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SERVICEMAN'S LOG
The TV set that smoked
Warranty work is not always plain sailing,
due mainly to the over-reaction of some
customers when a near-new set fails. And in
this case the set didn’t help matters much by
first smoking and then working normally.
Mr and Mrs Clarke were not happy.
Their nearly new NEC FS-5185 TV
(MM-2 chassis PWC4034A) had thoroughly disgraced itself and had had the
audacity to not only stop working but
actually smoke in their living room.
Anyway, the set was under warranty
and though they would really have
preferred a new set, they had to have
it fixed straight away.
On these occasions, one has to be as
polite and understanding as possible,
whilst remaining aloof and firm as to
what can and cannot be done under
warranty.
One way to reduce the acrimony
is to attend to the problem as soon as
56 Silicon Chip
possible. The set was actually under
an extended warranty with a major
department store, for which we are
the service agents.
So it was with trepidation that I
removed the back and had a good
look around before connecting it up,
switching it on and standing back.
Well, what an anticlimax – the set
came on and performed perfectly.
What’s more, when I examined the
one-piece motherboard, I couldn’t
really determine any components that
had burnt out at all. A number of resistors had got a bit warm and some of
the white/clear glue the manufacturers
now use to hold wires and components
in place while they are soldered had
melted, but really there was nothing
to write home about.
I got onto the blower to report the
situation to the Clarkes. They were not
amused. Apparently “clouds of black
smoke had come out of the set” and,
what’s more, they were definitely not
going to have that set back their living
room unless it was fixed properly. I
went back and gloomily examined the
hapless set once again.
First, I checked the HT rail and it
was spot on 115V which is correct.
Next I attacked the circuit board; I
shook it, froze it and heated it and it
kept right on going with a really good
picture. I then put it aside on test and
left it for three days. It didn’t miss a
beat.
I phoned the Clarkes again – “are
you sure that the smoke came out of
this set? Was it perhaps the VCR or the
stereo system which are nearby? What
about the power point? Does anyone
in the family smoke?”
Fig.1: the power supply circuitry in the NEC FS-5185 TV set. IC602 (bottom, right) monitors the HT rail and drives optocoupler IC601 to derive a
feedback control signal for chopper transistor Q601.
Mrs Clarke assured me that none of
these had been the case.
I was perplexed but even after a
week the set was still working perfectly and so I returned it to the less-thanunderstanding Clarkes.
The set bounces
It was nearly a fortnight later that
the set bounced right back into the
workshop with the same complaint.
This time, fortunately, the fault was
genuine – the set really was dead.
It didn’t take long to establish that
the 130V ZD621 R-2M safety zener
diode was short circuit across the
main HT rail. And for this to fail, it
meant that the HT rail had risen above
130V. I replaced the diode and the set
once more came on perfectly with
the correct voltage. However, when I
replaced the cabinet back and put it
aside to test, the set was dead again.
Ah, ha, I thought. It must be a faulty
back! But no; actually the zener had
gone short circuit again.
Rather than risk yet another, I foolishly decided to run the set without
it and initially all was OK. However,
predictably, as soon as my back was
turned, the B+ rose dramatically and
wisps of smoke (but not a lot) came
from numerous areas. This time, the
line output transistor (Q502, 2SD2499)
had gone, along with a couple of electrolytic capacitors (C313, C622). D631
had also failed by going short circuit.
After restoring everything, the volt-
age was constantly high and the set
could only be run for a few seconds
before the damage would re-occur.
Because of this, I used a Variac and a
lamp to keep the voltage down while
I sussed the problem out.
My first suspects were IC602
(SE115N) and IC601 (a PC817 optocoupler) which are at the heart of the
voltage control feed
back network.
Basically, IC602 monitors the HT rail
derived from transformer T601 and
drives optocoupler IC601 which in
turn varies the drive to chopper transistor Q601 (2SD1710). I replaced both
ICs and the associated 22kΩ resistor
(R623) and resoldered everything in
sight, in case a dry joint was lurking
somewhere but to no avail.
In fact, the control feedback circuit
of IC602 and IC601 was responding
to voltage changes and I could see
the mark-space ratio change on the
collector of the 2SD1710 chopper
transistor (Q601) as the Variac was
altered. However, it refused to lock and
there was no control of the secondary
voltage rails.
