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Five faults all at once!
A heading like that is hard to ignore. Likewise
the customer's claim as she went through the list
of faults. Superficially, at least, it was a
formidable list and I could only hope that it
wasn't as bad as it sounded.
The device in question was a
Panasonic TC-2969 TV set, an upmarket unit which first appeared about
four years ago although this particular set is only about three years old. It
is a 73cm model featuring, among
other things, an elaborate remote control unit.
·It has been a very popular set. My
dealer colleagues, between them, have
probably sold around 150 in this area
but the number of faults I have encountered has been extremely small.
This set belongs to one of my longstanding customers, an elderly widow,
but most of my dealings were with
her daughter and son-in-law, who
were better able to transport the set
and describe the various faults.
I have had one previous call to this
set but that was hardly the set's fault.
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====-
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for better
performance
from weak
UHF
signals.
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P.O. Box 678 (74Fulton Rd.) Mount Eliza 3930
Tel:03 787 2077 Fax:(03) 787 3460 ACN006893438
40
SILICON CHIP
and enjoying a TV program when the
screen suddenly went black.
But it wasn't a shut-down as in
fault number one because the sound
was still running and there were traces
of the brighter colours on the screen.
This was something new and it fi~
nally convinced them to seek my help.
So there we had it - five different
intermittent faults. Naturally, there
wasn't any point in trying to tackle a
problem like that in a lounge room; it
had to come into the workshop. Fortunately, the son-in-law had a ute and
plenty of help on hand to load the set
into it. And so it finished up on my
counter.
About 12 months ago, it was the victim of a lightning strike to the power
lines which, as well as damaging the
set, knocked out a microwave oven,
the electric stove and the hot water
system. In fact, about the only thing
that escaped in the household was
the video recorder - a typical Murphy
touch.
My first insight into the problem Related faults
was a phone call from the owner's
Now tracking down any one of
daughter, who proceeded to list all those faults could be a nasty job. But
the faults it was exhibiting. And, as was tracking down five going to be
already intimated, it was a formida- any harder? Not necessarily. In fact, it
ble list.
didn't need Sherlock Holmes to deThe first fault appeared not long duce that all five faults were almost
after I had serviced it and took the . certainly related; find the cause of
form of occasional spontaneous shut- one and you'd probably find the cause
down; ie, the set would simply turn of them all.
itself off. But while this was mildly
That was the bright side. The dark
annoying, it wasn't regarded as very side was that it was still an intermitserious; it needed only a touch of the tent and likely to take a lot of time to
remote on/ off control button to turn it track down. I started by simply turnback on.
ing the set on as it came in and, sure
Fault number two caused the set to enough, the number five fault just
randomly change channels, again in- described was still in evidence; ie,
termittently and also easily remedied normal sound but a dark screen with
via the remote control.
just a hint of colour. And, by using the
Fault number three could be al- remote control unit, I could change
most rated as two faults: random channels, as evident by the sound.
changes of brightness, or the same
It is necessary to digress here, so
effect on colour saturation.
that the reader can follow the next
Fault number four was a fairly re- step. As is common with many upcent manifestation. It had happened market sets, this set's remote control
on only a couple of occasions but was system offers a multitude of functions;
somewhat more serious. As the daugh- far more than most people find necester described it, it was " ... a kind of sary and more than some people can
blurting noise; frighteningly loud. handle.
Nearly scared Mum out of her wits".
In this case, the controls include
And then she went on to describe the usual on/off button, 10 channel
fault number five which had occurred selecting buttons, an up/down button
the previous evening. She and her which takes the set through the chanhusband had been visiting her mother nels one-by-one, a button to allow a
channel number - up to about 30 - to
be keyed in, volume up/down, stereo
sound enable/disable, video up/down
and audio up/down.
Suitably activated, the video up/
down provides a choice of brightness,
contrast and colour saturation control. The audio up/down operates
similarly, providing bass, treble and
balance control. And any settings selected by these controls are held after
the set is turned off, or even unplugged
from the mains.
There is also a TV /VCR switch
which allows the control unit to be
used on a Panasonic VCR. It provides
all the usual VCR control functions:
stop, rewind, fast forward, record, etc.
Then there is a another group of
five buttons. Button 1 switches from
TV to video input; button 2 is a timer,
to shut the set down after a selected
time; button 3 displays the channel
number in the top right corner of the
screen; button 4 provides sound mute
(now what would anyone want that
for?); and button 5 is marked "N" (for
normalising).
