Ray Robinson VK2ILV
7 Roland Ave
Wahroonga 2076 NSW
02-94898561
<ROBINSON@SHLRC.MQ.EDU.AU>
INTRODUCTION
The practice of repairing valve technology is disappearing. However,
it is not difficult, and can be easily understood. With a little care and
thought, most faults can be readily put right. This is because the components
used in valve technology are large, simple, still available, and usually
connected using "point-to-point" wiring methods. Keep safety in mind at
all times, as valve technology uses high voltages.
I will refer to a common 4/5 valve domestic broadcast
receiver, as an example, to describe common faults, and how to fix them.
I will introduce a little theory and safe operating procedures, to protect
you, and the radio.
The common superheterodyne from 1940 has 4 valves plus one extra for
the power supply, so an easy way to describe it, is to call it a 4/5 valve
radio. This typical radio has a 240 volt AC mains power supply, using a transformer
and valve rectifier. There are many variations. Please refer
to the circuits that show typical radios with different valve types depending
on the time it was built.
SAFETY
A radio is no good if you reduce it to a smoking ruin.
The idea behind fixing the radio, is to improve the performance.
Therefore, check the radio carefully for dangerous parts or damage and
fix them before powering it up.
Make sure it cannot fall over and break the glass valves, or something
fall into it and damage a component, or that balancing it upside down doesn't
break the dial glass.
Check filter capacitors so they don't get hot and explode.
Check mains transformers so that they don't heat up or burn up and can't
be replaced.
Keep the bench clear of rubbish, swarf, solder blobs, food, drinks and
any junk.
Only work on one radio at a time. If you can't fix it or get fed up,
draw a diagram or sketch, label the parts, put them all in a box, and put
it aside for a time.
Go on with something else.
A person is no good if they are lying dead on the floor.
If you can't fix a radio without killing yourself, getting a shock
or a burn, or using a laywer, then don't touch it. Get someone else to
fix it for you.
Turn the radio OFF when replacing components, turning it over, or adjusting
things. Use insulated screwdrivers, cutters, soldering iron, and alignment
tools.
Don't eat or drink while working, spilled fluids can ruin a radio and
conduct voltage to you. When finished working on a radio, wash you hands
before eating and drinking, as the chemicals, toxins, animal residue, or
manufacturing byproducts can affect you. Some older radios used radium on
their dials, some military radios used radium in their meters and in the
fluorescent paint.
If things don't make sense, give up, go away, come back after a sleep
or a rest, and things suddenly may be obvious.
CLEANING
Clean the underneath and inside of the radio, removing loose rust and
dirt.
Start with an air source and blow out the dust. Otherwise, use a soft
paintbrush, and assist with blowing. Don't inhale. Clean out all spiders,
wasps, cockroaches, mice, grass, newspaper and droppings. Don't cut yourself
as you can get an infection. I know of someone who got hepatitis from a
cut hand from a chassis infected with rat urine.
Replace any damaged components, including those wires or capacitors
that may have been eaten by mice.
Use a damp rag, and wipe away any stuck dirt. Dry in the sun, but not
direct sunlight as you might melt the dial or case.
So far we have avoided solvents.
If a radio has been in a kitchen or near someone who was a smoker, then
it may contain oil or grease. Use "Nifty" to dissolve the grease and wipe
away. This is a spray cleaner in a squirt bottle, that melts grease. Good
for hand marks around knobs too.
Never use metholated spirits (alcohol) ot turpentine, as these will
remove labels, stencils, transfers, and can attack the plastics, rubber,
and destroy a dial scale.
Test all cleaning agents FIRST, on a small area of the chassis or case,
that is not visible, like an area underneath.
Rust can be removed by scraping, and then sealed with a lacquer. Rust
converters or replating the chassis are not usually necessary, unless you
want to strip the chassis completely.
DIAL SCALES.
These are extremely important and can be easily damaged. A radio without
a dial scale is not worth a great deal.
