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SERVICEMAN'S LOG
All it needs is a new fuse
Some politician – whose name now escapes
me – once decreed that “life wasn’t meant to be
easy”. I don’t know who he was blaming for this
situation but from where I stand, there are two
who share some responsibility: TV set designers
& customers.
OK, so that’s a sweeping statement.
Only a few customers make life less
than easy and not all set designers are
to be similarly condemned. But there
are always some and when they get
together on the one job, it is no longer
“life wasn’t meant to be easy” – it is
life was meant to be hard.
What started this grouch? Answer:
the bloke who designed a Mitsubishi
projection TV set (model VS-360A)
and the proprietor of the local pub,
who owned the monster. And I use
that word monster advisedly; Frank
enstein couldn’t have done better.
Granted, projection TV sets are not
my strong point. More correctly, I have
never had to service one before this,
which probably didn’t help.
It all started about 12 months ago
with a phone call from the aforementioned establishment, explaining that
they needed to have the set serviced.
And the caller added, in a most
authoritative manner, that “all it needs
is a new fuse”.
Well, that started things off on the
wrong foot. People who insist that they
have diagnosed the fault, particularly
as stupidly as this, really annoy me. I
felt like saying, “well you replace the
fuse”. Instead, I replied that maybe a
fuse had blown but that this would
be only the result of a problem; the
cause of the fuse blowing was the real
problem.
But no, he wouldn’t have a bar of
that. All the set needed was new fuse
and he wanted me to come up and fit it.
Well, there was no way I was going to
be in that. I knew the set well enough
by sight and I can remember when it
was installed, about 10 years ago.
It is in one of the lounges and there
was no way that one could work on it
there due to the lighting and general
at
mosphere, to say nothing of the
equipment that I might need. So I had
to explain that the only way I could
service the set was in the workshop.
What’s more, he would have to arrange
delivery. I wasn’t being hard to get
on with, just practical; there simply
wasn’t enough room in my own van
for the monster. And, in any case, there
was no way I could handle the thing at
either end; it was far too big and heavy
for one person.
On the other hand, I knew that
the hotel had a utility and plenty of
manpower, so that it shouldn’t be a
problem for them.
Well, he hemmed and hawed about
that. He insisted that all it needed was
a fuse and that I should be able to fix
it in a few minutes. But I stuck to my
guns; it needed to be in the workshop
if I was to service it. He said he would
get back to me.
The monster arrives
Fig.1: the layout of the projection system in the Mitsubishi VS-360A, as
given in the manual. The internal shelf is not shown. It runs from the
back, about halfway to the front, above the projection tubes.
76 Silicon Chip
That was the last I heard of the
matter for about three months. Then
suddenly, one morning, he turned
up with the monster in the ute, along
with a couple of brawny blokes from
the local football club who manhandled it (the monster not the ute) into
the shop.
And he was still insisting that all
the set needed was a fuse. This record
was getting a bit worn by now and I
asked him on what grounds he based
this assertion. And then it came out.
It appeared that after I had insisted
that I could only service the set in the
workshop, he had called one of my
colleagues.
He had agreed to go to the hotel,
had found a blown fuse, and made the
appropriate replacement. And the set
had come to life. And that, of course,
proved that all it needed was a fuse.
The only snag was, it didn’t work
for long. I don’t know for how long
– he was a bit cagey about this – but
I strongly suspect it was only for a
few switch-on cycles. Anyway, all he
wanted now was another fuse fitted.
And try as I might, no explanation
would convince him otherwise. So I
said, “leave it with me.”
And so it was that I found myself
saddled with the monster. Initially, I
wasn’t quite sure how best to get inside
the cabinet, or what I would find when
I did. I had no technical data of any
kind and only the vaguest idea of the
likely layout.
The most obvious entry point at this
stage was a rear cover which extended
from near the bottom of the cabinet to
a point just below the bottom of the
top mirror, as shown in Fig.1. All I
had to do was remove the 16 screws
holding it.
This provided some access but it
was still quite restricted. The main
restriction was a shelf running the
width of the cabinet from the back,
above the picture tubes, and about
halfway towards the front. Its main
role appeared to be to provide support
points for the three projection tubes.
These are high intensity types, about
150mm in diameter, and are mounted
in line. The various boards, including
the power supply, were mounted on
the cabinet floor or on the sides of the
cabinet.
To get any kind of view of these
boards, I had to get my head halfway
into the cabinet, after which I was
able to see two fuses on the power
supply board and, with some difficulty, was able to extract them. They
were 2A slow blow types located in
the mains leads and, yes, one was
blown.
I suspected that the fault might be
intermittent – or had been – so I fitted
a new fuse and applied power. But the
result was an instant splat. So much
for the amateur diagnosis.
