This is only a preview of the November 2001 issue of Silicon Chip. You can view 29 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "100W RMS/Channel Stereo Amplifier; Pt.1":
Items relevant to "A Neon Tube Modulator For Cars":
Articles in this series:
Items relevant to "A Low-Cost Audio/Video Distribution Amplifier":
Items relevant to "Short Message Recorder & Player":
Articles in this series:
Purchase a printed copy of this issue for $10.00. |
VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
Test Instruments For Vintage
Radio Restoration; Pt.1
The restoration of vintage radios requires a
range of skills, from cabinet restoration to fault
finding. Fault finding and fine tuning the performance of a piece of equipment is a skill that
is developed over time. It is dramatically enhanced by the use of test instruments.
A considerable number of vintage
radio buffs don’t have a technical
background in radio and electronics
and may therefore have problems restoring the electronics of their radios.
Run-of-the-mill faults can be found
fairly easily found with quite basic
instruments. In most cases, if a set
hasn’t been butchered and is in reasonable condition, it is probable that
the set can be restored to working
order without the use of instruments.
This does not mean plugging the set
into the mains or connecting batteries
and expecting the set to work properly.
Sometimes this is all that is required –
but rarely so and I never recommend
this approach.
Why is this so? The question has to
be asked: “why was the radio taken out
of service?” Usually, it was because it
had developed some fault.
If the aim is to get the set going
without any test instru
ments, it is
often possible to achieve this by replacing components that are known
to be particularly troublesome. The
key components to be replaced are the
automatic gain control (AGC) bypass
capacitors, the audio interstage coupling capacitor(s) and the output valve
plate bypass capacitor. In addition,
the electrolytic filter capacitors in the
power supply should be replaced in
case they have become short circuit or
excessively leaky.
With these components replaced,
the set may work and work well but
you cannot be sure if all faulty components have been replaced. It’s a bit
like working blindfolded.
On the other hand, the set may
still not work and it could have some
serious fault that could cause more
damage when power is applied and
to anyone who may touch the chassis.
Some people are comfortable with this
approach but I’m not, although it is
less risky that the first method. However, all is not lost, as with the use of
a few common test instruments most
faults will be found in receivers. This
month, we’ll start with the humble
multimeter.
The multimeter
This photo shows two typical 20,000 ohm/volt moving-coil multimeters. Analog
meters have an advantage if the measured reading is fluctuating.
80 Silicon Chip
An analog moving coil multimeter
or a digital multimeter (DMM) will
find most faults where voltage, current or resistance can be measured.
It is very helpful to have a circuit
when conducting measurements on a
piece of radio equipment, or any other
electronic gear for that matter. A good
circuit diagram will list the voltages
www.siliconchip.com.au
that can be expected at various points
throughout a receiver. In older circuits,
it will even specify the characteristics
of the multimeter, usually 1000 ohms
per volt.
Measuring voltages
Until the early 50s, most multimeters had a rating of 1000 ohms per
volt. This meant that if the meter was
set on the 250V range, it had a total
resistance between the two probes of
250,000 ohms (250kΩ), while on the
50V range it had a resistance of 50,000
ohms (50kΩ). It is most important to
know this when making measurements.
For example, the first audio stage
may have a 250kΩ plate resistor
connected to the 250V supply rail
– see Fig.1. In order to measure the
plate voltage, the multimeter can
be switched to the 250V range and
the probes connected between the
plate of the valve and earth/chassis.
However, the maximum reading that
can be obtained would be 125V, even
if the valve drew no current. This is
because of the “loading” effect of the
multimeter’s internal resistance.
In effect, this internal resistance
forms a voltage divider with the 250kΩ
resistor connected to the +250V rail,
so the reading is much lower than expected – in this case, half the expected
reading.
Similarly, if the multimeter was
switched to the 50V range, the maximum reading would be 42V. And
if the 10V range were selected, the
meter would read a maximum of 9.6V.
That’s because the internal resistance
Because of their much higher input impedance, digital multimeters (DMMs) are
much more accurate than moving coil types for making voltage measurements.
They also often include capacitance measurement, transistor gain and diode
check ranges.
of the meter would be 50kΩ and 10kΩ
respectively, and so the loading effects
are much greater.
