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SERVICEMAN'S LOG
A screw loose somewhere?
It was a screw tight actually. My first story this
month is relatively simple but there are still
enough puzzling aspects to make it worth the
telling. And from down south comes a story
which must be close to the ultimate in servicing
by remote control.
This story concerns a Sharp model
CX1020; basically a portable colour
TV set but which also incorporates
an AM/FM radio and a cassette tape
recorder. Although a portable unit, it
is designed for mains operation only.
It measures about 45cm wide, 30cm
high and 23cm deep. The picture tube
is around 22cm.
The owner is a retired man who
had travelled around Australia a lot
during his retirement and had bought
the set specifically with these travels
in mind. The set is around 10 years old
now and had given him good service
during that time.
But now a fault had developed
which brought the set onto my bench.
But not directly. The fault was a failure
in the cassette recorder section and the
owner, who is something of a handyman type, decided to open the set and
look for anything obvious.
At least, that was the idea until
he took the back off the cabinet. He
progressed as far as sliding the TV
chassis out – which is quite easy –
then took one look, recoiled in horror,
and decided that access to the cassette
section was far too complicated for
him to tackle (he’s right; it really is
a nightmare). In fact, he decided that
the recorder wasn’t that important
after all; he didn’t use it a great deal
and he had a separate unit available
anyway.
So he slid the chassis back into
place, refitted the back on the cabinet, and wrote off the recorder. The
trouble was, having done that, the
TV set wouldn’t work any more.
And that was how he turned up at
the workshop with it, along with the
above history.
Exploded view
In order that the reader can better
understand what fol
lows, I am including some exploded views of the
unit, taken from the manual. The main
one (Fig.1) shows the cabinet, with
the picture tube opening on the left.
Above the picture tube is a straight
line dial, calibrated in VHF and UHF
TV channels. Tuning is by means of
the knob on the left which, via a dial
cord assembly, operates a pot which
sets the voltage fed to a varicap diode;
a popular arrangement with portable
TV sets. The brightness, contrast and
colour controls are on the top of the
cabinet.
Immediately to the right of the picture tube is the speaker grill and to
the right of that the cassette recorder.
Above this is the radio tuning dial,
with the tuning knob on the righthand
end of the cabinet. Other radio controls
and the cassette controls are on the top
of the cabinet. The smaller drawing
(Fig.2) is of the TV chassis, which I
will deal with in due course.
When the customer related his story,
I immediately plugged the set in and
turned it on. And it was just as he said;
quite dead and so he left it with me.
Later, I pulled the back off. It is held
by four screws and it is also necessary
to remove two screws holding the AM/
FM telescopic antenna assembly.
An unexpected cure
Fig.1: this exploded diagram shows the general layout of the cabinet used in the
Sharp CX1020. Note the chassis supports which are visible through the lower
righthand corner of the picture tube opening.
66 Silicon Chip
I had a good look and prod around
inside and could see nothing obvious.
So the next step was to turn it on again
– whereupon it immediately leapt into
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The only fault I could find was that the
tuning was very touchy, which turned
out to be due to the tuning pot feeding
the varicap.
A dose of cleaning fluid sprayed into
this soon put that to rights and we had
a nice smooth tuning action. But why
was the set dead when I first tried it?
I had another look and prod, without
actually moving the chassis, but could
not recreate the fault.
I decided to refit the back and see
what happened. In fact the set was running while I fitted the back. I had it face
down on a felt pad on the bench, with
the bottom of the cabinet towards my
body and, initially, I simply pressed
the back into place.
The set continued to chortle away
so I fitted the top lefthand screw, the
bottom left one, then the top right one.
There was nothing planned about this
sequence; it was just convenient, and it
had no effect on the set. Finally, I fitted
the bottom right screw. And this had
no effect either – initially. It was only
as I gave it a final tighten that the set
suddenly stopped.
I slackened off the screw but the set
did not respond. It was only when I
switched it off, and then on again, that
it came good. What’s more, I was able
to repeat this sequence quite reliably.
