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VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
Choosing your test instruments; Pt.2
Last month in Pt.1, we looked at the most used
and most common test instrument of all, the
digital or analog multimeter. Most faults can be
found with a multimeter but the performance of
your restored radio can be considerably
enhanced by the use of other test instruments.
The importance of test instruments
to do the job can to a certain extent
be dictated by your preferences and
experience with various instruments.
Generally, it is good to keep your test
instruments as simple as possible and
to the minimum number necessary to
achieve a good result.
When I was younger and less experienced, my aim was to surround
myself with as many instruments as
I could afford to do the job. It gave
me a feeling of importance and I also
conned myself into believing that with
a large selection of equipment I could
find anything. However, things don’t
work out that way.
Not only is it desirable to have
appropriate equipment, it is also necessary to know what you are looking
for. In those earlier times, with only
limited experience, I couldn’t interpret
the equipment readings or the effects
of the instruments on the receiver
under test. So at that time, the use of
complex equipment would have been
a waste of money.
RF signal generator
The two instruments that I use
most today are a multimeter and an
RF signal generator. I was asked by a
vintage radio buff what he would use
a signal generator for. He hadn’t used
one so naturally he was unsure of
what it did and how it performed its
functions.
A signal generator is very much like
a miniature radio broadcast station. It
is usually capable of being tuned to
any frequency used by ordinary AM
radio stations. It supplies a tone signal
instead of voice or music, as this is
easier to produce and makes it easier
This home-made
audio transformer tester enables
transformers to
be tested for short
circuited windings
and turns ratio.
www.siliconchip.com.au
to gauge the performance of the radio
it is attached to. It has adjustable signal level output so that it can take the
place of a strong signal from a nearby
station or even a weak, faraway station.
It also has the advantage that it can be
set to the intermediate frequency (IF)
of almost any superhet radio a vintage
buff is likely to see.
Having noted what a signal generator does, perhaps a few criteria for
selecting a suitable signal generator
can be listed:
• It may be powered from mains or
batteries.
• Its radio frequency (RF) tuning
range should extend from around
150kHz (the lowest IF that you are likely to see is 175kHz) to at least 30MHz
when dual-wave or multi-band radios
are being tested. If only broadcast band
receivers are going to be tested, an
upper frequency of 2MHz is all that
is needed.
• It needs to have audio modulation
either by a tone of 400Hz or 1000Hz, or
both. If possible, the level of this tone
should be adjustable and it needs to
be able to be switched off.
• The audio tone should be available
at the front panel and its level adjustable for testing the receiver’s audio
section.
• The radio frequency (RF) output
level needs to be adjustable so that
strong and weak signals can be simulated. This is an area that is often
poorly achieved in cheap generators
which are really just modulated oscillators. High-priced units have cal
ibrated output levels so that the user
can accurately determine the exact
performance of their receiver.
You may have seen in some of my
articles where I have quoted particular
sensitivities like 10µV (microvolt), etc.
This can only be done with instruments like the Hewlett Packard 606B
December 2001 83
This high voltage insulation tester was
built from an Altronics kit and is used
mainly for checking the insulation of
transformers.
shown in one of the photographs.
This facility is not available on
cheap instruments as it is costly to provide. Don’t despair as it isn’t essential,
just nice to have when a particularly
tricky fault is being traced.
• The tuning dial should be accurate enough for you to be confident
that the calibrations are within a few
kilohertz at the lower frequencies and
perhaps within 100kHz on a range up
around 30MHz. This isn’t precision,
because vintage receivers generally
have rather vague dial markings up
around those high frequencies anyway.
• The signal should not drift in frequency to any extent after an initial
warm-up period. For example, if you
were to tune to 3RN on 621kHz on
your receiver and adjust the signal
generator to exactly the same frequency, there would be no beat or
whistle coming out of the receiver.
If, after 10 minutes, this has changed
so that there is a whistle of no more
than around 1000Hz, the generator is
drifting a little but is OK for most purposes.
If you have to retune the generator
because there is a very high tone or
it is almost out of audibility, the unit
really has a little too much drift and it
could be a nuisance when aligning the
tuned circuits in a receiver. However,
this is rarely a problem.
