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Vintage Radio
By RODNEY CHAMPNESS, VK3UG
S
Restoration tips
& techniques
It’s not surprising that many vintage
radio enthusiasts don’t come from an
electronics background. In fact, prior
to taking up the hobby, most never got
closer to the subject than using the
external controls on various pieces of
electronic equipment.
84 Silicon Chip
OONER OR LATER, a
vintage radio enthusiast must decide which
technical areas to become
competent in so that they
can at least carry out some
restoration work. Some will
simply do the cabinet work
and clean the chassis but
leave the electronics restoration to someone else. By
contrast, others will want
to do the lot. The problem
is, electronic circuitry is a
complete mystery to many
newcomers.
So how can a novice
learn how to check and
restore electronic circuits? Well, we all have
to start somewhere and
that’s the aim of this article
– to provide a basic introduction. Of course, it won’t take you from
knowing nothing about electronics to
being an electronics wizard but at least
it will be a start.
Reading a circuit diagram
As an example, the Kriesler 11-90
AC mantel receiver is used as the
“guinea pig” for this article, as it
has a relatively simple circuit. It is
a broadcast-band receiver with four
valves, one of which (the 6GV8) is a
dual valve – ie, it has a triode and a
pentode in the one glass “envelope”.
As a result, the Kriesler 11-90 is functionally equivalent to a conventional
5-valve receiver.
The circuit diagram is shown in
Fig.1. As can be seen, it is well labelled, which makes checking things
within the circuit relatively easy.
A schematic circuit diagram is a
“shorthand” method of showing how
the parts are connected in a piece of
equipment. For this reason, it’s essential that you become familiar with
what the various symbols mean, in
order to understand how the circuit
works (if only at a basic level).
siliconchip.com.au
Fig.1: the circuit diagram for the Kriesler 11-90 AC mantel receiver.
It uses four valves and covers the broadcast band.
Let’s start with valves. These are
usually drawn with a heavy oval shape
which contains the various elements.
We will use the 6N8 as an example –
see Fig.1.
Pin 3 is attached to the “cathode”
of the valve and this element emits
electrons when it is heated. Pin 2
is the control grid and is shown as
a dashed line – it’s simply a grid of
wires. The electrons from the cathode
pass through the grid and are attracted
towards the positively-charged “plate”
which is attached to pin 6.
Pin 1 is the “screen” grid (it screens
the grid from the plate), while pin 9
is the “suppressor” grid. The latter
“captures” electrons which bounce
off the plate and takes them to earth
(chassis). Pins 7 & 8 are the cathodes
of the two detector diodes, which are
located close to the main cathode at
pin 3.
Note that the heater connections for
the valves are not shown in Fig.1. In
practice, these are connected between
pins 4 and 5 for most 9-pin miniature
valves (it is assumed in most diagrams
that you know this).
So basically, the shorthand drawing
of the valve is relatively close to what
the internals of the valve are really
like. Of course, the description here
is a simplistic version of what really
happens inside a valve.
Identifying valve pins
How do you identify which pin is
which? Simple, the valve socket as
viewed from below has a wider gap
between two of its pins. This is the
reference point and the pin numbers
start from the left as number 1 and
progress clockwise to number 9.
Other valve sockets are similar in
concept. For example, small 7-pin
sockets are read in the same way, while
octal sockets are read clockwise from
the keyway pin on the spigot. The
valve base diagrams usually make
this clear.
Other older valve socket types have
different layouts. Checking through a
valve data book will assist in identifying which pin numbers relate to which
pins on their bases.
Resistors & capacitors
Resistors are the items with the
“zig-zag” lines. For example, R10 is a
1MΩ (one megohm) resistor. The zigzag symbol always reminds me of a
tortuous path which restricts current
siliconchip.com.au
flow and in some ways, resistors can
be thought of as doing just that.
Capacitors, on the other hand, are
represented by two parallel lines –
eg, C8. The lines can be thought of as
being equivalent to the two parallel
plates that make up the capacitor.
However, this really is symbolic as
June 2004 85
up quite well unless it has really
been abused in some way or another.
Cleaning the set not only improves its
appearance but makes it much easier
and more pleasant to work on.