The scales are usually glass or plastic, and old and brittle. The plastic
is not flexible anymore, and the rubber covered clamps that hold the glass
may have hardened. The scales can be ruined, and easily made opaque with
alcohol or turps, or have their station legend removed with water. Clean
the front ONLY with a damp cloth. No Windex (alcohol based). If you really
have to, and I mean REALLY have to, clean the back carefully with a soft brush,
that is all. Even this may remove the lettering! Be gentle.
In the situation where the station legend has started
to fall off, you have nothing to lose, so get a spray can of clear lacquer,
like clear Estapol. Put the dial scale flat, with the legend side up. Spray
a 1 second duration VERY, VERY light mist from a LONG distance, Just a slight
mist, nothing more. Wait for it to dry. Spray another mist. Wait for it
to dry. Do this a few more times. If your mist coat is too close or too
heavy or the wind from the can too strong, the letters of the station legend
will move. If the spray coat is too heavy, the spray solvent will buckle
the lettering. The theory behind this method, is that a few tiny spots of
lacquer, will act like tiny spots of glue, and hold the letters on to the
glass. A solid total covering is not required. Good luck, as this method
does not always work.
If the glass is broken, you can make a colour scan of the dial
scale, using a computer, and then print the scan with a colour printer on
a transparency. This can be placed behind the glass or sometimes glued on.
GENERAL
Find out the history of the radio. Ask the seller, where it came from
and who owned it. Sometimes just knowing it had been bought new by someone's
grandfather is very interesting. Get a circuit diagram so that fixing it
will be a lot easier. Circuits are sometimes in the back of the cabinet, or
available from the web, or from the H.R.S.A. The manufacture's name, chassis
number, and valve types help identification. The coloured ARTS&P sticker
will help place the year of manufacture. The history behind this label is
interesting. There were patents on things like valve sockets and other parts,
so Australian law required that royalties were paid when using these items.
The stickers were sold to the radio manufacturer upon royalty payment and
each legal radio had one affixed to the chassis. The colour changed each year
so it is easy to determine the year of manufacture.
Draw diagrams as you dissassemble things, especially dial cords, component positions, and mechanical mechanisms. Perform a visual inspection for exposed wires, sharp edges, insects, animals. Remove anything that should not be there. Fix any mechanical faults. Lightly oil the moving parts. Quite often, someone has installed "improvements" and in some cases, I have fixed a radio by removing these improvements.
MAINS INPUT
This is very important and care and time spent examining this section
of the radio is not wasted.
The most dangerous part of an AC radio, is the mains circuitry. Check
this first before plugging it in. A common practice amongst radio collectors,
is to cut off the mains cord or plug, immediately that a new radio is acquired.
This method removes the temptation "to plug it in and see if it works",
which can quite often ruin a radio with a loud bang.
Examine the mains cord, to see if the cable is safe, not
perished, and no bare wires showing. Check for damage where it enters
the plug and where it bends entering the chassis. Renew (or shorten) the
cable if it is damaged, with cable of the correct vintage. Use some rubber
or cloth covered 3 wire cable from the junk box, or figure "eight" if the
radio uses that type. An earth wire may not be required on these radios
as they usually had a totally enclosed chassis and insulated knobs. Some
hardware shops have "cotton covered" 3 wire mains cord for use on domestic
clothes irons. This is suitable. Maroon coloured "cotton covered" mains
cord may be available from vintage radio shops.
Examine the mains plug for cracks and bare wires, especially "whiskers",
which may have escaped the wire holding screw.
Renew the plug, if it is cracked, burnt or dangerous, with a plug of
the correct vintage.
Check the grommet or clamp on the chassis for wear or damage. Fix or
renew it.
Check the ON/OFF switch with a multimeter to be sure that is is working
and has no short circuit to the chassis.
Check the power transformer connections.
Radios seldom have fuses, as they rely on the house fuse. Usually, this
is all that is necessary.
POWERING UP A RADIO
There are different methods for powering up a radio.