At this point it was obvious that I
could go no further without a manual.
I rang the Mitsubishi service department and contacted a very helpful staff
member. He confirmed that a manual
was available, price $36, plus $6 for
postage.
I also took the opportunity to
discuss access from the front of the
cabinet. I suspected that a front panel
could be removed and, in fact, had
noted that there seemed to be small
cover missing from the bottom of the
cabinet front and that there were two
large bolt heads in this area.
While it seemed likely that these
were holding the front panel, their
size suggested that they might be also
May 1995 77
SERVICEMAN’S LOG – CTD
Fig.2: this diagram shows the mains, power transformer & voltage
selector wiring in the VS-360A. The voltage selector selects the
appropriate tap on the power transformer.
holding something larger. I had no
desire to undo them and hear three
picture tubes crash to the bottom of
the cabinet.
I needn’t have worried. The Mit
subishi technician confirmed that
these were to ones to unscrew. He then
asked me what the problem was. When
I explained that the set was blowing
mains fuses, he didn’t seem all that
surprised. And he went on suggest that
I should check the power transformer
for a brownish goo or varnish with
which it might be covered.
Apparently this substance can cause
corrosion problems and, in fact, I can
recall something similar which caused
trouble in a video recorder. Anyway,
he suggested that this may have caused
a tracking problem and that it might
be cured by scraping away any obvious track.
Well, that was a help. I ordered a
manual and, while waiting for it, took
the technician’s advice and unscrewed
the two bolts in the front. This freed
the front panel which extends from
the bottom up to various user controls
beneath the screen area. This provided much better access but there was
still a mass of boards, interconnecting
cables, plugs and sockets, which I
had to sort out. I put it aside until the
manual arrived.
78 Silicon Chip
When it did, I was able to sort
things out in a little more detail. The
horizontal and vertical output boards
covered a large part of the cabinet floor.
And, looking from the front, the power
supply board was on the left, tucked
in behind them.
The main component, the power
transformer, weighing several kilograms, was bolted to the cabinet floor.
Other parts of the power supply were
mounted on brackets. Screwed to the
side of the cabinet, on the left, in front
of the power supply, was a large board
carrying the convergence circuitry,
and screwed to the side of the cabinet
on the right was the signal processing
board.
Power supply
Having sorted all that out, I faced
the real task of trying to track down
what was obviously a short circuit.
The accompanying circuit shows the
relevant part of the power supply.
To the right of centre is the power
plug, a two pole on/off switch, and a
plug/socket combination marked BA.
Next in line are the two fuses already
mentioned, while a voltage selector
socket at extreme left connects to the
transformer primary via plug/sockets
BC and BB.
There is also a second, smaller
transformer at extreme right, which is
fed via plug/socket BD. It is part of the
remote control system. It was one of
the first things to be unplugged.
The main transformer primary is
on the lower side of the core, with
200V, 220V, 240V and 260V tappings.
The secondaries on the upper side
connect to various plug/socket combinations.
This arrangement of plugs and sockets was doubtless intended to simplify
assembly and servicing and it does up
to a point. The difficulty was to identify all of these in cramped conditions,
at least for someone tackling it for the
first time.
As a result, I spent considerable
time and a fair amount of physical
effort – to say nothing of scoring a
few barked knuckles – sorting out all
these connections and making sure
everything was disconnected from the
transformer secondary.
When finally satisfied that this was
so, I pulled plug/socket BB apart,
which gave me direct access to the
primary tappings. I then made up a
dummy power lead, complete with
fuse, and connected it directly to the
common and 240V taps.
An instant splat
Then came the moment of truth. I
plugged it in and switched on. Again
there was an instant splat. And that
settled it; it was the transformer and
it had to come out. Whether it could
be salvaged or not still had to be determined.
Again, the problem was mainly
physical. As already mentioned, the
transformer was bolted to the cabinet
floor, which meant that I had to fit
a spanner at both ends; one in the
cabinet and one underneath it. There
simply wasn’t enough space under the
cabinet and I finished up having to tilt
it and prop one side up on a couple of
wooden blocks.
Even then it a was tedious procedure but I eventually had it free and
was able to pull it out and set it up
on the bench. And the Mitsubishi
technician was right; it was covered
with a brown varnish and there was
a dark patch between the primary
terminal lugs.
I spent some time scraping all this
discoloured varnish away, then tried
it again with the dummy lead. But it
was no use; there was another splat
and another fuse was written off. I
A shocked customer
I had some doubts as to whether the
hotel would accept this.
And I wasn’t far wrong. They were
clearly shocked at the figure but I
pointed out that there was really little
option. If they were not prepared to
spend that amount, then it was for
the scrap heap. Finally, they said they
would think about it.
