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SERVICEMAN'S LOG
Price isn’t everything
This month’s lead story concerns a rather
expensive, top-of-the-market European TV
set. Unfortunately though, its upmarket
status didn’t prevent it from being a right
proper stinker on the service bench.
Did you know that Loewe is a very
popular brand of TV set in Canberra?
This is because our Lords and Masters have them installed throughout
Parliament House which means that
they must be pretty good performers.
In fact, Loewe colour TVs have been
available in Australia since 1974 and
are highly-respected, German-made
sets with many advanced features.
However, this information really has
nothing to do with the Loewe Studio
70 (110C91 chassis) which Mr Canaris (not his real name) reluctantly
allowed me to take back to the workshop. He initially complained of the
sound dropping in and out, as well
as half vertical deflection, but when
I switched it on, it was dead.
“Oh yes, that too”, he replied –
offhandedly dismissing this all too
obvious fault when I phoned him back
to ask why it hadn’t been mentioned.
Fortunately, I was able to obtain a
service manual for the set (they are
available from a firm called Interdyne
in Melbourne and cost around $40).
That at least was a good starting point
and when it arrived, I wasted no time
in opening up the set for a preliminary
investigation.
The reason the set was dead was
that the power supply had blown, due
mostly to the electrolytic capacitors
(this set was now about 10 years old).
Getting it going again involved replac
ing IC611 (TDA4601), the switchmode
transistor, four electrolytic capacitors,
R613 and the start-up PTC resistor
(R622) – see Fig.1.
This done, the set came on perfectly. I adjusted variable resistor P633
Fig.1: the power supply circuit in the Loewe Studio 70-110C91.
26 Silicon Chip
(4.7kΩ) and set it for 155V at point
UB, as shown on the circuit. (Note:
some European manufacturers use
the designation “UB” to indicate the
main HT rail, while some German
manufacturers may sometimes just
use “U”).
Unfortunately, there was some initial confusion regarding this HT rail
value. The circuit involved is simple
enough, though. Diode D651 rectifies
the output from the switchmode transformer secondary at pin 18 and the
resulting DC is then filtered by R651
and C651 (47µF) to chassis. The HT is
then further filtered by coil L651 and
a second 47µF capacitor to chassis to
give the 155V HT rail (UB).
Although this value is clearly
marked on the circuit diagram, the
parts layout diagram indicated 145V
at what appeared to be the same point.
However, closer analysis of the layout
diagram clarified this; the 145V reading was the voltage at C651, whereas
the reading on the circuit indicated
the voltage at C652.
But this only created further confusion. Why was the reading across
the first filter capacitor less than that
across the second capacitor?
The logical explanation is that the
waveform across C651 still contains
sufficient ripple to upset the reading.
It needs L651 and C652 to produce
pure DC. Anyway, the set was working
for the moment and I put it aside for a
soak test, hoping that the sound and
vertical height faults would eventually show up.
Impatient customer
Unfortunately, I hadn’t counted on
the impatience of Mr Canaris who
was on the phone the very next day.
I explained that I had fixed the total
failure but hadn’t been able to observe
the other faults he had mentioned and
that obviously they were intermittent.
I would need time for them to show
up so that I could find and fix them.
He was very disappointed and although he didn’t actually say so, I got
the impression that he thought I was
incompetent. Apparently, I was supposed to wave some sort of magical
wand and all the unseen faults in his
set would disappear. Anyway, I told
him that he would have to wait a few
more days and that I would phone him
when I had fixed all the faults.
Unfortunately, the message didn’t
sink in because his wife phoned the
next day. Once again, I explained the
situation and said that I would ring
when the job had been completed.
The next day, I switched it on and
it came on with only half a scanned
picture. I heated it and I froze it and
suddenly it worked OK, before I had
time to work out which component
was responsible. Then Mr Canaris was
back on the phone wanting to know
when he could pick up the set; his
wife had said it would be ready today.
Feeling somewhat frustrated with
him by now, I told him that she had
misunderstood me and that the set
was still not ready. The next day
the fault was back, so I changed the
vertical output IC (I561, TDA8175).
The fault didn’t show again until
three days later but Mr Canaris was
still phoning every day. This was a
man who had trouble understanding
plain English.
Finally, I decided to replace all the
electrolytics in the vertical timebase.
After that, the set worked well and
there were no sound problems but I
Items Covered This Month
•
•
•
•
Loewe Studio 70 (110C91
chassis
Sharp VC-A200X VCR
Pye radio/cassette/CD player
Teac CT-M144 34cm TV set
was not convinced that I had solved
all the intermittent problems. The
manufacturers and agents are generally more familiar with difficult problems, particularly intermittents, than
individual servicemen, so I phoned
Loewe in Melbourne for advice.
