October 1960 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Capacitors come in a huge variety of package configurations,
chemical makeups, physical constructions and sizes, capacitance
values, and voltage and power handling abilities. Each has its
own strengths and weaknesses for a particular application. When
capacitors are functioning properly, life is great, but when
one decides to fail either completely or partially, troubleshooting
the cause can be a real challenge. The best kind of electrical
component failure from a troubleshooting perspective is one
where the component releases its life-giving internal smoke
and in the process leaving a clearly visible clue like a cracked
case or a nice black mark when none should be. Otherwise, the
job can get interesting. Being proficient at schematic reading
and tracing waveforms through circuits is a real benefit, but
that option is not always available, as in the case of undocumented
equipment. In that situation, experience and intuition might
be your only hope. This article gives some pointers that can
put another method or two in your collective bag of tricks.
Wayward Capacitor Woes
By Allan F. Kinckiner
Look out for "perfectly good" capacitors that pass all tests
but produce strange and confusing faults.
Every technician is confronted at one time or another by
tubes that pass every test on the very best tube checkers but
simply will not work properly in certain circuits, although
a substituted tube will. After a number of such experiences,
the technician adopts the following credo: If a tube is suspected,
don't rely on a check-substitute.
Another electronic component that often presents the same
baffling condition of causing circuit malfunction while passing
every test is our friend the capacitor. A look at what tests
these components are subjected to in the better capacitor analyzers
reveals the following: capacitance measurement (generally quite
accurate) and leakage (the amount of direct current the component
will pass). In the latter check, a substantial d.c. voltage
is fed into one terminal of the component, which is in series
with a voltage-reading meter. Some analyzers use eye tubes or
neon bulbs in place of the meter.
A more efficient leakage test is to use a source of several
hundred volts (depending on the component's rating) applied
to one terminal of the capacitor, which is series-connected
to a v.t.v.m., as in Fig. 1. Leakage resistance may then be
determined from the following relationship: Rc/Rm
= Ec/Em; where Rc is the leakage
resistance of the capacitor, Rm the input d.c. resistance
of the v.t.v.m. (usually 11 megohms), Ec is the voltage
across the capacitor, and Em is the voltage read
on the meter.
Fig. 1 - Sensitive capacitor leakage measurement.
In the example shown, the meter reading (Em.)
is .1 volt, which leaves 299.9 volts as Ec. Solving
for Rc, we we have a leakage resistance of almost
33,000 megohms. The leakage current, determined by Ohm's Law,
would come to less than a hundredth of a microampere. Insignificant
as these leakage figures seem, there are circuits in which capacitor
replacement would be warranted.
The advantages of checking leakage in this way are twofold.
First, the sensitivity is greater than in the test provided
by most analyzers. In the second place, leakage is more easily
read. A man could get eyestrain trying to judge the opening
or closing of an eye tube or the lighting of a neon lamp. However,
just as the tube checker should be neither condemned nor scrapped
because it doesn't bat one thousand, the capacitor analyzer
should not be demoted for its less-than-perfect score on the
It is true that the substitution of a suspected capacitor
is not so easily accomplished as that of a tube. Yet this will
often be the quickest path to a repair. However, the substitution
should only be considered after thorough circuit testing has
left the capacitor as the prime suspect, although on circumstantial
evidence. The capacitor fault may be so elusive that it will
not always be possible to, determine exactly what defect has
occurred, although malfunction disappears when a substitution
is made. Following are several accounts of skirmishes with capacitors,
to prove the points made.
Before going into the case histories, we should like to point
out another parallel between tubes and capacitors. The baffling
defects are most likely to occur in more critical electronic
circuits, such as low-level amplifiers, discriminating or limiting
networks (such as the sync stages), sine and other waveform
generators (such as TV deflection generators), and in other
pulse-handling or pulse-forming circuits.
The Sizzling Ceramic
A V-M model 711 tape recorder came in for service with the complaint
of being noisy. Inspection revealed that, with no tape threaded
on and with the "play" button depressed, a constant sizzling
noise could be heard. The latter could be attenuated by adjusting
the volume control, indicating that the trouble was in a preceding
stage (or perhaps in a later stage, but was being detected by
the sensitive preamplifier). A scope check quickly eliminated
the alternate possibility in this instance.
The preamplifier consisted of a 12A-X7 with the triodes in
cascade (see Fig. 2.) Further troubleshooting revealed that
grounding the second grid killed the noise, but grounding the
first grid had no effect. The sizzling noise sounded precisely
like a noisy resistor, so the resistors were substituted after
resistance and voltage checks in the order numbered, but without
Fig. 2. - Preamp in which noise was generated
by a "good" ceramic capacitor.