Fig.1: the Kriesler
11-90 was housed in
a plastic cabinet and
featured a simple
handspan dial.
Static tests
they may have many parallel plates,
with insulation (dielectric) of various
sorts between each plate.
For example, C3A and C3B are the
tuning capacitor sections and they
definitely have parallel plates that you
can see. The symbol for C3A and C3B
means that one series of plates moves
while the others remain stationary
(this is done to vary the tuning capacitance, so that the set can be tuned to
different stations).
Similarly, C4 is an adjustable (or
variable) capacitor which is used during
the alignment of the local oscillator (ie,
when the set was manufactured).
C12 and C13 are electrolytic capacitors and are different again. They have
fixed values (40µF & 20µF respectively) and are also polarised – ie, the
positive terminal of each capacitor
must go to the positive supply rail (or
more precisely, to a voltage rail that’s
more positive than that for the negative terminal).
Inductors & transformers
Inductors and transformers such
as L1 appear to look like coils, which
of course they are. The three parallel
series of dashed lines indicate that
it is wound on a ferrite or iron dust
core (a ferrite loopstick in this case).
Similarly, intermediate frequency
transformers IFT1 and IFT2 have
adjustable ferrite cores, again used
during the alignment of the set.
86 Silicon Chip
Note that in both cases, the IFT
windings are coupled together in close
proximity.
Audio and power transformers have
the same coil-like symbol but they differ by having two (sometimes three)
solid lines alongside each winding.
This indicates that they have an iron
core. Consider the power transformer
(T1), for example. This is a 240V
transformer with a primary winding
(on the lefthand side of the lines) and
two secondary windings (on the righthand side). These secondary windings
provide nominal output voltages of
115V AC (for the high-tension or HT
supply) and 6.3V AC (for the valve
heaters).
Note that many parts of the circuit
are connected to earth (also called
“common” or “chassis”). The most
common symbol for this is the one
used on the end of the line from pin
3 of all the valves except for the 6V4.
This symbol consist of three parallel
lines of progressively diminishing
length. In this set, all points with this
symbol are directly connected to the
chassis.
Starting restoration
The first step in any restoration job
is to give the set a thorough clean-up.
This involves not only cleaning the
cabinet but the chassis and the valves
as well.
In most cases, the set will come
As stated before in this column,
I never (or rarely ever) turn a set on
before carrying out a number of static
tests. It’s not nice having to repair a set
that sends up smoke signals as soon as
it is turned on. In fact, it really pays to
be over-cautious here, to circumvent
disasters before they happen.
A digital multimeter is all you require for these initial tests, although
an analog multimeter is also quite
OK provided it has a rating of at least
20kΩ/V (20,000 ohms per volt). In fact,
most common receiver faults can be
found using just a multimeter. Make
sure that the set is disconnected from
the power point before starting the test
procedure!
The first thing to do is to carefully inspect the chassis, the components and
all the interconnecting wires. Look for
shorts and broken wires, particularly
if someone has been there before you.
It’s also a good idea to test the soldered
joints by moving the wires attached
to them where possible, as some may
be what are called “dry joints”. These
are soldered joints where the solder
no longer properly adheres to the
leads and/or terminals it is joining. If
you do find any bad solder joints, the
wires (or terminals) should be cleaned,
re-tinned with solder and resoldered
together.
Next, make sure there are no shorting plates in the tuning capacitors.
Shorts can be detected by first disconnecting the leads to the fixed plates.
That done, you then connect a multimeter between the fixed and moveable
plates and vary the tuning capacitor
across its full range.
There should be almost infinite
resistance between the moving and
fixed plates. If the plates are shorting, it should be possible to bend the
moveable plates slightly to eliminate
the problem. This can be a delicate
job but it’s usually not too difficult
provided the tuning gang hasn’t been
seriously damaged.
The next test is to make sure that
the power transformer (marked T1
on Fig.1) has no short or partial short
from the mains active and neutral
wires (ie, the primary side) to chassiliconchip.com.au
This under-chassis view shows that all parts are readily accessible. Note that
using a knot to restrain the power cord is no longer legal.
sis. An ohmmeter on its highest range
should not show a reading of less
than 10MΩ between points A and B
on Fig.1. Most transformers test quite
OK but it’s imperative to find the fault
(or replace a faulty transformer) if a
short is found.
