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SERVICEMAN'S LOG
It was all a long time ago
I have two quite unusual stories this month.
For how long & by what devious means can
one keep a TV set working? And for how long
should one keep customers’ records? The other
story emphasises how lost we can feel when
away from our own workbench.
The first story really started over
11 years ago. It was little more than a
routine job then and I certainly did not
imagine that I would be resurrecting
it after all this time. But I did make
notes about it; not only in the normal
way for my own benefit but also for a
colleague who expressed some interest
in it. So I was able to recall the events
in reasonable detail.
It was all brought back by a recent
phone call. A lady at the other end of
the line introduced herself – the name
didn’t register immediately – and went
on to remind me that I had fixed her
TV set, an HMV, “some time ago”, and
that it involved a problem with the
green in the picture.
My memory isn’t particularly good
on some-time-ago jobs (one handles
so many jobs) but the mention of the
green problem rang a bell. I asked her
if she was living at a particular address
at the time – it’s funny the way one’s
memory works at times.
Yes, that was correct. And the whole
story came flooding back. In particular,
I remembered how accurately she had
described the symptoms; far more so
than most people. And I also registered
that it was more than some time ago; it
was long time ago. In fact, I had begun
to link it with other events and was
thinking in terms of six or eight years,
which wasn’t that far out.
But despite the fact that I remembered the set and its symptoms, I could
not recall the fault itself or what I had
done to fix it. Anyway, I regarded all
that as being of little importance and
asked what problem she had now. To
my surprise, she replied that the HMV
set was the problem.
Was that set still going after all these
years? Yes, it was but, on her own
admission, apparently not very well.
Her son, who works for the ABC in
another state, had visited her recently
and commented that the picture – and
particularly the colour – was rather
poor. In short, he realised that gradual
picture deterioration had crept up on
Fig.1: the RGB driver stage in the HMV 12642. The green driver
stage, X01, is at the top of the diagram & is direct coupled to the
green gun cathode. Doctoring this stage restored performance.
56 Silicon Chip
her over the years and suggested that
it was time she bought a new set.
And that was the purpose of the call;
not to initiate any service to the set
but to seek my advice regarding a new
set. In particular, she needed to know
which brands I handled on a warranty
basis because she wanted to be sure I
would be available to service the set.
Such loyalty can be quite touching
at times.
So that was more or less it. I
nominated the brands I handle on a
warranty basis and left it to her to
choose which ever one she fancied.
She thanked me and we left it at that.
How long ago was it?
And that should have been the end
of it. But I couldn’t get the set out of
my mind. Just how long ago was it? So
I began searching through my old files.
It turned out to be quite a search but I
eventually found the relevant file and
a copy of the invoice, dated September
1st, 1982 – over 11 years ago at the
time of writing. But that wasn’t the real
surprise; that was only appreciated
when I read my notes about the fault
and its cure.
The set was a 48cm HMV, model
12642 (which was also marketed as
a JVC 7765AU), and it was about five
years old at the time. The fault, as
described by the customer, was that
the picture had lost its green content.
And I had noted that this customer was
more astute than most; her description
was completely accurate.
In greater detail, she explained how
the set had turned on a display of multi-coloured fireworks on the screen.
This display lasted for a few seconds
and the set then behaved normally;
except that there was no green.
As recounted in my notes, there was
initially some confusion on my part as
to the exact cause. At first I suspected
some kind of green gun failure but, in
fact, I found I was able to produce a
green image using various brute force
tactics.
This led me to suspect a circuit
fault, whereby the tube was not receiving adequate green signals, and I
spent some time following this lead,
only to finally conclude that it was
false; that the fault had to be in the
tube. More specifically, I concluded
that the fireworks display had been
caused by a fragment – probably
loose cathode material – causing a
momentary internal short. And this
had damaged the green cathode,
reducing it’s emission to the point
where it was virtually cut off at the
normal bias level.
So was it worthwhile fitting a new
tube or was the set a write-off? It was
a marginal situation and the lady
was not very happy about spending
a couple of hundred dollars on a replacement tube, much less the cost of
a new set.