However, when reading from the
high tension (HT) line to earth, the
voltage reading on the meter will be
correct as the circuit has virtually no
resistance in series with the HT line.
To overcome the loading problem it
is most desirable that the meter used
have a rating of at least 20,000 ohms
per volt, which most moving-coil
multimeters have. The readings will
still be a bit low in high-impedance
circuits such as the first audio plate
circuit but not drastically so.
Remember that on circuits with
voltages shown as measured with a
1000 ohm per volt meter, the actual
voltage measured with either a 20,000
ohms per volt meter or a DMM will
be higher than the published figures.
A moving-coil multimeter of 20,000
ohms per volt rating cannot measure
the AGC voltage in a receiver as it acts
as a near short circuit on the AGC line.
On the 10V range, it has a resistance of
only 200kΩ between the probe points,
whereas the AGC filter resistor may be
2MΩ (two megohms). In this case, the
indicated AGC voltage reading will be
a tenth of normal.
Meterman. The Working Man’s Meter.
Meters that fit your job. Meters that fit your wallet.
Introducing Meterman, a hot new brand of test and measurement tools
that gives you the performance you need at a price you can afford.
Meterman is a line of more than 60 meters, clamps, and testers.
Each one designed with the right combination of features, functions
and accuracy to fit your application.
You work hard on the job. Get the tool that’s easy on your wallet.
Ask your local test and measurement supplier for the Meterman products
or contact Meterman on Locked Bag 5004 Baulkham Hills NSW 2153,
phone 02 8853 8812 or fax 02 8850 3300, or visit metermantesttools.com
TM
www.siliconchip.com.au
November 2001 81
Silicon Chip
Binders
REAL
VALUE
AT
$12.95
PLUS
P&P
These binders will protect your copies of SILICON CHIP. They feature
heavy-board covers & are made
from a dis
tinctive 2-tone green
vinyl. They hold up to 14 issues &
will look great on your bookshelf.
Having looked at the deficiency of
the moving coil multimeter in making
measurements in high impedance
circuits, it is time to look at ways of
overcoming this. A DMM with an input resistance of 10MΩ or more can be
used to accurately measure voltages in
all but the highest impedance circuits.
For normal valve receivers, it can be
used to measure all voltages up to
around 1000V.
Note that some cheap DMMs, such
as the one in the centre of the accompanying photograph, have an input
resistance of just 1MΩ which isn’t
good enough for some circuit measurements in valve receivers. Pay that
little bit extra; it’s worth it. I use both
digital and moving coil multimeters,
as each have their strong points.
Measuring current
Or fax (02) 9979 6503; or ring (02)
9979 5644 & quote your credit
card number.
Measuring current with a DMM
or a moving coil multimeter is not a
problem with either type. You will
need to break into the circuit so that
the meter leads can be placed in series
with the circuit.
When making measurements, make
sure that you start with a high current
range and then go lower. Meters do
not take kindly to currents or voltages
that send the needle or DMM well over
range. After you have finished, always
make sure that the meter is set back
onto a high voltage range (and the
meter probes connected to the voltage inputs), otherwise an expensive
mistake could be made by connecting
a meter that’s still on a current range
across the power supply.
Unfortunately, I’ve managed to do
this a few times.
Use this handy form
Measuring resistance
80mm internal width
SILICON CHIP logo printed in
gold-coloured lettering on spine
& cover
Buy five and get them postage
free!
Price: $A12.95 plus $A5.50 p&p.
Available only in Australia.
Silicon Chip Publications
PO Box 139
Collaroy Beach 2097
Enclosed is my cheque/money order for
$________ or please debit my
❏
Bankcard
❏
Visa ❏ Mastercard
Card No:
_________________________________
Card Expiry Date ____/____
Signature ________________________
Name ____________________________
Address__________________________
__________________ P/code_______
82 Silicon Chip
To accurately measure a resistance, one end of the component to
be checked should be lifted out of
circuit and then the meter probes
placed across the component (usually
a coil or a resistor). No adjustment of a
DMM is necessary to accomplish this
task (other than setting the unit to the
“ohms” range) but a moving coil meter
should be “zeroed” before trying to
measure a resistance.
Moving coil meters have very
cramped and rather inaccurate meter
readings at the higher resistance readings on each range. By comparison, a
DMM is much easier to read.