Well, they “thought about it” for
some six or seven weeks and I was
heartily sick of having the thing cluttering up the workshop. But finally
they rang and said, “fix it.”
So I ordered the replacement transformer, which arrived in about a week,
then set about putting it all back together. This wasn’t quite so bad; after
all I’d been there before and learned
some of the tricks and traps.
Then came the moment of truth;
would it work? Well, it did and quite
well in fact – for a projection system.
I gave it a grey scale and convergence
routine and the hotel sent down their
ute and another team of footballers
to take it home. And I made one final
check when it was back in the lounge,
giving the convergence a final tweak.
It appears that this adjustment is quite
sensitive and the journey in the ute
had upset it a little.
So they paid the bill – just on $600
– along with some general comments
about how expensive everything is
these days.
Well, it was a pretty expensive fuse.
The rejuvenated National
My next story is about a National TV
set, model TC-2138, using an M14H
chassis. These sets first appeared
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probed around a bit further but I was
obviously flogging a dead horse. As
far as I could see, the winding was
damaged internally. Not only was it a
write-off but I now felt quite sure that
it had originally been intermittent,
leading to the pointless replacing of
fuses.
So it was back to Mitsubishi to
determine the availability of a new
transformer and its price. I was concerned about this latter point, feeling
that it would be quite costly. And I
was right. It was available but the cost,
including freight, was just short of
$400. Add in the labour costs – what
had been done and what was yet to
be done – and it was making a mess
of $600.
May 1995 79
SERVICEMAN’S LOG – CTD
somewhere around 1987 and this was
one of that vintage. It belongs to a family that are long-standing customers
and I have serviced it for a couple of
routine faults over the years. It gave
pretty good service until about 18
months ago. Then it failed completely,
due to some minor fault, and landed
on my bench.
A dismal performance
As I say, the basic fault was minor,
and I soon had it up and running. But
it gave a pretty dismal performance.
In simple terms, the picture tube had
“had it”. It was so bad that I felt there
were only two possible approaches: to
try rejuvenating it or, if that failed, fit
a new tube. I talked it over with the
customer and suggested that since the
tube was so sick, there was little to lose
by trying rejuvenation.
Granted, there are always risks with
this approach. One is that it simply
won’t work; that the tube will be no
better off after treatment. The other
is that the tube may suffer further
damage and be a complete write off.
But, either way, the final result is
much the same; a new tube will have
80 Silicon Chip
to be fitted, if not immediately, then
in a few month’s time when the user
can no longer tolerate it.
I explained all this to the customer
and he saw the wisdom of rejuvenation. So I fished out the rejuvenator
and went ahead with the job. Basically, it consists of applying 700-800V
between grid 1 and cathode and this
should, hopefully, blast any accumu
lated rubbish off the cathode (usually
with a display of fireworks). Of course,
this is a somewhat simplified explanation but it will suffice for this story.
And it worked this time. Granted,
the performance was short of “as-new”
but it was still a vast improvement
on what it had been. And when the
customer and his wife saw the result
they were highly delighted; they had
obviously forgotten what a good pic
ture looked like.
But I had to add a final word of
warning; I had no idea how long the
improvement would last. I’ve known
it to last for a couple of years but
I’ve also seen it pack up after a few
months. There is no way to tell and
the customer accepted this.
With this in mind, I told him that
if – or, really, when – it failed again, to
bring the set back; that I could probably do a good deal on a picture tube,
adding that I had one on hand from a
wrecked set. And I emphasised that his
set was otherwise in good condition
and worthy of a replacement tube.
The set that fell
And that brings me to part two of
the story – a part which has its humorous side. This started about five
years previously. It concerns another
TC-2138 set, only about 12 month’s old
at the time, which belonged to a local
club. This was located in a convenient
viewing position on a small shelf, a
couple of metres above the floor, in
one of the club’s lounges.
Don’t ask me how it happened –
lots of strange things happen in such
establishments – but someone knock
ed it off the shelf. And apparently it
landed on a pretty hard surface which,
combined with the 2-metre drop,
didn’t do it any good.
The first I knew about this was when
one of the club’s staff walked into the
shop carrying the set’s original cardboard carton Naturally, I expected a
TV set requiring conventional service.
He put the carton on the counter
and said. “Can you fix this for us – it’s
had a fall.”
Even then I didn’t wake up; I imagined it had been simply knocked
over or, at worst, taken a gentle tumble
due to someone losing their grip while
carrying it.
Until I opened the carton, that is.
And what a sight. It was just a mass
of bits. These sets are housed in the
usual chipboard cabinet and all four
sides were now separate pieces, the
glued joints having failed. The plastic
back was broken, as was the mask
around the tube. And when I took a
closer look at the innards, I realised
that the mother board was cracked
right across.
