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TYPE 1263-B AMPLITUDE-REGULATING POWER SUPPLY
6.2 TUBE REPLACEMENT.
Refer to the tube-location chart (Figure 7) inside the dust cover.
Tube requirements are not particularly critical although some selection
may be required.
If the instrument fails to perform in accordance with
the specifications, one or more tubes may require replacement. Failure
to deliver rated output current is usually due to a defective V6 (6V6GT),
although failure of V1, V3, V4, or V5 may also be indicated. Failure or
poor performance of the voltmeter is usually due to a defective V2. Fail-
ure or poor performance of modulation is usually due to V4 or \'5. If V1,
V3 , or V5
is
replaced, check that the output of an oscillator operated on
cw from the power supply is zero at the counterclockwise limit of the
OUTPUT VOLTAGE control. If it is not, check that R2, accessible from
the top of the instrument, is fully counterclockwise and measure the volt-
age from TP 1 (pin 5 or V6) to GND.
This voltage must be at least - 45
volts with a 20,000-ohm-per-volt meter and a line voltage of 125 (or 250)
volts.
Turn R3 clockwise slowly and note that the voltage remains at
least -45 volts before decreasing rapidly to zero and beyond (checks
margin of cutoff bias voltage).
If
the above test is not satisfactory, in-
terchange V1 and V3 and, if necessary, replace one or both tubes. Re-
turn R2 to its counterclockwise limit. If the voltage from TP 1 to GND
is greater than - 45 volts and the oscillator output is not zero, replace
V6.
The usual defect found in replacement tubes for V1 and V2 is ex-
cessive de drift with changes in line voltage. First, V2 should be checked
as follows:
a. Set the selector switch to ZERO.
b. Adjust the ZERO control to obtain a meter indication between 0.1
and 0.2 volt.
c. Vary the line voltage from 105 to 125 (or 210 to 250) volts.
d. The total shift in indication should be less than one scale division.
e. Readjust for zero indication.
V1 can now be checked for drift as follows:
f. With an oscillator connected and adjusted for proper cw operation
as described in Section 4, set the output voltage to 0.5 volt.
Figure 7. Tube Location Chart (rear view
of the instrument) for Type 1263-B
Amplitude-Regulating Power Supply.
17
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