This is only a preview of the April 1995 issue of Silicon Chip. You can view 29 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Build An FM Radio Trainer; Pt.1":
Items relevant to "A Photographic Timer For Darkrooms":
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SERVICEMAN'S LOG
Sets aren’t made of rubber, but...
Nobody likes to have a set bounce. But
let’s face it; it’s an occupational hazard.
It happens to all of us sooner or later but
it’s still a blow to our professional pride &,
potentially, to our reputation.
Occasionally, a set bounces by
reason of our own careless
ness or
lack of experience with a particular
brand. But most of the time, it is just
plain bad luck. A second fault occurs
shortly after the set is returned to the
customer, probably producing similar
symptoms, and the customer expects
an explanation.
To be fair, most customers are reasonable but once in while one will go
off his brain. And it sometimes takes
fair bit of diplomacy to quieten them
down.
But they are not the worst. The worst
ones are the ones you don’t hear about,
except much later on the grapevine,
when the damage to your reputation
has been done.
Naturally, all those thoughts were
prompted by a recent experience.
In fact, none of these nasty things
happened but they could have, and it
served as a reminder that this threat
is always there.
The story is about an AWA model
C3423 colour TV set, a 34cm model
which is actually made in Korea
by Daewoo. It belongs to one of my
long-standing customers.
His complaint was straightforward
enough – distorted sound on all channels – and I imagined the cure would
be quite simple. And initially, this
appeared to be the case. When checked
on the bench there was no doubt about
the validity of the complaint; the distortion was really severe.
And, as I had expected, the cause
was simple enough; failure of one of
the two transistors in the audio output
stage. These are designated on the
circuit as Q601 and Q602 and both
carry the type number KTC2230Y. In
this case it was Q601. Fortunately,
I had a replacement in stock but it
appears that a 2SC2230 is, as far as I
can determine, the same device, the
KT prefix and Y suffix being a Korean
version.
Anyway, I had the specified type
number, so I simply fitted it. And that
cured the fault. I finished the job late in
the afternoon, and left the set running
on the bench for an hour or so until I
closed the shop for the night.
When I switched it on again the
next morning, it performed quite
normally and so I rang the customer
with the good news. I subsequently
unplugged the set and pushed it
aside when I needed the bench space
but later turned it back on again to
demonstrate it to the customer when
he called in.
It’s back again
Fig.1: the audio output stage in the AWA C3423 colour TV set. The audio
drive comes from pin 3 of IC101 (top) & is applied to the base of Q602
which apparently operates as a single-ended class-A stage, with Q601 as
a cascode. The output appears at the junction of Q601 & Q602 & is fed to
the loudspeaker via a transformer.
56 Silicon Chip
So that was another job finished
– or so I thought until it bounced. A
couple of days later, the owner was
on the phone with the bad news that
the sound was still distorting. He was
quite reasonable about it though, because he realised that it wasn’t exactly
the same fault as before.
While the original fault was obvious
the moment the set was switched on,
the set would now run normally for an
hour or so and then would gradually
begin to distort. At the end of about
two hours, it was really bad. And I
gathered that the owner had prob
ably been trapped in the same way I
had been, by initially using the set for
relatively short periods.
So the set finished up back on the
bench. Initially, I let it run for about
two hours, by which time it was quite
intolerable. I then decided to check the
audio feeding the output stage, on pin
3 of IC101. This was easy enough to
do using a small audio signal tracer
and it confirmed that the signal was
perfectly clean at this point.
My next thought was to make some
voltage checks but I didn’t have much
to go on. The circuit is one of those
that a colleague calls “a street directory
with no street names”; or, in this case,
no voltages. Well, there was one, the
supply rail to this stage, at 103V.
Assuming this figure was correct I
reckoned there would be about 50V
across each transistor. It also seemed
reasonable to expect that there would
be around 0.5V or 0.6V between the
base and emitter of each transistor.
So in spite of the circuit limitations, I
was able to build up a fair picture of
the likely voltages.
After allowing the set to cool down,
I switched it on again and confirmed
that these voltages were correct. The
supply rail measured the indicated
103V rail, there was roughly 50V
across each transistor, and there was
about 0.5V between the base and
emitter of each transistor. Having
confirmed this, I let the set run until
the distortion reappeared, then made
another voltage check.
It was a different story this time.