Complicated? You bet it is. And
many a viewer has got himself into a
hopeless mess trying to cope with all
those facilities. In particular, it is all
too easy for the beginner to black out
the screen or mute the sound, and
then forget how to undo it and go into
panic mode.
Hence the above mentioned "N"
button, which is almost an admission
of the extreme complexity. It effectively cancels all the control instruc-
TETIA TV TIP
Samsung TV chassis No. PSOF &
RM101 (used in Samsung, Tempest, Akai and Toshiba models)
Symptom: picture size is small and
pulsates, never quite filling the
screen. The 125V rail is low but
shows no sign of fluctuation. Reducing the picture contrast stops the
pulsation but does nothing to increase the size.
Cure: STR30125 voltage regulator
IC defective. This is the main cause
tions that have been fed in and takes
everything - brightness, contrast, colour saturation, sound settings, etc back to their mid-position (or "normal") settings. From there, the user
can start all over again.
Back to the interruption
That brings us back to where I interrupted myself; with the set on the
bench with' normal sound but only a
few spots of colour on a black screen.
I hit the "N" button and up came
the picture with normal brightness,
contrast and colour. I then tried something else. Using the remote control, I
activated the brightness function and
wound it down to minimum, then did
the same for the contrast. And that
exactly duplicated the condition of
the set when it came in.
All this was quite significant. It sup-
of the small picture - the pulsation is
a secondary effect produced by the
low rail voltage on the Automatic
Beam Current Limiter. Resetting the
contrast has the effect of reducing
the beam current, so the pulsation
stops.
TETIA TV Tip is supplied by the Tasmanian branch of the Electronic Technician's Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston
Bay, 7015.
ported a theory I had been toying with
from the start: that the fault was not,
as might have at first been imagined,
in the remote control system itself.
Rather, something was "telling" the
microprocessor to do these things - in
this case to turn down the brightness
and contrast. That "something" could
be almost anywhere in the set. All I
had to do was find it.
I let the set run all day and every
day for about a week. During that
time, it exhibited almost all of the
faults listed above; all, in fact, except
fault number four (the distressing
blurting noise). More importantly, I
confirmed that these effects were totally random; they didn't appear to be
related to temperature, period of operation, operation of other appliances
on the same power circuit or, in fact,
anything that I could pin down.
But that very randomness created a gut feeling that it was due to a
dry joint somewhere in
set. The back was already
off the set and so, armed
with my favourite
sledgehammer, I set to
work. (The sledgehammer is really just a large
screwdriver, the plastic
handle of which is the
right weight to deliver
an appropriate blow to
the circuit board).
Looking into the back
of the set, there is a large
horizontal board, about
350 x 200mm, in the bottom of the cabinet. This
carries the power supply and the horizontal
and vertical output
MAY 1992
41
SERVICEMAN'S LOG - CTD
stages . On the lefthand side there is a
similar size signal processing board
which is mounted vertically. This carries the tuner, IF strip, stereo decoding, remote control processing (but
not the receiver), and the audio/visual
inputs and t;mtputs.
On the righthand side there is a
smaller board, about 150 x 100mm,
which carries additional horizontal
components and the pincushion correction components, etc.
I tackled the horizontal board first,
starting on the lefthand side, tapping
moderately hard and watching the
screen in the mirror. No joy. I made
my way across to the righthand side ,
again without any result until I tapped
the horizontal output transformer,
which is at the right rear of the board.
Suddenly, there was a reaction. I
can't remember which fault it was, or
whether there was more than one,
except that it probably changed channels. Anyway, it was what I was after.
Well, that was encouraging. The
only snag was , I couldn't repeat it. No
matter how hard I bashed after that,
there was no response. I left it for a
while , tried again, left it, tried again,
and so on for more times that I can
remember. The result was, at best,
"iffy"; a couple of times I achieved a
briefresponse but nothing consistent.
Nevertheless, it did seem to suggest
that the fault was not on the board,
but somewhere nearby, and that the
vibration I was imparting to the board
was being transferred to it.
So what was closest? There is an
"L" shaped aluminium plate near the
horizontal output transformer, best
envisaged as lying on its side, with
the longer section, about 200mm long
and 80mm high, running from the
back towards the front of the set. The
shorter section, about 40mm long, runs
parallel with the back of the cabinet
and provides the support.
