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SERVICEMAN'S LOG
Most customers are reasonable
Servicing produces a wide variety of customer
personalities. Most are easy to get along with
and some are even apologetic for the “trouble
they are causing”. But at the other extreme is
the odd one who is aggressive right from the
start; convinced that all servicemen are rip-off
merchants.
My first story involves a typical customer type; easy to get along with and
prepared to pay what was necessary
to solve any awkward problems. The
set was a Masuda MGV28AV, bought
from the now defunct Brashs chain
of shops.
The Masuda was a Chinese-made
set and I explained to Mr Bull that
there could be problems obtaining
parts or service information. However, he was eager for me to try; as I
said, he was easy to get on with and
prepared to pay.
The problem was a very annoying
intermittent brightness variation – it
sometimes became brighter when the
set got hot. Well, the first thing was
to confirm that the set was exhibiting
this fault and so I put it to one side
where I could monitor the picture.
Nothing happened for the first two
days but on the third day, towards
closing time, the fault began to show,
the picture gradually becoming
brighter and brighter. I couldn’t do
anything about it just then, although
I did remove the back and set it up
on the workbench so that it would be
ready for me to tackle the next day.
The next morning, while waiting
for it to misbehave again, I tried accelerating matters by covering it with
a blanket. And I rummaged among my
circuits to see if I had a diagram that
might match. As luck would have it,
I found a circuit for a 1993 Teac CTM715B, which is very similar.
Where does one start? I needed
to make measurements consistent
with the symptoms so I started by
measuring the screen voltage to the
picture tube (pin 7) to see if it varied
when the fault occurred. When it did
occur some hours later, the voltage
was rock steady, so I wasn’t looking
at an EHT fault.
Eventually, I determined that it was
some kind of video fault, because the
tube cathode voltages were drifting
lower when the problem occurred. I
even went so far as to trace this drift
back to IC304, a TDA3504 which is
well known for causing problems
(usually resulting in loss of picture).
This was encouraging but the job was
still proving to be extremely frustrating because it took so long for the
fault to show.
Calculated gamble
As a result, I took a calculated gamble and plumped for replacing IC304.
Unfortunately, it wasn’t meant to be,
so I spent yet another day leaving a
meter connected to pin 17 of IC304,
the brightness control input. This was
normally at about 2V but under fault
conditions it was all over the place.
Where to from here? I followed the
beam limiting signal path from pin
17 via R355 (56kΩ) to R433 (150kΩ),
then to the 143V rail and R403, C425
& D406, but these all measured OK.
I also checked D302 which links the
contrast DC tracking, before going on
to check R322 and transistor Q302.
Once again, I drew a blank.
That left the brightness control
circuit itself which comes out of pin
54 Silicon Chip
Items Covered This Month
• Masuda MGV28AV TV set.
• Blaupunkt IS70-33VCT TV set
• Mitsubishi CT-29ATS(A)TY TV set
3 of microprocessor IC601 and also
involves pin 25, the mute line, which
controls transistor Q302.
I wasn’t getting anywhere – I knew I
was in the right area but I couldn’t isolate the exact cause. Finally, I decided
to check the sub-brightness control
circuit involving VR301 but initially
couldn’t find its location. The reason
was that it was, rather ridiculously,
situated under a large resistor which
in this instance had been bent down
so that it was literally touching the
plastic former of the control.
Naturally, the heat from the resistor had distorted the control former,
resulting in its function being intermittent when hot. A new control
fixed the problem completely and
I also relocated the resistor so that
it wouldn’t happen again. The only
tricky part now was telling Mr Bull
how much all this cost – plus the GST!
But he didn’t baulk.
Crook Blaupunkt
My next customer, Bill Strong, is
another reasonable bloke; not the sort
to whinge for no reason. He brought
in his Blaupunkt IS70-33VCT complaining about the sound – or lack of
it. He was very apologetic in admitting
it was extremely intermittent.
He wasn’t wrong. I put it on the
soak bench with the sound on low
while I hunted up a circuit. This set
used an FM310.32 chassis with part
No. 7663 700. The nearest I had was
for IS70-31VT with part No. 7660 800
but it would have to do.
The fault didn’t occur for the first
couple of days but did on the third
day and continued to give trouble.
Unfortunately, I couldn’t do anything
about it because I had my hands full
with other more pressing jobs.
On the fourth day, I transferred the
set to the main bench and took the
back off. But much to my frustration,
it worked perfectly all day. By the
fifth day, I was losing patience with
the set – I needed something I could
work with but it refused to play up. In
the end, I switched it off and pulled
the chassis out, looking for bad connections and faulty joints but could
find none.
I switched it back on and tapped
the chassis with a screwdriver handle, then tried heating and freezing
it. Nothing I did made any difference
at all. Fed up, I decided to put it back
on the soak bench and switched the
set off to do this. But – you’ve guessed
it – as soon as it was back on the soak
bench, it started playing up again.
By now, I was beginning to notice
that, at times, the fault would occur almost from the moment I switched it on
until I switched it off. At other times,
when I switched it on, it would come
good and stay good until I switched
it off. Bells were beginning to ring.
Perhaps it was the way I switched
it on and off. I was using the master
switch on the soak bench but perhaps
it was the set’s switch itself?
I tried switching it on and off many
times and it did seem that there was
something about the switch that was
causing the problem. But what was
it and more importantly, why did it
only affect the sound?
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November 2000 55
I removed the switch (S601) and
checked the contacts. The main AC
power contacts were fine but the
standby or “Temp contact” (as Blau
punkt calls it) was intermittently
sticking on. This momentary switch
controls T801 (BC548C) and the U
WISC line P1.0 to pin 15 of the microprocessor I811 and, if left permanent
ly on, mutes the sound!
