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SERVICEMAN'S LOG
History, symptoms & observations
These are the three basic rules behind any
approach to a service job – whether we are
conscious of them or not. But interrogating a
customer is not always easy. Most are helpful, but a few adopt a defiant “you-find-out”
attitude. And some are so vague and inept
that they are more hindrance than help.
My first story was a typical example
of the last point, although I didn’t
realise this immediately. Instead, I
took the customer’s observations at
face value.
Mrs Edwards needed a house call
for her TV set, since it was a 95cm
(37-inch) model that was too big to
bring in. And to help make the house
call worthwhile, she also had another
TV set which needed fixing. Both sets
had sound faults.
The larger set turned out to be a
1992 Grundig ST 95-775/9 PIP/TOP
with a CUC7890 chassis. The other
was a 1987 Philips KS786 with a
KL9A-3 chassis. But how hard can
a sound fault be? If nothing else, I
could fix the Philips and bail out of
the ageing Grundig, so I agreed to go.
The lady had switched both sets on
before I arrived and neither showed
any problem at all. I was somewhat
miffed at this but she insisted I stay
until the faults occurred. Fortunately,
the Philips didn’t take long – there
was a crackling coming from the
loudspeaker and when I removed the
back, I could see the EHT had been
sparking from a very dirty ultor cap.
I removed and cleaned the EHT lead
and made sure it was making good
contact before sealing it with silicone.
A piece of cake, I thought – now for
the Grundig which was running in
another room. This, too, was begin
ning to play up with slightly distorted
crackling but only in the left channel.
Using the remote control and se
lecting the audio menu, I moved the
60 Silicon Chip
stereo balance to the right and the
problem stopped. I told Mrs Edwards
that if she wanted this fixed, she
would have to wait while I ordered
the circuit manual. I explained about
the balance control and she seemed
quite happy with this.
The service manual duly arrived a
fortnight later and I phoned to make
an appointment. However, Mrs Ed
wards said that the set was now work
ing OK and that my services were no
longer needed. Such is life – I filed the
manual and moved on to other jobs.
It didn’t last, of course. Mrs Ed
wards was back on the phone three
months later, saying the Grundig was
“still playing up”. Patiently I asked
her to be more specific. Well, she
said, the sound was distorted in both
speakers, as before!
Because I now had the manual
and the audio output ICs in stock,
I decided to give it another go. But
I was more than a little concerned
about how vague Mrs Edwards was
with some of her comments.
When I arrived and checked the set
out, the distortion was still only in
the left channel – it was just that the
Items Covered This Month
• Philips KS786 TV set.
• Grundig ST 95-775/9 TV set.
• Panasonic NN-C855B microwave oven.
• Akai CT2869AUS TV set.
balance control had been returned to
the centre. Initially, I decided to re
place the left channel audio output IC
(IC4200, TDA2502H) and solder any
suspect joints I came across to see if
that fixed the problem.
The first obstacle I had to overcome
was the need to swing the TV set away
from the wall. Unfortunately, it was
sitting on a stand that didn’t have
castors, weighed about 100kg and had
to be slid across thick shag-pile car
pet. However, I eventually managed
to achieve this and remove the back.
The next problem was to figure out
how to remove the sound output mod
ule, which was held in with a series
of ingenious plastic clips (the service
manual doesn’t mentioned these).
After a struggle, I finally managed to
remove the module and replace the
left-channel IC. I then put it all back
together again and switched on but it
was still distorted in the left channel.
I then did what I should have done
when I first arrived and that was to
play around with the accessories on
this rather complicated TV set. The
problem was trying to familiarise my
self with such a complex set without
altering any settings that I might not
be able to restore.
I checked whether the TV system
had been correctly set as B/G for
Australia, which it was, but when I
selected the video via the AV (Audio/
Video output), I found that the sound
was fine in both channels. This proved
the problem wasn’t in the audio am
plifier stages at all.
Next, I tried to determine wheth
er the fault was the same in mono
transmission as well as stereo but
I couldn’t get any stereo reception!
The stereo indicators on the display
and in the menus wouldn’t alter at
all. The problem seemed to be in the
stereo decoder which was, as far as I
could see, part of the multi IF module.
By now, I had spent a lot of time on
this set without achieving anything,
so I decided I would go back to the
workshop and have a good think. If
only Mrs Edwards had been a little
more observant and more detailed
in her description of the fault – after
all they watch the set all the time.
And if only the set wasn’t so big and
heavy, so that I could get it into the
workshop.
Finally I contacted the agents to
find out the price and availability
of the ICs and modules. The parts
were all available but expensive and
I didn’t really have a clue as to what I
might need. I asked if I could speak to
technical support and was delightful
ly surprised to find out that not only
could I speak to someone but that he
actually knew – or was pretty sure he
knew – what the fault was.
In this set, there are two surface
mounted BC858 PNP tran
s istors
(CT2524 and CT2527) on the copper
side of the sound output module PC
board – one for the left channel and
one for the right – and these were the
nominated culprits. Armed with this
sort of 24-carat knowledge, I decided
to give it one more try in the house
and made an appointment after the
ordered transistors had arrived.