Well, to cut a long story short, I was
removing and checking every component on the primary (“hot”) side of the
chopper transformer when I noticed
that the board had been slightly modified by the factory. Some of the PC
board tracks had been cut and a number of components had been mounted
on the print side of the board but the
circuit was still the same as published
by NEC.
A closer examination revealed that
some of the parts on the component
side around this area were rather
awkwardly mounted and so I decided
to remove these parts one by one for
checking. When I desoldered one leg
of R699 (120Ω, 1W), I noticed that
the other leg simply fell away from
the board.
In other words, one leg of this component had not gone right through the
hole but was actually just touching
the solder pod on the other side. The
resistor checked out OK so I reinstalled
it, this time making sure that both its
leads were correctly soldered.
And that solved the problem. This
time, when the Variac was wound up,
the mark space ratio on the collector
of Q601 became fixed when the HT
reached 115V and the power supply
was once again stable.
I soak tested the set for another week
whilst fending off the ever-persistent
Clarkes. I had, in fact, given them a
loan set when the problem re-emerged
but it is the nature of some customers
to be impatient.
The set was eventually returned but
the Clarkes remained totally unimpressed with my efforts to help them.
You win some and you lose some.
The Beovision gear
Bang and Olufsen, or B&O, are
among the “Rolls Royce” brands
when it comes to producing home
entertainment equipment and this is
reflected by their prices. Their products are beautiful to look at, even when
switched off, and their performance is
definitely up-market – all of which you
would expect for the prices demanded
for this luxury equipment.
B&O has been quite innovative
with some of their technology over
the years and often have neat little
features built into their equipment;
eg, wave your hand in front of the CD
player and the drawer will open! Their
Beolink system, which allows you to
have different selected music in every
room of your house, was one of the first
designs of this nature.
On more familiar turf, their TV
sets introduced automatic grey scale
adjustment years ago. And of course
they have an amazing ability to cram
electronics into very thin, unobtrusive cabinets with hardly any wires
protruding.
But I digress – I am beginning to
February 1998 57
Serviceman’s Log – continued
sound rather like one of their salespersons.
Mrs Smythe-Jones was from the old
country and lived in an exclusive suburb along with her pedigree Siamese
cats and of course, her Beovision Type
3854 2502 stereo TV, her Beocord
VHS91 type 4493 VCR and her Beolink
1000 hifi system. These approximately
10-year old items had been reinstalled
by B&O when she arrived some years
ago and now, because of their age,
they were beginning to show signs
of trouble.
How much is it?
Despite her obvious affluence (or
perhaps because of it), she immediately enquired about the price of servicing the equipment and wasn’t too
impressed with my answers. However,
she was in too deep with her investment in B&O and so, reluctantly, she
instructed me to go ahead and sort out
the problems.
Living in an exclusive suburb does
have its drawbacks, one of them being
poor TV reception due to the tall trees
that characterise the area. Installing
a separate high-performance UHF
antenna certainly improved things a
lot but her main complaint was poor
reception through the Beocord hifi
video recorder. In particular, she complained about the picture which had
a vertical line down the screen about
5cm from the lefthand side. And the
video tuner gave a snowy picture with
lots of patterning.
I decided that the only way to
handle all this was at the workshop,
as both the TV set and the VCR are
connected via a SCART lead. In addition, the remote control for the TV
set controlled the VCR and the stereo
hifi sound.
The first thing I did was track down
B&O’s service and spare parts section
in Melbourne and order in the service
manuals and instruction books which
cost $120. To give them their due,
they were nicely bound and thick.
Unfortunately, what ever they gained
in presentation they lacked in detailed
substance. For example, there was no
circuit description and no PC board
component layout diagram for the TV
set although there was for the VCR.
As it turned out, the latter was made
by Hitachi for B&O and is similar to a
1986 VT-860E(AU).
The B&O service manager was very
helpful and suggested various courses
of action, including fitting the various
service/modification kits.
I started with the Beocord VCR
which I connected to a different TV
and tuned in on approximately Ch37.
The patterning and snow was easily
fixed by replacing the electrolytic
capacitors (as was suggested) in the
inverter power supply on the VS
tuning board. The main culprit here
appears to be C715 (100µF, 25V) which
is on the +A17V rail to transformer
L701 (this rail comes from the power
regulator board).
