operating
with in spec it is broke.

But this is a whole other philosophical discussion.

Just my opinion,
Paul P.