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SERVICEMAN'S LOG
A story to support Murphy's Law
Traditionally, by Murphy's Law, any servicing
short cuts we attempt will always bypass the
fault. And conversely, when we find a fault by
conventional means, we can see an obvious
short cut. But let's not take Murphy too
seriously.
The truth is, we live on short cuts
of one form or another, from recognising the symptoms from previous jobs
to the replacement of complete boards
in order to save time. Generally, these
short cuts work. It is only when they
don't work that we blame Murphy (or
bad luck).
.
This story would seem to support
Murphy's Law, at least initially. It concerns a Thorn Model 9007 colour TV
set, which uses the Mitsubishi G series chassis. This chassis was also
marketed as the AWA model C608.
These sets are now about 15 years old
and this one belongs to a local motel,
the owner being a relatively new customer.
The owner brought it in with the
complaint that it had lost colour and
an initial check on the bench confirmed this. However, I found that it
was possible to brute force it into
colour - of a sort - by deliberately
mistuning in the direction of the
sound carrier with the colour saturation control wound full up.
I used the term "colour - of a sort"
because there was something funny
about it; that being the only way I
could describe it initially. To put it a
little more scientifically, I knew it
wasn't right but couldn't work out
what was wrong with it visually.
But there was more wrong with the
set than that. The first thing I noticed,
apart from the loss of colour, was that
the picture was grossly over-scanning;
mainly horizontally but also vertically
to a somewhat lesser degree. There
was also some obvious convergence
error.
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64
SILICON CHIP
Colour bar pattern
I fed in a colour bar signal from a
TV pattern generator and, by careful
mistuning, was able to bring this up
in colour. But the degree of mistuning
was very critical and it produced a
strange effect. While each colour bar
displayed the correct colour, it did so
for only part of its width, starting
with monochrome on the left- hand
side and graduating to colour on the
right. This probably accounted for the
"funny" effect I had observed on program material.
.
My first reaction to the colour problem was to speculate on a possible
front-end fault. Perhaps the tuner or
the IF system was upsetting the response curve. However, I put these
thoughts aside for the moment and
decided to investigate the overscan
problem.
HT voltage
The first thing I check in such cases
is the HT rail voltage and this paid
off. Normally, the HT for this set is
105V but it was actually sitting on
138V. This was far too high and suggested a fault in the power supply
regulator system.
This set is made up of a main board
("PCB-MAIN") and a number of auxiliary boards which plug into this via
edge connectors. Fortunately, this being an old set, I have accumulated a
fair stock of these auxiliary boards
from scrapped sets and these prove
very useful. First, they can help pinpoint a faulty board quite quickly by
simple substitution. Second, leaving
the substitute board in place is often
the quickest and cheapest solution;
an important factor in sets of this age.
So I simply fished out a spare "PCBPOWER REG" board and plugged it
into the main board. That didn't help
much because the HT voltage was
virtually identical. This made me suspect that one or both of the power
~~?sr,
i"RA't>l1IONAU-'-f, 'B'-f MURPHY'S LAW,
A~Y SE"R.V\CIN6 SHO'R.T"CUTS W~
A1TE:M?r W\U... At..WAVS "BVPASS
™e:.
FAuL--r.. o
regulator transistors , Q901 and Q902
(2SC935), might be leaky. These are
mounted off the board, via a plug and
socket connection.
The plug and socket arrangement
makes it easy to check them but I
could find nothing wrong. I replaced
the original power board and went to
the "B-ADJ" pot, VR901, to see if (1) it
was still working and (2) whether there
was adequate adjustment available.
Well, it was working and I had no
trouble resetting the HT to the correct
105V with a perfectly reasonable pot
setting. In short, all the indications
were that there was nothing wrong
with the power supply, except that
the pot had been incorrectly set. Why
this was so was a mystery at that
point. And why hadn't the replacement board clarified the problem? It
turned out that, for some reason, now
lost in history, it was also wrongly set.
In fact, I put it back in the set and was
able to reset it to 105V as well.
