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SERVICEMAN'S LOG
Those summertime lightning blues
Every summer, after the hot humid days,
there follows the inevitable storm and
whether it is the El Nino effect or not, these
storms seem to be getting more violent. Or is
it that electronic devices are becoming more
vulnerable?
One would have thought that, after
all these years, lightning would be
better understood but it doesn’t appear
to be. I watched a program recently
that went on to show that lightning
does indeed strike the same place
twice – sometimes even more. And
with older TV sets, the high humidity
causes insulation breakdown in the
EHT transformers and focus circuits.
The day after a storm is always hec
tic, with many confused people on the
phone. Some are surprised that their
electronic equipment isn’t working
and fail to make any connection with
mother nature’s major disturbance the
night before. Most of them just want
their gear fixed immediately but of
course, that’s not always possible.
For my part, I organise a stack of loan
sets so that people don’t become too
impatient.
I never cease to be amazed by the
path lightning takes through a set –
it never seems to be logical or in a
straight line to earth. Sometimes the
damage can be particularly serious on
a set that did not appear to be near the
strike, while others right next to the
hit remain totally unaffected.
The fireball
One such case occurred when Mrs
Williams anxiously asked me to call.
She was certain that the whole lot
would be a complete write-off. When
I arrived at her house, she told me that
she was walking into the lounge room
when the lightning struck, hitting the
garage roof only a few metres outside
the front door.
28 Silicon Chip
And, according to the lady, a
fireball had come through the front
door and moved horizontally until
it hit the TV set. I must admit that I
was a bit sceptical about the fireball
bit but, even so, I half-expected the
AWA 2C6346 to be burnt to a frazzle.
I removed the back at arm’s length to
avoid getting soot on my clothes but,
to my surprise, it was exceptionally
clean for an 11-year old set.
I poked about, removed the chassis
and looked everywhere. There was no
sign of any damage and the fuse was
intact. In the end, there was nothing
for it but to switch it on. Fearing the
worst, I hit the power switch and
stood well-back but it was a total
anticlimax. The set came on perfectly
except that it gave a snowy picture,
which was fixed as soon as I recon
nected the antenna.
I then checked the VCR, a Panason
ic NV-G16A. Neither the controls on
the unit, nor on the remote control,
were working. I switched it off while
Mrs Williams brought in a cup of tea
and after 15 minutes I switched it
back on again.
The controls were now working
perfectly, as indeed was the remote
control. The one explanation appears
to be that a lightning surge can tem
porarily disable a microprocessor, by
applying false voltages to a wide range
of pins. And if one is lucky, it will
restore itself after a few on/off cycles.
Either that or there is a subtle in
termittent lurking in it somewhere.
In either case, there wasn’t much I
could do about it then. If, or when, it
reappears it will be time to try again.
So Mrs Williams was exceptionally
lucky; the only thing that had failed
completely was the Foxtel STV-100
cable decoder, which was Telstra’s
responsibility.
A weak effort
The next customer was lucky too,
as mother nature’s effort here was also
a pretty weak one. This time, the set
was an Italian-made Fujitsu General,
model FGS211 (BS950 chassis). This
too was dead and I soon discovered
that the mains fuse (F451) had blown.
There was no visible damage and
the cause turned out to be switching
transistor T401 (BU508A), which was
shorted. The bridge rectifier hadn’t
been damaged, as I would have ex
pected.
There was, however, one weird side
effect. The sound was fractionally low
and this turned out to be transistor
TR702 (BC547C) in the audio preamp
circuit. This transistor had become
leaky and so that problem was easily
fixed.
No lightning
The next problem was not quite so
easy. There are some customers who
insist that their equipment was dam
aged during a storm but subsequent
investigation proves that this was not
the real cause.
In this case, the customer brought
in a VCR that was now showing fine
horizontal interference lines when
playing tapes. Initially, it looked to
me as though an electro had dried out
in the power supply, thereby causing
ripple in the power supply to the
video head preamplifier. As it turned
out, the true cause was rather sneaky.