Always make sure that there are no
charged capacitors in circuit when
measuring ohms. Not only will the
readings be inaccurate but damage
to the meter may occur. Return the
meter to a high voltage range after
measuring resist
a nces so that no
meter damage occurs when voltages
are next measured.
Selecting a multimeter
(1) Analog multimeters: the AC and
DC voltage ranges need to extend to
1000V. The lowest range with fullscale deflection is likely to be 10V for
AC and 2.5V for DC. DC current only
can be measured with these units and
can start from as low as 50µA full scale
deflection (FSD) and go to as high as
10A FSD.
The resistance ranges should start
at around one ohm per division and
measure as low as 1Ω. The maximum
resistance that can be measured (or,
more accurately, estimated), is in the
region of 10-20 megohms. They are
quite inaccurate at the high end of the
measurement ranges.
The meter movement needs to be
rated at 20,000 ohms/volt (or higher).
An analog meter shines particularly
when the parameter being measured
is varying, as the trend of adjustments
can easily be seen. The claimed accuracy of most of these meters is around
±4% FSD.
Examples of units that meet the
above criteria are the Altronics Q1025,
the Dick Smith Electronics Q1025 and
the Jaycar QM-1020. The Altronics
and DSE models appear to be identical units. There will be similar units
Fig.1: this diagram shows a 1000
ohm per volt meter measuring
the plate voltage in a circuit
with a 250kΩ plate load resistor.
It shows 125V on the 250V range
(ie, 50% the correct value), 42V
on the 50V range and 9.6V on
the 10V range. By contrast, a
DMM with an input impedance
of 10MΩ gives a reading of 244V
– 97.6% the correct value).
www.siliconchip.com.au
Photo Gallery: Music Masters Mozart
Looking for an old valve?
or a new valve?
BUYING - SELLING - TRADING
Australasia's biggest selection
Also valve audio & guitar amp. books
SSAE DL size for CATALOGUE
ELECTRONIC
VALVE & TUBE
COMPANY
Music Masters Radio Company, Brisbane, produced the “Mozart” in 1940.
The set is a superhet with the following valves: 6A8-G frequency changer;
6U7-G IF amplifier; 6B6-G first-audio/detector/AVC rectifier; 6V6-G output and 80 rectifier. Photo & information courtesy of the Historical Radio
Society of Australia.
from other suppliers too, so have a
good look around to find a meter that
satisfies your needs.
Digital multimeters
The selection of a suitable DMM is
not as simple as selecting an analog
meter as there are just so many more
to choose from, with a myriad of features.
The first thing I look at is the input
resistance and this should be at least
10MΩ or even higher, so that high
impedance circuits are not loaded
excessively when measurements are
being made. Most meters costing more
than about $45 are likely to be suitable.
The voltage ranges should start at
about 200-400mV AC & DC and extend
to 700V AC and 1000V DC. On AC,
the maximum frequency that can be
applied to the meter without affecting
the measure
ment accuracy varies.
A couple of mine will still read the
correct voltage at frequencies up to a
least 2kHz.
The current ranges should start at
around 200µA on AC & DC and extend
www.siliconchip.com.au
to 10A or maybe even 20A AC and DC.
The resistance ranges should be able
to measure to below 1Ω and up to at
least 10MΩ or 20MΩ.
One facility I find very handy are
capacitance measurement ranges.
However, not all meters with such
ranges will accurately measure low
capacitance values. It is desirable to
be able to accurately measure values
down to 10pF and up to around 20µF
or more. Meters with a range of 4nF
(.004µF) or lower will usually measure
down to around 10pF with reasonable
accuracy.
Always make sure that a capacitor
is discharged before trying to measure it, otherwise damage to the meter
may occur. With some capacitors, it is
necessary to use clip leads to connect
them to the meter. If this is done, note
the reading of the meter before the capacitor is connected and subtract this
from the total reading to compensate
for the lead capacitance (note: this only
applies when measuring very small
value capacitors).
Sometimes, when measuring a
capacitor in a receiver, such as a tun-
PO Box 487 Drysdale, Victoria 3222.
