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SERVICEMAN'S LOG
How many symptoms from one fault?
I believe it was Henry Ford who made the profound (?) statement that “history is bunk”. But
someone else, whose name escapes me, made
the rather more realistic statement that “he who
ignores history will be made to relive history”.
What has all that to do with servicing TV sets? Well, it turned out to
be singularly appropriate in regard to
the story I’m about to relate, though
I doubt whether either of the afore
mentioned philosophers was thinking
of anything so trivial (to them) as TV
servicing.
It all started with a Teac CT-M515S
colour TV set, a 51cm model about
three years old and featuring stereo
sound, Teletext, and remote control.
The owner’s complaint was straight
forward enough; it was completely
dead. And so it appeared to be at
switch-on – no sound, no picture and
no light on the screen.
Until I advanced the brightness
control, that is. Then the real symptom became obvious. There was no
vertical deflection, the set displaying
the classic thin bright line across the
centre of the screen.
A routine voltage check immediately produced a vital clue – there was
no 12V rail. The 12V rail is derived
from pin 4 of the horizontal output
transformer (T402) via a 0.68Ω fusible resistor (R423), diode D404, a
6.2Ω 3W resistor (R422), zener diode
ZD402, and a 1000µF filter capacitor
(C421).
The immediate cause of the supply
rail failure was ZD402, which had
broken down and taken out R422. But
that was not all. The vertical output
IC (IC401 - TDA2653B) had also been
destroyed. Which had come first and
destroyed which? There were no clues
on this but I regarded it as of secondary
Fig.1: part of the horizontal output circuit in the Teac CT-M515S colour TV
set. A 12V rail is derived from pin 4 of the horizontal output transformer,
via 0.68Ω fusible resistor R423, diode D404, 6.2Ω resistor R422, zener
diode ZD402, and 1000µF filter capacitor C421.
40 Silicon Chip
importance anyway. More to the point,
replacing those three components
was all that was needed to get the set
working again.
And it worked very well. I gave it a
thorough once-over, made some minor
setting-up adjustments, let it run for
a couple of days, and then returned it
to the customer. And that should have
been the end of the story.
Here we go again
It wasn’t, of course. A month went
by and the set was back in the shop.
Well, that was bad enough but the
really nasty part was that it was the
same components which had failed.
Which meant that I had treated only
the symptoms, not the cause. And I
had to find the cause.
I replaced all the damaged components again (it was becoming a costly
exercise) and the set came back to
life. But of course I couldn’t leave it
at that; I had to find what caused all
this destruction.
In general terms, I suspected an
over-voltage condition of some kind,
either high amplitude short term,
or lower amplitude continuously. I
couldn’t do much about checking for
the former but at least I could check
the latter.
So I made a complete voltage check,
looking for any values which were
even marginally high. This achieved
nothing directly; all values were virtually spot on.
But it did help indirectly, even
though I did the right thing for –
initially – the wrong reason. While
making these checks, I paid particular
attention to the high tension rail. This
rail is derived from pin 5 of the switchmode transformer (T901) via D904 and
normally sits at 113V, as measured at
test point TPB+.
However, between diode D904
and TPB+ there is a network of three
transistors: Q907 which is directly in
the HT rail line, Q906 which controls
Q907, and Q905 which controls Q906.
I didn’t recognise this network immediately. I assumed it was a voltage
regulator and, on this basis, wondered
whether a fault here could have been
responsible.
I was clutching at straws but decided to check all three transistors. And I
struck oil! Q907 was short circuit. But
gratifying though this was, it didn’t
altogether make sense. If it had ceased
to function as a voltage regulator, why
did the rail still measure 113V? Why
hadn’t it gone high?
I took another look at the network
and realised my mistake. It wasn’t a
voltage regulator at all. Instead, it was
a switching network, used to switch
the set on and off via the remote control system.
In greater detail, the switchmode
supply runs continuously while ever
the mains supply is on. The remote
control switches Q905 which in turn
switches Q906 and ultimately Q907 in
the HT rail to turn the set on and off. In
addition, the remote control switches
various signal paths.