So that cured the gross overscan
and a minor touch up with the appropriate controls produced a well proportioned picture. But we still had
the colour problem. In fact, it was
now worse than ever, in the sense that
it was much harder to brute force than
before (a lesson there, had I but realised it).
The next logical board to check was
the chroma board, "PCB-CHROMA". I
have several on hand and it is a simple un-plug, plug-in operation. But
no joy. Remembering how I had been
caught with a faulty substitute power
board, I tried a second one. But it was
no use; this was not the source of the
trouble.
So what next? Still convinced that
the best approach, at least initially,
was board substitution, I picked what
seemed the next logical one - the
"PCB-VIDEO-CHROMA-OUT" board.
Again, it was a simple operation, or at
least it would have been ifl'd had one
in stock. I didn't as it turned out and I
finished up borrowing one-from a colleague.
Unfortunately, the effort was wasted; there was no joy here either. And
that just about exhausted the board
swapping approach. All the major and
likely sub-boards had been changed,
without result. The two suspect areas
remaining were the main board and
the tuner.
Swapping the main board was
hardly a proposition, at least at this
stage, but I had considered the possi-
bility of a tuner problem causing a
faulty response curve. Fortunately, I
am well set-up to check tuner problems. As mentioned in my last set of
notes, I have a self-contained tuner ·
and power supply which makes an
ideal substitute in such situations. It's
an old turret type VHF tuner from the
monochrome era but it still works perfectly.
The substitution was simple. The
set's own VHF tuner connects to the
IF system, the "PCB-HF" board, via
pin 1 of a 4-pin plug (FA). I simply
pulled the plug off and clipped the
substitute tuner output lead to pin 1.
And that disposed of that theory
quick smart! There was absolutely no
change in the set's behaviour. And it
also put paid to the idea of a simple
fix by substitution. I was going to
have to start working now and track
down the fault the hard way.
So, harking back to my opening
paragraphs, had my short cut attempts
been a waste of time? Not at all; they
had ruled out a very large part of the
set, involving hundreds of components, and forced me to narrow my
investigation to relatively few components.
That said, I wasn't quite sure where
to go from there. The only thing I
could immediately think of was the
colour burst and/or the associated
gating pulse. It was a long shot, because failure here normally means
total loss of colour. However, the CRO
quickly ruled out that theory, as everything was normal here.
What next?
So what hadn't I checked? Perhaps
I had missed something around the
chroma board area and I so I began by
checking the supply voltage. The supply to this board is a nominal 20V
which is derived from the deflection
out board ("PCB-DEFL-OUT") and fed
in on pins 1 & 10.
This voltage was spot on but I then
began wondering about any bypass
capacitors associated with this supply. ThE;lre is one on the chroma board
(pin 10) but this was ruled out because the board had been changed.
Further checks revealed a second bypass capacitor (C579) on the deflection out board, at point "B5". But
again I drew a blank as changing it
made no difference.
I was feeling rather frustrated by
now. In desperation, I went over the
FEBRUARY
1993
65
a problem. It looked like I would need
access to the control board but this,
not to put too fine a point on it, is a
swine of a thing to remove. What was
needed was some other way to conFig.1: the faulty circuit
firm my theory before I took such a
section in the Thom 9007
colour TV set. Transistors
drastic step.
Q623 & Q624 are the first &
So I tried another tack. By suitably
second chroma amplifier
juggling the chassis, I was able to reach
stages respectively, while
the underside of the main board and
VR671 (top) is the colour
bridge pins 15 & 16 of the edge consaturation control. This pot
nector using a clip lead. This was
connects to the chroma
electrically equivalent to turning the
board via a plug connector
saturation control fully up which,
to the main board (labelled
mechanically, was where it had al"MN") & then via pins 15 &
ready been set.
15 of an edge connector
The result was that the picture imsocket.
mediately jumped to full colour saturation. Not only that, but all symptoms of the fault had vanished. I could
now bring in colour - albeit oversaturated - on all channels, without
resorting to the critical tuning procedure previously required.