Back at the Edwards’ residence, I re
moved the sound output module and
sat down with it at a table. The first
problem was finding the two transis
tors, as they aren’t marked and there
is no component layout. Eventually,
I found them under a metal screen
soldered on the top lefthand side of
the PC board.
The next drama involved remov
ing these surface-mounted devices
(which are soldered and glued) with
out damaging the tracks on the board.
After that, I had to hold each of the
tiny new transistors in position with
a pair of tweezers while I soldered
their legs into place without applying
too much heat.
Finally, I reassembled everything
and was ever so relieved to hear clear
stereo sound from both channels.
Because the circuits don’t show the
interior block diagrams of the associ
ated ICs, I can only speculate on their
roles. IC2530 appears to be part of the
digital surround sound preamplifier
processor, while IC2350, IC2520 and
IC2510 make up the stereo decoder
(both Nicam and Zweiton) – this being
a multi-system TV set. Why the two
transistors failed I don’t know – I was
just happy to get out of there with the
job done.
A spooky microwave
Many electronic items are now so
cheap that they are no longer econom
ical to repair at all. Unfortunately, I
am old enough to remember fixing
6-transistor AM radios, whereas a
modern “boom box” – usually con
sisting of a CD player, dual cassette
and stereo AM/FM tuner – is no longer
worth fixing.
Video cassette players are border
line too, with new VCRs retailing at
just over $200 and the hifi models
starting at about $300. It is hard ad
vising someone who paid in excess
of $1000 many years ago that a better
performing machine is now available
at a third of the cost.
The same can also apply to comput
ers and to microwave ovens. I recently
saw a microwave oven advertised for
just $120! One wonders how they
can purchase the raw materials for
December 2000 61
Serviceman’s Log – continued
that! At the same time, the power
output has increased in these ovens
to almost double what it once was,
although many are now so small that I
wonder whether my coffee cup would
fit inside.
So what is all this leading up to?
Betty Carver brought in her rather
up-market Panasonic microwave
oven, somewhat grandly called “The
Genius Dimension 4”. This particular
model (NN-C855B) also includes a
convection oven, is about five years
old and would cost in excess of $600
to replace.
The problem, according Betty, was
that it intermittently “cooks by itself”
62 Silicon Chip
(ie, turns itself on) and sometimes she
couldn’t set the clock.
With all the safety circuits built into
ovens, I found it hard to believe that
it could turn itself on unaided and
suspected there were some two-legged
gremlins in her house. But she swore
that she had been woken in the mid
dle of the night by the audible alarm
noises that the oven makes when it
has finished cooking. Apparently,
she even went into the kitchen once
to find it on and cooking! More spe
cifically, the display read “7/8 Beef/
Pork”.
Intrigued by this somewhat spooky
story, I agreed to have a look at it. This
oven has a “Word Prompting Display”
with an operation guide built into the
microprocessor. When I switched it
on it said, “Welcome to Word Prompt
ing”, and I found that everything from
setting the time to convection cooking
all worked perfectly for me.
I left it on the soakbench and days
later, when I was about to phone Betty
and tell her that I could find nothing
wrong, it finally started its tricks. I
was in another part of the shop and
suddenly I could hear the oven beep
ing, followed by the noise of the relays
cutting in, the magnetron powering
up and the tray rotating. It was all
very X-files stuff, as no one else was
around. And why had it selected the
food category “8 Pork”.
I tried to stop it but it wouldn’t
immediately. After four tries it did
stop and then the oven would work
normally. It didn’t play up for the
rest of the day, and I was puzzled as
to how this had happened. Normally,
selecting “Pork” would require, as a
minimum, someone to press the 7/8
button twice and then press Start.
The next day, when powered up, it
wouldn’t let me change the clock or
do anything much. In the days that
followed, I found that this apparition
was also likely to occur when the
oven was switched on in the morn
ings, intermittently giving exactly the
symptoms Betty had described.
I opened the oven and found it to
be exceptionally clean. I then decided
to stop our “ghost” in his tracks by
disconnecting the membrane switch
panel from CN4 on the Digital Pro
grammer circuit board.
And this, at least, provided a clue.
The oven no longer operated by itself
but neither could I make it operate.
I then left it connected and the next
time it tried to cook by itself, I un
plugged it before it could complete
its sequence. Gradually, by plugging
and unplugging it, I convinced myself
that the source of the phantom cooker
was indeed the switch panel itself.
I ordered a new one and when it
arrived, I dismantled the front panel
and carefully pulled off the old switch
panel. I tried to examine the internal
parts of the switch membrane assem
bly (visible through the transparent
underside) but everything looked OK.
Finally, I pulled the paper backing
off from the new panel and glued it
to the front escutcheon before reas
sembling the unit. The unit was then
soak tested for three weeks and it now
worked perfectly.