As it turned out, they were very
helpful. In particular, they suggested
I replace IC I441 (APU2471) and fit a
special kit to the horizontal output
transformer to connect the ferrite core
to chassis, which might be flashing
over. I had already reworked the entire
chassis for dry joints.
I acquired and fitted IC I441 plus
the special kit as suggested, but then
just when things were looking good,
the next disaster hit – the set went
dead again. In the meantime, the
Canaris were still phoning every day
and getting very shirty, which didn’t
help matters.
This time, both the horizontal
output transistor T534 (S200AF) and
diode D536 BY228 had gone short
circuit. Replacing these restored the
picture but the horizontal system was
overscanning and there was no east/
west (E/W) pincushion correction.
By going into the service mode with
the remote control, I found that I could
adjust the picture but not enough to
correct the problem. I also noticed that
the E/W output transistor’s heatsink
November 1999 27
was getting very hot. By now, Mr Canaris was no longer phoning me but
complaining long and hard to Loewe
in Melbourne, who then did their best
to help me.
The voltages seemed correct everywhere and so did the waveform on
pin 24 of IC I511, except for some
horizontal pulses superimposed on
the lower part of the parabola. I removed and checked all the transistors
and replaced a number of electrolytic
capacitors (including C558, C546,
C583, C581, C512 and C542) but it
was all to no avail. I then replaced
C594, C536, C537, C538, C541 and
C531 but still no joy. It wasn’t until I
noticed some of the old brown goo on
L538 (incorrectly marked as R538 on
the circuit) that I realised I had a clue.
This large coil (1.6mH but marked
14323) measured shorted turns when
checked on my shorted turns tester,
so I acquired a replacement from
Melbourne and fitted it. Success at
last – the picture was good and the
only thing left was transistor T594
(BD537B) in the E/W correction circuit, which (I felt) was running far too
hot. I replaced it and its other half of
the Darlington pair, transistor T593
(BC546B) and rechecked the voltages.
There was 15V on the collector and
0.65V on the base. In addition, the
waveform on T594’s collector was
100% correct.
I rechecked all the transistors in
the E/W correction as well as all the
resistors and everything was correct.
28 Silicon Chip
In the end, I felt I had taken all possible steps to solve this problem and
the only lame idea I had left was to
add more aluminium to the heatsink
to get rid of the heat (it was literally
too hot to touch). This was done and
after soak testing for another 24 hours,
I finally agreed to let the set go home.
Mr Canaris complained long and
hard about the service, the cost,
the delay and how I didn’t know
what I was doing. For my part, I was
thoroughly fed up with him and did
little to hide my annoyance. In fact, I
doubt very much that I will hear any
more about this set. There’s so much
bad blood between us that even if
it does fail, I’m sure he will take it
somewhere else.
That’s a pity really, as I would like
to know if any more faults subsequently showed up. Some customers
really are their own worst enemies.
Perverse inanimate objects
I’m in a whinging mood at the
moment. I don’t usually whinge, at
least not in print. But fair dinkum, I’m
getting fed up with components that
keep giving different measurements.
During the last few months, I have
been beset by several such frustrating
experiences. Perhaps they are due to
what an acquaintance calls “the perversity of inanimate objects”. His philosophy was that some objects have
mind of their own and that if you want
them to behave in a certain way, they
will do all they can to frustrate you.
A questionable philosophy? Well,
maybe it is. But have you ever tried
to fit a nut to a screw, in an awkward
corner of a chassis? Or have you tried
to fit a pigtail through a hole in a PC
board from the hidden side?
Of course, these are relatively simple mechanical situations. It’s when
these inanimate objects are part of an
electronic circuit that the fun really
begins.
This month, I had a Sharp VCA200X VCR with no display. I didn’t
have a circuit but I felt it shouldn’t
matter as the circuit is so simple.
Apart from disassembling it, it wasn’t
difficult to establish that there was
-28V on the segment legs of the fluorescent display panel but no filament
volts on the ends.
Moving along to the switchmode
power supply, I measured the voltages
on each diode. There were appropriate positive or negative voltages on all
diodes with respect to chassis, except
for diode D921. As I quickly discovered, this diode rectifies an output
from the switchmode transformer and
feeds the fluorescent display.
The output from D921 is filtered by
a 100µF 6.3V capacitor, C921. There
was no voltage across this capacitor,
the capacitor wasn’t short circuit and
it made no difference when I connected another capacitor across it.