So what was I to make of that?
I took the back off again and went
straight to the offending screw position, thinking it might be touching
something. But no, I had to rule that
out. Or was there a lead being pinched
somewhere? No, that was out too. The
next theory was that the back was
pressing against the chassis, distorting it, and aggravating a dry joint or
hairline crack in a board.
Fig.2: the chassis layout for the Sharp
CX1020. The two side runners mate
with the chassis supports inside the
cabinet.
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July 1994 67
SERVICEMAN’S LOG – CTD
plastic supports in the front of the
cabinet, one of which can be seen
through the bottom righthand corner
of the picture tube opening. There is
a similar assembly in the bottom left
corner, part of which is just visible.
Now, somehow or other, the owner
had misaligned the chassis on this
support system. Don’t ask me how – as
I said before, I lost this evidence when
I pulled the chassis out.
I refitted the chassis and made some
attempt to recreate the fault by twisting, prodding and pounding it. It was
all to no avail; nothing I could do had
any effect.
So I screwed the antenna back in
place – the holes for which now lined
up exactly – and refitted the back. And
this time all four screws were tightened without any ill effects. And that
really was the answer. The set hasn’t
missed a beat since.
Unanswered questions
In order to investigate these theories
I needed to pull the chassis out of the
cabinet; the first time I had moved it.
But in preparing to do this I suddenly realised that there was something
amiss with the chassis position. It was
not sitting exactly level in the cabinet;
one side was slightly lower than the
other. The error was not very great
and was easily overlooked with casual
observation.
But it wasn’t right and I then realised that this probably explained
something else I had noticed. When I
had replaced the two screws holding
the antenna, I found that the holes
did not line up exactly. I did not pay a
68 Silicon Chip
great deal of attention to it at the time.
Errors like this are not unusual, it was
very small, and I was able to juggle
the screws into position quite easily.
Suddenly everything started to
make sense. When the owner had
pulled the chassis out to investigate
the cassette recorder, he had not
replaced it correctly. I’m not exactly
sure what he had done, because I
destroyed the evidence when I pulled
the chassis out, but the drawings of the
cabinet and chassis give some idea of
the setup.
The chassis drawing (Fig.2) shows
that it is fitted with two side runners.
And these are designed to mate with
But it does leave some questions up
in the air. I don’t know exactly how
the chassis was misaligned and I don’t
know how this misalignment caused
the set to fail when the back was fitted.
I can only assume that, somehow, it
was triggering a protective circuit
which shut the set down and could
only be reset by turning off the power.
Anyway, the owner was quite happy
to have the TV set back in operation.
By mutual consent, we did not investigate the tape cassette problem as it
would have been an expensive job to
get at.
But it was not a totally wasted effort.
I had cleaned up the touchy tuning
system and I also repaired the supports
for the ferrite rod (AM) band antenna,
which had broken away. The owner
was fully appreciative of both repairs
and so it all ended happily.
Remote control servicing
And now to the story I mentioned
earlier: the problem of servicing by
remote control – and an old colour
TV set no less.
We’ve all encountered this situation – at least potentially. It is nothing
new for a customer to present the bald
statement and question, “My TV set
(or something) doesn’t work; what’s
wrong with it?”
And it often takes a good deal of
diplomacy to explain that no such
simple diagnosis is possible. And even
then, one is not always believed.
Fig.3: the horizontal & vertical drive circuitry in the Rank C2205. Transistor TR412 is the righthand one of a group
of three at the left of the drawing. Its class B mate, TR409, is above it & to the right, & is connected into circuit via a
plug & socket. The auxiliary board, PWC470, is at the top righthand corner.
A serviceman I once worked for,
back in the old valve radio days, had a
simple approach. He would nominate
the first component that came to mind,
usually one of the more expensive
ones, like the power transformer or
the speaker.
He reasoned it was bad policy to
admit that one didn’t know and that
an answer didn’t have to make sense
to satisfy the customer. Mind you, he
often had to do a lot of faking when
making out the bill.