If the radio had been dormant for many years, it may not have been working
when it was put in storage so it may have a 50 year old fault, as opposed
to a fault caused by age.
However if the radio was working and has just stopped, then normal fault
finding can be attempted.
A SIMPLE AID
A 100 watt incandescent light globe can be placed in series with the
mains cord during testing. This will act like a fuse. If there is a short
circuit, the bulb will shine at full brilliance. If the radio is working
normally, the bulb will glow just a little.
FOR THE FIRST TIME (SLOWLY).
A variable transformer called a VARIAC is a useful device for slowly
bringing a radio to life. You can slowly turn the knob and apply the mains
voltage, from zero to 240 volts over several hours. The range from 180 to
240 is crucial, as the rectifier suddenly starts to work and the HT is applied.
Be careful and take your time.
FOR THE FIRST TIME (STEP BY STEP)
Another method, is to test the radio section by section, and turn the
radio OFF between each test.
Start by drawing a diagram of what valves go where. Then CAREFULLY remove
the valves and place them in a box with tissue paper. Grip them by the base,
not by the glass, as this may break the bond between the base and glass.
Pull them straight up, do not wiggle. If the valves have no makings,
attach a label to their base, with the type or location. Don't assume the
valves are correct, or that they are working, or in the proper socket, someone
may have attempted to fix the radio before you.
Check the mains wiring, cord and socket as described.
Put a meter across the primary of the transformer, set to 250 volts
A.C. or greater.
Turn the radio OFF.
Turn the power socket switch OFF.
Plug in the mains plug.
Turn the power socket switch ON.
Check for smoke. Listen for sparking.
Turn the radio ON
Check for smoke. Listen for sparking.
Check there is 240 volts AC reading on the meter.
Turn the radio OFF and the power socket switch OFF.
Move the meter to read the heater voltage (a convenient place is across
the audio output valve socket). Set the meter to 10 volts AC or greater
for 6 volt valves. Set it appropriately for different valves.
Turn the radio ON and the power socket switch ON.
A reading of the correct heater voltage, means that the power transformer
is working, and that there are no short circuits on the heater wiring.
Do the same thing for the high tension winding, which is usually 250
AC (or 385 AC) and can be checked at the rectifier valve socket.
If this is all correct, then plug in the audio valve ONLY.
It should light up correctly.
If this is all correct, then plug in all the other valves, but not the
rectifier.
They should light up correctly with a dull red glow.
Now for the tricky part.
Plug in the rectifier, attach the meter to the high tension point, with
the meter is set for 500 volts DC.
Turn ON the radio, and watch the meter needle climb to 250 volts (or
above).
If this doesn't happen within 20 seconds, turn the radio off quickly.
Start fault finding in the power supply.
Immediately check the filter capacitors for heat (they should be cold,
not hissing, nor sparking, nor warm).
Turn the radio OFF quickly if any noise (apart from the speaker) is
heard.
If this is all working then start fault finding in the other stages.
Continue checking the filter capacitors and power transformer for heat,
during the fault finding.
GENERAL FAULTS
Valves account for less than 10% of faults.
Most faults are due to capacitors, particularly filter capacitors, bypass
capacitors, and coupling capacitors. I usually don't bother to even check
the valves. I go straight to the capacitors, and if they are elderly, will
often replace all the paper and electrolytics, even before powering a radio
up.
Sometimes I will check the resistors, but unless they look burnt, I
leave them alone.
Fading: If the radio works, then slowly fades away, then slowly comes
good again, it could be an intermittent connection to a valve heater, causing
it to warm up then cool down. Check for a dry joint on the valve socket,
or a dirty socket.
For instability, check shielding, particularly the I.F. amplifier. Sometimes
corrosion on the shield rivets or bolts means they are not earthed.
If the radio produces a funny burping sound (called "motorboating"),
it is usually feedback between stages, coupled by the HT or AGC line, so
suspect faulty screen, high tension or A.G.C. bypass capacitors.
