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SERVIC
'SLOG
Found dead in a motel room
That heading was the gist of a garbled phone
message from one of the motel employees, under
instructions from the manager. Unfortunately, it
wasn't immediately clear just what had been
found dead.
Fortunately, it wasn't the body I
had momentarily envisaged; just one
of the TV sets. And, of course, it was I
who would have to track down the
fault and bring it back to life.
The motel manager is a long-standing customer. Most of his TV troubles
are routine and I didn't expect that
this would be anything unusual. But
it was, in a couple of respects.
For a start, it was almost a new set,
a Samsung model CB5012Z, one of a
recent batch bought by the motel and
only about six months old. More to
the point, it was a model I had never
handled before.
Second, it turned out be an extremely rare fault; the kind of thing
that is investigated only because there
is nothing left to suspect.
As with most modern TV sets, it
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36
SILICON CHIP
was fitted with remote control and
this brings me to a point I have been
meaning to mention for some time.
This fault did not involve the remote
control, so the following remarks are
more of a general nature, although
they are indicative of changing set
design.
Remote control is, of course, nothing new. TV manufacturers have now
used it in their sets for many years. In
most cases, they use a microprocessor
which is operated via the front panel
controls or by an infrared remote control unit.
Until recently, when a customer
brought a set in with its remote control unit, I would always hand the
control unit back to him - unless, of
course, the fault obviously involved a
remote control function. Experience
had taught me that it was just one
more piece of paraphernalia to keep
track of and that it could easily be
temporarily misplaced or forgotten
when the set was picked up, thus
causing a deal of inconvenience. This
is not good for PR and handing it back
was the simplest solution.
But not any more. With many devices these days - both video recorders and TV sets - it is essential to have
the remote control unit in order to
work on them. Some cannot even be
turned on - at least in the normal way
- without the remote control and they
certainly cannot be programmed without it. And so, the remote control
must now be left with the set and that
means more bookwork and labels to
keep track of everything.
Anyway, after that little digression,
let's get back to the body in the motel
room. It was duly delivered to my
workshop, complete with its remote
control. I set it up on the bench and
turned on the master (mains) switch.
Nothing happened, so I tried operating one of the channel selector
switches on the front panel. Still no
joy. Finally, I tried turning it on via
the remote control. Again no response;
it was very dead.
I pulled the back off and went
through what is a fairly common routine: mains fuse; power supply; horizontal output stage for short circuit
transistor, ICs, etc. This only takes a
few minutes and is time well spent
even though, in this case, it revealed
nothing obvious.
Brownie points
Now, to award Mr Samsung a couple of Brownie points, he has made it
possible to separate the receiver from
the remote control unit to some extent: He has provided two pins on the
chassis, complete with a printed caption: "To start set, bridge these two
pins".
Suitably intrigued, I wanted to know
just what these two pins did. Unfortunately, reference to the circuit didn't
help; they weren't shown. But a spot
of tracing provided the answer. One
pin goes to chassis and the other to
pin 41 of the microprocessor IC, RIC01
(top right of IC).
From there, it was easy to see what
it did. The on/off function is controlled by transistor RQ11- to the right of
the IC. This has its collector connected
to the 16.5V rail, via resistor RR51
(2700), while its base is fed from pin
41. The collector of RQ11 provides
another supply rail - called simply
"POWER" -via a 100n resistor, RR52.
When pin 41 is high, it turns on
RQ11 and pulls its collector down
towards the emitter, which goes to
chassis. In other words, the POWER
line is turned off. But when pin 41
goes low, QRl 1 is turned off and the
collector rises towards the 16.5V rail,
thus energising the
power line. Fair
enough, so what happened when I bridged
the pins?
Well, it did turn the
set on - at least to the
point where the screen
lit up. But there was no
sign of a picture; just a
bright screen as when
on a blank channel.
There was no sound either, not even noise, but
that was normal. This
set features a muting
circuit to turn the
sound off when there
is no signal.
So, at least the major
part of the set was up
and running but that
was as far as it would
go. It would not respond to any of the user
controls, such as channel selection, brightness, colour, etc. In
short, the microprocessor was not working,
either in itself, or because of some associated component.
When a microprocessor fails, the
most likely suspect - and the easiest
to check- is its 5V supply, in this case
at pin 42. But it wasn't going to be that
easy; the 5V supply was intact.