Tel: (03) 5257 2297; Fax: (03) 5257 1773
Mob: 0417 143 167;
email: evatco<at>mira.net
Premises at: 76 Bluff Road,
St Leonards, Vic 3223
SMART FASTCHARGERS®
2 NEW MODELS WITH OPTIONS
TO SUIT YOUR NEEDS & BUDGET
Now with 240V AC + 12V DC operation
PLUS fully automatic voltage detection
Use these REFLEX® chargers for all your
Nicads and NIMH batteries: Power tools
Torches Radio equip. Mobile phones
Video cameras Field test instruments
RC models incl. indoor flight Laptops
Photographic equip. Toys Others
Rugged, compact and very portable.
Designed for maximum battery capacity
and longest battery life.
AVOIDS THE WELL KNOWN MEMORY EFFECT.
SAVES MONEY & TIME: Restore most Nicads with
memory effect to capacity. Recover batteries with
very low remaining voltage.
CHARGES VERY FAST plus ELIMINATES THE
NEED TO DISCHARGE: charge standard batteries in
minimum 3 min., max. 1 to 4 hrs, depending on mA/h
rating. Partially empty batteries are just topped up.
Batteries always remain cool; this increases the total
battery life and also the battery’s reliability.
DESIGNED AND MADE IN AUSTRALIA
For a FREE, detailed technical description please
Ph (03) 6492 1368; Fax (03) 6492 1329; or
email smartfastchargers<at>bigpond.com
2567 Wilmot Rd., Devonport, TAS 7310
November 2001 83
Photo Gallery: Stromberg-Carlson
Model A22 3-Valve TRF Receiver
An auto-ranging facility is also
useful in some circumstances but can
be confusing where a range changes
unexpectedly. If you believe auto-ranging is for you, make sure that you can
manually select the range that you
want as well.
Just about a all units these days have
a claimed accuracy on the voltage
ranges of 0.5% (or better) ±1 count. Of
course, this applies only if the meter
has not been abused in any way. Accuracy greater than this is not necessary
for routine work.
Analog or digital?
Made by Stromberg-Carlson, Sydney, in 1930, the Model A22 is a 3-valve
TRF receiver housed in an elegant long-legged wooden cabinet. It was
fitted with an 8-inch (200mm) loudspeaker and used the following valves:
B443 detector, E415 output and UX280 rectifier. Photo & information courtesy of the Historical Radio Society of Australia.
ing capacitor, it is desirable to swap
the test leads over to get the correct
reading. The actual capacity of the
meter circuitry may cause erroneous
readings if near the receiver chassis.
An audible continuity facility is
another useful feature, as this makes it
unnecessary to watch the meter when
making continuity tests. A diode test
84 Silicon Chip
range is a handy range too – this will
measure the forward voltage drop in a
solid state junction, whether it be in a
diode or a transistor. It’s also handy for
checking that there is no conductivity
in the reverse direction and for determining whether a transistor is PNP
or NPN type and whether a diode is a
silicon or germanium type.
This is really your personal choice.
Analog units are more suitable if a
reading is varying and some people
prefer to see a needle moving across
a meter scale. That said, digital multi
meters (DMMs) are much better value
for money, are more accurate and have
a greater selection of measurement
facilities.
What’s more, it doesn’t matter
which way around you connect the
probes when making measurements
on a DMM. Analog meters, on the
other hand, must be connected with
their positive (red) lead to the more
positive voltage point when measuring
a DC voltage. The same applies when
measuring current.
Which ever meter you select, make
sure that there is an insulated collar
around each probe shaft just above the
probe tip. These collars are designed so
that if your fingers slip along the probe
(eg, in humid weather), they will not
come in contact with the metal part
of the probe (which could give you
a shock).
What do I use? I use both digital and
analog meters, although most of the
time I prefer a digital meter.
A multimeter, whether it is an
analog or digital model, is by far the
most important test instrument that
you will use for fault-finding and testing vintage radios. Select wisely and
you will have a versatile instrument
that will last you for years.
However, while a multimeter will
allow you to find most faults in a receiver, there are some problems that a
multimeter will not be able to detect.
Under these circumstances, other test
instruments are needed. We’ll look at
some of these instruments in future
columns, including signal generators,
signal injectors, signal tracers, transSC
former testers and so on.
www.siliconchip.com.au
|