One of this plate's functions is as a
heatsink for the horizontal output transistor, which is mounted towards the
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rear. It also carries a heavier, finned
heatsink at the front for the power
supply regulator IC and a couple of
large resistors. And when I tapped
this, ever so gently, the set "went
mad", changing channels and varying brightness, contrast and colour.
But then it, too, settled down and I
went back to the main board again.
But no joy here either. So over the
next few hours, in between other jobs,
I went through several tapping sessions. I achieved some briefresponses
but, more importantly, I concluded
that the aluminium bracket was the
more sensitive of the two, although
the point to be tapped was most critical.
And it was while I was establishing
this critical point that I encountered
fault number four; the blurting sound.
And it was only then that I fully appreciated why it had "nearly scared
Mum out of her wits". With my head
half inside the cabinet while I was
probing, it must have hit my eardrums
at about lOdB above the threshold of
pain and it came close to having a
more drastic effect on me than it had
on "Mum".
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What happened next was
I
')
more fortuitous than scientific.
I,,
The set was on a bench near
one of the windows and, in
the late afternoon, the sun
~
shines directly into my eyes. I
have a blind fitted to take care
of this situation and this darkened the area considerably but
.
.~
it was still light enough to see
,~-what I was tapping.
And so I had another go at
q~:==::.i--:;;-~·"V'·>
.•
the aluminium bracket. After
-~~~
a couple of tries, I found the
sensitive spot again and the
set gave forth with another
raspberry. This time, fortunately, I had moved back a
little, so it wasn't quite so traumatic.
oWl-,-H r,/\'-( 1-tG:Ai;> l~SI?~ "fl1E:
But, more importantly, I
INE:"i" J: E-Na::>ul\J"'t1::;..~e;D
happened to be looking at the
L.:
i 1\-\~ 1)\-Sl'f<G:S'SlNG
right spot at the right time.
"BLURTING NOi
The horizontal output transistor is in a TO-3 case, connected
quick fix was out of the question. And
to the main board via three flying SC5326, which uses the "Q" chassis.
leads. The base and emitter leads are The "Q" chassis is a mite long in the so the customer had no option but to
soldered directly to the appropriate tooth now; it first appeared at least 10 leave it with me.
pins but the collector lead is con- years ago and remained current, with
A little later, I turned the set on
nected to the case via a solder lug minor variations, for several years.
again and studied the problem more
secured under one of the mounting
This particular set has been through leisurely. I still couldn't make sense
the workshop several times, mainly of the hold control behaviour, so I
screws.
At the instant the system took off, I for minor faults, but is still in good decided to disconnect the line to the
was looking at this solder lug. And nick and has a few years of life left in sync separator and let the vertical circuit float. Normally, this would enthere it was - a tiny spark, clearly it yet.
Initially, the owner contacted me able me to roll the picture slowly in
visible in the subdued light. From
there on it was routine. Closer exami- by phone and, from the symptoms either direction and even hold it
nation confirmed that what was sup- described, it appeared that the pic- briefly (with a little care).
posed to be a soldered connection · ture was rolling very rapidly. When I
The vertical functions are provided
between the lead and the solder lug suggested that he try adjusting the by IC401 (LA7800) and the video and
was no such thing; it was a classic dry vertical hold control - on the front of sync pulses come in on pin 14 via
the set - he replied that he had done number of components, including
joint.
And that was it; a few moments that, but that it didn't seem to make R402 (220Q) which was fairly easy to
get at. It was also a convenient access
work with some flux and a hot iron any difference.
This clearly indicated that it was
point for the CRO and this indicated
produced a proper joint and all the
problems were solved. It was almost not a simple case of maladjustment, that the waveform was correct. So I
lifted the resistor and tried the hold
an anti-climax, except that I had the so I said "bring it in". When it arrived,
I switched it on for a preliminary check control again.
satisfaction ofknowing that I had fiwhile the customer was still there.
nally cracked it.
The result was rather strange. IniThis confirmed the customer's obser- tially, the effect was much the same as
But there was one puzzling aspect.
vations and also seemed to confirm before, with the picture rolling rapI would normally expect such a fault,
idly upwards. But it was now floating
in that position, to take out the tran- my interpretation of them; that the
sistor. So why had it survived? I dunno picture was rolling very rapidly up- and, by backing off the vertical hold
please; ask Murphy.