Having set down the criteria for a
signal generator, the next question
is “Where do you get them?” Until
recently, Dick Smith Electronics had
a good general-purpose RF signal
generator which was quite suitable for
the job. I cannot find any generators
advertised in the general electronics
magazines, so where and how can a
vintage radio enthusiast get such a
device?
Fortunately, there are quite a few
signal generators for sale at amateur
radio conventions/communications
days and the like, and a quite reasonable Leader LSG10 or LSG11 (such
as shown in one of the photographs)
can be obtained at a reasonable cost.
Amateur radio equipment has become
so sophisticated that generators of this
standard have become of little use to
amateurs.
There are of course other brands
available such as Advance and Taylor in the general-purpose range and
both manufacturers produced good
equipment. Units like the Hewlett
Packard 606B are excellent and
Fig.1: this transistor multivibrator is powered from a single 1.5V cell. It produces a 1kHz square signal which can be used as an audio signal while
its higher harmonics can be used well up into the broadcast AM band.
84 Silicon Chip
sometimes appear on the disposals
market. They are magnificent pieces
of equipment but be aware some of
the parts may be a hard to source if
they become faulty.
Is there any other way of obtaining
a signal generator? Yes, some of the
old modulated oscillators that are
collected by vintage radio buffs can
be restored and used for their original
purpose. Additionally, it is possible to
build your own. Admittedly there are
few designs around but back several
years ago, ETI and EA did have some
designs. I’m personally looking at
building a relatively simple generator but it will be some time before it
becomes a reality.
High voltage tester
Testing the insulation quality of
power transformers, capaci
tors, RF
transformers and other devices that
rely on the integrity of insulation is
important in getting the best out of
a restored vintage radio. I have often
laboured the point of testing the insulation of power transformers, between
windings and windings to earth. It is
very much a safety issue. I make no
apology for this. The unit shown in
the photograph is a device built from
a kit put out by Altronics.
I believe that it is no longer available but a replacement kit also sold by
Altronics (K2557) has been available
for as low as $45. It is a SILICON CHIP
design featured in May 1996 and is
more versatile than the model I have.
Signal injector
This is a very simple little device
that is suitable for testing valve and
transistor radio equipment. It is a
signal generator that puts out a basic
audio frequency of 1kHz or there
abouts and harmonics of 1kHz well
up into the tuning ranges of domestic
radios.
It consists of a pair of transistors
in a multivibrator oscillator circuit.
It generates square waves at the basic
fre
quency of 1kHz (nominally) and
these are applied through a probe to
RF, IF and audio circuits throughout
a receiver, often pinpointing the stage
with the problems.
In a set that is performing properly, applying the probe to the aerial
terminal will produce a loud tone at
the receiver output. By the time the
probe is applied to the plate of the IF
valve, the output level will be noticewww.siliconchip.com.au
ably lower than when it was applied
to the aerial, as there is no radio frequency gain after the plate of the IF
valve. Placed on the detector, a loud
tone should again be heard which is
controlled by the volume control, and
the level will reduce as the probe is
attached to audio stages nearer to the
speaker.
While it is a very useful little instrument, misleading results can occur
due to signals getting into stages that
they were not intended to get into.
However, Fig.1 shows a typical circuit
for readers who wish to make one.
They have to be tiny. An old marker
pen can be used to house it and an
AAA cell, or you could use a torch
shell suited for AA cells. It’s tiny so
a small circuit board or matrix board
is needed to build it on. Some clues
on construction can be obtained from
the articles in the December 2000 and
May 2001 issues of SILICON CHIP, on
LED torches.
One precaution necessary with the
signal injector is to make sure that the
transistors do not receive pulses from
the circuit being tested that exceed
their breakdown voltage.
For example, when the earth lead is
connected to the chassis and the probe
is touched on the HT line, a pulse will
be sent through the probe’s injection
capacitor (as it charges) to the collector
of transistor Q1. This will exceed the
breakdown voltage of the transistor
and destroy it. To overcome this prob-
Photo Gallery: Peter Pan FJJ
Manufactured by Eclipse Radio, Melbourne, in 1949, the Peter Pan FJJ is
a 4-valve superhet receiver employing virtually identical circuitry to a
number of companion “Astor” models of the day. Housed in a substantial
Bakelite cabinet and fitted with a 6-inch speaker, the Peter Pan FFJ was
capable of good RF and audio performance.
lem, a neon (NE2) and two strings of
diodes are used to clamp this pulse
to a level that will not harm the transistor.
