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Vintage Radio
By RODNEY CHAMPNESS, VK3UG
Reforming electrolytic capacitors
Capacitors are the most troublesome parts in
vintage radio receivers. This month, we look
at the various capacitor types and describe
an easy-to-build circuit that can be used to
reform electrolytics.
I
N “VINTAGE RADIO” for October
and November 2004, we looked at
paper capacitors and described the
problems that they can cause. Those
articles also described how paper
capacitors could be tested for leakage
and described the circumstances under
which they should be replaced.
In practice, the decision whether or
not to replace a leaky capacitor often
depends on where it is located in the
circuit. In many cases, leaky capacitors in non-critical positions (eg, with
low voltages across them) can be left
in circuit, as they will have negligible
effect on performance. By contrast,
capacitors with high voltage across
them or in certain critical positions
(eg, AGC bypass capacitors and those
in bias circuits) should be replaced
if leaky.
In this article, we’ll look first at electrolytic capacitors and describe how
they can be reformed (or re-polarised).
We’ll then take a look at mica, polyester, styroseal (polystyrene), ceramic
and air-dielectric capacitors.
Electrolytic capacitors
Electrolytic capacitors are usually
used as power supply filters and as
bypasses in valve receivers. They are
also used as coupling capacitors in
low-impedance sections of transistorised receivers.
Polarised electrolytics have positive
and negative terminals and must be
connected into circuit with the cor-
Electrolytic capacitors are commonly used in
valve receivers for power supply line filtering
and as bypasses.
98 Silicon Chip
rect polarity. By contrast, bipolar or
non-polarised electrolytic capacitors
can be connected into circuit either
way around, however they are seldom
found in radio receivers.
Note that the capacitance values
marked on electrolytic capacitors are
only approximate. In practice, they
and can vary from about 10% low to
as much as 50% high. So don’t get
too upset if the measured value of a
nominal 16mF capacitor turns out to
be anywhere between say, 14mF and
24mF.
Main problems
Electrolytic capacitors suffer from
two main problems: (1) loss of capacitance and (2) excessive leakage
current. The first problem, that of
reduced capacitance, occurs because
the electrolyte inside the capacitor
tends to dry out over the years. As a
result, the capacitance of a nominal
16mF power supply filter capacitor
may reduce to virtually zero. This will
result in hum and/or “motorboating”
in the audio output of the receiver and
replacement is the only answer.
As for the second problem, electrolytic capacitors always have some
leakage – usually be less than 1mA.
However, an electrolytic capacitor
stored for a long period of time can
become depolarised. As a result, it
will draw considerable current (greater
than 40mA in some cases) until it is
reformed (by applying a polarising
voltage across it).
So how do you reform an electrolytic capacitor? There are three different
methods and I’ll describe the pros and
cons of each. Note that some capacitors will not respond to the reforming
process and will need replacement.
Reforming method 1
Regrettably, some vintage radio colsiliconchip.com.au
Polyester capacitors became available in the late 1960s,
towards the end of the valve era, and are very reliable.
lectors try the brute force method of
reforming electrolytic capacitors – by
giving the set a “smoke” test without
first checking the power supply and
for faults on the HT line. In many
cases, this is exactly what does happen – smoke appears as soon as power
is applied.
Often, a set will have been put aside
because it has a fault and subsequently
stored in less than ideal conditions
which leads to further deterioration.
This makes it extremely risky to turn
any old set on before checking it thoroughly. There may be shorted capacitors or capacitors that are so leaky that
they may explode after a short time.
In the process, they may destroy the
rectifier and perhaps even the power
transformer.
A leaky paper audio-coupling capacitor could also cause the audio
output valve to draw excessive current, destroying the valve in the process. In short, turning a set on without
checking it can produce some rather
expensive smoke.
Reforming method 2
Over the years, I have often used
a method that some people consider
risky when it comes to reforming electrolytic capacitors. First, I check that
there are no short circuits on the HT
line and that the minimum resistance
from the HT line to chassis is at least
10kW (the actual value will depend on
the circuit). In addition, if an initial
physical check shows that any capacitors are bulging or leaking electrolyte,
I replace them.
That done, I connect a multimeter
via insulated short jumper clip leads
across the first electrolytic capacitor
siliconchip.com.au
Mica capacitors usually have relatively low values and
are typically used as RF bypasses, in tuned circuits.
and observe the rising voltage as the
set is turned on for a brief period. This
period is around 20 seconds for a set
with an indirectly-heated rectifier and
just a few seconds with a directlyheated rectifier.
In practice, I let the voltage rise to
about a quarter of the expected HT
voltage and then turn the set off. If the
rectifier shows any sign of distress (red
colour on the plates or sparks inside
the works), I turn the set off immediately and recheck for shorts.
