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Section IV
Paragraphs 4-6 to 4-14
Model 450A
4-12.
TESTING POWER SUPPLY OPERATION.
4-13. The amplifier employs an electronically-regu-
lated power supply with very low line-frequency ripple.
To test operation of the supply, proceed as follows:
circuit, power transformer windings, or in the trans-
former primary circuit. The resistance of each trans-
former winding is as follows:
8 ohms
8 ohms
RESISTANCE
COLOR
Black- Black/yellow
Black/red-Black/green
Brown-Brown
0.14 ohm
Yellow- Yellow
0.06 ohm
Green-White
0.18 ohm
Green-Red
0.12 ohm
Red- Yellow
105 ohms
Yellow-Red
105 ohms
*Fil. #4 part of winding for Fil. #3.
WINDING
Pri. #1
Pri.
#Z
Fil.#l (6.3 v)
Fil.#2 (5 v)
Fil.#3 (9 v)
*Fil.#4 (6.3 v)
H. V. #1
H. V.
#2
less than 0.5% distortion and a distortion meter such
as the Hewlett-Packard Model 202C Oscillator and
330B Distortion Analyzer.
The frequency range of
the 330B is considered adequate for this application.
Measurement of distortion at higher and lower fre-
quencies requires rejection filters not readily avail-
able.
A variable line transformer is required to
produce line voltages from 100 to 130 volts.
4-6.
TROUBLESHOOTING.
4-7. The first step in servicing a defective amplifier
is to inspect for any sign of overheating, physical
damage, or wear. The second step is
to
attempt oper-
ation to see if the fuse blows, pilot lamp lights, and
if the amplifier can be operated without damage.
There are two sets of operational tests: power supply
checks (see paragraphs 4-10, 4-12 and 4-18) and
amplifier checks (see paragraphs 4-15, 4-20 and
4-22).
Suspect electron tube failure first, then as-
sociated circuitry. Look for intermittent and margi-
nal malfunctions. These types of failures can some-
times be found while troubleshooting, by physical
shock and by applying low and high line voltages
while making the tests.
4-1.
TUBE REPLACEMENT.
4-9. The best way to test a tube is to replace it with
a new one, noting any change in amplifier perform-
ance while measuring noise and distortion in the amp-
lifier output.
If
the replacement tube does not im-
prove performance, return original tube to socket to
avoid complicating the troubleshooting procedure.
Make the test at low and high line voltage to see if
malfunction is marginal.
If
a tube tester is used to
check tubes, consider its indication semi-final
if
it
shows "good", final if it shows "bad". Tube testers
do not measure second order effects such as exces-
sive change in transconductance, plate current and
grid current with changes in heater voltage, noise,
microphonics, heater-cathode leakage, etc, which
may be important in certain circuits.
Remember
that most tube failures occur during the first hundred
hours of operation. After this period tubes age slowly
and should not be replaced prematurely as part of
routine maintenance.
4-10. LOCATING SHORTS IN POWER CIRCUITS.
4-11. Check the amplifier for shorts whenever ap-
plication of line power causes the fuse to burn out, or
whenever operation causes the power transformer or
other part to overheat. Proceed as follows:
a. Replace blown fuse, remove V5 and again attempt
operation.
If
the amplifier no longer blows fuses,
the trouble is located in the circuits which follow the
regulated power supply; check C8C, C8D, C2 and C4.
b.
If
amplifier continues to blow fuses with V5 re-
moved, remove V4 also and again attempt operation.
If
the amplifier no longer blows fuses, the trouble is
located in the power supply filter; check C8A and C8B.
c.
If
the amplifier continues to blow fuses with V4
removed, the trouble lies either in the tube filament
a. Connect the amplifier to an adjustable line trans-
former which can supply from 100 to 130 volts.
b. Remove the amplifier bottom plate and connect the
negative leads of the multimeter and ac voltmeter to
the amplifier chassis.
c. Set the line voltage to 115, turn amplifier on and
allow 3-minute warmup.
d. Measure the ac and dc volts at V5 pin 5. The dc
voltage must be about 15 volts less than that measured
in step g; the ac voltage must be about 1/10 that mea-
sured in step g. Excessive dc voltage drop indicates
excessive current being drawn by the amplifier cir-
cuits or filter capacitors.
Insufficient attenuation of
ripple indicates filter capacitors low in capacity.
e. Measure ac and dc voltage at V5 pin 8. The dc
voltage must be between 205 and 215 volts; the ac
voltage must be less than 3 millivolts. The value of
R25 can be selected to obtain exactly +210 volts.
f. Increase the line voltage to 127 volts; the dc volt-
age must remain within 1 volt of that read in step d;
the ac voltage must not increase above that in step d.
g. Decrease line voltage to 103 volts; the dc voltage
must remain within 1 volt of that read in step d; the
1lC
voltage must not increase.
h. With line voltage set to 115 volts, measure the ac
and dc volts at V4 pin 8. The dc voltage must be close
to +390, the ac voltage less than 3 volts.
4-14. Possible trouble symptoms in electronic voltage
regulators include rectifier tube which does not deliver
full voltage to the series regulator tube, which in turn
prevents good regulation at low line voltages.
The
same symptom may be observed with a weak series
regulator tube. Another indication of this same trouble
4-2
00001-1
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