Underneath a small metal can,
soldered as a component screen onto
the printed board, was some of the no
torious brown goo. It was completely
concealed and it had corroded and
shorted the tracks underneath a drop
out compensation circuit. Removing
it and cleaning the board restored the
picture, so the problem was hardly
due to a lightning strike!
The 76cm Toshiba
There was no doubt about the next
job. Mr Johns owns a 1989 76cm
Toshiba (model 329P8A) and lives on
the first floor of a block of units. This
is a huge set and weighs in at around
50kg. Faced with this, I elected to go
to the mountain.
The fault was described as a
horizontal line across the screen. I
removed the back and quickly dis
covered that R327, a fusible 6.2Ω
1W resistor feeding 27V to pin 7
of the vertical output stage (IC303,
AN5521), was open circuit. Access
to most of this circuit is not easy but
nevertheless, I decided to replace the
resistor and IC together to save time.
When I switched it back on, it only
took a second or so before the resistor
burnt out again. Naturally, it was the
last resistor of that type in my toolbox.
This time, I looked more carefully at
the set and soon noticed that IC361
(TDA8145) on the U905B DPC-2
board was very hot. In addition, the
PC board was dark all around it. I
decided to leave the set where it was
and lend Mr Johns a portable until I
could order in the necessary parts.
A few days later I returned and re
placed IC361 and resistor R327. This
time the resistor lasted a few seconds
longer before failing and the IC was
still getting extremely hot. It was then
that I spied what had to be the real
culprit: coil L464 on board U905C
DPC-1. This coil (TLN3061) had be
come so hot it had melted its plastic
insulation and was burnt black.
Well, I knew I didn’t have a replace
ment coil and it might be a long wait
for another one. Because the part was
in the horizontal deflection correction
circuit I decided to try it without the
coil in circuit to see if I had some sort
of vertical deflection.
This turned out to be a colossal
mistake. Initially, at switch-on there
were a few noises I hadn’t heard be
fore, then a very loud one. After that,
there was silence and all life ceased
to exist inside the set.
I had blown the line output transis
tor (Q404) and vapourised the 145V
rail which feeds Q404’s collector via
resistor R444 (0.82Ω, 2W) and pin 2
of the horizontal output transformer.
Again, the thought of negotiating a
76cm TV set down a flight of stairs,
lifting it into a vehicle big enough to
carry it and then getting it into the
workshop was too horrible to con
template. I left the set where it was
and went home.
The next day I ordered the parts
which arrived almost immediately.
Third time lucky? No, you guessed
it; there was more trouble to come.
Despite replacing all the parts, the
line output transistor (Q404) got very
hot and failed yet again. There had to
be another short nearby and after a
while I found that diode D440 (ERC0615) in Q404’s collector circuit was
shorted. I didn’t have one of these to
hand but I did have an FR307, another
line output transistor and a 0.82Ω
resistor for R444.
Pincushion distortion
This time, success at last! The pic
ture was restored and I checked the
HT rail, refitted the back and recon
nected the external speaker and AV
connections. With it all back together
again I was enjoying a nice cup of tea,
when Mr Johns and I both noticed
that the tennis wasn’t quite right.
No, I don’t mean that someone was
chucking a wobbly; instead, I mean
March 1998 29
Serviceman’s Log – continued
lightning strikes. One way of quoting
for such sets, where damage appears
to be severe, is to price a replace
ment chassis rather than fixing it to
component level. Sometimes it is a
safer proposition but in some cases,
the cost of a replacement chassis is
simply too high to make it econom
ically viable and the set either has
to be repaired at component level or
written off.
The unluckiest victim
that the picture was all wrong in the
east/west direction, with noticeable
pincushion distortion.
“Oh bother!” I said (rather untruth
fully) as I removed the back again.
I quickly found the three east/west
controls – R358 width, R356 keystone
(trapezoid) and R359 DPC (pincush
ion) – and marked their positions
with a felt-tip pen before twiddling
each one in turn. This had no effect;
all three appeared to be totally inop
erative.
This seemed to suggest that IC361
had failed again; either that or I was
indeed going to have to manhandle it
back to the workshop so that I could
use the CRO to trace the waveforms
around this circuit. Please – not that!