While the other voltag
es remained
as before, the base-emitter voltage of
Q601 had dropped significantly. I left
the meter connected and let the set
run. The voltage continued to drop
as the distortion increased until, after
about two hours, it had dropped to a
mere 0.05V.
Well, that was a clue but that was
all it was; I still had to find the cause.
Fortunately, there is only a handful
of components in this section: six
resistors, six capacitors, and the two
transistors.
I was inclined to ignore the transistors. After all, Q601 had just been re-
placed and the chances of two failures
in a row seemed remote. But statistics
can let one down. I had more spares
on hand and it was only a few minutes
work to change both.
And that promptly ruled out that
possibility; it made no difference. The
resistors did not seem to be a high risk
but were easy to check anyway. And
again I drew a blank.
That seemed to leave only the capacitors – two low value plastic types
and four electrolytics. Of the latter,
C608 (22µF) served as a decoupler
for the 103V rail. However, I couldn’t
relate a fault here with the observed
symptoms.
All things considered, including
the change in Q601’s base-emitter
voltage, the most likely suspect was
C610, a 3.3µF coupling capacitor to
the loudspeaker. It was an electrolytic,
of low value, and in what appeared to
be the fault area.
It was simple matter to pull it out
and test it. Its capacitance measured
3.3µF as marked and there was no
significant leakage. But it was just
as easy to fit a new one anyway,
whereupon the set produced good
clean sound. More importantly, it
April 1995 57
continued to do so for the rest of the
day, after which I consid
ered the
point proved.
So I’m not sure what was wrong
with the capacitor. Normally, there are
three likely faults in a capacitor: loss
of capacitance, leakage and internal
series resistance.
Since it appeared to have correct
capacitance and no leakage, that left
only internal resistance, which is not
quite so easy to measure. On the other
hand, there seems little doubt that it
was a temperature sensitive fault and it
is sometimes difficult to duplicate the
exact temperature conditions when
making measurements.
So, all things considered, I’d put
my money on leakage. After all, one
side of it connects via the output
transformer (T601) to the 103V rail
and the other side to Q601’s emitter.
So, if it was leaky, the effect would be
pretty drastic.
So it all ended happily. But it was
a nasty trap and I’m not sure whether
there were two quite separate faults
or whether the faulty capacitor was
the cause of Q601’s failure in the first
place. In any case, I fell into the trap.
With the benefit of hindsight I should
have given the set a longer soak test.
But this is not always convenient and
58 Silicon Chip
there were no symptoms to suggest that
it would be advisable.
How does it work anyway?
Finally, having solved the problem,
I couldn’t help but wonder about
that output stage configuration. It is
not an uncommon arrangement and
I must have looked at it many times
in various makes and models of sets.
And despite having replaced faulty
components in these circuits, I have
never bothered to think much about
the arrangement.
Until now, that is. It must have been
the need to service it twice in quick
succession, and the need to work out
voltages, which prompted me to start
wondering about how it operates.
The first point to note is that the two
output devices are of the same type
number and, therefore, of the same
polarity. Compared with the popular
complementary symmetry pair configurations, I find this arrangement
puzzling.
And the more I look at it the more
confused I become. I simply cannot
grasp how the circuit works. And those
colleagues I have consulted appear to
be equally as confused. Some made
suggestions based on other circuits
with which they were familiar but
nothing seemed to add up.
As already noted, the two transistors
are effectively in series in the DC sense
and operate from the 103V rail. The
audio drive is from pin 3 of IC101 and
the output is taken from the junction
of the two transistors and capacitively
coupled to the speaker transformer,
the other side of which connects to
the 103V rail.
It also appears that the output is at
relatively high impedance, hence the
speaker transformer. There is also a
feedback network into pin 2 of IC101.
Beyond that, it is not clear how
the circuit works. It would appear
that Q602 operates as a single-ended
class-A stage, with Q601 as a cascode.
But the biasing arrangements for Q601
are something of a mystery since the
base of this transistor is tied one diode
drop below its emitter.
So there it is; an ultimately successful job but one which left a frustrat-ing
circuit puzzle. If anyone can throw any
light on this circuit, I would be happy
to pass it on to readers.