If the radio appears to be working but there are no stations, scratch
the aerial wire with a screw driver or to chassis. If a scratchy noise is
heard, then the mixer oscillator is not working or is off frequency.
POWER SUPPLY
The power supply normally consists of a transformer, rectifier, filter
choke and filter capacitors.
The secondary of the transformer, is usually a centre tapped winding
with 250 volts AC per side. The two ends are connected to the rectifier plates,
to provide full wave rectification. A separate heater winding of 5 volts
AC is supplied to the rectifier (type 5Y3). This is where the rectified
voltage appears and there is a smoothing filter capacitor of usually 8 microfarads
rated at 350 volts DC. The high tension then flows through a smoothing choke,
to another filter capacitor. In newer radios, there may be a high wattage
resistor used as the choke. This can burn out. In older radios, the speaker
used an electromagnet, so this winding was used as the filter choke. In
these radios, there is a large voltage drop across this coil, so the rectifier
supplies about 400 volts DC, and the capacitor is rated at a higher voltage.
There is usually a separate heater winding of 6 volts AC for the remainder
of the valves. On more modern radios, a new rectifier was used (type 6X5),
that had a separate cathode (with a high isolation between cathode and heater),
so that only one heater winding was required on the transformer.
Check the voltage across the filter capacitors for about 250 volts DC.
Common transformer faults can be: burnt out (replace or
rewind the transformer), open circuit (replace or fix), buzzing (tighten
the clamps or screws, or if a short circuit is causing this, remove it).
If open circuit, determine which winding. Sometimes the tinning or soldering
process can break a wire where it enters the transformer. This can sometimes
be carefully rejoined.
Common rectifier faults can be: burnt out (replace the valve), sparking
(check for a short circuit in the high tension line).
The rectifier valve should have a dull glow for the filament or heater.
If the plate is glowing red, then there is a short circuit on the HT line.
Turn it off immediately and locate the short.
Common filter choke faults are: open circuit (replace or look for broken
wire or damage), or short circuit to choke body (insulate the short). Chokes
can be rewound.
Common filter capacitor faults: open circuit (replace), short circuit
(replace), low capacity (replace), hot (turn OFF quickly, and keep trying
each 24 hours, they may reform), exploded (clean the mess and replace).
There is a less brutal method of reforming filter capacitors, by limiting
the current to them, by using a high value resistor, until they recover.
To do this, place a 470k resistor in series with the capacitor and apply
250 V DC. Measure the voltage across the capacitor. When the capacitor
"reforms", the voltage will be the same as applied. While the capacitor
is reforming, it will be a lot less.
SPEAKER
A speaker consists of a paper diaphragm, with a coil wound on the apex
of it, called a voice coil. The coil is placed in a magnetic field, and
when a current passes through the coil, the diaphragm will move and produce
sound waves in the air. The magnet can be a normal permanent magnet on modern
speakers, but may be an electro-magnet on older speakers. The electro-magnet
coil can also double as a filter choke for the power supply. In this case
there may be a small additional coil connected in series with the voice coil,
called a "hum bucker" to prevent any hum from the electromagnet producing
sound. There may also be a speaker transformer mounted on the back of the
speaker.
In radios with removable speakers (like radio grams or consoles) the
HT line is often routed through the speaker plug with a link, to prevent
high tension being supplied to the radio if the speaker is unplugged.
Distortion can occur from a torn diaphragm or torn edge suspension. Repair it with a thin layer of silicone rubber. If there is dirt or iron filings in the coil, remove them. A pair stainless steel tweezers (non-magnetic) are useful. If the voice coil is touching the magnet, called "poling" adjust it if you can. Put a piece of 35 mm film in between the pole and voice coil as a spacer, and loosen the pole bolt, adjust and retighten. Remove the film. If the electromagnet is open circuit, find the break, which is normally near the start or outside. Reconnect and reinsulate. Even taking off a few hundred turns will not significantly affect the operation of the speaker or the filter function. H.R.S.A. members can rewind the electro-magnet, and even recone a speaker, if required. Checking the primary of the transformer with a multimeter on the ohms scale should cause an audible click to be heard in the speaker (the radio should be off).