Diode D921 (FR103) couldn’t really
be measured in circuit because of
the low impedances everywhere, so
I unsoldered one end and found that
its forward resistance was too high for
my liking. This looked like the culprit
but when I removed it completely and
measured it out of circuit it measured
perfectly.
Still, I really didn’t have any other clues so I fitted another diode, a
BYV96E, in its place. This immediately restored the 3.5V rail needed to
drive the 3V filament and the display
with the word “SHARP” came up at
full brilliance.
So that solved that problem. But it
really cheeses me off, having to keep
remeasuring components because of
the uncertainty that the first reading
was correct.
So that’s my whinge for the month.
I know it’s nobody’s fault and there is
nothing I can do about it. And having
had my whinge, I feel better already.
And now here is a story from a colleague, P. K. I’ll let him tell the story
in his own words.
The Ghettoblaster
This story concerns what is often
referred to as a ghettoblaster; in this
case, a Pye radio/cassette/CD player
with two tape cassettes. The unit had
originally come in for service about
two weeks previously and I diagnosed
the problem as being in the CD player,
which required cleaning and testing.
This time it was a cassette problem.
I replaced a fuse which had blown,
after which there were some signs of
life. The radio worked, as did the CD
player and the “A” cassette. But when
I pressed the play button for the “B”
cassette, everything went dead.
Closer inspection revealed that a
switch associated with the play button
had failed. It was a leaf switch and
one of the two leaves had broken off
at the base and was hanging loose.
And in order to understand the implications of this, a brief description of
the switch’s associated mechanisms
should help.
The play button for each cassette –
the “B” button in this case – activates
the mechanical loading functions,
moving the tape against the head,
closing the pinch roller against the
capstan, etc. At the same time a lever –
at chassis potential – activates the leaf
switch, which is suitably insulated,
closing its two contacts. The intact
leaf carries the 12V supply, while
the broken one connects to the load;
the motors, the audio, oscillator and
other circuits.
Fig.2: this simple circuit was
used as an electronic switch to
replace the broken mechanical
unit in a Pye cassette player.
But now, when the button was
pressed, the lever contacted the remaining, live 12V leaf, taking it to
chassis. The result was inevitable; a
blown fuse.
It was a simple enough diagnosis
but what could be done about it? I
contacted Philips but I was advised
that a replacement was not available.
But even if one had been available, it
would have been a major job to pull
the unit apart to fit it.
My next idea was to simply bridge
the two leads. This would mean that
the “B” cassette drive would function
continuously, while ever the set was
switched on. This was not as a wild
an idea as it sounds; a number of
other model cassette players use this
arrangement.
Well, it was worth a try. And at
first I thought that it had worked. In
fact it had, to the extent that the “B”
tape worked perfectly. But now the
“A” tape would not play – the wheels
worked but there was no sound. When
i disconnected the two switch leads
which I had bridged, the “A” tape
came good but, of course, the “B” tape
was dead again.
So I could make one or other cassette work, but not both at the same
time. Why? – I don’t know; it would
have been too time-consuming to
figure it all out. All I knew was that
while the 12V rail to the “B” cassette
was activated, the “A” cassette would
not work.
Doubtless, given the time and
enough technical backup, one could
analyse the device in sufficient detail to work out how it functioned
and perhaps find a solution. But, at
a practical level, this approach was
out of the question. Considering the
age of the unit, I was beginning to fear
that the customer might be forced to
cut his losses and settle for only one
cassette player. After all, he could
only use one at a time!
Then I had another thought. Was
it possible to substitute an electronic
switch for the faulty mechanical one?
In fact, this looked to be relatively
simple.
I selected a BC327 PNP transistor as
the switch, connecting the emitter to
the 12V supply rail and the collector
to the load. The base was connected to
the emitter via a 1kΩ resistor, which
would ensure that the transistor
was switched off unless other
wise
instructed.
Obviously, an “instruction” would
be needed to turn the transistor
on when the “B” play button was
pressed. And this could have been
tricky. Fortunately, the remaining
switch leaf came into its own. It was
no longer connected to anything but
still made electrical contact with
the lever when the play button was
pressed. So the leaf was connected to
the base via a 3.3kΩ resistor, applying
forward bias to the base and turning
the transistor on.
Did it work? Yes it did – just like a
bought one! And I had another happy
customer.
Spring crisis
When it is a glorious day in spring,
one arrives at work feeling euphoric,
convinced that nothing could possibly spoil your day.
So it was last Tuesday – the birds
were singing, the tem
perature and
humidity were just right and I was full
of bonhomie when I booked in Mrs
Townsend’s Teac CT-M144 34cm TV
set. All that was wrong was a broken
RF socket on her tuner and she still
had the broken parts. It promised to
be a simple fix for a simple lad on a
sunny day – if only life could always
be this sweet.