A much better test for the power
transformer is to use a 1000V highvoltage tester across points A and B.
If the high-voltage test is successful,
with no apparent leakage, then the
transformer is OK (at least as far as
leakage to chassis is concerned).
If the set only has a 2-wire power
lead (and has a transformer), consider
fitting a 3-core lead to earth the chassis, as this is a safer option. Of course,
this work must only be carried out be
someone who knows exactly what
they are doing – a mistake here could
prove deadly. Make sure too that the
new cord is properly anchored – tying
a knot in the cord to restrain it (as was
commonly done many years ago) is no
longer legal!
Warning: if you are a novice, stay
well away from hot-chassis (trans
formerless) sets, which have one side
of the mains directly wired to chassis.
They really are potential death traps
for the unwary. If in doubt, ask someone who’s qualified to give advice.
Next, check resistor R12 to make
sure it is about 120Ω. It should be
replaced if it has drifted in value but
note that an allowance of ±20% in any
resistor or capacitor value is generally
siliconchip.com.au
OK. However, this doesn’t include
electrolytic capacitors, which can have
very wide tolerances; eg, +100% and
-50% for the very old types.
Similarly, check resistor R11 by
measuring the resistance between
points HT1 and HT2 on Fig.1 – you
should get a reading of 3.3kΩ. If it is
high, the resistor has drifted high in
value and should be replaced if it is
beyond the accepted tolerance range.
Conversely, if it is low, it’s possible
that either or both C12 and C13 are
leaky and need replacing. However,
before doing this, you could try “reforming” the two capacitors, as described later.
The next step is to check the resistance between the HT2 and BIAS
points. Initially, the meter should read
up the scale then gradually increase in
value to in excess of 50kΩ.
Also, check the resistance between
HT1 and chassis. You should get a
similar value to the previous measurement. If either of these reads low – ie,
below 50kΩ – it indicates that there is
a partial short on the high-tension line.
Either C12 and/or C13 could be leaky
or there could be a problem elsewhere.
This can be diagnosed as follows.
First, removing all the valves will
quickly indicate whether one or more
of them has a problem. Valves rarely
develop shorts, although some rectifiers do; eg, the 6X5GT. Next, measure
all the resistors with an ohmmeter and
if all is well, they will all be within
10% of their marked value. The only
exception is R2, which shunts a low
resistance winding in IFT1 – it will
have to be checked with one lead
disconnected from circuit.
Similarly, disconnect one lead of
each electrolytic capacitor (C12 & C13)
and check them using an ohmmeter.
Replace them if you get readings of
less than 50kΩ.
Now measure between pin 6 of
the 6GV8 and the chassis and if this
shows a short circuit, it is likely that
C11 has short-circuited. You should
also check capacitors C6 & C7, which
are on the HT line near IFT1. If there
is no indication of a short but the
HT line measures just a few ohms to
earth (chassis), then it is necessary to
disconnect sections of the circuit until
the shorting part is found.
Output transformer
The audio output (or speaker) transformer is a component that often gives
trouble, as the primary winding has a
habit of going open-circuit. To check
it, measure between HT2 and pin 6 of
the 6GV8 – you should get a reading of
about 150-200Ω. However, depending
on the impedance of the transformer,
the resistance can be around 500Ω in
some sets.
A further quick check of the output
transformer can be done using an
analog meter. Select a low ohms range
and connect the leads between HT2
and pin 6 of the 6GV8 – a click should
be heard in the speaker. This indicates
that all is probably well with the transformer and loudspeaker. Note: digital
June 2004 87
capacitors are all located in parts of
the circuit where leakage cannot be
tolerated.
Capacitors C6, C7 & C11 can be
mildly leaky without this being a trouble in the set. However, C11 occasionally shorts in this position and it is a
good idea to replace it anyway. If any
capacitor gets warm after the set has
been running for a few minutes (switch
the set off and pull the power plug from
the wall socket before testing), it is too
leaky and should be replaced.
somewhere near 140-150V, as there’s
no load on the power supply.