·
wards.
control, I was able to make it roll
I tried the vertical hold control but quite smoothly and under perfect conConfusing symptoms
it had only a very slight affect on the trol. Well, that was better than before.
My next story involves a 53cm AWA upward movement in one direction.
But it was when I tried to roll it
colour TV set and some very confus- Rotating the vertical hold control in slowly upwards that it baulked. I could
ing symptoms. Indeed, symptoms are the other direction then caused the hold the blanking bar at the bottom of
not always what they appear to be at picture to suddenly reverse direction the screen and move it slowly upwards until it was about half way up
first glance. And if you don't get the and roll rapidly downwards.
Well, I hadn't seen anything like the screen. At that point, it started to
symptoms right, well ...
The set involved is an AWA model that before and any suggestion of a jitter, as though the bar was bouncing
.
~
. ..
. be
~
~~
se::
a
MAY 1992
43
• C4,Z
3.Cf
41,3
sov
scv
3~
~30_..u
040
,.,_,z /2S0
/2SC20
66 o/2SC21
Fig.1: part of the vertical & horizontal output circuitry in the AWA SC5326. Transistors Q402 & Q403
form the vertical output stage & drive the vertical scan coils (at left) via capacitors C412 & C413.
against some obstacle. And the further up the screen I forced it, the
faster it jittered.
At this point, I suddenly realised
that I had been misleading myself.
What I had imagined to be a fast roll
was, in fact, a fast jitter. Of course, the
difference was academic as far as the
viewer was concerned but it was important diagnostically.
In any case, I wasn't particularly
worried. There isn't a great deal to
suspect in this part of the circuit and
I had already mentally cleared part of
it anyway. Most of the circuitry is in
IC401, which contains the sync separator, vertical oscillator, blanking circuit and the vertical drive.
The latter comes out on pin 6 and
goes to the vertical output pair, Q402
& Q403. These, in turn, drive the vertical scan coils. And it was everything
from pin 6 to the scan coils that I had
mentally cleared, on the simple basis
that both the height and linearity,
checked on a test pattern, were perfect.
So was it the chip? I did a voltage
check of all the pins and found nothing abnormal. I then checked the various minor components around the
chip but again drew a blank. That
threw suspicion on the chip and, since
I had some in stock, it was easy enough
to fit a new one. But, alas, still no joy.
As far as I could see, that didn't
leave anything else to check. It was
44
SILICON CHIP
time to seek help. I rang one of the
service technicians at AWA, described
the symptoms and told him what I
had already tried. He wasn't at all
fazed by my story and immediately
suggested two other components: capacitor C421 (220pF) across the scan
coils and C588 (1000µF) across T573
in the pincushion circuit. He explained that both these components
could give trouble and create symptoms similar to those I had described.
So, full of hope, I changed both of
them. But I drew another blank; it
made absolutely no difference. And
that really put the ball back in my
court. I could hardly ask for more
help; it was obvious that this was all
that was known.
But the advice had done some good.
... A CON\/G:t-.llli.N""i'
AcCG:~':S ?DIN, FOR
.,.-1-1e;. C. 'RO .. o
It had broken through a mental barrier which had excluded anything in
the output stage from my thoughts. If
one component in this section could
cause these symptoms, then why not
another? And that brought me to capacitors C412 & C413 (330µF, 50V) in
series between the output pair (Q402
& Q403) and the scan coils.
I've often puzzled over this arrangement. Some models used only one
capacitor, others the two-in-series arrangement (not back to back). Anyway, in desperation I pulled them both
out and fitted two new ones. And
bingo! That was it; everything was
now back to normal.
But the reason remains a mystery. I
checked both capacitors on the capacitance meter and they both read
full value. Nor did there appear to be
any leakage, at least at the test voltage. Nevertheless, I suspect that leakage may be the answer - after all,
what else can go wrong with an electrolytic? Or was one capacitor partially open circuit?
And what was the reason for the
two ·capacitors in series? Had leakage
proved a problem in early versions,
with only one capacitor, and had
someone decided that two would reduce the risk? The reasoning, of
course, would be that one could fail
without creating a problem, while the
chances of both failing would be quite
small.
SC
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