Using oscilloscopes 77
diagram can be displayed in colour, with, say, single or low count
pixels shown in shades of blue, through the spectrum to red for
the pixels with the highest counts. Additionally, the data can be
further processed to show histograms illustrating the 'openness'
of the eye in various ways, Figure 5.10(c).
The oscilloscope in servicing
Several of the facilities of a good scope have been discussed above
in connection with specific applications. The rest of this chapter
looks at other particular areas of use for a scope. First, TV
servicing is considered briefly; for a more extensive treatment of
the topic reference should be made to one of the many excellent
books available dealing specifically with this subject.
It is important to pay due regard to safety when working on
any type of mains operated equipment. This is doubly true when
working on TV sets, as some of them do not have the circuitry
and chassis isolated from the mains. The circuitry of the
ubiquitous 12 in black and white portable set is designed to run
from 12 V d.c. in order to permit operation from a car battery
when required. For mains operation a step-down transformer,
rectifier and smoothing supply the required 12 V d.c. Thus only
the transformer primary is at mains potential, the rest of the set
being isolated. Larger mains-only colour TV sets may have a type
of switchmode power supply providing full mains isolation, but
this is by no means invariably so. To avoid drawing a d.c.
component from the a.c. mains (which was quite normal in the
days of valved TV sets), non-isolated sets use a fullwave rectifier:
as a result the set's circuitry and chassis can be at approximately
half the mains voltage.
The only safe way to proceed when working on a TV chassis is to
run it from a mains isolating transformer of a suitable rating. A
500 VA transformer should be more than adequate. The television
set's chassis should be firmly earthed, as is the case of the
oscilloscope. Even then, one must be very wary of the high voltages
present in the line deflection and e.h.t, sections of the receiver.
No one should work on a TV set without adequate knowledge
and expertise. Even apart from the safety aspect, many faults will
prove difficult or impossible to rectify without the full servicing
78 Oscilloscopes
~__=0
I0
_i-l_fi.I-Lftfi_fi.fi~LllJlJ-l~~
.F-! FL! LI-L.I L! L
0
I
Trig
,.,
Clock to Trigger
Random Data to Ch. 1
(a)
256
pixeis
J
// i
t/ I:
i
I
F
t
!
I
I
/ / 11
512 plxels
A sample fell on this
pixei 27
times
(b)
Using oscilloscopes 79
Figure 5.10 Measurements on eye diagrams, see text (courtesy Tektronix UK
Ltd)
data for the particular model. Occasional loss of colour, for
example, can be due to a variety of causes, and adjusting the
controls in the wrong sequence can easily give you permanent
loss of colour!
The most convenient type of scope for TV servicing has built-in
line and frame sync separator circuits, e.g. the Fluke model 3094
80
Oscilloscopes
featured. i.n Chapcer
3.
These are handy when examining t.h.e
operation
of
line
and
frame dellrt:iion circuits respectively,
particularly
when
the set
is
receiving live programme
material.
The
TV
sync circuits enable the scope
to
be triggered stably from
the output
ol
the video detector. However, it should be possible to
trigger any good oscilloscopc from thc linc sync component
of
the
video waveform by selecting normal trigger, positive or negative
slope as required, and adjusting the level control to trigger on the
tips
of
the video, i.e. the sync pulses. Problems may be
encountered in the case
of
a cheaper black and white set (with
mean level AGC applied
to
the vision
i.f.)
on programme
material, as the sync level will change with scene changes, the
video being a.c. coupled. This problem is easily solved by using
instead the signal from a greyscale or colour bar generator.
Servicing hi-fi equipment
is
a
less complex task than servicing
a
TV
set.
A
low-distortion sine wave
a.f.
signal generator and
a
scope should enable sufficient testing for all practical purposes to
be
carried
out.
Dummy
load resistors,
of
a suitable rating, to
replace the loudspeakers during
full
powcr testing can be
described
as
a
necessiiy rathcr than a luxury.
A
sine wave
t.est.
signal can bc followed through
the
various stages
and
any
gross
distortion observrd and pinpointed
10
the offending stage. In
addition to clipping the
signal on
posi~ive or negative peaks
(usually a sign
of
a
faulty
bias network),
a
widehand scope may
reveal that the amplifier
brcomcs
unstable with bursts
of
oscillation on one
or
both peaks
of
the waveform at full drive,
while behaving normally at lower Ievels. On live programme
material, this can give rise
to
a
nasty tearing noise appearing
in
loud passages only. Quite apart from these extreme forms
of
distortion, an amplifier (or
much
Iess often
a
preamplifier) may
exhibit
1
or
2
per cent distortion, usually more noticeable at
higher volume levels.
It
is very difficult
to
detect even several per
cent
of
third harmonic distortion simply by examining the output
waveform
on
a
scope,
birr
the diagnosis is much easier
if
it
is
possible
to
display
the
undistorted test sine
wave
input
on
the
other trace
of
the scope
for
ct)mparism. Unlike third harmonic
(and other higher odd-ordcr) distorticin c~jrr~pone~~ts, second and
other cvcn-ordcr harrnonic
disrortion
affect. the positive and
Using oscilloscopes 81
negative half-cycles of the waveform differently, usually making
one flatter and the other more peaky. Consequently, with care
even 1 per cent of second-order harmonic distortion can be seen
by examining the trace. Of course, even 1 per cent of distortion
completely disqualifies any amplifier from any pretence to the
title 'hi-fi', but it is surprising how many of the less expensive
amplifiers on the market, especially those forming part of a cheap
packaged 'music centre', do little better than this, particularly at
the extreme bass and treble ends of the audio range at full power
output. In many hi-fi outfits, the power amplifier for each stereo
channel has been condensed into a single power IC (integrated
circuit); often indeed both channels are contained within a single
IC. So here, it is simply a case of servicing (when needed) by
replacement. Preamplifiers are more likely to be amenable to
servicing, in that there will often be separate, identifiable stages-
for input equalization, tone controls, balance, etc.
To measure the distortion in a 'real' hi-fi amplifier a scope will
not suffice. A total harmonic distortion (t.h.d.) meter is required,
to remove the original sine wave from a sample of the amplifier's
output and measure the relative amplitude of the residual signal.
This consists of harmonics, noise and, very often, 100 Hz hum from
the mains power supply. Many t.h.d, meters make the residual
signal available for examination on an oscilloscope, which can be
very informative. For example, once the fundamental is removed,
it is very easy to see whether second or third harmonic
predominates, while the presence of little pips of alternate
positive- and negative-going polarity indicates 'cross-over' distor-
tion in a class B amplifier. Class B amplifiers are the norm
nowadays, only the most expensive amplifiers working in class A.
Often also, class B amplifiers show considerable 100 Hz ripple in
the residual at full power output, due to penny-pinching in the size
of the smoothing capacitors of the power supply. At low volume,
the class B output stage draws little current, so there is little ripple
voltage appearing on the supply rails, while at full output the
designer relies on the loud progamme content to mask the hum.
Provided it has sufficient bandwidth to cope with the signal, an
oscilloscope can be very useful when developing or trouble-
shooting radio frequency circuits. The main point to watch for
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.