It would be no problem to replace
the transistor but would this bring
me any closer to the real problem?
Well, it did. Deep down in the brain
cells, something stirred. Mr Ford’s
disparaged history was proving to be
anything but “bunk”. Rather, a whole
lot of historical bits were coming
together. So much so that I began deriding myself for not realising sooner
what might be wrong.
I went straight to C909, a 47µF 25VW
electrolytic on the base of switching
transistor Q904 in the power supply,
and reefed it out. I replaced it with a
high temperature, higher voltage type
and modified the mounting somewhat
to keep it as clear as possible from
heat sources.
Been there, done that
So what was the connection? This
switchmode power supply is virtually
identical to one produced by Siemens
many years ago – almost back to the
beginning of colour TV in Australia
– and which has been used by many
manufacturers since then. It was used
in some early HMV receivers (C211,
C221 series, etc) and more recently
in the Fujitsu-General FT-1411 and
FT-2011 receivers and the Sanyo
CTP6626, among others.
And it was memories of the Fujitsu-General FT-1411 which stirred first.
It all happened many years ago and,
Fig.2: the switchmode power supply in the Teac CT-M515S. The HT rail
comes off pin 5 and goes to switching resistor Q907 at top right. C909 is
at lower left.
what with my memory cells being
somewhat sluggish these days, I had
completely forgotten it. But as I recall
it now, the complaint was that it could
not be switched off properly via the
remote control.
And I use the customer’s term
“properly” because, while there was
no picture, there was still a raster on
the screen; ie, full line structure but
no video. It looked a simple enough
problem initially. The setup was almost identical with that of the Teac – a
transistor (Q606) in the HT rail (109V),
controlled in turn by Q605 and Q608,
the latter fed from the remote control
system. And Q606 had gone short
circuit. (As an aside, Q606 was a type
2SC2335, which is the same type as
Q906 used in the Teac).
Anyway, the problem was easily
fixed – a new 2SC2335 and the set was
back to normal. The trouble was, the
set bounced. I thought it was just bad
luck the first time but when it bounced
again I knew I was in strife.
I won’t bore the reader with all the
details as to how I finally cracked it
but, as I recall, it was a combination of
good luck and some physical evidence.
The physical evidence was signs of
corrosion around two electrolytics
in the power supply, C607 and C620,
both 100µF/25V. They were connected
in series to give 50µF and fed the base
of the switching transistor, Q604, in
August 1996 41
was all that was needed to put the sets
back in operation and minimise the
recurrence of the fault.
Which is pretty much where we
came in. And, no Mr Ford, history isn’t
bunk; it’s a very good teacher.
The money-hungry customer
the same manner as C909 in the Teac.
I can’t explain the reason for the
series arrangement. If it was to increase
the voltage rating, it wasn’t a very good
effort; there was no resistor network
to equalise the voltage distribution.
Anyway, I substituted a 47µF capacitor
with a higher voltage rating and a high
temperature rating and that finally
solved that problem.
Then there was the Sanyo CTP6626
which uses an 80P chassis (or more
correctly, there were several sets with
the 80P chassis). And, once again, this
uses what is virtually a Siemens type
switchmode power supply. In fact,
this story goes back even further and
would have been my first encounter
with this particular fault.
In this case, however, the story of
one fault is essentially the story of
them all. Apart from minor variations
42 Silicon Chip
(some sets were intermittent), they all
produced the same symptoms from the
same fault. It was real beaut at the time
because I quickly learned to handle the
situation. But it did little to prepare me
for the variations on the theme which
occurred in the Fujitsu-General and
the Teac several years later.
In essence, the problem presented
itself as a destroyed horizontal output
transistor (Q451), caused by a dramatic
rise in the main HT rail due, in turn,
to the failure of capacitor C314. C314
was a 47µF electrolytic capacitor in
the power supply and fed the base of
the switching transistor (Q304). And,
in some cases, Q304 would also be
destroyed.