So what next? One of the disconcerting facts about the manual and
the circuit - and for which I will have
to recall those Brownie points - is
that nowhere in either are there any
voltage references or waveforms. Nor
are there any indications as to the
state of the microprocessor pins - ie,
whether high (5V) or low (OV) - for
any particular function or operating
condition. Some makers do supply
this information and it can be very
useful.
Component checks
I went over the surrounding circuitry, checking individual components on the basis that a failure in one
of them could have upset the microprocessor. But I found nothing and, in
the absence of any more specific data,
I was eventually forced to the conclusion that it was the microprocessor.
Naturally, it was one that I didn't
have in stock, so it had to be ordered.
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It arrived in a few days and I went
through the routine of pulling the old
one out and fitting it. And a fat lot of
good it did; the set was the same as
before.
I took another long hard look at the
circuit and mulled over the problem
while I attended to some routine work
on the bench. This is a technique
which sometimes helps to get the grey
matter into gear and it helped on this
occasion.
A vital part of the microprocessor
circuit is its clock and the pulses from
it. In this case, the clock operates from
a 10MHz crystal (RXOl) which is connected between pins 31 and 32.
Could the crystal be at fault? That
was a long shot; a very long shot because crystals are very reliable devices. In fact, I have never encountered a faulty one in any of the microprocessor controlled sets that I have
handled over the years. Still, stranger
things have happened.
I reached for the CRO leads and
connected them across the crystal. The
result was somewhat inconclusive.
There was something there but I
needed maximum CRO sensitivity -
down in the millivolt range - to produce it. More to the point, I couldn't
resolve any waveform out of it, in
spite of my best efforts at setting the
timebase and sync controls. In short,
it appeared to be nothing more than
mush.
Been there, done that
Nevertheless, I felt that the time
had come ·to seek some help. I rang
the Samsung service department and
contacted one of technicians, who I
know fairly well. After identifying the
set, I put the problem to him along the
simple lines that the microprocessor
seemed to be totally inoperative. His
response was immediate: "Changed
the microprocessor?"
"Yeah Bill, been there; done that".
"Bet you haven't checked the 5V
rail".
"Yeah Bill, been there; checked
that".
"Oh. Er, well ... " I could sense that
he was puzzled. "What about the crystal", I prompted.
"Could be, I s'pose; but not very
likely. They don't usually give trouble".
APRIL 1992
37
of all or part of a low voltage rail. The
luminance amplifier transistor sometimes causes this symptom, or an open
circuit luminance delay line can remove the picture, though this usually
results in a white, rather than black,
screen.
Next, of course, the brightness control circuit might be faulty-that could
cause a black screen.
Anything else you can think of?
Well, there is something else but I'll
not reveal it until the end of the story.
See if your guess is correct.
The story concerns a Sony KV1830
TV set, one of the earlier versions of
this model with mostly discrete components. The lady complained that
the picture just disappeared suddenly.
The sound was OK but there was no
sign of a picture.
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Fig.1: part of the microprocessor circuitry in the Samsung CB5012Z. Pin
41 of the microprocessor (top right) controls transistor RQ11 which in turn
controls the POWER rail. It wasn't the transistor that caused the problems,
however.
"OK, but what waveform voltage
should I be getting across the crystal?" He thought for a moment: "About
two volts".
''.OK, I can't get anything like that;
not even a readable waveform. Better
send me another crystal".
Well, that was it. The new crystal
duly arrived, was fitted, and all the
control functions snapped back into
action.
With hindsight, I suppose, I was a
bit hasty in assuming that the microprocessor was at fault. But then, as
my mate at Samsung agreed, crystal
faults are very rare. And it might just
have been a little easier had the
manual supplied more details. It might
at least have prompted me to check
the crystal.
Southern Sony
And now, for a change of scene,
here's a story from my colleague, J. L.,
in Northern Antarctica, who appears
to have come out of hibernation after
a long absence. His story is also about
38
SILICON CHIP
a set that wouldn't produce a picture.
Here's how he tells it.
How many different ways can you
think of to kill the picture - ie, create
a black screen - on a TV set?
First, and least likely, there's a tube
failure. Next, there's an EHT or horizontal output stage failure. After that,
one is getting into the odd faults: loss
of luminance output voltage and loss
lr
In the workshop, I confirmed her
story but with one minor addition.