In some circumstances, the earth
lead is not needed and hand capacity
is sufficient to act as an earth. Experimentation and experience will soon
show you the best way to use the
injector. Try it out on a few good sets
first.
Signal tracers
Just as we can inject signals into
a radio and assess the results, it is
quite practical to do the reverse. We
can listen to the signal as it progresses
through a radio with a device called
Above: if you are going to do alignment work on vintage
radios, you do need a good RF signal generator, such as this
Hewlett Packard model 606B which has the advantage of a
calibrated output attenuator and output metering.
Left: while many vintage radio enthusiasts would probably
like to have a valve tester, this AVO MkIII Valve
Characteristic Meter (Mutual Conductance Valve Tester) is
really only likely to be used by someone is who designing
valve circuits.
www.siliconchip.com.au
December 2001 85
“Radio & Hobbies” magazine described the Senior Signal
Tracer in May and June 1954. It is essentially a tuneable
superhet radio.
Left: somewhat cheaper than the HP unit and with a good
deal fewer facilities, the Leader LSG11 Signal Generator is a
good instrument for vintage radio enthusiasts.
a signal tracer. For example, a signal
tracer can have its probe attached to
the aerial terminal of a set and signals
listened to at that point. Depending on
the type of tracer, one or many signals
(usually weak) may be heard as they
don’t usually have the performance of
the receiver.
At the grid of the first valve, hopefully only one signal is heard, being
the one the set is tuned to. In the
grid or plate circuit of the IF stage,
the signal will be much louder in the
tracer’s output. At the detector there
is a choice of listening for RF or AF
signals. Following the detector, the
audio signal will get louder as each
stage is probed with the tracer.
There are two types of signal tracer.
There was an untuned type described
in SILICON CHIP for June 1997 (errata
on this appeared in the August issue).
These units are cheap and work quite
satisfactorily in tracing run of the mill
faults. They are switchable from RF
to AF tracing and have gain controls.
To use them, it is only a matter of
putting the probe onto various signal
points throughout the receiver and it
will soon be obvious at which stage the
signal disappears or becomes weaker
instead of stronger. It is then a matter of
testing that section of the set in depth
to locate the fault.
A tuned signal tracer is more versatile and more expensive. In effect, a
tuned signal tracer is a special super86 Silicon Chip
het or TRF receiver, with attenuators
to cut the gain of various stages so that
it is not overloaded. It also usually
has a meter to show the strength of
signals as they are traced through the
receiver. As the name suggests, it can
tune to any of the frequencies that the
set tunes to as well as the intermediate
frequency (IF).
In addition, it can detect the oscillation from the local oscillator, usually
without even touching any part of the
oscillator circuit – just as long as the
probe is near the circuit. It will also
assist in determining if the oscillator
is tuned to the correct frequency.
I haven’t seen any of these available ready made or in kit format for
many a long year. Occasionally a
vintage signal tracer shows up in a
collector’s display. The one featured
in a photograph is a Radio & Hobbies
design from the mid 1950s. It works
extremely well, being more versatile
than the untuned version. A solid
state version would be a much better
device for today I’m sure. Regrettably
the market for such an instrument is so
small that it would be up to individual
collectors with time on their hands to
design and build such a unit.
Transformer tester
I purchased a quantity of unmarked
audio output transform
ers. I really
wasn’t looking forward to laboriously
testing each one with a haywire sys-
tem. It would take me ages to test them
all, so I did some lateral thinking and
came up with a simple device which
will do all the testing I needed. It will
determine the turns ratio and by looking up a table, the likely impedance
ratio. The inductance of the high-impedance winding and the approximate
efficiency of the transformer can also
be obtained on other ranges.
It uses just one quad operational
amplifier IC and cost around $20 for
bits. I can now test all the parameters
listed above in around a minute per
transformer. It is a bench instrument
with an accuracy of around 10%; quite
adequate for the job it is intended to
do.