After about a minute, I then repeat
the procedure, this time letting the
voltage rise a little higher. If the electrolytic is reforming, the voltage across
it will rise to the expected HT voltage
after a few cycles of this procedure.
Note that it’s necessary to check the
second filter capacitor as well. I’ve
sometime found that one capacitor
would reform but not the other. Note
also that more modern electrolytics
don’t seem to need much reforming.
If an electrolytic capacitor shows any
signs of overheating, it should be
discarded as it obviously has far too
much leakage.
What are the advantages of this
method? It will successfully reform
capacitors over a period of a few
minutes of on-off switching. It has the
advantage that no capacitor has to be
removed from the set to do the reforming. If used with care in the manner
described above it would be rare for
any damage to occur in the receiver.
What are the disadvantages? It is a
bit harsh and if care is not taken the
end result will be damage similar to
that which occurs with the previous
“smoke test” method.
(Editor’s note: we regard this method
as decidedly risky. While initial resistance checks may indicate nothing
amiss, when the voltage across a suspect capacitor rises to a critical value,
the leakage current may suddenly
increase or it may become short-circuit
which can immediately damage the
rectifier. If the capacitor then suddenly
leaks all over the chassis, you then
have a major clean-up job. And the
smell is something you will remember
for the rest of your life! Finally, an
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October 2006 99
Styroseal capacitors became available around the same
time as polyester capacitors and are quite reliable.
exploding electrolytic capacitor poses
an extreme risk to your eyes!)
Reforming method 3
Method number 3 is much more
benign and involves using a special DC
power supply. This supply should be
voltage regulated (so that the applied
voltage can not exceed the peak voltage rating of the capacitor) and should
feature current limiting.
In operation, the capacitor is connected to the output and the current
limiting set to 10mA. This current
limit applies whether the voltage
across the capacitor is 5V or 500V
(or what ever the maximum working
voltage happens to be).
Forming Electrolytic
Capacitors
So what is this “forming” process?
Basically it refers to re-forming the aluminium oxide layer on the aluminium foil
electrode in the electrolytic capacitor. In
essence, the aluminium foil is the positive electrode and the aluminium oxide
layer is the dielectric of the capacitor.
The conductive electrolytic in conjunction with another small aluminium foil
and the aluminium can then forms the
negative electrode of the capacitor.
In applying the “forming” current to
the capacitor we are setting up a controlled chemical process between the
conductive electrolyte and aluminium
foil to re-anodise or oxidise the aluminium surface. This heals any breaks in
the oxide layer (the dielectric) and thus
reduces the leakage current.
100 Silicon Chip
Early ceramic capacitors were not very reliable but later
types gave few problems.
When the capacitor has reformed, the
voltage across it will be at the selected
reforming voltage, while the current
will have reduced to a fraction of a
milliamp in most cases. However, if
the current remains at about 10mA and
the voltage doesn’t risen to the selected
reforming voltage, the capacitor is suspect and should be replaced.
You can get a good idea as to just
how well a capacitor is holding a
charge by disconnecting it from the
supply and observing how quickly the
voltage across it decreases with just a
digital multimeter in place.
(Be careful though – a capacitor
charged to a high voltage can deliver
a fatal shock. Always make sure that
a capacitor is fully discharged before
touching it).
This method of reforming has a
couple advantages. First, provided
it’s done properly, with the voltage
increased in stages, no undue stresses
are placed on either the capacitor or
the test instrument. Second, it shows
just how good a capacitor is and gives
an indication as to whether it should
be used or not.
What are the disadvantages? If the
capacitor is “new old stock” and is out
of circuit, there are no disadvantages.
However, if it is in-circuit, it may need
to have one lead disconnected.
A simple and very effective repolariser/reformer test instrument is
described later in the article.
Mica capacitors
Mica capacitors usually have relatively low values and are typically
used as RF bypasses, in tuned circuits
and as vibrator buffer capacitors, etc.
They are usually quite reliable but
they can develop faults that give some
strange effects in receivers. For example, local oscillators can drift or jump
in frequency, while the audio output
can have annoying crackles in it.
A high-voltage tester will usually reveal if a mica capacitor has noticeable
leakage and if this leakage resistance
fluctuates. Most mica capacitors were
made as a “stack” interleaved with
sheets of tin foil and mica clamped
together and then encapsulated.
Sometimes the contact between some
metal foils and the clamps becomes
intermittent and so the capacitance
will vary.
If you don’t have a high voltage
tester, the easiest way to test whether
a mica capacitor is at fault is to replace
it and see if this makes a difference.