Intrigued as to what caused the
problem, I checked for high resistance
between the contacts on the flex cable
but couldn’t find any problems. So
what happened? I can only speculate
that Betty used detergent and water
to wipe the front panel and some had
penetrated around the edge and crept
inside by capillary action. After that,
depending on the temperature and
humidity, this would intermittently
activate some of the switches, causing
the oven to start cooking.
A crook Akai
Long, long ago (even before Star
Wars), Akai Japan used to make
up-market reel-to-reel tape recorders,
then moved into VCRs and TV receiv
ers. They became very big and other
factories in Korea (Samsung) started
making TV sets for them, after which
they bought sets made in China. Most
of these were for the small-screen
budget market and Akai felt that
there was a gap they could fill by also
supplying upmarket large-screen TV
receivers.
Akai turned to Europe to produce
these and they were made by ITT
(also Nokia/Salora/Schaub Lorens/
NAD/Dual/Luxor, etc). Only a dozen
of these sets were sold in Australia
and these were early digital types (the
term “digital” is used very loosely in
TV technology and is mostly confined
to the tuning and control end of the
set – the scanning and audio is essen
tially analog. Most use the Philips I2C
bus system).
The problem is that when I encoun
ter these sets, I am intimidated by
the often unrecognisable – so called
– interna
tional symbols, the sheer
number of features and the need to
figure out how to use them before
the function times out. Often, I wish
that they would print an instruction
booklet for the instruction book – in
English!
Anyway, there I was behind Mr
Smithfield’s Akai CT2869AUS TV
set, grateful that this was only a ba
sic “poverty-pack” model. The fault
ticket said succinctly that the set was
dead which was partially true – there
was no sound or picture, just a ghastly
expectorant sort of noise from the
rear, as though the horizontal output
transformer was dead or in the pro
cess of dying. Connecting the CRO to
the collector of the horizontal output
transistor (T501) revealed significant
ringing in the horizontal output trans
former – that is before T501 decided
to cark it, right in front of me.
This was a not a good start.
Convinced that the horizontal out
put transformer was the main fault
I ordered a new one, only to find it
was no longer available. After a lot of
searching, I found that an HR6160 or
EHT3790 was probably an equivalent
and ordered one. When it arrived, I
duly slipped it in with supreme con
fidence, along with a new horizontal
output transistor, fully expecting that
that would be the end of the matter.
Well, I was wrong! – it made no
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December 2000 63
Serviceman’s Log – continued
difference at all and the set was still
dead and buzzing.
I subsequently spent an lot of time
checking the tuning capacitors around
the collector of T501 and the deflec
tion yoke, to see if any had changed
in value. After all, it takes a lot to
destroy a horizontal output transistor
which is rated at 1500V and 5A. The
horizontal pulses are normally 1200V
but if a capacitor changes in value
(either going lower or open circuit),
these pulses can go a lot higher and
exceed the collector-emitter rating of
the transistor, destroying it instantly.
Alternatively, if there is a short
circuit and the current exceeds 5A,
the transistor gets very hot and fails.
However, the switchmode power
supply normally senses the excess
current drain and will reduce its out
put – which was what was happening
here. In fact, the 155V rail was down
by about one third.
After some hunting about, I found
that D573, a 33V zener diode, was
short circuit. This zener diode is
in the collector circuit of the east/
64 Silicon Chip
west output transistor (T573) but not
shown on the diagram.
Another component not shown
on the circuit diagram was C516, a
470pF 1.6kV blue capacitor, which
appeared to be somewhat worse for
wear. Unfortunately, replacing these
two components had no significant
effect on the symptoms. However, the
situation improved when I removed
the deflection yoke and I subsequently
wasted a lot of time substituting the
yoke from another set only to find it
made no difference.
In the end, I turned my attention to
the power supply. This really hasn’t
changed all that much in 25 years and
is not all that dissimilar to that used
by Sunbeam Graetz TV sets in 1975!
The switchmode power supply is
based on a TEA2164/G control IC
(IC701) and chopper transistor T701.
Among other things, the output from
the transformer supplies the hori
zontal output transistor (T501). This
means that if anything happens to the
power supply, it is likely to have some
affect on the horizontal output stage.
With this in mind, I decided to
replace all the electrolyt
ics in the
switchmode supply, from C700 to
C734 (about 10 capacitors in all; the
others in the sequence are elsewhere
in the circuit). None of the electro
lytics on the primary side made any
difference but when I replaced C732
(1000µF) on the 31V rail, the set was
fixed – well nearly!
Now you could have knocked me
down with a feather at this point!
Why should ripple on the 31V rail
be so significant, especially as all
this rail seems to feed are the audio
output ICs?
I wasn’t quite out of the woods yet,
as the set still showed a picture that
was distorted with what looked like
hum. Hot on the trail now, I continued
replacing the electros and eventually
found that C722 (470µF) on the input
to the 5V regulator (IC721) and C522
on the output of the 12V regulator
(IC521) were the main culprits.
Finally, I set the 155V rail exactly
with R612, making sure there was no
picture (ie, minimum contrast and
brightness) during this procedure. I
also touched up a few potential faulty
joints before composing a suitable bill
SC
for Mr Smithfield.
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