Replacing the faulty transistors
and substituting a high temperature
electrolytic, mounted as far away as
possible from any sources of heat,
My next story, as fate would have it,
is also about a Sanyo TV set: a fairly
old set, a model CCC-3000, a 34cm
“Cosmo” portable, using an 80P chassis and, yes, the same Siemens type
power supply. But the story is just
about as far removed from the power
supply problems as it could be.
The customer was a European
gentleman with only a limited grasp
of English. But his grasp of money
matters suffered no such limitation;
he was as sharp as they come. So this
story is nearly as much about customer
relations, charges and the eternal problem of quotes, as it is about technical
problems. Inevitably, of course, the
two are interwoven.
The basic problem was simple; the
set had been dropped. Not particularly
hard apparently – there was no obvious external damage – but enough to
put it out of action.
Right from the start, and simply
on the basis that the set had been
dropped, the gentleman wanted me to
quote him to repair it. As a basic rule,
I don’t quote for repairs and certainly
not on the basis of such vague information. I will try to assess a particular
situation, based on the best available
evidence, but at best this is a guest
imation. There must inevitably be a
number of “ifs” and “buts” included
in such an assessment.
As a colleague once put it, “you
don’t really know what a job is going
to cost until it’s finished – and it’s a
bit late then to quote for it.” An exaggeration? Perhaps, but there is lot of
truth in that too.
Anyway, I explained that could not
quote him for the job and set out some
of the reasons. I told him I charged
so much an hour for labour, plus the
cost of any components which had to
be replaced.
The best I could do was switch the
set on and try to assess how much
damage had been done and, therefore
what kind of cost might be involved.
And, as I pointed out to him, I didn’t
even know whether the picture tube
was still working.
This didn’t seem to make much
Fig.3: the switchmode power supply in the Sanyo 80P chassis. It’s similarity
to the Teac supply is evident, both being based on an early Siemens circuit.
impression but I switched the set on
anyway. The result was more promising than I had expected. The sound
came up immediately and, as the tube
warmed up, there was some signs of
life on the screen – a bright horizontal
line. Well, this meant that the tube was
intact, the power supply was working,
and the horizontal output stage was
working. In fact, most of the vital
parts were working except the vertical
output stage.
On this basis, I told him that I
thought the most likely fault was a
cracked board. I couldn’t say how
serious this might be. It might be
possible to repair it or, if it was too
badly damaged, the only alternative
would be to replace it – assuming a
replacement was available and the
cost could be justified.
My most favourable assessment,
therefore, was that it would involve
at least two hours work. And that assumed that no components had to be
replaced, which I felt was a fair bet.
That still wasn’t good enough; he
insisted that I open the set, on the
spot, determine the exact nature of the
damage, and give him a firm quote for
a repair. I was equally insistent that
this was out of the question and that
the situation was not negotiable; take
it or leave it.
He hummed and hawed about this
but we finally reached a compromise.
I agreed to quote him for two hours
labour. If the job was going to cost more
than that I was to contact him and give
him the choice of either going ahead
with the job or aborting it, in which
case there would be no charge.
I wasn’t particularly happy with this
arrangement but felt fairly confident
that I could work within it. So it was on
to the bench and off with the back. My
immediate impression was that it had
obviously spent most of its life near the
ocean, because there was considerable
corrosion on the metal parts.
But I was looking for cracks. There
was nothing immediately obvious and
I removed the main board for a closer
inspection. The high risk areas would
be near the horizontal transformer and
where the board is supported by the
cabinet.
And this was where I found it;
from the transformer to the edge of
the board. It was a very fine crack,
about 10cm long, and not at all easy
to see. In fact, I suspected that at least
some of the copper tracks were still
be functioning, though obviously not
very reliably.
Anyway, it looked like a fairly
straightforward job, despite the fact
that a number of tracks were broken.
There was a fair amount of work involved in cleaning the board of dust
August 1996 43
Serviceman’s Log – continued
and grime, plus the original green varnish, until I was back to bright copper.
Then it was simply a matter of flowing
solder over the breaks.