But assuming that an attempt at
remote diagnosis is justified for some
reason, it calls for maximum co-operation and observation on the part of the
owner, to provide intelligent answers
to the questions the serviceman will
have to ask.
And when the only link between
serviceman and owner is by mail ...
well, that really makes it hard. But that
is gist of the story from my colleague,
J. L., from the Apple Isle. This is how
he tells it.
Regular readers of The Serviceman’s Log might remember that
until recently I conducted a feature
called “TETIA TV Tip”. One result of
having my name and address listed
each month was that I received a lot
of correspondence from readers, most
asking for help in solving their
TV set or VCR problems.
In most cases, I had to
ignore these pleas: I was too
busy on my own bench and
I could afford neither the time
nor the postage needed to reply. Those correspondents
who included a stamped
addressed envelope always
received an answer, even
if it wasn’t exactly the one
they hoped for.
July 1994 69
SERVICEMAN’S LOG – CTD
However, there has been one correspondent who has received much better service, since he usually enclosed
a $5 note and a stamped envelope
with his letter. (Not that $5 would buy
much time from a busy serviceman but
it was the principle that counted – he
appreciated that time was money!)
The following story comes from
this correspondent, whom I shall
henceforth identify by his initials N.
B., and it provides an opportunity to
discuss a common fault in a very old
but still-popular colour TV set.
On one occasion, N. B. wrote asking
my help in (1) identifying a particular
early model Rank Arena colour TV
set, (2) supplying a circuit diagram
for same, and (3) suggesting any likely
causes for vertical non-linearity and
bright retrace lines.
One of the difficulties in tracking
down information on these old TV
sets is identifying the model number.
The only model identification on the
old Ranks was on a small slip of paper,
pasted on the outside of the cabinet
back. Most have long since fallen off
and identification is a process of comparing chassis details with those from
all the likely manuals. In this case, N.
B. helped considerably.
He described the old set in considerable detail and from this information I
was able to identify it as one of the “14
70 Silicon Chip
pcb” (printed circuit
boards) models. In the
very early days, Rank
did not assign chassis
numbers to particular
models. Only the indi
vidual printed boards
were numbered and
these could change
unpre
dictably in the
various models.
But I needed further
details so I wrote back
asking for the numbers
of some of the boards;
in particular, the horizontal output board
number, since this was
where most of the circuit changes occurred
during the life of these
models.
N. B. replied with the
numbers of most of the
boards. Using a cross
reference published by Rank many
years ago, I was able to identify the set
as most probably a C2205. One or two
of the boards had different numbers
but that was par for the Rank course
in those days. The main identifying
feature was the horizontal output
board, in this case a PWC433 (one of
the later boards in this series).
I made a copy of the circuit for the
2205 and marked on it some of the
components that I have found to give
trouble in the vertical stage. Most are
electrolytic capacitors, as might be
expected. There are several electros in
the vertical oscillator, drive and output
stages, and failure of any of these will
cause bad linearity.
The degree of non-linearity varies
with the particular capacitor but the
most dramatic and common problem
lies with two small tantalum capacitors, C451 and C452. These are in series with the vertical linearity control,
so it’s not surprising that changes in
them cause odd faults.
On that subject, tantalum capacitors
were introduced as much for their
low leakage as for their tiny size. Unfortunately, they have not proved to
be stable and many 10µF tantalums
measure as low as 1µF. This doesn’t
matter in some circuits but it’s fatal
in linearity networks.
My final advice to N. B. was to check
the voltages around the circuit. Wrong
or missing voltages usually point to
some kind of total failure, not partial
failures. Since N. B. has no oscilloscope, a careful check of voltages and
capacitor values was about his only
course of action.
All of that went off to N. B. in one
of his prepaid envelopes and I heard
nothing more for several months.
Then I received a letter saying
“Thanks a million! It’s going again,
thanks to your suggestions”. He didn’t
say which suggestions were helpful
but from further discussions it would
seem that the voltage analysis did the
trick.