A stroke of luck
Well, it was at this stage that my
luck turned for the better. Earlier
in the piece, I had sensibly had the
foresight to fit a socket for IC361. This
30 Silicon Chip
meant that I could easily replace it
without having to first unsolder the
module that it was on (U905B) in or
der to gain access to the copper side.
As it turned out, the IC had failed
again and the new one finally fixed
the last problem.
But why had IC361 failed the sec
ond time? The answer is that it was
almost certainly a byproduct of L464’s
failure and/or any of the other failures
that occurred after replacing IC361
the first time.
I realigned the three controls using
a crosshatch generator and settled
down to a replacement cup of tea
before replacing the back. I then had
another cuppa to make sure that it
was still working – while watching
the tennis – and I would probably
have stayed for a third if Mr Johns
hadn’t made some subtle comment
about how busy I must be.
The above story just goes to empha
sise the good and bad luck aspects of
The unluckiest of the lightning
victims was undoubtedly Mr Evans,
with Yours Truly suffering collateral
damage on the side. He was watching
the spectacle that night when there
was a strike a few streets away. The
lights and power flickered and his
Fujitsu General TV set, an FG2012
with a Goldstar PC-04A chassis, went
completely dead.
When I called, I removed the back,
expecting just the fuse to have gone.
However, not only was fuse F581 in
tact but the switchmode power supply
was actually working. But that was as
far as it went and I noticed that some
of the printed wiring had evaporated.
That was enough for me; it was back
to the workshop.
Later, I found that the missing print
ed wiring involved the sub-switch
on the mains power switch, SW851.
The latter is a pushbutton toggle type
which opens or closes the two con
tacts directly in the incoming 240V
power line to the power supply. It also
performs several functions via the
sub-switch (P801). These functions
involve the various rails out of the
power supply and also include the
control of relay RL801 in the 112V
rail from pin 17.
The vaporised sub-switch wiring
led to four NPN transistors – Q704,
Q705, Q713 and Q714 – in the
standby/on switching circuit and
this had also been destroyed. All
this damage was duly repaired but
this only allowed the standby LED
LD701 to turn on. By this stage,
there should have been 112V on
the collector of the horizontal out
put transistor (Q402), applied via
relay RL801. However, the relay was
not being activated.
This set provides a test function
here although this is not shown on
the circuit. A 2-pin male connector,
P702, is connected between pin 41
of the main IC (IC701) and chassis
(pin 41 drives the damaged NPN
transistor circuit mentioned above).
By bridging these pins, the relay could
be switched on via Q801S and Q713.
Unfortunately, the horizontal output
stage started but then died.
My next stop was Q401, the hori
zontal driver transistor. A waveform
check at its collector indicated that
it was delivering only low ampli
tude pulses. The collector voltage is
derived from pin 6 of the EHT trans
former (T801) which should be at
28V. After following a number of false
leads, I finished up at D401 which
connects between the 18V rail from
the switchmode power supply and the
28V rail. This diode was short circuit.
At last, I seemed to be getting
somewhere, as a snowy raster now
appeared. Unfortunately, no control
functions were available at the front
panel or via the remote control and
I still had no sound. On the other
hand, I now had clear confirmation
that IC701 was faulty; pin 41 still
needed to be bypassed, whereas it
should have been doing the job itself.
So, regardless of any other faults in
the set, this IC it had to be replaced.
In fact, I finished up replacing two
IC701s. No, I didn’t replace the same
one twice; there are two separate ICs
marked IC701 on the circuit – the
original 42-pin PCA84C640P/030
device and an 8-pin PCD8572 device
connected to it. But just to confuse
matters, the 8-pin device in the set
was marked X24C02P and designated
IC702.
Anyway, I now had a picture but
there was still no sound and the au
tomatic tuning wouldn’t lock in. It
took the replacement of IC401 to fix
most of this. This IC is a TDA1940
and, among other things, provides
sync separation, a burst generator and
vertical and horizontal sync outputs.
Next on the replacement list was
IC601 (TBA-120T) which provides
the sound IF functions. Changing
this restored the sound. I now had a
working set – well, sort of.