So at last we had a breakthrough; it
physical arrangement. The
appropriate part of the cir- was somewhere in the saturation con•ce-•ow~ocus (
cuit is reproduced here as trol circuit. But where? It took only a
,
~
I._
Fig.1. The saturation con- moment to shift the clip lead from
trol is a soon pot, VR671, pins 15 & 16 on the edge connector to
which connects to four pins pins 2 & 4 on the main board, whereon the control board. Pins 1 upon the fault was back exactly as
and 3 connect to chassis, before.
pin 2 to the moving arm,
So what was there between the two
and pin 4 to the active end set of pins? Answer: just two copper
of the pot. Plug MN fits on tracks, about 70mm long, and the solthese pins and its cable ter- dered joints to them. I went over all
minates on a second MN these joints but could find nothing
plug which mates with four wrong. Just to make sure, I resoldered
~~i
pins on the main board.
them anyway but I wasn't really surWith me so far? Right.
prised when this had no effect.
The saturation control is
But I was hot on the trail now. By
eventually wired into the pulling the plug off the motherboard
chroma amplifier chain pins, I was able to make a resistance
aroµnd transistors Q624 measurement from pin 4 to pin 16.
and Q625. This is achieved There was no problem there but pin 2
chroma circuit again but without re- by wiring pins 2 and 4 on the main to pin 15 was another story - this
sult. Eventually, I finished up near board to pins 15 and 16 of an edge · measured no less than 100!2 And,
the top lefthand corner of the chroma connector and this, in turn, carries seeing that I had already cleared the
circuit, looking at a 4-pin plug on the the chroma board. Pins 1 and 3 con- soldered joints, this left a faulty copnect to chassis via the main board.
per track as the only possibility.
main board marked "MN".
I had traced out all this circuitry
Yet try as I might, I could see nothThis plug connects to the colour
saturation control which, along with because I wondered whether the fault ing wrong with it, even with a magnithe contrast, brightness, vertical hold was somewhere in the saturation con- fying glass. Granted, the green lacand horizontal hold controls, is trol circuit. Ifit was, there were plenty quer covering the print didn't help
mounted on a sub-board designated of places where it could occur: either and, in any case, I was well past car"PCB-CONTROL". It sits at the front of in the pot itself, in the plugs, between ing about the fine detail. All I wanted
the set, beneath the picture tube, and the chroma board and the edge con- was to get the job finished.
It was simple enough, of course. A
the controls are all connected to the nector, or in the soldered connections
appropriate parts of the circuit via to the boards.
short length of tinned copper wire
plugs, sockets and cables.
between the two pins effectively
In order to follow what happened Pure speculation
wiped out the offending resistance
next, it is important to get a clear
The trouble was, this was all pure for all time. I then put everything
picture of both the circuit and the speculation and proving it presented back together again, satisfied that the
SERVICEMAN'S LOG - CTD
r
66
SILICON CHIP
_____
fault had been positively identif.ied
and fixed.
Why was the HT high?
And that, as far as the job itself was
concerned, was that. But there is a
little more to the story than just the
colour fault. What was the history of
the fault and why had I found the HT
rail wound up to 138V?
As I mentioned at the beginning,
this motel is a relatively new customer and this was the first time I had
seen this particular set. Prior to that,
the motel had employed a serviceman from an adjacent suburb. I know
the party concerned. He's a nice
enough bloke at a social level and, I
have no doubt, is very kind to his
mother.
But his technical expertise is another matter. In short, I don't have a
very high regard for it.
So what happened? I can only
speculate, of course, but I think it
went something like this. The board
was probably cracked right from the
start but functioned OK while the two
edges of the copper track were bright
and touching. But eventually the copper corroded and this introduced
It had him beaten until he discovered,
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on shaft under centre pulley. The
pulley contains a spring loaded
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enough resistance to upset the colour
to some extent, at least on an intermittent basis.
It was too much for our serviceman
friend, although I would be the first to
concede that it was a very nasty fault.