The screen was normally quite black
but, when the set was switched off, it
flashed briefly with a very distorted
raster. It didn't last long enough for
me to see whether there was any picture or colour on it but it was enough
to indicate that the horizontal output
stage was working.
At this , I heaved a sigh of relief.
The horizontal output stage in this
Sony ·set uses an SG613 GCS (gate
controlled switch) which is quite expensive and cannot be replaced with
anything else. Knowing that this stage
was alive and working took a great
load off my mind.
I went first to the picture tube neck
board, designated board "C" by the
manufacturer. This provides access
to the picture tube operating voltages
and also to the red, green and blue
TETIA TV TIP
AWA C620 (G chassis)
Symptom: screen shows a small,
bright raster with all four sides curved
inwards. There is no sign of convergence anywhere on the screen. The
bottom edge of the picture shows
severe vertical foldup and the whole
picture is covered with flyback lines.
Cure: in spite of the complex nature
of the symptoms, the fault is quite
simple. It is caused by the loss of
the 150V rail. The usual reason for
the loss of this rail is that D575 (UF2) gees short circuit and takes out
the safety resistor R581 (4.70 O.SW
fusible) . A DYXSS/600 makes a good
substitute for the UF-2.
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.
Fig.2: this diagram shows part of the "B" board in the Sony 1830AS, with the
"C" (neck) board at right. The luminance chain transistors (Q451, Q452, Q453 &
Q454) are approximately mid-way up the "B" board, while the RGB output
transistors are at the bottom of the "C" board.
output transistors (Q701, Q702 &
Q703).
The picture tube G4 and G6 voltages
were close enough to normal but the
three cathodes and the Gl voltages
were quite wrong. The cathodes were
each at 220Vinstead of the 170V nominated on the circuit. And the Gl voltage was lower than the specified 30V,
which aggravated the effect of the
higher than normal cathode voltages.
It was quite apparent that the three
output transistors were cut off and
that this was the reason for the black
screen. All that I had to do was find
out why they were cut off.
The first thing I found when I
checked the transistors, was that they
all had 200V on their collectors - the
same as on the supply rail - rather
than the 170V shown on the circuit.
This simply confirmed - if confirmation was needed - that they were not
drawing any current.
The base voltages were wrong too
(2.1 V instead of 6.3V), as were the
emitter voltages (5.5V instead of 7V).
This was more of a headache than a
help, because either voltage could
upset the other. So which one was at
fault?
I decided that ifI could make one of
detector stages were all OK. Only the
brightness control had no effect but
that was not surprising, since the battery supply would o_verride any control from the brightness pot.
Froin there, I started backtracking
into the "B" board. As already mentioned, this carries the luminance
drive transistor (Q453) and this, in
turn, is direct coupled to, and driven
by, the luminance amplifier, Q451.
Unfortunately, because of the way the
set is constructed, it is almost impossible to get at these transistors while
the set is working.
However, since the luminance am-
the voltages right, by brute force, it
might give me some idea as to which
part of the system was working, even
if it did not tell me which part was
not. And the easier voltage to brute
force was the one applied to the bases.
This is common to all three bases and
comes from the luminance (Y) drive
transistor, Q453 , on the "B" board. It
finds its way to the "C" board via pin
6 of plug B5.
And pin 6 provided an easy access
point to this line,
where I could clip a
battery box into the circuit and wind up the
voltage . The idea
worked quite well. As
the battery supply
reached about 6V, up
came the brightness
and there was a picture. Not that anyone
would want to watch
it. It was overbright,
negative and covered
with flyback lines.
..,
But it allowed me to
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determine that the colour and contrast con'40M0 S~RVU S, NOR"T1t&=.RN
trols were working and
AN-r'A.RCTICI\, e.U\E.ltGING
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APRIL 1992
39
plifier is direct coupled to the luminance drive, and the luminance drive
stage is direct coupled to the red,
green, and blue output stages (the ones
with the grossly incorrect voltages on
them), it seemed safe to assume that
varying the input voltage - ie, the
voltage on the base of the luminance
amplifier, Q451 - would vary the operating conditions on the "C" board.
Or so I thought.
The input to Q451 is via pin 4 of
plug B2 on the "B" board, (left side of
the circuit) and the plug pins extend
through the board, making a very convenient contact point for the battery
box lead. Unfortunately, varying the
voltage at this point had no effect the screen remained dark.
So what was the voltage at this pin?