AUDIO OUTPUT
The audio output valve uses a speaker transformer to match the valves
high impedance to the speakers low impedance. The screen is normally
connected directly to the high tension supply. There is sometimes a small
capacitor to ground to prevent oscillation, or it may be connected to a
switch or variable resistor as a Tone control. The valve needs a grid negative
bias, and this is normally done by a cathode resistor which has a bypass
capacitor. Another bias method called " back bias" can be used. This method
involves grounding the cathode, and using a resistor in the power transformer
centre tap to generate a negative voltage. There is sometimes a capacitor
connected back to the grid as feedback. There may also be a resistor in series
with the grid as a "stopper" to prevent oscillation. All of the resistors
and capacitors in this circuit can be 50% out of tolerance, and there will
be little change in its operation. A quick check is to touch a screwdriver
to the grid, then touch it with your finger, this will introduce some hum,
and you should hear it in the speaker. To avoid a shock, do not touch the
chassis.
If the primary of the speaker transformer is open circuit
there will be no sound, and the screen will be red hot. Turn off quickly.
If the cathode bypass is open circuit, there may be distortion or low
gain.
A crackling noise can be a noisy anode capacitor, or a scratchy
Tone control. Spray CRC or WD40 into the control.
If the valve is glowing blue, it may have air in it (be "soft") and
need replacing.
Distortion may be caused by a "leaky" coupling capacitor from the previous
amplifier stage. This is one of the most common faults in all valve radios.
The audio grid should always be zero or negative volts. Any positive volts
means a leaky capacitor. Replace it. It should read "infinity" on the
ohms scale of a multimeter.
Also check that the resistor from grid to earth is not open circuit.
Hum: A short between the heater and cathode can introduce hum. Replace
the valve.
Another source of hum can be a faulty filter capacitor on the high tension
supply. Replace it.
If you are really driving the audio amplifier hard, and the speaker
is not connected, there can be arching between the screen and anode pins
on the valve socket. This normally burns the socket and it needs replacing.
AUDIO PREAMPLIFIER
The audio preamplifier can be a triode or pentode and as the valve contains
the detector and A.G.C. diodes as well, the circuit may appear complicated.
Treat it function by function, and the theory is not too difficult. The
anode of the valve is connected to the high tension supply, and a capacitor
from here, couples the audio to the power amplifier. Sometimes there is a
small capacitor from the anode to ground, to ensure none of the IF gets into
the audio to cause instability. The volume control wiper takes the detected
audio to the grid of the preamplifier. There is usually a grid resistor
to ground, and a coupling capacitor from the volume control. These seldom
fail. The cathode may either have cathode bias or be connected to ground.
A quick check is to touch a screwdriver to the grid, then touch it with
your finger, this will introduce some hum, and you should hear it loudly
in the speaker. The grid may be the top cap (in an older radio) or the wiper
of the volume control.
A crackling noise can be a noisy anode capacitor
to ground. Replace it.
Sometimes the anode or screen resistors can be open circuit, causing
no sound. An out of tolerance resistor is not a problem.
If sound can still be heard when the volume control is, turned all the
way down, the cathode bypass capacitor may be open circuit. Replace it.
The valve rarely fails.
DETECTOR
One of the diodes in the audio preamplifier valve is used as the audio
detector. The top of the secondary of the last IF transformer is connected
to the diode. The rectified audio appears between the bottom of the IF transformer
and the valve cathode. This is then coupled to the volume. Usually there
is a filter circuit consisting of a resistor and two small filter capacitors,
to smooth the audio and remove any I.F. component before it gets to the volume
control. The bottom of the volume control may be connected to ground or
the cathode. There is little to fail in this circuit, and faults here are
uncommon.
An open circuit IF transformer can cause the audio to fail, but the A.G.C. may still function.
