SECTION 5
UNUSUAL POWER SUPPLIES
A typical radio requires two different voltages, a high tension and a heater
supply. The power source available, and the valves that are fitted, will
determine the power supply. There are many variations. The radio power supply
must convert the locally available power, to a high tension for the valves,
and a low tension for the filaments/heaters, and sometimes a bias supply
as well. The early power supplies had naming scheme which depended on
the batteries used. This is a convenient way to refer to them. The filament
supply or Low Tension (LT) was called the A supply. The High Tension
(HT) was called the B supply. The bias supply, if there was one, was called
the C supply. If a different high tension was required, the higher one was
called the B++ or B1, and the lower one was called the B+ or B2.
One way to approach power supplies, is to look at the subject historically
and follow the developments of domestic mains power and how the radios
adapted.
Many houses in the 1920s did not have ANY mains 240 volts AC power supplied
at all, and used candles or gas for lighting. The radios ran from batteries.
As power was connected to homes, the battery radios used a Battery Eliminator
to supply the B voltage. Some houses had 100 volts DC, others had 220
volts AC, other countries had different voltages. The radios with an AC
supply could use a transformer to convert the mains voltage to A and B
voltages. Homes with a DC supply, required a different approach to generate
the B supply, often using a vibrator and series filament connections.
Imported radios from Europe or the USA sometimes had different voltage mains,
or 110 volts AC, and sometimes did not have a mains transformer. Farms
had a 32 volt DC system running from a diesel or petrol generator.
Cars had a 6 or 12 volt DC battery.
BATTERY POWERED SETS
These sets have a battery to supply the A and B supplies, and they
are usually individual batteries. Some sets had a combined battery, but
these were not popular as the A and B batteries would run down at different
rates. Commonly the A battery would go flat first. The definition of
a torch applies to a battery radio, "a place to store flat batteries".
Replace them. Ordinary D cells (1.5 V) are suitable for the A supply.
Ordinary 9 volt batteries connected in series (90 volts) are suitable for
the B supply. Use the existing polarised plugs. Disassemble the old battery,
and use the connector. Make a battery holder the same size as the old battery,
and cover it with the cardboard outer of the old battery. Common faults
are leaking batteries, which corrode the plugs, wires, and battery holders.
Sometimes they corrode the electronics, but manufactures would normally
place the battery where leakage would not harm the components.
BATTERY ELIMINATORS
When homes became connected to electricity, they could still use
their battery powered radio, by replacing the expensive B battery with
a mains powered supply, called a B battery eliminator. This usually supplied
the C bias as well. Flying leads from the radio were used with banana plugs,
to connect the B voltage which was usually 100 volts (and sometimes a B2
voltage of 50 volts). There was also a C lead and plug for the negative bias
(which was commonly minus 10 to 15 volts). The A battery eliminator was
not successful as large electrolytic capacitors had not been developed. It
was quite common to have a trickle charger connected to a large accumulator,
for the A supply. The A supply accumulators were usually 2 volt lead acid
batteries, and commonly provided 2, 4 or 6 volts for the filaments.
The Philips eliminator was made in Holland and was very popular in Australia,
costing 9 pounds and 15 shillings in the 1920s. Quite a few still survive
and are usually in good condition. The Philips 3003 model has a transformer
and a full wave rectifier valve (type 506), with a capacitor and choke
filter, to provide the B voltage. The C supply has an independent half
wave rectifier valve (type 3006), capacitor and resistors. The various
voltage tappings are provided by resistors. The Philips 3009 model has a
transformer and a full wave rectifier valve (type 506), with a capacitor
and choke filter, to provide the B voltage. The C supply negative bias is
provided by resistors in the HT negative lead, in the traditional back bias
method. The Philips 372 model has the valve visible on the top. The Philips
eliminators usually have a good transformer, rectifier valve and choke,
but the capacitors require a little time, before they reform. The resistors
for the voltage tappings are commonly open circuit and are made of
fine resistance wire wound on a glass rod. The wire can be rejoined.
