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tion when the Grid Signal toggle switch is indexed
to the "On" position and the Microamperes
toggle
switch is indexed to the "IS" position.
To correct
this select the next higher Gm range, as this will
reduce the grid signal.
Any attempt to check certain tubes under Fixed
Bias conditions that should be checked under Self-
Bias conditions, usually results in tube oscillation
or instability.
The Grid Current meter will also indicate to the
right of zero when the plate supply and grid bias
potentials are extremely low or zero.
INCONSISTENT G .m READINGS ON fiLAMENTARY TYPE
TUBES:
I nconsistent Gmreadings on filamentary type
tubes is caused by improper setting of the Hum Con-
trol. It is recommended
that Step 7:7 in the Step-
by-Step Procedure
on page 7 be performed more
carefully.
The operator might find it helpful to tem-
porarily set the G.mFactor range switch to the Xl
position to obtain the maximum
sensitivity
when
performing this operation.
VERYLOW OR NO G.m READING:
Assuming the elec-
trode potentials to be correctly applied and adjusted
the operator may find that the Gm reading is zero.
This may be caused by having the"Signal
Ampli-
tude" adjuster turned down to zero.
If the Self Bias Resistance
control is set a value
higher than zero ohms and the grid bias voltmeter
control has not been returned to the extreme counter-
clockwise position, the total grid bias applied to the
tube will be that indicated by the Control Grid V olt-
meter plus the bias developed in the self bias resis-
tor. This condition would cause the plate current to
be very low and the Gmto be either very low or zero.
Other factors causing low Gm readings
are de-
scribed in the section on Maintenance.
PRECAUTIONS
Don't Attempt to Test a tube unless it has been found
cleared in the short test.
Don't attempt to patch the circuit unless the Plate
Supply toggle switch is in the "Off" position.
Don't Insert a Tube until you are certain that the
Heater Voltage switch is set to the correct value
for that tube. Check with the Tube Data Chart.
Don't Insert a Tube in the Test Circuit Unless All Volt-
age Adjusters Are Rotated to the "Off" Position.
This may be waived if testing a group of identical
tubes.
Don't Test Rectifiers beyond their maximum current
carrying capacity.
Don't apply more than 10 volts to diode detectors or
their emission may be lost.
Don't Fail to Bring Up the Grid Voltage First, the
Plate Voltage
Second and the Screen Voltage
Third, otherwise the cathode or the screen may
be damaged.
.
Don't fail to completely Shut Off the device when
testing is completed.
MAINTENANCE
RECTifiER TUBES:
It is recommended that the several
components in the device be checked at two year
intervals so that the equipment is maintained
in a
completely satisfactory operating condition.
If the power supply will not deliver 300 volts at
100 milliamperes
at a measured line voltage of 120,
the 5U4-G rectifier is failing and should be replaced.
A 6L6 or a type 2A3 tube may be used as a load by
applying 300 volts to the plate and lowering the grid
bias from the specified value until the element cur-
rent meter indicates 100 milliamperes.
This will pro-
vide a convenient
load for the power supply for
checking the rectifier.
Each of the small miniature type tubes, 3A4, OB2,
and 6X4, can be checked by removing it from its
socket and testing in the Model 686.
ELECTROL
YTiC CONDENSERS:
E very two to three
years the electrolytic condensers
in the Model 686
should be checked with a capacity analyzer to deter-
mine whether they are in a satisfactory condition. It
is necessary to remove the back cover to check these
condensers.
The low voltage
1,000 microfarad
condenser
is
located in a clip near the right hand section of
the top panel when looking at the instrument from
the back. The capacitance
of this unit should not be
allowed
to fall below 500 microfarads
as it will
cause Gm indications to be very low under self bias
conditions.
The 40 microfarad
450 W.V. electrolytic conden-
sers located in clips under the shelf at the left side
when looking at the back of the instrument, should
not be allowed to fall to a value below 30 micro-
farads, nor have a power factor exceeding 30 percent.
High Gm indications
can be expected
on tubes
when the capacity of the 40 microfarad condensers
fall below 30 microfarads.
ORDERING INFORMATION
If an occasion
should ever arise requiring
the
ordering of parts, be sure to give the Model Num-
ber, Type Number, Serial Number, Voltage and Fre-
quency Ratings.
A description
and location
of the part in the
equipment should be as complete as possible.
Address all inquiries to Weston Electrical Instru-
ment Gorporation, 614 Frelinghuysen
Avenue, New-
ark 5, New Jersey.
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