A new switch fixed the problem
completely.
An overbearing customer
And now for a change of scene.
Mrs Ruddock did not strike me as a
reasonable type and the screaming
and demanding children she brought
with her only made matters worse.
Anyway, she arrived with her Mit
subishi TV set in the back of the station wagon. She said that it wouldn’t
56 Silicon Chip
come on and asked if I could fix it.
I said that I was sure I could – not
realising that she meant immediately.
But having lifted the 68cm 46kg TV
set into the workshop, all by myself, I
wasn’t in the mood to return it to the
car when she made that point obvious.
And she reinforced this idea by closely following me into the workshop
and standing there expectantly.
Somehow or other, I had allowed
myself to be painted into a corner –
almost literally. For lots of reasons
(including safety), I regard it as an
unwritten rule that clients do not to
come into the workshop. Yet here I
was plugging the set in and switching
it on. And before I knew it, I had the
back off and was making measurements.
This Mrs Ruddock had powers
beyond my understanding. Indeed,
I felt as though I was part of some
Greek mythology, where I was being
controlled by one of the Gorgons –
probably Medusa. I even tried to avoid
Mrs Ruddock’s gaze; after all, I didn’t
want to be turned to stone!
Anyway, this was a 1993 Mitsubishi
CT-29ATS(A)TY with an ATMT691
chassis. This set employs a lot of advanced features and is a real bells and
whistles job. It even had a motorised
swivel stand so one could rotate it by
remote control.
Apparently, the fault had been
intermittent but had pro
gressively
become worse until now it was completely dead. I had never come across
this model before and didn’t have a
circuit. However, I did have a few
notes from a trade meeting I attended,
at which this type of dual switchmode
power supply had been discussed
although not in great detail.
I could measure voltages all over
the place and all the fuses were OK,
so I knew this wasn’t going to be easy.
But Mrs Ruddock was still pressurising me to fix it now.
I located the standby switch transistor (Q9132) and was trying to measure
the voltage on its base when suddenly
the set came on. At the time, I didn’t
have a clue why and could only conclude that my shaking hands had accidentally shorted something. What’s
more, the set now came on perfectly
each time it was switched on, with
all functions working (including the
power/on/off/standby). And nothing
I could do would recreate the fault.
Mrs Ruddock was initially pleased
that it was working but wanted more
– basically, a lifetime guarantee was
the kind of thing she had in mind.
However, her slight sign of pleasure
released the psychological hold she
had over me; the image of Medusa
instantly vanished and she was now
just plain Mrs Ruddock. The spell
had been broken and I took full advantage of it.
Emboldened, I told her that whatever I had done could only be considered temporary and that, sooner
or later, the set would give trouble
again. I also told her that the only
way to fix the problem was to leave
the set on test while I acquired the
correct service manual (which costs
$60), so that the real fault could be
tracked down.
She immediately tried to reassert
control but failed. I had won; well,
sort of. In any event, I was happy
to replace the back of the set
and put it back into her car.
But I wouldn’t budge from
my position and I didn’t
charge her.
That was the last I heard of
the set until some nine months
later, when the set suddenly
reappeared with the same
fault. The lady’s attitude had
improved a little. She said that the
fault had recurred within a few days
(just as I had predicted) and so she
had taken it to a Mitsubishi agent. She
also complained that they had kept it
for eight weeks before they fixed it.
I am also a Mitsubishi agent but I
resisted the temptation to ask her why
she had taken it elsewhere. Nor did I
ask her why she hadn’t taken the set
back there now. But I insisted on my
basic ground rules – I must have a
circuit, time and space!
This agreed to, I proceeded to connect the set and see what was cooking.
Well, to begin with, the set came on
straight away with severe hum and
picture distortion and with the picture
jumping. Mrs Ruddock thought I was
splitting hairs when I mentioned these
faults, pointing out that it came good
after five minutes.
I then asked her what the other
Mitsubishi agent had done. Their bill
was produced, which showed they
had changed micropro
cessor IC701
(M50436-566SP) but, in fact, they had
done more. I discovered this when
checking the -30V rail on the power
supply – capacitors C9F1 and C9E9
had both been changed.
My insistence on getting the correct
service manual had paid off but it was
irritating to find different voltages
marked along the same supply rail.
This makes it difficult to be sure of
the correct value. In the end, I found
that the 12V rail was significantly
low, with the CRO showing significant
ripple on it. The culprit turned out to
be C9E5, a 470µF 25VW electrolytic
that was leaking badly.
Because I had to take the board
out to change this capacitor, I took
the opportunity to examine the rest
of them. C9E1, another 470µF 25V
electrolytic was also leaking badly
and I could see that most of the other
electrolytics were in poor condition.
Before changing all 25 or so, I thought
that I would confirm that the two I
had just replaced were the significant
ones. I was gratified to see that they
were, the set coming on immediately
with picture and sound.
I then set about replacing all the
remaining capacitors and cleaning
up the damage caused by the leaking
electrolyte from the C9E1 and C9E5.
These had even corroded the heatsink
a few centimetres away.
Finally, when it was ready, I replaced the board and checked the
six main voltage rails (130V, 33V,
15V, 12V, 9V and -30V), all of which
were OK. With soak testing I had the
set turned around within one week
but received only moderate praise
from Mrs Ruddock. I guess there is
no pleasing some people but maybe
she learnt something. Who knows, she
might even call on me the next time
SC
something fails!
November 2000 57
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