Capacitor Cx was disconnected at the second grid
and checked with an analyzer, which passed it as being faultless.
Cx was also checked as per the technique discussed
in connection with Fig, 1; not the slightest leakage was indicated.
Since every other component in the circuit had been substituted,
Cx was now temporarily replaced with a tacked-in
unit, and dawg-gone if that didn't cure the trouble.
This capacitor, a black .01-μf. disc ceramic, subjected
to all types of further testing on our brand X capacitor analyzer,
was also checked on a fellow service technician's brand Y analyzer.
It passed without even the slightest indication of fault.
A TV set came in with the complaint that raster height was
insufficient, with the compression occurring on the bottom.
The condition occurred only after the receiver had been operating
an hour or more, and would get progressively worse. After the
saw-tooth forming capacitor in the vertical circuit had been
replaced, the bottom of the raster was easily stretched out
to fill the bottom of the CRT screen with normal linearity.
Furthermore, the raster remained constant in vertical size,
without needing later readjustment.
The capacitors that produce the not uncommon defect noted
here are generally of the waxed paper type. Their capacitance
may tend to increase as their temperature goes up. While a capacitor
analyzer is quite capable of indicating an increase in capacitance,
it can do so only if the suspected component is heated to the
temperature at which it works in the receiver. Thus, tacking
in a replacement is the quickest way to make a satisfactory
The horizontal Synchroguide circuit is one of those where
capacitors can really raise havoc. For example:
The fellow service technician previously referred to as the
owner of the brand Y analyzer sought help on a tough dog. It
was an RCA KCS34B that would not hold horizontal sync for more
than twenty minutes. In answer to questioning, he insisted that
he had disconnected and tested all the capacitors in the circuit
and that all read up to par on his analyzer. Knowing him to
be a thorough technician who normally makes the necessary resistance,
voltage, and scope checks when he has trouble, we advised him
to tack-solder capacitor substitutions.
About one hour later he phoned with the information that,
"after replacing the .002 μf. that feeds the sync and sampling
pulses to the grid of the a.f.c. triode (C1 in Fig.
5), the trouble was corrected." While he was happy that he had
repaired the set, he was also extremely unhappy because his
expensive analyzer would not indicate anything wrong with the
replaced unit. The .002-μf. unit was one of those black,
In line with this experience involving Synchroguides, one
set, an RCA KCS84F, operated relatively normally except that
the horizontal sync was critical. It was noticed during troubleshooting
that the frequency slug of the oscillator transformer adjusted
at an extremely withdrawn position. Component and voltage-checking
tests revealed nothing, so the various frequency-determining
capacitors were temporarily replaced. When C2 in
Fig. 5 was replaced, not only did the frequency slug adjust
to a more orthodox position, but the horizontal saw-tooth waveform
increased from 130 to over 160 volts, peak-to-peak, as a scope
check showed. As a result other improvements occurred; the width
increased, as did the high voltage, with improved focusing.
C2 was also a black, plastic-encased tubular;
it too passed all tests, including leakage, and its measured
capacitance was within ten percent of nominal value although
it was only rated at twenty percent. We are not exactly sure
what the elusive fault is that occurs in units of this type,
but suspect that the pulsed nature of the voltage to which they
are subjected causes them to react erratically in a way that
does not show up on static tests.
Watch That "Q"
On to case 4: The stabilizing network in the Synchroguide
circuit is a tank that generates a sine wave at approximately
the horizontal sync frequency. This plays an important role
in maintaining synchronization in the presence of random pulses
that might otherwise trigger the oscillator falsely. The network
consists of an adjustable coil shunted by a capacitor.
In Fig. 5, this network consists of C3 shunted
by Lp, with the latter being known as the phasing
coil. Stabilizing efficiency is affected by the over-all "Q"
of the tank, which is affected by the "Q" of C3 specifically.
This factor may decrease over a period of time, reducing sine-wave
Analyzers will not indicate this lowered "Q" factor, but
it can be determined with the scope. The waveform of Fig. 3A
was noted at point C of a Synchroguide used in a '53 Philco.