Finally, I decided to take a punt on
a mild form of butchery. As matters
stood, the tube and – potentially – the
set were both a write off. But suppose I
were to doctor the bias voltage; could I
brute force the green back to a normal
level? And, if so, how long would such
a trick last?
The first question was easily answered. Reference to the circuit (see
Fig.1) shows that the collector of the
green driver transistor (X01) is directly
coupled to the green gun cathode and
sets it at about 142V. By shunting R01,
a 1.2kΩ resistor in the emitter circuit
of X01, the current through X01 would
be increased, its collector voltage reduced, and the bias on the green gun
reduced. And it worked. After some
trial and error, I settled for a 5.6kΩ
resistor across R01, which produced
virtually normal green performance.
The second question was another
matter. I had no way of knowing how
long it would last but I reckoned a
minimum of 12 months would be a
reasonable guess. Anything after that
would be cream on the custard. And
so, on that basis, I felt that this ap
proach was justified. It would give the
customer time to assess her financial
situation and decide on a new tube
or a new set.
I was most careful to explain what
I had done; that it was a short-term
measure that could only be justified
on a nothing-to-lose basis. Assuming
she was happy, I would leave it like
that. She said she was and so I did.
As I recall, I checked the situation
about a year later and the set was still
going strong. And that was the last I
heard of it – until now. But another 10
years? That’s a lot of cream!
Curiosity killed the ...
Having sorted out all those memories, another thought occurred to me.
Would the lady let me borrow the set,
to check it and satisfy my own curiosity as to just how well it had stood
up over all those years?
So, a week or so later, when making
January 1994 57
a call in the area, I took a punt and
knocked on the door. When the lady
answered I explained that I was simply making a courtesy call in case she
needed any more advice about a new
set and to ask whether I could have
a look at the old set out of curiosity.
She greeted me warmly and invited
me in to see her new set. It turned
out to be a 48cm Samsung and she
was very happy with it. Then a tray
appeared carrying the inevitable cuppa accoutrements – teapot, cups and
saucers, and chocolate biscuits. And
so we sat and chatted for a while.
Eventually, I raised the matter of the
old set again. Yes, of course, it was in
the back room. And I could have it if
I wanted it; she was a loss to know
what to do with it. I went through
the motions of protesting but she was
adamant; take it away.
And so, after a pleasant interlude,
I came away with the old set. I didn’t
regard it as valuable in the financial
sense but I valued it for what I might
learn from it.
Back at the shop, I lost no time in
setting it up. In most respects, its performance was first class – a good sharp
picture, excellent geometry, normal
sound, no tuner problems and no noisy
controls. It was quite remarkable for a
16-year old set, with only one service
job in that time.
But the colour – yes, that was crook.
I fed in a colour bar pattern and, as
58 Silicon Chip
in the original case, the green was
very weak. Also, the red was flaring
but the blue was about normal. Just
for the heck of it, I went through the
motions of grey scaling and this improved things a little. But it was still
very poor.
Picture tube rejuvenation
So what now? Did I have any more
tricks? Well, there was one other possibility but it was a Sydney-or-the-bush
approach. What about a spot of picture
tube rejuvenation?
There are various devices for this
but the basic principle is pretty much
the same. Normally, the heater is run
at its rated voltage, although some
authorities recommend overrating it
somewhat. I prefer not to. After that,
a voltage of between 600V and 700V
is applied to the grid for about two
seconds. This typically creates some
brief fireworks around the cathode
area and the idea is to keep applying
short bursts until this activity ceases.
It’s very much a gamble. Sometimes
it works and sometimes it doesn’t. And
even when it does, there is no guarantee as to how long the effect will last.
But what did I have to lose?
So I set up the booster and went
over each gun in turn. Significantly,
I needed to give the green gun about
eight bursts before all the fireworks
ceased. For the others, one or two
bursts were sufficient.
Next, I went through the grey
scaling process again. It was much
improved now; almost good enough,
in fact. But there was one limitation
which is common with weak tubes,
whereby the grey scale tends to vary
with the setting of the brightness
control.
In this case, turning the brightness
down would increase the green level
in the lowlights, while turning it up
had tended to emphasise the red. The
best that one can do is try to balance
things at what would be regarded as a
typical brightness viewing level.