Mica capacitors can really cause
headaches because they can produce
very obscure symptoms in a receiver.
In fact, it’s not uncommon to find that
the faulty component is nowhere near
where you expected to find it but is in
a different section altogether. Faulty
local oscillator grid coupling capacitors have led me up the garden path
more than once.
Polyester capacitors
Polyester capacitors are usually
available in 160V, 400V and 630V DC
ratings and take the place of paper
capacitors. The most common style
became available in the early sixties
towards the end of the valve era.
I don’t think I have ever had to
replace one of the yellow-coloured
Philips units – they are just so reliable.
In fact, it’s a pity they weren’t availsiliconchip.com.au
able many years earlier - valve radios
would have been so much more reliable without paper capacitors.
“Greencaps” and MKT capacitors
are also polyester types. However,
their voltage ratings can differ from
those quoted above.
Photo Gallery: Peter Pan GKL 4-Valve Radio
Styroseal capacitors
Styroseal (polystyrene) capacitors
became available around the same
time as polyester capacitors and from
my experience, are quite reliable.
They have been used have been used
in much the same way as polyester
capacitors and also in tuned circuits
to some extent.
Ceramic capacitors
Some early ceramic capacitors were
not considered particularly reliable,
whereas later types gave few problems.
They generally come in two types.
One type is used more as a bypass
where the exact value is unimportant,
whereas the other type is more precise
in value and is often used in tuned
circuits.
In addition, ceramic capacitors can
be manufactured with negative, zero
(NPO) or positive temperature coefficients, so that frequency drift in tuned
circuits can be compensated for with
changes in temperature.
Ceramic capacitors come in a
range of voltage ratings from 50V up
to several thousand volts. However,
they are not usually used in valve receivers, with some exceptions. I now
commonly use 47nF (0.047mF) 50V
ceramic capacitors on AGC lines as
replacements for leaky paper capacitors. They are small and can often be
hidden which helps keep the set looking original.
Air-dielectric capacitors
The air-dielectric capacitors we see
in vintage radios are the tuning and
trimmer capacitors. And although
these items do occasionally have problems, the faults are easily detected.
The problems to look out for are
usually just mechanical. In tuning
gangs, for example, the rotor (movable)
plates may have been bent slightly
so that they scrape against the stator
(fixed) plates. This will show up as erratic tuning and crackles as the tuning
gang is operated. It’s easy to track the
problems down by removing all connections to the stators, connecting a
multimeter (set to ohms) between the
siliconchip.com.au
MANUFACTURED IN 1946 by Eclipse Radio, South Melbourne, the GKL was
a compact 4-valve reflex superheterodyne receiver housed in a bakelite
cabinet. These sets were produced in a number of colours, the pink example
shown here being quite rare.
The valve line-up was as follows: 6A8-G frequency changer, 6B8-G reflexed
IF amplifier/1st audio amplifier/detector/AVC rectifier, 6V6-GT audio output
and 5Y3-GT rectifier. Photo: Historical Radio Society of Australia, Inc.
stator and the frame and then operating the tuning. As the unit is tuned,
any shorts will soon become evident
on the meter reading.
By placing a light behind the gang
and looking along the plane of the
plates, it should be obvious which
plates are touching each other. The
shorting plates can then usually be
carefully bent back to where they
should be to clear the shorts.
Sometimes, the meter may show
that a short is present but no evidence
of plates touching can be seen. In this
case, there is probably a small sliver
of metal that is shorting the gang. The
best method to deal with this problem
is to burn the short out.
First, check that the gang is still
isolated from the circuit, then connect
a 47kW 1W resistor from the receiver’s
HT line to the stator. That done, turn
the set on and rotate the tuning control
from one end to the other and if there is
a small sliver of metal causing the trouble, there will be some intermittent
sparks between the capacitor plates.
This should clear the problem but keep
in mind that you are playing around
here with a high voltage, so be careful.
If you don’t understand exactly what
you are doing, then don’t do it!
Another problem that commonly
occurs is the rotor shaft not making
good contact with the frame. This
can cause jumps in frequency as the
receiver is tuned. It can also cause
crackles and the set may stop operating. Most, if not all, tuning capacitors
have either a metal spring bearing
onto the shaft to the gang frame or
other spring-loaded contacts to ensure
good contact is maintained between
the frame and the tuning shaft. If any
of these are missing, erratic tuning is
almost a certainty.
There is one last problem and that
is where the gang has virtually fallen
to pieces. This occurs with very old
gangs that have been made from poor
quality metal and the only answer to
this problem is replacement.
OK, now let’s take a look at the reformer circuit.
October 2006 101
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