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Last updated 13th March
2003
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Troubleshooting
The AC Power Supply
Function:
A power supply can be a set of
batteries that supply the A, B, and C voltages for a
receiver. However, if a receiver is to be operated from
the AC mains, a means must be provided to convert the 115
VAC mains voltage to the necessary DC voltages required
by the tubes, and to supply the proper AC voltage to
operate the tube filaments. The AC transformer power
supply accomplishes this.
Theory of Operation:
Refer to the diagram
below.
The power supply consists of:
- the power transformer (T-7),
to step up or step down the input AC voltage;
- the rectifier (5Y3), to
change the secondary high-voltage AC to pulsating
DC;
- the filter (C-15, C-16
and L-15), to smooth out the pulsating DC
into unvarying B+ voltage;
- the voltage divider (R-15
and R-16), to subdivide the B+ voltage as
needed in various plate and screen circuits.
The 115 VAC input is applied to the
primary L-16 of the power transformer T-7.
The secondary windings of the transformer step the AC
voltage up or down to provide the proper voltages. The
amount the voltage is stepped up or down depends upon the
number of turns in each secondary winding.
Transformers are wound with a turns-per-volt
ratio. Transformers for radio work are usually designed
to operate at 2 to 4 turns-per-volt. Assume a 4 turns-per-volt
transformer; for an input of 115 volts, the primary would
have 115 X 4 = 460 turns. Each 4 turns of a secondary
winding will produce 1 volt. So for a filament winding to
supply 6.3 volts it would have 6.3 X 4 = 25.2 turns; the
5 volt winding for the rectifier filament would have 20
turns. The high-voltage winding, usually around 700 volts
would have 2800 turns with a center tap at 1400 turns.
Secondary winding L-17 is the winding for the
filaments of the receiver stages and L-19 is the
filament winding for the rectifier. Winding L-18
is the high-voltage winding and is center-tapped for use
in a full-wave rectifier circuit. Filament voltages will
vary depending on what type tubes are used in a receiver.
The voltage of the high-voltage secondary is applied to
the plates of the 5Y3 rectifier tube. This tube has a
directly heated cathode (the cathode and filament are the
same element), so the pulsating DC is taken off of pin 8,
which is one side of the filament. The plates act as a
one-way gate to allow current to flow only when the plate
is positive with respect to the cathode. Because the high-voltage
winding center tap is grounded and is the zero reference
point, each plate is positive only on one-half of the AC
cycle. During one-half of the cycle, plate #1 will be
positive with respect to the cathode, and during the
second half of the AC cycle, plate #2 will be positive
with respect to the cathode. This causes the voltage on
the cathode of the rectifier to be a pulsating DC voltage.
The filter circuit smoothes out the pulses to form the
unvarying B+ voltage. The electrolytic capacitors C-15,
and C-16 along with L-15 form the filter
circuit. L-15 is the field coil of the speaker and
acts as the filter choke for the supply. The inductance
of the coil helps oppose the pulsating component of the B+
voltage and the magnetic field that is developed by the
current flow is used as the magnetic field for the
speaker. The filter capacitors charge up to the peak
voltage level of the pulsating DC and during the time
between pulses, begin to discharge to supply current to
the load between pulses.
The voltage divider subdivides the B+ to provide a second
voltage for the screen circuits.
AC (Transformer)
Power Supply
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Troubleshooting:
A normal operating power supply
should exhibit the following characteristics:
- all tubes light
- no sign of overheating
- all voltages check within
specifications
- hum level is normal
Common Troubles In The Power
Supply
All components in the power supply
are subject to failure, even the rectifier tube socket.
Dirt between the plate pins can cause arcing of the high-voltage
and burning of the socket. This can easily be seen by
visual inspection.
Power Transformer - One of
the chief causes of failure of the power transformer is
overheating due to overload caused by shorts in the
windings, or by external shorts. Just a few shorted turns
in the high-voltage winding, while not having a great
affect on the B+ voltage, can cause a heavy drain from
the primary and cause overheating. Even with the reduced
B+ voltage, the radio may continue to operate, but the
transformer would eventually overheat.