Now turn the set off and monitor
the voltage at pin 3. It should decrease
slowly, unless the electrolytics require
“reforming”. To do this, turn the set on,
let the rectifier (6V4) warm up, wait a
few seconds until the voltage on pin
3 appears to have stabilised, then turn
the set off again and let the capacitors
discharge. Repeat this several times
with a gap of a minute or so between
cycles, until the capacitors discharge
quite slowly.
If the rectifier plates glow red during this procedure, then either the
electrolytics are faulty or some other
component is breaking down when
the voltage is applied. In that case,
the set should immediately be turned
off. Disconnecting various sections of
the set will then help to isolate the
defective component.
If the HT voltage still “disappears”
within 10-15 seconds, it means that
one or both capacitors have excess
leakage and cannot be “reformed”.
By disconnecting one capacitor at a
time from the rectifier output, it is
possible to determine which capacitor is faulty (ie, the faulty unit will
discharge quickly compared to the
good one when the power is removed).
Note, however, that most modern
electrolytic capacitors require little if
any “reforming”.
Dynamic tests
Installing all the valves
OK, now for the smoke test! First,
remove all the valves, then plug the set
into the wall socket and turn it on. The
dial lamp is still in circuit so it should
light up unless it has blown. Try a new
one in its place if it has failed.
Now keep an eye on the set while
you run it for about 30 minutes. After this time, the power transformer
should only be slightly warmer than
the chassis. If it gets hot, then you
have a faulty transformer. Fortunately,
this is rare.
If the transformer appears to be OK,
the voltages on the two secondary
windings can be measured. These will
be about 10% higher than the voltages
measured when the set is fully operating. Take care when measuring the
high-voltage secondary – it’s capable
of delivering a fatal shock!
The next step is to install the 6V4
rectifier but switch the set off first. Now
turn the set on again – the voltage on
pin 3 of the 6V4 will probably rise to
Once the power supply is working
correctly, it is time to fit the rest of
the valves. That done, turn the radio
on, tune it off-station and measure
the voltages at all the various points
shown on the circuit. If everything is
working correctly, these should all
be within about 20% of the indicated
values. Note that all voltages are measured with respect to earth, so it’s a
good idea to use a clip lead to attach
the earth lead of the multimeter to
the chassis.
If the voltage at HT 2 is much lower
than 110V and the BIAS voltage is also
low, it indicates that the 6V4 is low
in emission and should be replaced.
Conversely, if the HT 2 voltage is appreciably higher than 110V and the
BIAS is noticeably less than -5V, this
may indicate that the pentode section
of the 6GV8 has lost emission and
should be replaced.
The voltage on pin 1 of the 6GV8
should be around 30V when checked
It’s a good idea to thoroughly clean the chassis before checking the parts
and starting restoration. Be sure to make a note of the valve positions before
removing them from their sockets.
multimeters usually don’t have much
current flowing through their test leads,
so a click may not be heard.
All of the wound components (coils
and transformers) should have continuity with reasonably low resistance.
For example, the aerial, oscillator
and IF transformers should not have
more than a maximum of 100Ω across
any winding and quite often are less
than 10Ω.
Paper capacitors
Now let’s look at those components
that often give trouble but are not easily detected using a multimeter.
First, Ducon and UCC paper capacitors (from the 1960s) became
renowned for problems. The Ducons
became leaky and the UCCs often
became intermittent and sometimes
leaky. By “leaky”, I mean that they
had relatively low resistance across
them compared to a good capacitor
– eg, a few megohms for a faulty one
compared to 200-1000MΩ or more for
a good one.
Unfortunately, a “normal” multimeter will not normally detect this
leakage, as it usually does not become
apparent until a considerable voltage is applied across the capacitor in
question.
Note that some leakage can be tolerated in some capacitors but C2, C9 &
C10 should all be replaced with modern polyester or similar capacitors of
the same ratings. In fact, this should
be done without question, unless
you have a high-voltage tester. These
88 Silicon Chip
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with a digital multimeter. If it is lower
and resistor R8 is the correct value,
the valve may be drawing too much
current. If it is higher, the valve may
be low in emission. Once again, try
replacing the valve.