On removing the covers, I felt I
might be able to resolder the coax
socket onto the tuner in situ. Unfortunately, I soon discovered that I
would have to remove the tuner and
its covers to resolder the centre pin
to the PC board.
I pulled the chassis out and placed
it upside down, happily whistling
a little ditty while I prepared the
solderwick to desolder. Mrs Serviceman wasn’t quite so happy – I don’t
know whether it was because she
didn’t like my ditty, because it was
out of tune and rather repetitive. Or
November 1999 29
perhaps it was because I was happy
and she wasn’t.
Anyway, all this was about to
change because, unbeknown to me,
the set had been switched on in the
last 12 hours or so and when I placed
the solderwick braid across the PC
board pattern, there was a bright flash,
a spark and a crack. Whoops! Well,
there was nothing I could do until I
had replaced the tuner.
It didn’t take long to do this but
a degree of anxiety was creeping
into me and my whistling ditty had
stopped. Where had the spark come
from and more importantly, what had
it struck? I was praying it was just a
direct short across an electrolytic capacitor but unfortunately this wasn’t
the case. When I switched on there
was no sound or picture.
There was EHT and voltage on the
CRT filament heaters but not much
other activity. Fortunately, I had a
schematic diagram and I soon established that all the obvious voltage rails
were intact (115V, 24V, 15V, 12V, 5V,
etc). And the source of the spark was
eventually traced to a residual voltage
across C260, a 2.2µF capacitor associated with the video output transistors
(Q601, Q602 & Q603). But it was one
30 Silicon Chip
thing to know where the discharge
originated and quite another to know
what it had struck.
By turning up the screen control, I
established that the raster was scanning correctly and touching the pins
of the audio IC (IC205) produced noise
in the speaker. At this point, I wished
I had insisted on having the remote
control. Because the fault had seemed
so simple, I hadn’t seen the need for
it when the set was brought in. Now I
hesitated to ask the lady for it, in case
she suspected the worst.
The front controls were having no
effect and there was no effect when
external signals were applied to the
SCART socket on the rear; neither
was there any on-screen display. By
now, I was beginning to suspect that
the microprocessor IC201 (TMP47
C434N-R214) and/or the EEPROM
had been damaged.
I checked that Vcc of IC204 (vertical output) and Vdd of the microprocessor were both getting 5V, and
that crystal XT201 was oscillating
correctly at 4.19MHz. I changed IC202
(TC89101P) first as it was cheaper and
simpler but even replacing IC201 as
well made no difference. The CRO
confirmed that the video was getting
to the TA8717 jungle IC (IC206) from
the SCART socket but no further.
By switching the IC to the TV mode,
I could also put the set into the preset
tuning mode and tune stations, using
the CRO to monitor the video input to
the jungle IC on pin 16. But, as before,
the signals were going no further. I
checked the voltages to IC206 and
then used the CRO to check crystals
XT202 and XT204. Unfortunately,
this provided no clues and it was
now obvious that I had overlooked
something, but what and where?
I went back to microprocessor
IC201 and decided to check each pin.
I discovered that even though nothing
could be seen or heard, most functions
were working and responding to the
front controls, and these could be
measured on the appropriate
pins.
I finally checked pin 26,
marked HD, and found nothing
on it. I did not know what HD - or
indeed VD next to it - stood for but
they suggested horizontal and vertical pulses. I followed the horizontal
circuit back to the collector of Q218
and then checked the base circuit.
This was fed from horizontal output
transformer T201 (pin 10) and so I
expected to see horizontal pulses - but
there weren’t any!
Following the circuit further, I
found a branch feeding diode D233
(MTZ208) and resistor R330 (10kΩ) to
pin 17 of the jungle IC (IC206) which
wasn’t getting any pulses either. It
took some time to follow the PC track
to find where D233 was situated on
the mother board but I finally found
it nestled right next to connector
CN203. And guess what was on pin
1 of this connector?
Yes, the 200V rail to the video
output transistors (Q601-Q603). This
rail is derived from pin 3 of T201 via
diode D229 and my old friend capacitor C260. Obviously, the desoldering
braid connection had shorted this rail
directly to D233, the residual voltage
in C260 instantly destroying it and
turning it into a short circuit.
The next step was to identify D233
and I worked out that it was a 20V zener diode. Fitting a new one restored
all the set’s functions.
Mrs Townsend was spared my
anguish and so remains blissfully
unaware of the trials and tribulations
involved in fixing her wretched anSC
tenna socket.
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