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VINTAGE RADIO
By JOHN HILL
Testing capacitors at high voltages
using a megohm meter
Faulty capacitors can cause lots of problems in
old valve receivers. Although this topic has been
covered in previous Vintage Radio columns,
other aspects keep arising which suggest that
another session on capacitors is in order.
When it comes to valve receivers,
we are not looking at one particular
type of capacitor. There are high and
low-voltage paper capacitors, high and
low-voltage electrolytics, and standard
mica and silvered mica capacitors.
There were even a few “late model”
valve receivers fitted with polyester
capacitors.
Discarding all paper capacitors
when restoring a receiver has been a
standard procedure for me for a long
time. Although I do this, there is no
reason to replace all paper capacitors
as even (slightly) leaky ones will work
OK in some applications; eg, when
used as a cathode bypass capacitor.
If one is prepared to properly test
paper capacitors, many can be reused
although not always in their original
positions.
Leaky capacitors should not be used
in any high tension application or the
AGC (automatic gain control) circuit,
for example.
Personally, I prefer to replace all
This Altronics megohm meter is assembled from a kit. It tests at 500V and
1000V and is powered by a 9V “AA” battery pack.
86 Silicon Chip
suspect capacitors in an old radio set.
That way, I can be absolutely certain
of elimi
nating one common source
of problems. However, the following
information will be of use to those
restorers who like to retain as many
of the original components as possible
in their radios. This means replacing
only those capacitors which really are
faulty. When this is the case, those
capacitors that are not replaced should
be thoroughly tested and their serviceability properly established.
Leakage problems
When restoring a radio receiver, I
often find that the high tension voltage
can rise by as much as 100V after the
paper capacitors have been replaced.
This gives some insight as to the
amount of leakage that can be involved
with faulty capacitors.
Without going into details, capacitor leakage can seriously overload a
variety of components such as valves,
screen resis
tors, chokes, field coils
and output and power transformers,
to name just a few.
This means that any paper capacitors left in the high tension circuit
must be carefully tested for leakage.
What’s more, they should also retain
their original capacitance and that may
not always be the case. Sometimes a
paper capacitor with no leakage also
has little or no capacitance, due to
internal open circuits.
Quite simply, it boils down to
this: all capacitors must be carefully
checked for both leakage and capacitance before using them in a high
tension circuit.
I was recently embroiled in a debate
over how capacitors should be tested
for leakage. This is a difficult subject
Above: this Megger is self-contained and requires no batter
ies. “Megger” is a registered trademark used by Evershed and
Vignoles Limited, England.
Right: a modern electronic Megger. Gone is the old crank handle
used in early design to generate the test voltage. The trademark is
faintly visible at right, just below the meter scale.
for some to appreciate, especially
when they are accustomed to low voltage circuits. It is difficult to appreciate
the stress placed on a capacitor when
it has hundreds of volts across it.
My opposition claimed that all that
was needed was a ca
pacitance test
and, if the capacitor was leaky, then
the capacitance reading on the meter
would slowly drop away.
It is incredible the things some
people say when they have not even
tried out such a theory. In fact, it was
immediately disproved by testing a
known faulty capacitor with a multimeter set to capacitance. The reading
remained static for quite some time,
then it increased slightly, a characteristic of that particular meter. Yet the
same capacitor measured about 2MΩ
when tested with the same meter set
on the ohms x 1000 scale.
My counter argument was that a
high voltage megohm meter – such as
a “Megger” – should be the ideal instrument to conveniently test suspect
capacitors. If the dielectric can withstand a 400-500V potential without
showing leakage on the meter, then
there would be little to worry about
if that capacitor were to be put back
into service.
Again, unproven theories were
thrown into the discussion on the basis that a Megger was never intended
to test capacitors. According to my
opponent, “a Megger would produce
high voltage spikes that would blast
holes in the dielectric, thus rendering
what may have been a perfectly good
capacitor totally useless”. Well, that’s
what I was told!
Now before going any further, let’s
clear up the terminolo
gy regarding
the word “Megger”, which is often
carelessly and incorrectly used.
The word “Megger” is in fact a trade
name for a particular brand of megohm
meter. The old familiar type used a
black bakelite cabinet fitted with a dual
range meter scale and used a folding
crank handle that was used to spin a
small generator! Anything else – without the Megger trademark – is simply
a megohm meter.
Trial runs
Doing a few tests on a range of
capacitors with a borrowed megohm
meter seemed to support all my
assumptions. Testing ca
pacitors at
400V clearly showed up any leakage
problems. Good capacitors kept the
meter needle hovering around the infinity mark, while the not-so-good ones
showed various amounts of leakage in
megohms, or zero ohms in the case of
a shorted capacitor.