No remote control
The one thing left not working was
the remote control. This involves
a 3-terminal package (PA1) which
contains an infrared sensor and a
preamp
lifier. One terminal goes to
chassis, one is fed from a 5.6V source
and the other delivers pulses to pin
35 of IC701.
At least, that is what should hap
pen but no pulses could be seen on
the CRO.
I removed the PA1 and disassem
bled it. It was fitted with a GL3274
IC (IC1) which is not obtainable.
However, I was able to purchase the
entire package for $32 plus tax.
This fitted, the pulses could now be
seen at pin 35 but there was still no
remote control action. I checked the
5V rail for ripple and replaced ZD701,
a 5.6V zener, as it was down to 4V but I
was getting nowhere. I confirmed that
the remote controller was working
properly and transmitting the correct
data patterns, using another TV set.
And this is where the exercise
came to an abrupt halt. I was forced
to the conclusion that the faulty pre
amplifier (IC1) had damaged the new
microprocessor (IC701), this probably
involving the internal circuitry asso
ciated with pin 35. And this meant
that the microprocessor would have
to be replaced a second time to restore
the remote control function.
But enough was enough. This IC is
an expensive item and is time-con
suming to fit. And even if I did fit it, I
could not be 100% sure that that was
the only fault still involved.
So I had to call it quits. It was a
difficult decision, con
sidering the
amount of time, money and frustra
tion that had been expended but I
had to make it. I told the owner that
I couldn’t economically repair the
set and advised him to settle with his
insurance company. I would make no
charge, of course; the loss was mine.
By mutual agreement I acquired the
set which was working but without
the remote control function. I hoped
that I might be able to salvage it in
the long term and recoup some of my
losses but right now, it was a write-off.
In fact, this is a classic example of
how a repair can get out of hand; the
time taken to diagnose and repair each
successive stage eventually exceed
ed the value of this 1992 48cm TV
set. Sometimes, it is hard to decide
where to stop; the next component
replaced may be the last one needed
to complete the job. And it is virtually
impossible to estimate the total cost
of the repair without actually making
it. Talk about a catch 22 situation!
It’s not the end
End of story? Well, I thought so
when I put the set aside a few weeks
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March 1998 31
ago and then, more recently, when I
had finished writing the above notes which I have left exactly as I originally
wrote them – ready to go to the Editor.
In fact, it was only the intervention
of a weekend which held them up.
Boots and all
And then, quite by chance, a similar
model set came in for a minor service.
I was sorely tempted – could I solve
the mystery once for all? It wouldn’t
cost anything, except time, so I
jumped in, boots and all. I “borrowed”
IC701 from the new arrival and fitted
it to the written off set.
The moment of truth had arrived;
I switched it on and tried the re
mote control. It still didn’t work –
everything was the same as before
with normal pulses into pin 35 of
IC701 but with no response. Obvi
32 Silicon Chip
ously, the fault wasn’t in the IC as I
had thought.
But where was it? The preamp had
been replaced and was generating
normal pulses; or was it? More to the
point, did I have the correct preamp?
I began to recall some discussion over
the part number. Fujitsu’s spare parts
division had been most helpful and
had quoted me for a type 106-042A.
I noted at the time that the unit in
the chassis was a 106-042B and this
was also shown in the manual. I did
draw attention to this difference but I
was assured that the “A” version was
a direct replacement. Nothing unusu
al about that; it happens all the time.
But now I was more than a little
suspicious. I went back to Fujitsu
and began delving a little deeper. In
fact, it didn’t take long to confirm
my suspicions; the two units were
not interchangeable. They both did
the same kind of job and generated
similar – but not identical – pulses.
In greater detail, the pulses from
the type “A” preamp are inverted
compared to those from the type “B”
unit. That’s fine for an IC designed
to accept that type of pulse train but
not for IC701.
Fujitsu were most apologetic and
promised to send a re
p lacement
immediately. But I couldn’t wait. I
“pinched” the preamp from the set
on the service bench and fitted it in
my set.
And bingo, it worked – at long last
the set was complete and working
normally. It’s hard to describe one’s
feelings in situations like this. After
all the hard work, there was finally a
happy ending and I would be able to
SC
recoup some of my losses.
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