Oscilloscope
This is the device to use to find that
really elusive fault. It can be used to
measure voltages, particularly AC
voltages, and is useful for looking at
signal waveforms at various points
throughout the circuit to determine if
there is anything unusual that a multimeter cannot detect. Such things as
supersonic oscillations in audio stages
come to mind here. Some of these
nasties occur only on audio peaks,
for instance.
If you can afford it, a CRO is often
well worth the money. They are excellent instruments but one that the
average vintage radio restorer may turn
on only once a year – just to see if it
www.siliconchip.com.au
works or to look at the pretty patterns!
Valve testers
Photo Gallery: Mastertone 5-Valve Superhet
These instruments were all the go
years ago, particularly the emission
testers. In reality, I found it was just
as easy and usually more accurate to
try a replacement valve rather than test
the one from the set. They have their
place but the average restorer will find
little use for one.
However, if a restorer is into design
work as well, the purchase of a mutual conductance valve tester will be
money well spent. From the variety of
readings that such an instrument can
give, a very high performance piece
of equipment can be designed. They
are cumbersome and slow to use, and
like their simpler cousins, not something most restorers would consider
important to have.
Made by Burnell of Perth,
the Mastertone 5-valve
superhet receiver used
the following valve types:
EK2-G frequency changer; 6D6 IF amplifier, 75
1st audio/detector/AVC
rectifier; 6F6-G output
and 5Y3-G rectifier.
Test methods
Often a fault is more readily diagnosed when replacement parts are
tried in a receiver.
The most obvious and easiest to try
is a replacement valve – as long as you
have one of the same type, known to
be good, or you can obtain one at a
good price or on a loan basis. Only
do this after you have determined
with the multimeter that all is well as
far as supply voltages are concerned.
Valves at times can appear OK as far as
static measurements with a multimeter
are concerned but may still not work,
so a replacement often cures a tricky
problem. TV servicemen in the black
and white days often carried a stock
of known good valves to try in place
of suspect valves. It saved a lot of
mucking around (although it did lead
to some of them being called “valve
jockeys”).
Strange crackles and distortion in
the audio can often be cured by trying another speaker transformer and
speaker on a trial basis. In the 40s and
50s, it was possible to buy a universal
speaker test set. It had a multi-impedance speaker transformer which
matched impedances from around
2.5kΩ ohms up to around 15kΩ, with
several low impedance taps so that
the various speaker impedances could
be matched. It also had a choke and
resistor arrangement that could match
most speaker field coils.
This was a handy device in its day
but is hardly necessary to deal with
the occasional restoration.
www.siliconchip.com.au
A spare receiver can also be used to
test the local oscillator in a receiver. To
conduct this test, tune another receiver
to a weak station near 1000-1600kHz,
with its aerial lead close to the set
under test. Then slowly tune the set
being tested from the 530kHz end up
to around 1200kHz.
As you tune the set under test, at
some point a high-pitched whistle will
be heard which drops to a low pitch
as you continue to slowly tune the
set. It should then disappear as tuning
is continued, then increase to a high
pitch and finally disappear from the
test receiver. If the whistle is heard,
the oscillator is operating.
Going further than this by trying
various replacement parts is neither
quick nor effective and is not to be
recommended. Get help from someone
who has more experience than you
have – it will save a lot of time.
Summary
So there you are, a run-down on test
instruments from the essential down to
the “nice to have, but don’t think I’ll
bother” types. My order of importance,
but not necessarily yours, is:
(1). Digital Multimeter (and an
Analog Multimeter);
(2). RF Signal Generator;
(3). High Voltage Insulation Tester;
(4). Signal Tracer – preferably a
tuned type;
(5). Signal Injector;
(6). Oscilloscope;
(7). Valve Tester – Mutual Conductance type.
Finally, while I did list the transformer tester in this article, it may not
be a device that many would need,
SC
although I find it handy.
If you can afford
an oscilloscope for
your vintage radio
work, this 2-channel
20MHz Hung Chang
model from Jaycar is
more than adequate
for the job. (Photo
from Jaycar).
December 2001 87
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