In the beginning
My next story takes us back a few
years; some 20 years in fact, to the
beginning of colour TV in Australia
in 1975. More particularly, it involves
Fig.2: the power supply for the Kriesler 59-1. The two mains fuses (F101 & F102) are at left, while fuse F120 is to
the right of the bridge rectifier. TR120 is the chopper transistor.
one of the first colour sets of that era. I
refer to the model 59-1 made by Kriesler which, in various modified forms,
was popular for many years.
And while this particular set may
not necessarily be 20 years old, it
would be pretty long in the tooth. It
belongs to a lady customer who moved
into my district a couple of years ago
She first sought my assistance about
a year ago. On that occasion, the main
problem was due to some dry joints,
of which this set had its share. In addition, I made a routine modification
to permit the set’s use with a video
recorder. It had been a long time since
I had done this and I had to dig out
the appropriate modification note to
refresh my memory.
The modification involves the
horizontal oscillator circuit. In greater
detail, it involves modifying the time
constant of the automatic frequency
control (or flywheel sync system). In
these early Kriesler sets and in some
Philips sets of the same era, before the
advent of the domestic VCR, this time
constant was relatively long. This was
perfectly satisfactory for the highly
stable off-air TV signals but was too
severe for some video recorders.
The modification is relatively simple. It involves the Line Control Unit
(CU701) and pins 3, 10 & 11. Pins 3
and 10 must be connected together,
while pin 11 is connected to chassis.
With that done, and the dry joints
repaired, the set was returned to the
customer.
When it came in this time round it
was completely dead and I had a gut
feeling that it was power supply failure. There was no life of any kind; not
even a hiccup to suggest an overload
shutting down the power supply.
My first check was at the fuses.
The two mains fuses (F101 and F102)
were intact, but fuse F120, a 2A type
between the bridge rectifier and the
chopper transistor (TR120), was
blown. So it looked like a fault on the
board itself, most likely TR120.
Fortunately, I still have a fair stock
of boards for this model, salvaged from
sets scrapped for other reasons. So it
was a relatively simple job to pull out
the power supply board and substitute
a known good one. This would at least
confirm my suspicion and clear the
rest of the set.
And it did; the set came to life immediately and put up quite a creditable
performance, considering its age. Even
the picture tube looked as though it
was good for a few more years.
OK, so the fault was on the power
supply board. If it was as simple as
I suspected, it would be well worthwhile repairing. Naturally, I went
straight to the chopper transistor pins,
on the underside of the board. And a
quick check with the meter confirmed
my suspicion – it was shot, base to
emitter.
I unscrewed the mounting nuts,
then turned the board over to pull
the transistor clear. And this was the
first hint of something unusual. One
glance was enough to indicate that
there had been “a certain amount of
mucking about going on”, as one of my
colleagues often puts it. Sticking out
from under the transistor were some
pieces of black insulating tape as used
by electricians.
It was now clear that TR120 had
been replaced on a previous occasion.
This was no surprise – faults of this
kind are common enough in all sets.
But the nature of the repair was. The
insulating tape had been used in place
of the isolate mica washer that’s used
to separate the transistor from its
heatsink. In fact, two strips of tape had
been used, with one overlapping the
other to provide the necessary width.
A real shocker
Such a bodgie repair was a real
shocker. At that stage, I had no idea
when, or by whom, the repair had
been done. I could only assume that
someone had been caught out in the
field without a washer and had taken
this way out to do a quick repair and
avoid a return visit.
Well, that would be an explanation,
if not an excuse. But it is a pretty
rough approach. For one thing, as we
all know, insulation tape degenerates
with time, particularly in a heated
situation such as this.
And, in any case, it would provide
very poor thermal conduction compared to a standard mica washer. The
standard washer is made as thin as
possible, consistent with adequate
electrical insulation, in order to
provide maximum thermal conductivity, usually aided by a heatsink
compound.
Insulation tape is thicker and, in this
case, there was a double thickness of
tape where the two strips overlapped
in the middle of the transistor between
the two pins. In fact, I took a few minutes off to check these thicknesses with
a micrometer.
A typical washer is of the order of
.005in, while a single thick
ness of
this tape was .008in, making a double
thickness of .016in (pardon the imperial measurements; my micrometer goes
way back.) So the poor old transistor
must have been running much hotter
April 1995 59
SERVICEMAN’S LOG – CTD
Fig.3: a previous “serviceman” had isolated the chopper transistor using two
pieces of electrical tape instead of a proper mica washer. It’s a wonder it lasted
as long as it did.
than it should have been since the
repair was made.
