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. (See waveform diagram)
The voltage divider subdivides the B+ to provide a second voltage for the screen circuits.
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:
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
known 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. For dealing with a defective field coil, see "Electromagnet Dynamic Speakers" on the "Tips, Hints, & Kinks" page.
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 a modern 630 volt unit, as voltage spikes on the AC line can be quite high.
|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
|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
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)|
|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|