Eico 950B - Circuit Analysis; Circuit Description

both tests, start with the "VOLTAGE" control at "0". The capacitor in both
tests is connected across the right-hand binding posts just as in capacitance
measurements. Be certain that the positive lead of an electrolytic capacitor
is connected to the binding post marked plus and the negative lead to the bind-
ing post marked minus. Otherwise, not only will the leakage test have no
value, but the electrolytic may be seriously damaged. Observe the electron-
ray tube and note that the shadow angle is at maximum, which is the "normal"
or "no-leakage" indication in the leakage tests. Now turn the "VOLTAGE"
control from "0" to the rated de working voltage of thecapacitor. The shadow
angle should contract instantaneously as the capacitor charges and thenslowly
expand. If the shadow angle completely disappears and does not reappear,
the capacitor is excessively leaky. The shadow angle need not expand to the
maximum as before in order for the capacitor to check good. As these tests
are very sensitive, a little leakage in capacitors with values above. 1mfd may
result in only a small shadow angle reappearing. This is normal with a good
capacitor and the reappearance of even a very small shadow angle means that
the capacitor is good. Only complete failure of the shadow angle to reappear
indicate an excessively leaky capacitor. CAUTION: Discharge the capacitor
before disconnection from the instrument terminals by turning the "VOLTAGE"
control down to "0".
Note on Leakage Testing of Electrolytic-Capacitors: Initial leakage current
in an electrolytic capacitor is a function of shelf life; it does not show the
condition of the capacitor. When making the leakage test, enough time must
be allowed for the leakage current to reach the normal value. The instructions
of the capacitor manufacturers are to measure the leakage current of the ca-
pacitor at the rated de voltage, after the rated dc voltage has been applied
to the capacitor for five minutes plus one minute for each month of shelf storage.
circuit description
The operation of the Model 950-B may be best understood by examining the cir-
cuit formed at each position of the range switch. In each of the bridge circuits
Pl varies the resistance in two arms. Capacitor C5 or C6, resistor R10, and
the electron-ray tube V2 compose the null indicating circuit. The 54 volt
winding is the secondary winding of the power transformer that supplys the ac
power to operate the bridge. RI is a current limiting resistor.
The B plus voltage required to power the electron-ray indicator and the neg-
ative dc voltage required for leakage testing of capacitors are obtained from
the 6X5 tube (V1) connected as a half-wave rectifier, in conjunction with the
high voltage secondary winding of the power transformer and the filter and
voltage divider networks composed of R8, R7, P3, Cl and C9.
NOTE: The electron-ray indicator is preferable to headphone or a metersince