Sometimes, if a crack is bad enough,
I fit a wire bridge but I didn’t consider
it necessary in this case. In fact, the
end result was very satisfactory, both
visually and mechanically.
via a 6.8kΩ resistor (R410) to pin 15,
which is marked as 12.7V. And this
voltage was spot on.
There wasn’t much left to suspect,
except the IC itself. Had I destroyed
it in some way while making tests? I
hoped not but the only way to prove
the point was to replace it. I pulled it
out and, because of possible doubts
The big test
Unfortunately, when I switched
it on, the result wasn’t satisfactory
at all; the vertical deflection stage
was still not working. My first reaction was to suspect that a supply
rail had been lost, perhaps because
of a crack I had missed.
I pulled out the circuit diagram
and began checking all the rails
which, at least at their starting
points, appeared to be correct. So
I began tracing them. And, since
I don’t like running a set for long
periods with a fault like this, I
would switch it on briefly, check
a voltage, then switch off while I
lined up another check point.
Then suddenly, when I switch
ed the set on, the white line had
vanished. And not because I’d
cured the fault but because the set
was now completely dead. This
was a really revolting develop
ment; instead of finding the fault
I had seemingly created another
one.
I went over the board again,
looking for any missed cracks, but
drew a blank. There was still the
full 110V on the main HT rail from
the power supply but no secondary
voltages from the horizontal output
transformer. And the CRO confirmed
that there was no horizontal activity
of any kind; nothing at the output
stage (Q451) and nothing at the driver
stage (Q450).
Further checks revealed that the
voltage on Q450’s collector was high.
Instead of the indicated 64.7V, it was
sitting at the full rail value of 110V. It
was obviously turned off and the CRO
confirmed that it was not being driven
from pin 3 of IC401, which contains
both the horizontal and vertical oscillators.
So why wasn’t IC401 working? This
IC takes its voltage from the 110V rail
44 Silicon Chip
why were there no other symptoms
due to the cracks?
Without backtracking and identifying every broken track, I can only
guess. However, it is possible that
there were other symptoms which
were masked by the vertical failure.
There may have been no video or no
colour, for example.
And why did IC401 then suffer a
further failure? This may have been
due to my test routine but I don’t
think so. Closer examination of the IC
revealed quite a lot of corrosion on
the pins, particularly where they
enter the plastic body. The pins
were firm enough mechanically
but it’s possible that some corro
sion had made its way inside the
body.
As a check, I refitted the original
IC in the socket and gave it a bit
of a bashing for good measure.
But it was completely dead. As I
say, these are questions for which
I have no answer.
Ungrateful customer
about my diagnosis, fitted a socket to
the board and plugged in a replacement IC.
And that fixed it. Not only did the
set come back to life when switched
on but the vertical scan had also been
restored and we had a full picture on
the screen.
Unanswered questions
All of which leaves a lot of unanswered questions. If the vertical failure
was due to a fault in IC401, rather
than the crack in the board, what had
caused it to fail?
Was the set running when it was
dropped and was there a voltage surge
when the copper tracks fractured? And
More to the point, in practical
terms, the job had now gone outside the terms of the cost agreement. As well as the two-hour
labour charge – which had been
exceeded but which I would carry
– the customer was now up for an
extra $20 for the IC.
Sticking to the agreement, I
rang him and advised him of the
situation. More aggro; he didn’t
want to go beyond the origi
nal
labour charge. I refused to budge.
I pointed out that I had kept my
part of the agreement and it was
up to him to keep his. And I added
the clincher – if he didn’t want to
pay the extra $20 I would put the old
IC back in the set and he could come
and collect it, no charge.
That did it. Knowing that I had the
set running on the bench but that I
could easily disable it was too much.
He agreed to pay the extra charge,
albeit reluctantly.
It was over a week before he turned
up and during that time the set never
faltered. But would you believe it,
when he came to collect it, he tried to
beat me down again.
I didn’t even argue with him; I made
it clear it was take it or leave it. He
took it – and I hope I don’t see it or
him again. Some customers are really
SC
not worth the trouble.
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