A transformer that has overheated
can usually be identified by melted tar that has run out
of the transformer and/or a burned smell, that once
smelled is not easily forgotten.
If an overheated transformer is suspected, a check must
be made to see it the problem is internal to the
transformer or an external short somewhere in the
circuitry. A quick check of the transformer can be made
as follows:
- remove all tubes from the
radio
- plug the transformer into a
variable transformer (Variac). A means to measure
the primary current is necessary for this test.
- bring the input voltage up
slowly, a good transformer will show very little
primary current (a few milliamperes - see note),
or cause the lamp to just glow at rated input
voltage
- any short will cause heavy
current to show on the meter or cause the lamp to
glow brightly
- if a short is indicated,
remove the leads from their connection point, one
winding at a time, and test each time, to
determine if the short is external to the
transformer. If the short is external, the
primary current will drop to a normal level, and
the secondary circuit should be traced to find
the short. If removing the leads does not reduce
the primary current, the trouble is internal to
the transformer.
Note: Of four transformers
tested, the primary current ranged from .09 to .180
amperes with no load on the secondary windings.
The Rectifier Tube - The
most common problem with rectifier tubes is either low or
loss of emission or an open filament. Low emission can
cause dergraded performance of the receiver due to lower
B+ voltages. Low or loss of emission can be verified by
checking the tube on a tester, or substituting a know
good one. Occasionally the rectifier may become gassy and
glow with a purplish light. In this case the receiver may
not operate at all. The only cure is to replace the tube.
This applies only to high-vacuum rectifiers, as it is
normal for the glow to appear in gas rectifiers such as
mercury-vapor rectifiers like the type 82 and 83.
An open filament is evident by a dead receiver and no
glow of the filament elements.
The Filter Choke (Speaker Field)
- The common problem with the filter choke, speaker field,
is that the winding opens. This will be evident by
missing B+ voltage on filter capacitor C-16, and a
higher than normal reading on C-15. When this
condition is found, C-15 must be discharged when
the receiver is turn off since there is no discharge path
through L-15 and the capacitor will remain charged
to full voltage. An ohmmeter check will verify an open
field coil. A good field coil will measure somewhere in
the range of 800 to 2500 ohms.
The Input Filter Capacitor -
The input filter capacitor C-15 is another common
cause of trouble in the power-supply stage. This is a
high-voltage, high-capacity electrolytic capacitor and
may be either the wet or dry type. Over time,
electrolytic capacitors loose capacity and open. When
this is the case the B+ voltage will tend to be low and
there will be considerable hum in the receiver output. A
quick check to confirm this is to bridge a new unit
across the old one and noting any imporvement.
The input filter capacitor also has the highest DC
voltage across it as it is tied directly to the output of
the rectifier. There are large voltage surges across it
as it is receiving the unfiltered output of the rectifier,
and this makes it subject to voltage breakdown and
shorting. When this happens, the B+ goes to zero and the
plates of the rectifier tube become red hot from the
heavy current drain. The rectifier tube and/or the power
transformer may be damaged if this condition is allowed
to exist for any period of time. A leaky capacitor will
have reduced filtering capacity, draw excessive current
and the hum level will be increased.
When this capacitor is replaced, the capacity and voltage
rating of the replacement should match the original as
close as possible, with particular attention to the
voltage rating. Never replace with one of lower voltage
rating, higher is OK. Be sure and observe polarity when
replacing any electrolytic capacitor.
The Output Filter Capacitor
- The output capacitor C-16 is subject to the same
problems as the input capacitor. If it opens, there may
little or no effect on the B+, but there may be an
increase in the hum level, squealing, or motorboating in
the receiver output. Substituting a new unit and
observing the results is a quick check of this capacitor.
If it shorts, the B+ voltage will be zero and excessive
current will be drawn. Since it is after the choke (field
coil L-15), the excessive current may cause damage
to the coil. If the B+ voltage at the output filter reads
zero, do not automatically assume the culprit is the
capacitor, as the problem could be somewhere in the
receiver circuits, such as a shorted plate or screen by-pass
capacitor. When replacing this unit, observe the same
cautions as the input capacitor, and always remove the
old units from the circuit.