It was all I could
do not to burst out
laughing; surely he wasn ’t
serious? It reminded me of
all the stories
I’d heard about
amateur clock repairers, who finish up with
a cardboard box full of bits on the
local jeweller’s counter. I never did
believe those stories but here was
something that would top them all.
I’m afraid I recoiled in disbelief and
this was probably obvious. “You don’t
seriously expect me to repair this?”
It was the customer’s turn to express
disbelief. “Aw, y’should be able to glue
the cabinet back together.”
Well, I had to agree that, yes, one
could probably glue the pieces back
together but when I tried to explain
about the cracked board and other obvious damage, I realised I was battling
against the wind.
So I took the diplomatic step of
saying, “leave it with me, I’ll have a
look at it”. That at least gave me time
to think.
A hopeless case
In fact, I did go through the motions
of taking another look. Of course, it
was hopeless. The motherboard was
cracked from front to back, taking an
erratic path around the horizontal
output transformer, with a number
of secondary cracks radiating from
this area.
But these were only the visible
faults. Even assuming they could be
repaired – or replaced – there was
always the risk that more damage
would come to light as one progressed.
And what about the picture tube?
Strangely enough, it was physically
intact but what had such a fall done to
its insides? Even if it still functioned
electrically, there was a real risk that
the shadow mask had been distorted
and it would also be a write off.
So I rang the club, contacted someone who seemed better able to appreciate the seriousness of the accident,
and laid it on the line. There was no
way that the set could be repaired and
they would be well advised to cut their
losses and buy a new set. This advice
was accepted and I was thanked me
for my trouble.
Well, that was the end of that
part of the story, except that I
was left with a carton of broken
bits. There was almost nothing
left worth salvaging and I sent
most of it off to the tip. But I
hung on to the picture tube. It
was intact, the scan coils were
still fitted (also apparently intact), and shaking it failed to reveal
any internal rattles.
And so I tucked it away under the
bench, hoping that I might get another
TC-2138 through the shop so that I
could test it. Well, it was over a year
later but such a set did turn up. And
as soon as I had it working, I fished
out the spare tube and propped it up
on the bench with its back facing the
back of the set.
It was the simplest possible operation. All the set’s leads were long
enough to reach my tube without
any fiddling and, in no time at all, I
had the whole arrangement up and
running. And not only did the tube
and assembly work but everything
was spot on and the picture bright
and sparkling. I gave it a purity check
and even this failed to reveal any
problems.
So, not only did I have a spare tube
in good condition, but one complete
with deflection coils and all adjustments spot on. It was, literally, a
perfect plug-in replacement.
So it went back into stock and I
thought no more about is until my
aforementioned customer turned up
with his sick tube, and I went through
the rejuvenation process. Had this not
worked, then the tube from the junked
set was a good candidate. But it did
work and I thought the customer might
as well get his money’s worth for a
few months.
In fact, it was some 18 months
before I heard from the family again.
Then the wife was on the phone one
morning and opened the conversation with the rather matter-of-fact
statement that the old TV set had given
up the ghost at last. But this was only
a prelude to the main reason for the
call, which was to ask my advice about
buying a new set.
Probing a little deeper, I learned
that “we stuck the old set down in
the garage because we can’t use it any
more”. Apparently, my suggestion
about a replacement tube had been
forgotten. I immediately enquired as
to whether the set was still actually
working and was assured that it was,
but that the picture was now quite
unwatchable.
I then reminded the lady about my
offer of a replacement tube and suggested that they bring the set in and
let me look at it. At the same time, I
pointed out that I could probably restore the set for a good deal less than
the $700 or more that they would be
looking at for a new set.
The set returns
And so the set duly landed back on
my bench again. A quick check confirmed the situation. Yes, the set was
still working and yes, the picture was
crook – real crook.
I pulled the old tube out, fitted the
spare tube and connected the various
cables. The whole operation took less
that half an hour and resulted in a firstrate picture. And, since the scan coils
were still as originally assembled and
adjusted, no convergence adjustments
were required.
All I had to do was reset the grey
scale, these adjustments having been
juggled for best results after the original tube had been rejuvenated. This
turned out to be perfectly routine and
the end result was excellent.
When the owners came in to view
the result and collect the set, they were
delighted because the performance
was virtually that of a new set. But
I was perfectly honest with them,
pointing out that it was a secondhand
tube with about 12 month’s use. I also
explained how I had acquired it.
How much did it all cost? I charged
them $150; $75 for the tube and $75
labour, which I felt was reasonable.
And the custom
ers thought so too;
they were more than happy with the
whole transaction.
So everybody was happy; they had
a first-class working set for a modest
outlay and I had made a few dollars
on a tube I had scored and carefully
SC
stored for several years.
May 1995 81
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