(Editorial comment: our oldest contributor recalls that one of the first jobs
he was given when he entered the radio
industry back in the mid-30s – yes,
mid-30s – was to help test hundreds
of paper capacitors, as they came from
the makers. And his job was to crank
the Megger while another operator
applied the test prods to rows of the
capacitors laid out on the bench. There
was never any suggestion that this was
detrimental).
The interesting aspect of the high
voltage test is that a crook capacitor
that shows a leakage of around 1-2MΩ
at 400V will appear quite good when
checked with a normal multimeter set
to the ohms x 1000 range.
Leakage and straight resistance are
two different things. A 10MΩ resistor
will measure the same on both types
of meters but leakage through a capacitor will usually increase with the
voltage and that is why capacitors
require a high voltage test. If an old
paper capacitor is going to be operated at several hundred volts, then
it needs to be tested at that voltage
or more.
Many of the old capacitors from
the early 1930s have the test voltage
June 1996 87
A close-up view of Altronics meter. Despite the 1000V warning, there is little
sensation at the test terminals but always be sure to discharge fully-charged
capacitors before touching their test leads, or you could get a nasty shock.
selected by a rotary switch –see photo.
These voltages can be varied to some
extent by internal adjustment.
Although the megohm meter tests
at potentials as high as 1000V, one
can hold the test leads and not feel as
much as a tickle.
This is because there is a 10MΩ resistor in series with the test leads and
this restricts the current flow to such
a degree that the instrument is quite
shock proof, even though it carries a
warning referring to the 1000V potential at the test leads. But a charged
capacitor is another story and they
can really bite!
The 10MΩ resistor also overcomes
another of the “anti megohm meter”
comments made during the great
debate. Because of this high value
resistor, there is no great inrush current to internally damage any delicate
capacitor. As a result of this resistor, it
takes about 20 seconds to fully charge a
0.47µF capacitor. But there is no doubt
about the effectiveness of the high voltage test if a capacitor is shorted. The
discharge spark can be clearly seen and
heard. ZAP! (Editorial note: shorting a
charged capacitor is not good practice,
as it can cause internal damage).
If the capacitor is left connected
after testing, it will discharge through
the meter. Larger capacitors need
longer discharge times, so be careful
here.
Mica capacitors
Most 100V greencaps will withstand a 1000V test. That indicates that they
should work OK in a lot of valve radio situations without much trouble.
clearly marked on them and a 400V
capacitor was often tested at 1500V
– well above its normal operating
potential.
Using a borrowed megohm meter was a great help in establishing
whether or not it was a suitable test
instrument for capacitors. But someone else’s Megger is not mine, so I set
about finding an equivalent for my
own use.
A kit-based meter
After a period of unsuccessful
searching, I came across an advertise88 Silicon Chip
ment for a megohm meter in kit form
for $80 from Altronics. There was a
minor problem with the kit, with the
parts layout diagram and circuit board
showing the wrong battery polarity.
However, that problem has since been
rectified if you are thinking of buying
one. So, despite the minor hiccup,
I eventual
ly had myself a working
megohm meter.
The Altronics kit seems to be a good
design and is an electronic type, not an
electromechanical device like the old
Megger. It has two test potentials (500V
and 1000V) with the desired voltage
One very interesting test procedure
was carried out on a couple of known
faulty silvered mica capacitors that
were causing a distinct crackle in a
receiver.
When tested with the megohm
meter, the needle continually fluctuated up and down the scale. In other
words, the problem could be clearly
seen when the capacitor was subjected
to a high voltage test. However, when
one of these capacitors was reversed,
the needle swung over to infinity and
held steady.
This test seems to indicate that
whatever happens inside a faulty
silvered mica capacitor can create
a rectifying effect whereby there is
current leakage in one direction but
not the other.
The next time I have a crackly mica
capacitor problem, I will reverse the
capacitor to see if that cures the fault.
Although there is some possibility that
it might, whether it lasts is another
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All of these old capacitors failed the high voltage test. They have varying
amounts of leakage, with some registering less than a megohm of resistance
when tested on a megohm meter. A good capacitor should not show any DC
current leakage.
question. While I am not suggesting
that this technique be adopted as a
recommended practice, it will be an
interesting experiment.
In conclusion, it would appear
that a high voltage megohm meter is
an entirely suitable instrument for
checking capacitors for leakage. It
works on all types of capacitors, except
electrolytics, provided that the voltage
rating of the capacitor to be tested is
appropriate for the test voltage.
Finally, high voltage testing of pow
er transformers, chokes, field coils, etc
can also be carried out using a meg
ohm meter.
Footnote: the High-Voltage Insulation Tester described in the May 1996
issue of SILICON CHIP is also ideal for
testing capacitors for leakage. It has
a 10-step LED bargraph display and
provides five selectable test voltages
ranging from 1000V down to as low
SC
as 100V.
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June 1996 89
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