Naturally, I fitted a new TR120,
complete with the correct washer,
whereupon the set came back to life.
There had been no other side effects
from the failure.
But the bodgie repair raises the
question as to why this transistor
failed. Maybe it was due to fail anyway but there are two far more likely
possibilities. One was that there had
been an electrical breakdown between
the transistor case and the heatsink, as
the tape did not fit too snugly around
the mounting bolts. Alternatively, the
lack of adequate heatsinking may have
finally taken its toll.
Who did it?
But regardless of the reason, that is
no way to repair a TV set. I was curious
as to how it had happened so, when
I rang the lady to advise her that the
job was finished, I raised the matter of
the previous service – after all, it did
involve the same component.
In fact, she was most helpful. It
transpired that, before moving into
my area, the set had been covered by
a service contract with a large service
organisation. And when she came in
to collect the set, she brought all the
relevant documents with her, including the job sheet for the service in
question.
And this produced another surprise.
There was no suggestion of an emergency repair in the house, as I had
envisaged. According to the dockets,
it had been taken to the company’s
workshop and the job done there. So
60 Silicon Chip
how on earth could such a bodgie job
be justified?
The documents also pinpointed
when the job had been done, which
was about six years previously. So it
had lasted rather longer than I would
have expected. But that’s no excuse.
What firm was it? No, I’m not saying.
I’ve seen and heard only one side of the
story. There could be an explanation
which completely absolves them, so
we’ll let it rest there. But it was a nasty
act on somebody’s part.
The intermittent VCR
And finally, here is a story from a
reader, J. S. of Portarlington, Victoria.
Here’s how he tells it:
After reading the Serviceman’s Log
in the August issue of SILICON CHIP
about the NV-370 and NV-600 VCRs,
it rekindled my memory of an NV-470
I had fixed two months earlier.
This was one of those intermittent
faults. Don’t you just love those?
This particular problem seemed to
involve the power switch. At times,
one could keep pressing it and get no
response whatsoever. Even shaking
the whole unit, or prodding the board
around the power section, would not
revive it.
And then, for no apparent reason,
it would come good and remain so.
Once again I repeated the shake and
prod tests, with no result.
I waited for it to reappear of its
own volition. When it did, I took the
covers off and removed and replaced
a couple of the 3-pin wire connectors (PJ1003 & P1002 ) on the power
section of the board. And bingo, the
problem vanished. A dirty connector?
I subjected the unit to another shaking
and prodding test and, as it did not
fail again, I more or less accepted that
this could have been the cause of the
problem. At that time, I did not have
a circuit diagram with which to check
the layout.
But not being 100% satisfied that
the problem was solved, I kept it for
further observation. Sure enough,
some eight days later it happened
again. It was time to get serious
and get a copy of the circuit. Thus
equipped, I realised that at least one
of the connectors I had changed,
PJ1003, had little to do with the power
supply circuit.
On closer examination of the copper
side of the board, in the power supply
region, I noticed some discoloration,
apparently due to overheating, around
transistor Q1001 (2SD1275), the voltage regulator for the 12.7V rail. At the
same time, I had my finger on Q1001’s
heatsink and as I applied pressure, it
sank towards the board.
My interest aroused, I wiggled it
and watched it from the solder side.
The unit was plugged in at the time,
and I noticed arcs being emitted from
Q1001’s collector and its copper
track. Sure enough, the fault could be
induced and corrected by wriggling
Q1001’s heatsink.
Closer examination of the copper
tracks around Q1001 revealed that
the collector track had broken due
to the size of the heatsink. This was
attached directly to the transistor
body, without any anchoring pins into
the board. Q1001’s base and emitter
pads were also beginning to lift off
the board.
I soldered a substantial piece of
tinned copper wire to each lead of
Q1001 and along their corresponding
copper tracks, which gave the transistor and its hefty heatsink a solid base.
Hopefully, this will solve the problem
for the life of the unit.
This fault clearly illustrates that
one should always start any diagnosis
with a thorough visual inspection.
The telltale signs could be very time
saving, as in this case, especially as I
was looking around and at the fault
right from the start.
Thank you J. S. for an interesting
story. Your point about a thorough
visual inspection is well taken. I’ve
been telling myself that for years but
SC
I still get caught.
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