HP 400D, Инструкция по эксплуатации и обслуживанию

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4-9
POWER SUPPLY TROUBLE
LOCALIZATION
If
trouble is localized to the power-supply section,
the following checks will be helpful in trouble shooting.
1)
Set line voltage to exactly
115
volts.
2) The dc voltage at plate pin 9 of 12B4A series
regulator tube V7 or at cathode pin 8 of GAXS rec­
tifier tube
V6
should be between
400
and
420
volts.
Low voltage at this point may be due to rectifier
tube
V6,
power transformer
TI,
Or input capacitor
C30 being defective.
To check V6. connect a dc voltmeter between pin
8
of
V6
and chassis. Reduce line voltage from
llS to
103 volts. The dc voltmeter reading should drop
immediately when the ac line VOltage is reduced
and then remain steady.
If
the dc voltage continues
to drop at a slow rate, rectifier
V6
is
probably weak
and should be replaced.
3) The dc voltage between pin
I
of tube V7 and chas­
sis should be
2S0
volts,
±S
volts.
If
the regulated voltage is not within this range, re­
placement of V6, V7, VS and/or V9 may be neces­
sary.
If
changing these tubes does not correct the
dc voltage level, resistors
R62
and/or
R64
may
have changed value and should be replaced.
4} Check power supply regulation and
de
heater volt­
age as outliMd
in
paragraphs 4-7B and 4-7C.
4-10 AMPLIFIER TROUBLE LOCALIZATION
To localize trouble by de voltage analysis,
the
fol­
lowing checks will be helpful.
1)
With a line voltage of exactly 115 volts, check
the dc plate (pin
5)
and screen (pin
6)
voltages for
VI, V2, V3, V4, and VS.
Plate and screen
of
Vi are tied together and should
have a dc voltage of
+
165 volts, ±S volts.
Tubes
V2, V3,
and
V4
should have a plate voltage
of at least
+
90
volts but no higher than
+
135
volts.
Plate voltage for
V S
should fall between
+125
and
...
14S volts.
Sect.
IV
Page
13
Screen voltages for V2, V3, V4 and V5 shOuld be
+135
volts,
±IO
volts.
2) Reduce line voltage to 103 volts. Again measure
V2 through V5 dc plate and screen voltages.
If
a tube is weak, plate and screen voltages will
drift to a higher value after the initial slight change
following the reduction in line voltage. This effect
will be more noticeable at the plate of each tube.
3) Reset
line
voltage to
US
volts. Check that voltage
from pin
3
or
V3
or
V4
to chassis measures
12.6
volts;
readjust RG6. if necessary. Measure dc voltage
at grid pin 1 of V2, V3, V4, and VS. A reading of
zero volts will normally be obtained for
V2, V3,
and V4. The grid of VS wUl be approximately
...
0. 6
volt with respect to the chassis.
A
high-impedance
vacuum tube voltmeter such as the �Model
410B
must be used for this measurement.
Leaking coupling capacitors from a preViOUS stage
will cause a more positive de grid-voltage for V2,
V3, V4,
or
V5.
Replace any coupling capaCitors
in doubtful condition.
Occasionally a coupling capacitor will be found that
is slightly leaky only when the instrument is oper­
ated in its case. When the instrument is operated
with its case off, components are permitted to cool
off to below normal operating temperatures.
4) A cathode voltage of approximately + 1 . 4 volts
is normal for V2, + 1. 1 volts for V3 and V4, and
+
1 .
6
volts for
VS.
An open cathode resistor will result in a relatively
large increase in these voltages.
A
weak tube or
a defective cathode bypass capaCitor can result in
low cathode voltage.
S)
A cathode voltage of approximately + 58 volts
and grid voltage of + 56 volts is normal for the grid
of
VI.
Measure the grid voltage at the junction of
RI (lIOK)
and
R7 (220K)
at terminal of capaCitor
Cl behind the pilot lamp socket (see Figure
4-1).
The actual values will vary slightly, but the cathode
voltage will always be about
2 volts positive com­
pared to the grid.
With
a weak
VI
the bias will less than 1 volt which
will show up as instability when the range switch
is rotated.