Note that, while the phasing coil is adjusted properly, the
sine-wave amplitude is about 15 percent of the composite waveform's
total amplitude. The waveform of Fig. 3B was noted at the same
point after C3 was changed. Note now that the sine-wave
amplitude is nearly 25 percent of the composite amplitude. This
change improved horizontal synchronization in this particular
Fig. 3. - Low (A, top) and proper (B, bottom)
sinewave height in Synchroguide waveform.
In multivibrator-type horizontal oscillators, the stabilizing
tank again consists of an adjustable coil (usually called the
ringing coil) shunted by a capacitor. The coil's action parallels
that of the phasing coil in that it generates a sine wave each
time the oscillator plate switches on to draw current. The tank
is invariably in series with a resistor in the plate lead of
the multi vibrator's controlling (first) triode. The stabilizing
efficiency of this network is similar to that of the one in
Fig. 5, and similar problems may occur.
Fig. 5. - Synchroguide oscillator with sinewave
generating tank (Lp and C3).
The relative "Q" factor of the tank in these circuits can
also be determined with the scope. Fig. 4A was taken at the
plate of the first triode of a horizontal multivibrator in a
Motorola. Note that the amplitude of the sine wave is about
30 percent of the composite waveform's amplitude. Fig. 4B was
taken at the same point after the tank's capacitor was changed.
Now the sine wave scopes better than 50 percent of the total
waveform height. The replacement cleared up a complaint that
setting of the horizontal hold control was too critical.
Fig. 4. - Low (A. top) and proper (B, bottom)
sine-wave height in multivibrator waveform.
The ratios given here for sine-wave amplitude to overall
waveform height are those most often used in original design
for Synchroguide and multivibrator circuits, although they are
not universal. In general, where marginal horizontal-hold is
the problem and no other defects exist, approximating these
ratios will produce enough improvement to satisfy an unhappy
The change in the capacitor, over a period of time, that
produces this reduction in sine-wave amplitude sheds light on
the tendency of older sets to develop more critical-sync.
Capacitor or Rectifier?
A Philco TV about nine years old was benched for drifting
vertical lock. The hold control had to be readjusted every ten
minutes until it reached the end of its rotation, after which
rolling could not be stopped. Trouble of this nature is due
to gradual changes in such frequency-determining oscillator
components as resistors (including the control), the blocking
oscillator transformer, and, of course, the coupling and timing
capacitors. In this case, replacing a capacitor corrected the
Suspected of leakage, the capacitor had been checked on an
analyzer but no leakage had been found. When it was checked
again after the replacement had worked, there was considerable
leakage. Further checking showed that, when the capacitor was
connected to the tester in one way, there was still no leakage.
However, when the capacitor leads were reversed, leakage was
clearly indicated! Evidently the component had begun to act
like a semiconductor, passing current in one direction only.
It was weird but it happened. The unit was a .01-μf. capacitor
encased in plastic.
An Eccentric Electrolytic
The villain in case 6, unlike the smaller units involved
with the other histories noted so far, was the big brother of
the capacitor family, an electrolytic. A Sylvania TV (Model
540) came in with the complaint that it was erratically blowing
a 2.5-ampere fuse. One fuse might last several days; but the
next might only survive for one hour.
In a bench check, line current was metered at about 1.5 amperes.
Receiver operation was entirely normal, with good picture and
sound. Instrument checks revealed no unusual conditions. However
a visual check showed a suspicious chemical staining at the
metal band used to mount a 150-μf., 200-volt electrolytic
filter to the chassis. Unfastening the metal band by removing
the self-tapping screw that held it to the chassis caused the
line current to fall to about 1 ampere.
Fig. 6 shows the staining on the cardboard case of this unit,
with the metal band removed to render the symptom more visible.
This type of electrolyte leak-through on cardboard-cased units
was more common in prewar radios, where it often led to puzzling
hum problems. Service technicians can be grateful that the difficulty
doesn't arise so often these days, but they should keep in mind
the fact that it can occur. One end of this capacitor was connected
directly to one side of the a.c. line in a voltage doubler using
two selenium rectifiers, which is why the leakage blew fuses.
However, an analyzer would not have indicated abnormal leakage.
Fig. 6. - The pointer indicates stain on capacitor case from
Thus we close the file on wayward capacitors. In each of
the cases described here, the defects were of the kind that
would escape detection with capacitor analyzers or other direct
instrument checks. In each, secondary evidence was the only
indication that the capacitor might be at fault. In closing,
a few words of commendation might be said for that old standby,
the oscilloscope. As in several instances recounted here, its
role in revealing the secondary conditions that lead to apprehension
of guilty capacitors with off-beat defects is important.
Posted March 25, 2014