So this was what I did. I then let it
run in the workshop for a couple of
days to see how it would hold, this
being the critical aspect of tube boosting. And it did shift, again involving
the green.
I gave the green gun a second short
boost and grey scaled it again, which
again improved things a little. So
that’s where it stands at the time of
writing. I hope it might stabilise a little
more with time but I know I’m being
optimistic.
So of what use is it now? Well,
not much for serious bench work,
although it could serve as a loan set
in an emergency. But what I am really
hoping is that I might score a suitable
tube from a set written off for other
reasons. If that should happen, I may
have a set that’s good for a few more
years – but not 11!
Serviceman’s holiday
And now for a complete change
of scene – literally. After several
months of planning, I recently set off
on a leisurely trip up the New South
Wales north coast on what was partly
a holiday and partly a business trip.
Among other things, I had been invited to stop over for a few days with
a family I have known for many years
but had not actually seen for quite
some time, although we have kept in
touch. And I had little doubt that they
would have an array of electrical and
electronic jobs lined up for me.
And so I had packed as much
gear as was practical – multimeter,
soldering equipment, small tools
and an assortment of likely minor
components. Outside of that, I could
only hope.
And so I found myself settled in and
we spent some hours mulling over
old times and catching up on all that
had been happening. But then came
the practical problems of the present,
in the form of a video recorder which
had begun to play up just a few days
before I arrived.
It was a Sharp model VC-505X, a
model with which I have had very
little experience and, without a man
ual, I was starting behind scratch. The
problem itself was that, at times, the
machine would go into the play – or
record – mode briefly, then shut itself
down. And it was similarly erratic in
fast forward or rewind modes.
Anyway, the recorder was taken
out of its cabinet and set up on a
small table. Since the problem was
a mechanical one, there was no need
for a TV set connection at that stage. I
pulled the cover off, pushed in a tape,
and set it running.
It loaded and ran normally the first
time and for the next several tries
but then suddenly baulked. And the
primary reason was immediately obvious; the take-up reel had stopped and
the take-up reel sensor had shut the
machine down. We then tried the fast
forward and rewind functions, with
the same results.
Well, there was obviously a fault
somewhere in the reel drive mech-
anism and this was the first hurdle.
As I mentioned earlier, I am not
very familiar with this machine; all
I knew was that it used an unusual
reel drive system. Most machines use
a rubber-tyred idler wheel and this
is supported on the end of a short
arm which toggles to one side or the
other and engages the wheel with the
appropriate reel drive.
Instead of the tyred wheel, this
machine has a gear wheel mounted on
the end of the arm. This toggles from
side to side in a similar manner and
engages a matching gear in the reel
drive train. But that’s not all; as well
as moving from side to side, the arm
carrying this gear also moves up and
down. This movement is probably necessary to ensure smooth engagement
of the gears.
That much established, I took a
breather. In addition to the main
problem, I was also aware of several
routine things that needed to be done.
One was a general clean-up of the
capstan, heads and guides, which
were a trifle grotty. Another was to
fit new belts.
There are only three belts in this
machine; one from the capstan motor
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to the capstan drum and flywheel and
two associated with the loading motor.
They weren’t in bad nick but since it
would probably be long time before I
serviced the machine again, it seemed
prudent to replace them if possible.
Of course I didn’t have any such
belts with me, or any cleaning alcohol.
So, when the family announced a visit
to the nearest large shopping centre,
I jumped at the opportunity to try for
what I needed.
Medicinal alcohol
Well, I was lucky. I found a TV
service centre which had suitable
belts and an obliging chemist who
dispensed a small bottle of medicinal
alcohol. Thus equipped, I completed
the various routine tasks without
incident but the mechanical problem
remained a mystery. The problem was
that the mechanism would not function at all unless there was a cassette
in it and when there was a cassette
in place, it was impossible to see the
mechanism.
The solution was crude but effective. My friend had several old tapes
on hand which had been put aside due
to wear or other faults. So I pulled one
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January 1994 59
of these apart, removed the tape, and
cut a small opening in the bottom. The
result was a crude version of the commercial transparent dummy cassette I
use in my workshop.
With this in place, it all became
clear. The reason the gear was not
engaging was simply because the arm
holding it was not lifting through the
full distance. Well, that took me one
step closer but the reason for this failure still had to be determined.