AUTOMATIC GAIN CONTROL (A.G.C. or A.V.C. )
The second diode in the audio amplifier valve can be used as the A.G.C.
bias rectifier. The diode is connected to the anode of the I.F. amplifier
by a capacitor, so that it does not cause audio distortion. There is
a resistor to ground from the diode which is the diode load, and the negative
voltage is developed across this. The stronger the signal, the higher the
negative voltage. This is coupled forward to the grids of the I.F. amplifier
to reduce its gain on strong signals. The A.G.C. is sometimes connected
to the mixer grid as well, but this may cause unwanted frequency change
to the oscillator. In domestic sets this is not critical. In communications
receivers, the audio preamplifier may also have A.G.C. applied. The A.G.C.
line has large capacitors and large resistors to connect it to the preceding
stages. These reduce the voltage as it goes forward and prevent any coupling
which may cause oscillation. High value resistors can be tolerated.
A leaky coupling capacitor can cause positive voltage
to appear on the A.G.C. diode, and so generate a positive A.G.C. voltage.
Replace the capacitor.
Open circuit capacitors can cause instability. They may also cause
the A.G.C. to act too fast.
I.F. AMPLIFIER
This amplifier is the most important amplifier in the radio, as this
is where most of the gain comes from. It can also be where the most instability
is found. Good shielding and bypassing is essential. Sometimes this amplifier
may have "negative" feedback or "neutralising" on it to make it stable.
It is a simple amplifier, with a tuned circuit on the input and output.
It often has cathode bias, and always A.G.C. control. The tuned circuits
act like a filter to reduce reject adjacent signals, and to ensure the local
oscillator is removed. The I.F. transformers can be tuned with a signal
generator. They may also tuned by ear, if you are careful and use a weak signal,
to make the tuning sharp and to make sure the A.G.C. has no effect. Sometimes
resistors and capacitors are conveniently (or inconveniently) mounted inside
the I.F. transformers.
Common faults are mis-tuning of the tuned circuits, open
circuit windings (broken wires) or broken slugs.
If the I.F. amplifier tunes up correctly, and then suddenly looses sensitivity
when you turn it upside down, the slug is probably broken and moving around
freely inside the I.F. transformer. Most radios have fixed capacitors and
ferrite slug to tune them. Some older ones may have variable capacitors.
In this case, use an insulated screw driver to adjust them as there will
be HT on them. Don't short them to earth.
Instability can be due to faulty bypass capacitors or failed shielding
due to corrosion.
MIXER
The mixer (sometimes called the first detector) is usually a valve that
has many screens, or it may have a separate triode inside. The screens
or triode act as an oscillator. The oscillator has its own tuned circuit
and tuning capacitor (part of the main tuning capacitor). The oscillator
normally runs at a higher frequency that the received station, and the
difference is always the I.F. frequency. The oscillator in this way, selects
the desired station, for the I.F. amplifier. The oscillator signal is mixed
with the received signal inside the valve, and the coupling is by virtue
of the components being inside the same valve. Communications receivers
that have a separate oscillator valve, require a coupling capacitor to
inject the signal into the mixer. The settings of the stations on the dial
is achieved through adjusting the oscillator.
The aerial signal is connected by a tuned circuit to the mixer grid. There may be A.G.C. connected to the bottom of the tuned circuit. The main tuning capacitor has a separate section to tune in the stations. Each tuning capacitor section has a "trimmer" capacitor across the main capacitor to adjust the frequency of the tuning, but this trimmer only has an effect when the gang is nearly fully open, that is, at the high frequency end of the dial. There is sometimes a "padder" capacitor in series with the main tuning capacitor, but this padder only has an effect when the gang is nearly fully closed, that is, at the low frequency end of the dial. If a padder is not fitted, then the tuning can be adjusted with a ferrite slug inside the coil. Use this at the low frequency end of the dial. Always adjust the low frequency end first. Re-checking and readjusting, may be required several times as they interact.