VIBRATORS
Radios that have a power source of low voltage DC require some method
to generate the HT voltage. A common method, is to use a transformer
with a HT secondary, but with a low voltage primary and a device to generate
the AC that the transformer requires. A simple electromechanical device
uses a vibrating reed to generate the AC voltage. This device is called
a vibrator. The vibrator comes in several types. First there is the
voltage rating, which is commonly 6, 12, 28 or 32 volts. Next there is the
reed type, which can be non-synchronous, synchronous, or split reed. Most
vibrators are not sealed and therefore contain air, but there are some heavy
duty vibrators that are filled with an inert gas.
The vibrator is normally mounted in a sound treated tube with a valve
base, so it can be changed easily. There is a coil to energise the
flexible reed and a set of primary contacts to switch the DC to the transformer
windings. At power on, the coil is energised, which causes the reed
to swing to one side, which breaks the coil contact, and so it swings back.
The cycle repeats at a frequency of about 100 cycles per second. A set
of contacts switches the DC to the transformer to generate an AC voltage.
The reed is usually in the negative (earth or common) lead, and the positive
(hot) lead is connected to the transformer centre tapped primary winding.
The contacts earth each side of the centre tapped primary, causing current
to flow each way in it, similar to AC. The difference is that the current
is not an AC sine wave, but a square wave.
The centre tapped secondary of the transformer is connected to a full
wave rectifier valve, in the normal way, and this rectifies the AC to
generate the HT voltage, which is then smoothed and filtered. This method
uses a normal vibrator (called a "non-synchronous" vibrator) and has 3 or
4 pins.
The rectifier valve is not necessary if the vibrator contains
a second set of contacts, which will switch the secondary voltage in synchronism
with the switched DC input. This is a "synchronous" vibrator and usually
has 6 pins. The HT is filtered in the normal way. If the radio design
requires "back bias", then the secondary reed cannot be earthed, and must
be brought out to a pin on the base. This is called a "split reed" vibrator
and commonly has 7 or 8 pins.
Old vibrators that are not sealed, can be easily opened and cleaned.
The contacts are usually hard tungsten, and can be cleaned with emery
paper or a file. After cleaning, check with an ohm meter to ensure that
no emery cloth is caught between them. The contacts can have burnt during
use, or vapor from the internal padding can have deposited on them over
the years. The fibre base can arc over and burn due to dirt. Remove the
carbon with a rotating hand tool, and seal with silicone rubber. Re-adjust
the coil contacts, so that it buzzes correctly, and starts properly (under
load as well). Re-adjust the primary contacts so that they switch properly,
and that they are a "break-before-make" contact.
There are several capacitors that should be inspected and changed. The
HT filtering capacitors should be checked, and changed if necessary. The
timing capacitor should be ALWAYS renewed. The capacitor is in the order
of 0.0068uf and 1000 volt rating. This is important, as this capacitor
determines the time constant (oscillation or "ringing") that occurs when
the contacts open. Sometimes there is a resistor in series with the capacitor
to get the correct time constant. The value is chosen with regard to the
transformer inductance and vibrator frequency (and to a smaller extent the
load components) so that the voltage decay coincides with the next contact
closure, and (if perfect) there will be no voltage across the closing contacts,
so there is no arcing or burning and consequent RF noise. This capacitor
may be located on the primary or secondary winding. A cheap source of the
the timing capacitors are old CRT computer monitors, as there are normally
three 1000 volt capacitors in the EHT section. There are RF noise filtering
capacitors in various places, the primary winding to earth, the secondary
windings to earth, and possibly some input filter chokes as well. These capacitors
are often mica capacitors and seldom fail.
Common vibrator faults are, open circuit coil, faulty contacts (burnt,
dirty, mal-adjusted). Other faults can be, burnt or open circuit transformer,
shorted or open timing capacitor, faulty HT filtering components, faulty
noise filtering components.