In the event, this was the easy part.
Closer examination revealed that the
vertical shaft on which the gear arm
moved up and down had become
gummy due to some kind of lubricant,
applied either during production or
subsequently.
A good swabbing down with alcohol
cleaned this away and that was the
answer. I chose not to lubricate it again;
it was a plastic bearing moving up and
down on a metal shaft and I seriously
question the need to lubricate such a
simple movement.
Broken tape
So that was that problem solved.
But hardly had I heaved a mental sigh
of relief than another one landed on
my plate. It was a broken tape. More
specifically, it was a recorded program
60 Silicon Chip
which, while of no longterm value, was of considerable immediate interest.
But they had seen only the
first 15 minutes or so of it
when the tape broke.
As nearly as I could
work out, the tape had
been running for about 15
minutes when it became
necessary to stop play for
some reason. But then the
recorder went cranky and
would not start again. In
the process, it had formed
a small tape loop before
the machine shut down.
This loop then became
tangled and the tape broke
when the cassette was
ejected from the machine.
While not a common
problem, it is not the
first time a customer has
turned up with a broken
tape, begging for help. On
my own bench it would
be no problem. After the
first couple of incidents,
I invested in cheap tape
splicer. It is a simple jig which holds
the tape with the broken ends overlapping. An angled slot then guides
a sharp blade to make a clean cut
through both layers. The two ends
now left butting together are joined
with an adhesive patch.
In spite of the jig’s low cost, it works
quite well and has helped several
customers. Prior to that, I had resorted
to a more primitive approach. This
involved making a simple overlap
joint using an acetone-based adhesive, on the understanding that this
joint would be used only to play the
remainder of the tape and that it would
not pass over the heads or capstan.
Crude though it was, this idea
worked too and it seemed that I would
have to resort to a similar trick in this
case. My friend’s workshop yielded a
tube of clear acetone based adhesive
and I went to work on the break, finishing up with reasonably neat joint.
But I had no time to test it. It was time
for me to leave and I wanted the joint
to be left overnight to set properly.
And so I departed, with an invitation to stop over again on my way back
in about a week. I had an idea that there
might be more jobs waiting for me then
but I didn’t anticipate anything like
what actually happened.
It appeared that the jointing operation had been a complete success
and the tape had been played to its
conclusion. But when another tape
was subsequently played, there was
trouble. Although it was a 3-hour tape,
at the end of two hours the take-up reel
was chock-a-block full, to the point
where excessive tape had fouled the
bodywork, stopped the reel, and shut
the machine down.
Crinkled tape
The basic reason was easy to see –
the tape was crinkled, taking up much
more space than normal. But why? I
could only assume that something in
the tape transport path was damaging
it. So off came the cover and I went
straight to guides, heads and capstan.
And one glance at the capstan was
enough. It looked as though it hadn’t
been cleaned for years, even though
I had cleaned it thoroughly only a
few days before. More specifically, it
displayed the two characteristic dark
oxide rings (one tape width apart) that
normally occur after prolonged use.
Even more puzzling was the fact
that attempts to clean it using alcohol
and a tissue proved fruitless; it simply
wouldn’t budge. Again, this is what
one would expect from severe fouling
over a long period – often requiring
that the capstan be carefully scraped
to remove the rubbish. But why after
only 3-4 hours of playing?
Anyway, I attacked it with the first
thing handy – a fine screwdriver blade.
I know that sounds drastic but it isn’t
really, unless one is woefully heavy
handed. And in this case only the
lightest touch was necessary; all the
fouling moved as one piece and came
away as a tubular shell with the oxide
rings at each end. The capstan was now
as clean as I had left it.
So what was it? The adhesive I had
used to join the broken tape. And why?
Because the tape had been rewound
and, contrary to my advice, the join
had passed over the capstan.
How it happened I don’t know, and
diplomacy dictated that I not stress
the point. No real harm had been
done apart from the damaged tape and
nobody was upset about that. So it all
ended happily in the long run.
But I think the lesson is that trying to improvise away from one’s
own workbench can be fraught with
danger. It doesn’t always pay to be
SC
too clever.
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