By the way, the circuit indicates
that this voltage is measured with a
1000Ω/V analog meter. However, this
is probably a mistake as 20kΩ/V analog
meters were common in 1962.
On my set, I measured 22V with a
1000Ω/V meter and 26V with a digital
multimeter. Resistor R8 is within tolerance and as both readings are below
the indicated voltage, it would appear
that the valve in my receiver is drawing
more current than others of the same
type. However, the receiver’s performance is quite satisfactory so replacement of the 6GV8 is not warranted.
Both the 6AN7 and the 6N8 should
have plate voltages of about 80V,
while the screen grids should be at
approximately 45V when the set is
tuned “off-station”.
What if it doesn’t work?
By now, it is quite likely that the
receiver is showing signs of life and
you may even be able to tune stations
in. In fact, at this stage it’s not unusual
to find that the receiver is performing
quite well. But what if it isn’t? Here
are a few tests that can be conducted
now that normal voltages are appearing around the circuit.
First, turn the volume control fully
up and put your finger on the top terminal of the volume control (but DO
NOT do this with a live-chassis set). Be
careful here, as the back of the volume
control in this set carries terminals
which are connected to the 240V AC
mains (the pot functions as a combined volume control/on-off switch).
If the audio output stage (based on
the 6GV8) is functioning correctly,
a healthy “blurt” will be heard from
the loudspeaker. If not, you’ve got a
problem in the audio stages.
If you’ve carried out all the tests
suggested previously, then it is likely
that the valve is defective and another
should be tried in its place.
If there are still no stations to be
heard after getting the audio section
working, the next thing to check is the
local oscillator. This can be done by
lifting the “earthy” end of R1 and connecting a multimeter (set to milliamps)
between it and earth. When the set is
turned on, the meter should show a
siliconchip.com.au
Photo Gallery: 1937 Healing 447M
Manufactured by Healing in Melbourne in 1937, the Model 447M was housed in
a stylish timber cabinet and tuned both the medium-wave broadcast band and
the 6-18MHz shortwave band. The valve line-up was as follows: 6A8-G frequency
changer, 6D6 IF amplifier, 75 audio amplifier/detector/AVC rectifier, 42 audio output
and 80 rectifier. Photo: Historical Radio Society of Australia, Inc.
reading of about 0.2mA and this reading should change slightly as you tune
the set across the band.
If this happens, it indicates that the
local oscillator is working. Conversely,
if there is no reading, it is likely that
the 6AN7 is defective or there are
shorted turns in the oscillator coil(s).
If necessary, a 6AN7A valve may be
substituted for a 6AN7 with no circuit
changes. Don’t forget to resolder resistor R1’s lead to earth after removing
the multimeter.
If the local oscillator is working
but the set still refuses to operate, try
changing the 6N8.
It’s worth noting that I find very few
faulty valves and I probably average
less than one replacement per set. Note
too that some valves can become microphonic and you can quickly track
down the culprit(s) by gently tapping
each valve in turn with a pencil or the
plastic handle of a screwdriver. If a
valve is microphonic, it will produce
a noise (possibly a “ringing” noise)
when tapped.
Valve sockets can also cause problems, For example, the contacts may
be dirty or they may be loose and not
making proper contact with the valve
pins. In addition, the sockets and
switches may need to be lubricated
and cleaned with a proprietary contact
cleaner.
Other problems
If someone before you has twiddled
with the cores of the various coils, it
may be necessary to re-align the set
using a signal generator before it will
operate. Other possible problems include faults inside the RF, oscillator
and IF coils that cannot be determined
by pure resistance measurements.
Another trap to be aware of is that
someone else may have replaced parts
with incorrect values, or even installed
parts in the wrong locations. As a result, simply checking the components
may not show where the problem is.
The way around this is to carefully
check the receiver against the circuit
diagram.
Finally, more complex receivers
can also be tested using the same
techniques described here – it will
just take longer. However, it’s best to
start with the simpler broadcast-band
radios first and then work your way
up to more complicated units as you
SC
gain experience.
June 2004 89
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