Getting a meter prod onto it wasn't
quite so easy but an extension lead
solved the problem. And it came up
at 2.1 V, which was close enough to
the 2V specified on the circuit. This
suggested that the fault, whatever it
was, was back along the chain, in the
direction of the "C" board. In other
words, I had probably overshot.
So what about the brightness control? The brightness control works on
the base of the luminance drive transistor, Q453. The control is a 20kn
pot between the base and chassis. Its
connection is made via pin 3 of plug
B4. It was a simple matter to confirm
that this control was properly connected and working.
·
Blanking circuitry
There are two other transistors associated with the luminance stages
on this board. They are an ABL (automatic beam limiter) transistor, Q452,
and the BLK (blanking) transistor,
Q454. (Initially, I took BLK to mean
black, of which I had more than
enough. It took me a moment to translate it as blanking!)
And, by a process of elimination, it
was looking more and more likely
that the fault was in one of these two
stages. I pulled the "B" board out and
checked all the resistors and capaci-
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40
SILICON CHIP
tors associated with the ABL circuit.
In particular, there were several low
value electros, which are always suspect. I removed these from the board
but they all tested as perfect.
Nevertheless, I replaced them because I've been caught before with
electros that test OK - at least as far as
conventional test equipment is concerned - but simply will not work.
But fitting the new ones was a waste
of time and money; the screen remained as black as ever.
That left the blanking stage, Q454. I
pulled the board out again and
checked this transistor and its associated components thoroughly. There
was nothing wrong that I could find
but I changed the transistor on the off
chance that it might have been one of
those funny ones that check OK but
will not amplify properly.
Next I tried measuring the base voltage on this transistor, using a clip
lead from pin 6 on plug B3. This is
shown as -lV on the circuit but my
measurements were quite meaningless. Depending on which meter I used
(analog or digital), the reading ranged
from zero to 50V!
I couldn't find a convenient point
to break into the base circuit of this
transistor, so I applied my battery box
in parallel with whatever was supplying bias to the transistor. By adjusting the input voltage, I was able to
produce a white screen, but without
any trace of a picture. I wasn't sure
whether this test was pointing me in
the right direction but I decided to
investigate the blanking drive to see if
there might be something along that
road.
The horizontal and vertical blanking pulses are developed on their respective boards, and are combined in
a network of resistors and capacitors
on the "D" board. The resultant drive
exits on pin 6 of plug D4 (not shown)
and enters the "B" board via pin 6 of
plug B3.
The vertical pulses are taken directly from the vertical output, with
very"little processing. This was easy
to check and revealed nothing unusual.
The horizontal pulses, on the other
hand, arise on the "E" board and undergo considerable processing. The
network (lower right quarter of the
"E" board circuit) includes blanking
rectifier D808; blanking zener D809;
blanking amplifiers Q801 and Q802;
Fig.3: the "E"
board in the Sony
1830AS. The
horizontal
blanking
components
(transistors Q801
& Q80Z, etc) are
in the lower right
hand corner.
three fusible resistors; and C816, a
4. 7µF 250V electro.
There were plenty of opportunities
for trouble in that lot. My first checks
were on the diodes and transistors
but in-circuit meter tests, while not
conclusive, were not so unusual as to
suggest that they should be replaced
at this stage.
I would not have been surprised to
find one of the fusible resistors open
circuit - except that these units were
one and two watt types and I have yet
to find one of these go open spontaneously.
This brought me to C816, the 4.7µF,
250V electrolytic. An in-circuit test
told me that it was not shorted and so
I decided to remove it for a capacitance check.
Funny thing though; as soon as I
unsoldered the negative lead, C816
fell off the board. It had become a oneterminal device and the positive lead
was still attached to the circuit board.
And that was the cause of all the
trouble. In one way or another it had
turned the blanking circuit hard on.
After I had finished the job, I realised that I had seen something similar
26
E A· I.U.!11-05.J,A
once before. On that occasion, the set
was an HMV CZ 11 and half the screen
was blacked out because a faulty transistor couldn't switch fast enough.
This Sony was a different story altogether but, from now on, I'll have to
remember that there are more ways to
black out a screen than I had previously considered.
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Thank you, J. L., for an interesting
story and a useful insight into the
workings of this circuit. I have had
several of these models through the
workshop recently but with nothing
like the fault you described.
Anyway, congratulations on an arduous piece of detective work and a
successful outcome.
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41
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