If the radio appears to be "live" by scratching the aerial
terminal or grid with a screwdriver and causing a noise, but there are
no signals, the oscillator has probably stopped. A weak valve is often
the cause. Replace it.
A negative voltage on the oscillator grid means it is oscillating. This
is hard to measure, as touching the grid with a multimeter or oscilloscope
probe can often cause it to stop. One sure way to check, is to lift the
earthy end of the oscillator grid resistor and check for negative grid current.
It will be a few micro amps. If no current, then it is not oscillating.
If the stations do not agree with the dial, first check the dial pointer
and dial scale, and mechanically move it if necessary or possible. Only
then, adjust the oscillator if further correction is needed. Always set
the low frequency end first.
If dial readings cannot be set, check the trimmer and padder capacitors.
Also check that the oscillator is on the correct side of the received frequency.
Some communications receivers, have the oscillator on the high side for
the low bands, and then change to the low side for the high frequency band.
Common faults can be burnt aerial coils due to lightning
strikes. Also broken slugs and capacitors from previous attempts to repair
or tune the radio. Look for bent tuning capacitor plates, or corrosion
"whiskers", if the radio only works for half of the dial. Check for open
circuit coils and I.F. transformers, and open circuit resistors.
If the radio is a dual band radio, check the band change switch and
clean it if necessary. Poor performance on short wave is often caused when
a wrong type mixer valve is fitted.
Broken dial cord can be restrung with fishing line if really necessary,
but it slips on small diameter shafts. Real dial cord can still be
purchased.
If A.G.C. is applied to the mixer stage, through the bottom of the grid
coil, the tuning gang may be insulated from the chassis with rubber mounts.
These may be old, brittle or missing, Grommets are a suitable replacement.
CONCLUDING REMARKS
There are other designs in radios, some are better than others,
and some are easier to fix than others. Take it carefully and slowly and
try to understand what you are doing and why you are suspecting a component.
Logically determining a fault, is the best way to achieve success.
Your repairs should be generally neat, and soldering tidy. Avoid
burnt wires, solder splats, and modifying the radio in any way. Try and
use period components and wire. Don't use modern screws, old imperial screws
can still be purchased. Don't clean the values off valves or components.
If in doubt, stop and think about it. Ask someone. There is a lot of people
in the Historical Radio Society of Australia who have knowledge and are
willing to help.
There are many other topics that are not covered here.
HOW A VALVE FUNCTIONS
R.F. AMPLIFIERS
SUPERHETERODYNES
T.R.F. (Tuned Radio Frequency)
REGENERATIVE
SUPER REGENERATIVE
REFLEXING
CRYSTAL SETS
HEADPHONES
SPEAKERS
TRANSFORMERS
MICROPHONES
MICROPHONICS
AERIALS
OSCILLATORS
OSCILLATIONS
SENSITIVITY
IMAGING
BATTERY POWERED SETS
FARM RADIOS
CAR RADIOS
TRANSFOMERLESS SETS (AC/DC or HOT CHASSIS)
BARRETERS
REGULATORS
COMMUNICATIONS RECEIVERS
TRANSMITTERS
AUDIO AMPLIFIERS
RECORD PLAYERS
MILITARY RADIOS
DYNAMOTORS (GENEMOTORS)
PARTS
Clark Rubber: Grommets, rubber feet, rubber tube, rubber edging.
Brian Smith's Wireless Workshop: cotton wire, cotton covered mains cable,
capacitors resistors, dial cord, dial scales. <http://www.users.bigpond.com/Brianswireless/page42.html>
WES components: Dial cord, alignment tools.
Lee Bros (Paramatta): Imperial screws.
Information and help: Historical Radio Society of Australia: <http://www.hrsa.asn.au/>
Transformer Rewinding: H.R.S.A. members.
Circuits: H.R.S.A. or the web.
Information: Ray Robinson <http://www.shlrc.mq.edu.au/~robinson/museum>
Information: Ian O'Toole. < http://users.froggy.com.au/vk2zio/museum>
19 September 2003
end