N. B. advised that all the electros
had been replaced and produced no
improvement. He then found that
TR412, one of the vertical output
transistors was completely open circuit. Replacing this restored normal
linearity but did nothing to improve
the retrace lines.
This part of the story puzzles me,
since TR412 is one of a class-B output pair and open circuiting one of
these should collapse the picture
to little more than a line across the
centre screen. Yet N. B.’s description
of “non-linearity” implies a far less
dramatic symptom. I shall have to
experiment with that next time one of
these sets comes into my workshop.
(Serviceman’s comment: yes, you’ve
raised an interesting point about that
class-B output stage, J. L. And note that
it is not a symmetrical arrangement.
TR412 is a small power transistor
(2SA653) which is mounted directly
on the board, whereas its mate is a
much larger unit (2SC1104) which is
mounted remotely on its own heatsink.
And while I can’t be sure, the implication is that it contributes the major
portion of the vertical scan.
So, if the failure of TR412 only partially reduced the scan, and someone
tried to correct this by simply winding
up the height control, the result might
well be poor linearity – to the extent
that the trick worked at all. Just a
thought, J. L. – carry on).
Retrace lines
The rest of N.B’s story centres
around the retrace lines. He solved
the problem almost by accident but
doesn’t really know what he did!
He found a small printed board,
PWC470, mounted on the top of the
horizontal output board. It held only
two transistors and a few other components, but oddly enough, it was not
connected into the circuit. A 3-pin
plug had been disconnected and left
hanging loose near the board.
When he reconnected the small
board, the retrace lines vanished but
he was greeted with an array of broad
black lines moving up the screen and
a degree of vertical rolling. Removing
the plug immediately stopped the
black lines but restored the white
(retrace) ones!
N. B. removed the small board for
a closer examination and found that
it had been worked on extensively at
some time or another in the past. In
particular, the two 2SC945 transistors
had been replaced with two BC547s.
So he decided to restore the proper
types, if for no other reason than that
substitutes can often introduce faults
of their own.
And that was all that it took. When
he replaced the board, the retrace
lines had disappeared and there was
no sign of the black lines. In fact, he
claims that the set is giving as good a
picture as any set of its age that he’s
ever seen.
So what did he do? What connection does PWC470 have with vertical
linearity? Well, none that I know of.
I believe that N. B. had two different
faults and the retrace lines are a fault
that is quite common and easily explained.
On the various early Rank horizontal output boards, vertical blanking
pulses were picked off the vertical
output and fed into one of the low level
video amplifier stages. This system
relied on the DC stability of the video
amplifiers since any drift altered the
black level of the picture and in some
circumstances allowed the appearance
of retrace lines.
In the C2205 model, vertical blanking was applied much later, at the video output stage. Because the blanking
was now added to whatever DC level
had already been established, variations in DC level made no difference.
However, a much more substantial
blanking pulse was required compared
to earlier systems. This was the purpose of PWC470.
It was a simple 2-stage amplifier
which was used to boost the vertical
blanking pulse amplitude. Unfortunately, it was also very critical as to
circuit values and in some circumstances it could turn itself into a very
effective multivibrator.
In this condition, it would produce
blanking pulses at three or four times
the correct rate, hence the black lines
on the screen – they were synthetic
blanking pulses manufactured by a
faulty PWC470.
An easy cure
A cure was ridiculously easy – just
replace any component on the board! I
usually replaced one of the transistors
but I have also solved the problem
by replacing one of the resistors or
capacitors. All that was needed to
create the fault was to upset the critical
balance of component values – and
as far as I could tell any component
could do this.
In N. B.’s case, someone had found
that pulling the plug was an easy way
to stop the black lines. Apparently
the retrace lines were less annoying.
I dunno; it takes all kinds!
Nice going J. L. – a most interesting
story and it makes a very important
technical point. But you won’t become a millionaire serviceman that
SC
way!
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