Interestingly, the Beolink 1000 remote control would not operate the
VCR in a standalone configuration
–only when connected to the TV. However, when I used a generic programmable remote control (Quadrant Plus)
with the B&O numbers programmed
into it, the VCR worked.
I was about to install the service kit
(3375102 at $129 plus freight) when
a routine check on playback showed
the hifi sound to be intermittently
distorted and critical with tracking –
all indicative of worn heads (normal
mono sound and picture was OK). This
was the death knell for this VCR, as
the heads (8600097) cost a cool $600
trade price.
It was too hard to work out the
equivalent Hitachi part number and
the risk that it wouldn’t work properly
was too high. B&O advised us that the
unit was too old to repair anyway and
that we shouldn’t proceed.
The TV set
So it was on to the TV set, with the
problem described as “curly beads”
(the aforementioned speckled line
down the screen about 5cm from
the lefthand side). This is usually
due to para
sitic oscillation in the
line output stage and can be fixed in
most cases by fitting a ferrite bead or
two to the emitter of the line output
transistor.
B&O had not heard of this problem
in their sets before but suggested
we install their service kit 3390454,
which prevents failure of the line
output transistor (4TR11). An S2000A
was fitted here but a BU508 is shown
on the circuit diagram. The kit of 10
components cost $30 plus freight and
included a BU508A replacement transistor. A ferrite bead (FE1) was already
installed in the set.
It was all fairly straightforward
to fit, the only exception being PNP
transistor 4TR15. This was originally
a BC328-25 but the replacement was
a BC369 which has a different pin ar
rangement (bce instead of ebc).
Wrong leads
Fig.2: the line output stage in the Beovision 3854 2502 stereo TV set. B&O
now recommend a BC639 for TR15, instead of a BC328-25 as originally
specified.
58 Silicon Chip
You guessed it – in the melee of
installing this and soldering any suspicious-looking joints, I inadvertently
connected its collector and emitter
leads around the wrong way. When
give the board a good going over by
re-soldering any suspicious joints.
I also spent some time cleaning the
board and cleaned and lubricated
around the ultor cap.
A few days later, I received not one
but two BC328-28 PH27s instead of
the specified BC369 PH72. Anyway, I
fitted one of them and the picture was
back to normal. As an experiment, I
then refitted the original transistor
only to find that it too now worked
perfectly.
More questions than answers
I switched the set on afterwards, the
picture was perfect and the “curly
beads” were gone.
It was only when I was checking
my work and cleaning up afterwards
that I realised my mistake and quickly
corrected it, only to find that the fault
was back. I double-checked everything
to confirm that it was all as per the
instructions and even fitted another
BC369 I had in stock (without result)
before phoning B&O for clarification.
To cut a long story short, I was told
that I shouldn’t use a generic substitute
as all B&O components are carefully
selected for optimum performance – in
this case for its “slew switching rate”.
I was also told that they had done this
modification lots of times and that I
was extremely lucky I hadn’t blown
the line output transistor.
Suitably humbled, I delicately
asked if I could have a genuine B&O
selected BC369 (PH72) – to give it
its full title – to replace the original
BC328-25 PH27. The manager kindly agreed to send me out one free of
charge. In the meantime, I decided to
Well, despite fixing the problem,
this left more questions than answers.
First, which component or components fixed the fault? I replaced the
originals and undid any modifications
to try to find out but it made no difference. I now think that the problem
could have been caused either by a dry
joint, by sparking where the ultor cap
connects to the tube, or by sparking
at the CRT socket. In the process of
cleaning, soldering and lubricating
everything, I had inadvertently fixed
the problem.
I remain unconvinced of the necessity for highly graded components, as
the original line output transistor had
not failed in 10 years and it showed no
signs of stress, even with the collector
and emitter leads of 4TR15 reversed.
Besides, I was always told that a
well-designed circuit should operate
with any generic component.
Doesn’t the saying go something
like this? – “an engineer is someone
who can design something for five
bob that any damn fool can make for
a quid”.
I refitted the service kit again – after
all, it had been paid for – and put it
aside to soak test. We abandoned the
VCR and sold Mrs Smythe-Jones a
new Loewe-Opta hifi unit which did
everything and more than the original did except use the Beolink 1000
SC
remote control system.
February 1998 59
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