"Antonio Vernucci" wrote in message
. ..
Re my previous message, I have downloaded a GR bridge
manual from BAMA. It reports that:

- the bridge measures the series capacitance
- if D is low, the series capacitance almost coincides
with the parallel capacitance
- but if D is high, they differ significantly. A chart is
provided to convert series capacitance into parallel
capacitance.

This confirms that, if D is high and if the loss is caused
by a parallel resistance (as it actually is), one must
convert the measured capacitance value using the chart.

73

Tony I0JX

I think you are looking at a manual for a later model
bridge. My 650A manual has the formulas but not charts.
I remeasured a bad cap and calculated the parallel
capacitance, series resistance, and parallel resistance.
This is a paper cap rated at 0.02 uf. The values I got a
Cs = 4.8 uf
D = 0.3
Cp = 4.3 uf
Rs = 994 ohms
Rp = 12 kohms

Not a very good cap.

New plastic film caps measure very close to the marked value
and have a D which is below the residual of the bridge
(essentially zero)

While there is an error from the rather high D it is not
significant in terms of this measurement, that is, the value
of the cap measures nearly three times its marked value. I
have not dissected one of these but suspect the winding is
distorted. That would also affect the voltage rating. What I
mean is that the plates of the capacitor are closer together
than originally, probably because of loss of the wax
impregnant. I found other caps in this RX which had high
values so this one is not unique.

I have not measured the caps at RF but I seems like an
interesting project and a practical use for my Boonton
Q-Meter:-)

BTW, I think my math is OK but maybe not.


--

--
Richard Knoppow
Los Angeles
WB6KBL