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probably by accident, that he could
cure it by jacking up the HT. So that
was what he did, hoping that the resultant overscan would go unnoticed
- which it did.
Eventually, of course, the corrosion
increased and the fault re-appeared,
intermittently at first, then permanently. And that's where I came in.
Am I being too hard? I don't think
so. I would be the first to agree that
anyone can be stumped by a stinker
and I'm not criticising on that basis.
But faking a cure is another matter.
Apart from the fact that the customer
is being cheated, it simply doesn't
pay off in a purely business sense.
Sooner or later it backfires and the
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After my two previous stories about
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monitor repairs and before long I had
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FEBR{!ARY
1993
67
SERVICEMAN'S LOG - CTD
~
c::=:::::::,
~ c:>C::::.~
another customer appealing for help
with a suspect monitor.
It turned out to be another Acer
MM211 - a fact which made me feel
almost cocky about tackling it. After
all, I now knew where all the screws
were. On the other hand, it seemed to
be tempting fate to hope that it would
be another power supply fault. '
Even so, the symptoms as related
by the customer seemed to support
that theory. The fault had first appeared as a collapsed vertical scan,
resulting in a bright horizontal line
across the middle of the screen. It had
lasted only a second or so, the monitor then reverting to a normal display.
In fact, the customer at that time
wasn't even sure whether there was a
fault or whether he had simply observed a brief mains power failure.
When the fault subsequently reoccurred several more times, he knew
he had a problem. On the other hand,
he wasn't sure what to do about it. He
knew enough about intermittent faults
68
SILICON CHIP
to realise that tracking them down
could take days, or even weeks. And
as he was using the computer professionally, he couldn't afford to have it
out of action for a long period.
So he kept flogging it along while
ever it would go until, inevitably, it
failed completely. And that was the
best thing that could have happened
really. At least ·I had something to
work on.
With the cover removed, I went
straight to the 12V rail where it
emerges from the 12Vregulator. It was
dead, so I moved back to the input
and measured 20V which, from my
previous experience, was spot on.
Crook regulator
My immediate conclusion was that
the regulator had packed it in but I
checked the 12V rail for shorts, just to
be sure. There was nothing wrong
there, which left only the regulator.
This was mounted on a 75mm-long
finned heatsink which is attached to
the PC board by a couple of screws
tapped into one end. The regulator IC
was mounted near the bottom of this
heatsink, with its three leads passing
through holes in the board to the copper pattern on the underside.
The first problem was to identify
the IC. I can't remember the type
number but it was a new one on me
and so I sought the help of the friendly
man behind the counter at the local
Dick Smith Electronics store. It turned
out that he was quite familiar with
the unit and the circuit but not with
the regulator IC. No problem, though.
A quick phone call to the service department at headquarters and he had
the answer. The original type number
was not available but it could be replaced by a 7812. This is a regular
stock item and he pushed one across
the counter.
I expected that that would virtually
be the end of the story, the repair
involving no more than a routine replacement. But not quite. Not being
familiar with the faulty IC, I took the
precaution of tracing the copper pattern around the pin connections before I removed it.
And just as well. Having identified
the pin connections, I compared them
with the 7812 arrangement and found,
as I feared, that they were not the
same; two of the pin connections were
transposed.
This was an annoying development
because it meant a messy repair. The
IC would have to adopt a crossed leg
attitude and I would probably have to
attach an extension lead to one leg to
facilitate this.
I was trying to figure out how best
to do this when I noticed an extra set
of unused holes adjacent to those for
the existing regulator but which had
no apparent purpose. When I traced
the copper pattern around these holes,
it quickly confirmed what I had begun to hope. The makers had anticipated this situation and had provided
a second set of connections to suit the
alternative pin arrangement.
From there on, it was plain sailing.
I mounted the IC on the heatsink,
soldered its leads to the alternative
pads, and put everything back together
again. And, of course, it worked and I
had another happy customer.
But I swore him to secrecy. I don't
want any more computer monitors,
thank you. And they're making them
in colour now, aren't they? Ouch! SC
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