110 VOLT RADIOS
American made radios with 110 VAC transformers are conventional and
suffer the usual faults. They can be easily run on 240 VAC by using a readily
available external step down transformer. Some were made for export with
dual 110 volt windings, that enabled the two windings to be strapped in
series for 220 volts. Sometimes a resistor may be included in the 110 volt
radio or in a separate box to drop the voltage. These may burn out. Replace
them with a step down transformer. A method of adapting 110 VAC radios to
240 VAC in the UK, was to use a resistive mains cord. This dropped the voltage
in the same manner as a resistor does, so the cords got warm. As they
broke and were shortened, they got hotter, and the radios were subjected
to a higher voltage. Replace them with an external step down transformer.
TRANSFOMERLESS SETS (AC/DC or HOT CHASSIS)
The mains transformer in a radio is expensive, so it was avoided
in some designs. The rectifier valve was a half wave diode (or a
full wave rectifier connected as half wave), and connected directly to
the mains active lead, the other mains lead being connected to the radio
chassis. There was no mains isolation, so any mains borne noise could couple
into the radio. If the house wiring or the mains plug wiring became reversed,
then the chassis would become live (hot) and dangerous, as the earth or
aerial lead could have mains on it! They usually have a totally enclosed
bakelite or plastic case, with insulated knobs, so that you could never touch
a metal part. These sets could be 240 volt or 110 volt. The use of a 240
to 240v (or 240 to 110v) isolation transformer is a necessary safety item
when servicing one of these. A step down transformer or a VARIAC may
be an auto transformer, and if so, will not provide isolation. some TV sets
from the 1990s hadnosiy and dangerous switching power supplies, and to be
sold on the Australian market, they required an isolation transformer to
be fitted. Look for throw out TVs on the side of the road.
Other than the danger, the power supply was simple, and had the usual
faults. Be careful if using an oscilloscope or signal generator,
as the earth lead can touch a mains live part and cause a huge short circuit.
Another reason for the popularity of these sets in some regions, was
that a transformerless set could operate on a DC mains. The rectifier
would conduct normally. If the mains was reversed, it would not work
at all.
The filaments of the valves in these sets were connected in series,
and normal valves were used for the RF, mixer, IF and AF preamplifier.
The RF or mixer were normally at the earthy end of the series string. Since
these would only add up to about 24 volts or so, there would be a special
version of the audio output or rectifier valve. These have 50 or 110
volt heaters, and importantly, 110 volt cathode/heater insulation, as
they were at the top of the series heater string.
A common fault is for one of these to burn out, then all the valves
don't light. To determine the faulty valve, either remove them one by one
and check for heater continuity, or start at the earthy end with the radio
plugged in, the meter set to 240 VAC, and work up from the earthy end,
until you find voltage. Don't forget the pilot lights, as they are in the
chain as well, or may even be used as a fuse. If there has been a heater/cathode
short circuit, the cathode capacitor and resistor may be damaged.
BARRETTERS
The series string of heaters can be controlled by using another method.
The radio design can use valves with normal heater voltages, and a series
regulator valve to control them. The common controller is called a Barretter,
and is a current regulator (NOT a voltage regulator). Therefore, all the
valves must have the same heater CURRENT and any that are not the same,
will need a parallel resistor to compensate. The Barretter will glow and
adjust the current to the correct value, regardless of mains voltage. The
Barretter looks like a large light globe, with a very long filament that
glows dully. It is an iron filament in a helium atmosphere, and it regulates
the current, by changing its resistance with temperature. When checking
the heater voltage with a voltmeter, do not worry if the voltages are not
exactly right, as the current will be correct. Common faults are an open
circuit Barretter filament.
There is a device called a "ballastron" which is similar in function
to a barretter, but it is merely a resistor in a plugable valve like
case.