The Voltage Divider Resistor
- The voltage divider resistors in later model radios are
usually of the 1- or 2-watt carbon variety. The most
common problem with these resistors is that they open up,
or change in value. If resistor R-15 is open, the
set will not play and the screen voltages will be zero. A
resistance check should be made for a possible shorted
screen by-pass capacitor that may be the cause for R-15
opening up. When resistor R-16 is open, the screen
voltage will be high and the radio may oscillate. If
either resistor changes value, the screen voltage will be
abnormal and the radio may oscillate. A check with the
ohmmeter will confirm an open resistor or one that has
changed value. When replacing these resistors, attention
should be given to the wattage rating and replace with
one with at least the same or higher rating.
Sometimes the voltage divider resistor will be a tapped
wire-wound type. Wire-wound types seldom change in value,
but often open up. When replacing an open section, any
resistor of the proper ohmic value and wattage rating may
be use. The replacement resistor can be soldered across
the terminals of the bad section. If this is done,
unsolder the resistive element wire of the defective
section from the terminals, as the open may
intermittently heal and cause noise and fading problems.
The Line Filter Capacitor -
Line filter capacitor C-17 is a paper tubular type
with a usual voltage rating of 400 volts and a capacity
rating of .01 to .1 mfd. An open line filter capacitor
will sometimes cause "modulation hum", that is,
hum is heard when tuned to a station, but disappears when
tuning between stations. Bridging C-17 with
another capacitor is a quick check for an open capacitor.
When replacing this capacitor, it is recommended to use, at least, a
modern X2 or Y2 rated component, as voltage spikes on the AC line
can be quite high and this particular component serves an important safety function.
Service
Data Chart For The AC Power Supply
Sumptom |
Abnormal Reading |
Possible cause |
Tubes do not
light |
An ohmmeter
check from prong to prong on the AC plug, with on/off
switch on, shows open. |
Defective
line cord
Defective on/off switch
Open primary winding on transformer T-7
Open fuse |
Rectifier
tube plates glow red |
Chassis-to-rectifier
filament check shows short circuit with ohmmeter |
Shorted input
filter capacitor C-15 |
Rectifier
tube overheats |
B+ voltage
checks zero. Chassis to B+ checks short circuit
with ohmmeter |
Shorted
output filter capacitor C-16
Short in B+ circuit wiring |
B+ voltage
checks low, zero plate voltage on amplifier tubes.
|
Shorted plate
by-pass capacitor |
Hum
|
B+ voltage
checks low |
Open input
filter C-15 |
B+ voltage
normal |
Open output
filter C-16 |
All voltages
normal, all components of power supply check OK |
Hum-bucking coil leads on
speaker reversed. See Troubleshooting The
Audio Output Stage |
Oscillation
or motorboating |
B+ voltage
normal, or fluctuating with motorboat beats.
Screen voltage normal |
Open output
filter C-16 |
Weak
reception. No sign of overheating |
B+ checks low
|
Weak
rectifier tube |
No signal
from speaker. No sign of overheating |
B+ voltage
checks zero |
Dead
rectifier tube
Open filter choke L-15 |
No reception.
No hum. B+ voltage normal |
Screen
voltage checks zero |
Open voltage-divider
R-15
Shorted screen by-pass capacitor or both |
Oscillation |
Screen
voltage high |
Open voltage-divider
R-16 |
Modulation
hum |
Poor ground
Open line filter capacitor C-17, or both |
Fading
|
Screen
voltage changing due to defective voltage-divider
R-15 and R-16 |
Rectifier
tube shows purplish glow |
Gassy
rectifier tube (high-vacuum type) |
Typical
Voltage Readings
Rectifier filament-to-filament
|
5 volts AC |
Across other tube
filaments |
6 volts AC |
Chassis to rectifier
plates |
250-380
volts AC |
Chassis to rectifier
filament |
265-400
volts DC |
Chassis to B+ |
200-300
volts AC |
Chassis to screen |
90-100
volts AC |
©Bill
Harris 1997
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