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SERVICEMAN'S LOG
The topsy turvy world of remote control
A popular saying in my boyhood household was
that “laziness is no good unless it is well carried
out”. It was usually prompted by my tricks
that made my chores easier to do; a snipe with
the implication that making things easier was
cheating in some way.
By the standards of those days, the
modern TV set, with its remote control
unit, must surely represent the ultimate in laziness being well carried out.
Be that as it may, they are now a fact of
life and, to be fair, they offer more than
the opportunity for laziness. For the
hospital patient and anyone confined
to bed, or the disabled generally, they
are a godsend.
All of which is leading up to a story
about remote control units. I could
write innumerable stories about these
devices; hardly a day goes by but that
one of these turns up on the bench.
The faults are mostly routine – routine for remote control units, that is,
because they have a pretty hard life.
They are left on chairs and sat on,
which can be disastrous if it is a hard
seat; they are dropped on the floor,
which is bad enough in itself, but
worse if they are kicked or trodden
on; and the kids play “catchings” with
them. One that came in recently had
finished up in the washing-up water
in the kitchen sink.
Strangely enough, I was able to salvage that one. The lady of the house
scooped it out almost immediately
it hit the water and, although some
moisture found its way in, it wasn’t
flooded.
Nevertheless, the lady was very
diffident about approaching me,
convinced that it would be a write off.
I thought so too, until I opened it. To
my surprise, it didn’t look so bad, so
I gave it a solid spray with dewatering
compound, the kind of thing used to
dry out wet ignition systems.
I left it for a while to dry out, then
62 Silicon Chip
gave it a try. And it worked. More
importantly, several months later it is
still working. So that was one of the
happier accidents.
Conventional faults
The more conventional faults include various battery problems. It’s
not so much flat batteries though,
because most users fit new batteries
immediately there is a problem.
However, the battery contacts can
give a lot of trouble, mainly due to loss
of tension. And there can also be problems where the contacts are soldered
to the board pattern. This junction is
often weak me
chanically and, after
repeated battery replacements, the
joint fails.
Then there are the inevitable corrosion problems. Sometimes it is a
rogue battery that has leaked but more
often it is due to exhausted batteries
that have been left in a unit that hasn’t
been used for a while.
It is usually possible to re-tension
the contacts and to clean up corrosion.
The latter may mean removing the contacts, scrubbing off the corrosion, and
re-tinning them with solder. Fractured
joints can sometimes be repaired and
sometimes not.
And there is some pattern of inherent faults. One that comes to mind
seems to suffer more than a fair share of
keypad problems. Repairs are seldom
satisfactory and they are generally
written off.
Another problem area concerns
the ceramic resonator. In most units,
the ceramic resonator is mounted flat
on the component side of the board,
with its leads bent at 90 degrees and
taken through the board to the copper
pattern.
The weakness here is that, in most
cases, the ceramic resonator is supported only by its leads. And, eventually, vibration will take its toll; one
of the leads breaks. Fortunately, there
is usually enough lead left to salvage
the situation, after which a dab of glue
to secure the ceramic resonator to the
board makes for a better-than-new
repair.
An unusual fault
So much for the general background. The story that started all this
is something else again. The fault is so
unusual I was tempted to hold back
on the solution and offer a prize of a
free flight to the Moon for anyone who
picked it. Fortunately perhaps, some
distant Scottish ancestry caused me to
have second thoughts.
OK, down to the story. This particular remote control is an NEC model
RD-309E and is teamed with an NEC
model N3420 TV set. It is owned by
one of my a regular customers and
the complaint was simple enough and
fairly typical: “It doesn’t go”.
I checked the batteries (two AA
cells) and they were OK but closer
examination threw suspicion on the
battery contact tension, along the lines
already mentioned. It was a simple
job to bend the springs to provide adequate contact pressure, then replace
the batteries and try it with a test unit
I have.
No joy; it was still dead. So I opened
the case and set it upside down (ie,
keypad down) on the bench. As can
be seen from the photograph, this reveals the component side of the board,
with the IC, the ceramic resonator, a
transistor, the IR LED and a few minor
components at one end.
My first check was at the ceramic
resonator. The leads were intact but
the ceramic resonator was not secured,
so a spot of glue was applied to hold
it firm. I then set it up for further testing. I have found that the easiest way
to work on most of these units is to
leave them upside down on the bench
and feed them from a variable power
supply via a couple of clip leads. This
is often more convenient than trying
to use the batteries, which may not
be very secure when the case is open.
So this was the setup I used for
this one. And lo and be
hold, the
thing worked. I pressed a number of
keys from underneath and everything
seemed to be OK. The only snag was,
I didn’t know what I had done and
so I decided to press on and see what
happened.
I put the whole thing back together
again, refitted the batteries, turned it
right side up, and checked it again. It
was as dead as the proverbial dodo.
I pulled it open, hooked it up to the
power supply, and checked it again.
And it worked. So what was I doing
wrong? Seeking inspiration, I carefully
turned the whole thing over, checked
the front panel, and tried again. Once
more, it was dead.
It didn’t need many such checks
to confirm what I now suspected; it
would work when upside down but
not when right way up. Well, I had no
idea what was wrong but I suddenly
realised that I had probably been
caught out once before with the same
symptoms, without realising it.
A couple of months before, a customer had brought in exactly the same
model unit. And, initially, the complaint was the same: “It doesn’t go”.
And with good reason it appeared.
One of the ceramic resonator leads
had broken but with enough lead
protruding to allow the break to be
bridged with a blob of solder. After
then gluing the ceramic resonator to
February 1995 63
Fig.1: view inside the NEC RD-309E remote control, showing the PC board. The
components next to the IC, from left, are: the 47µF electrolytic capacitor, the
LED driver transistor, the ceramic resonator & the IR LED.
the board, I fully expected the device
to work.
It did too, when I mocked it up on
the bench upside down. I assumed that
that was the end of exercise, apart from
putting everything back together. As it
so happened, pressure of other work
caused me to put it to one side at that
point, the customer having indicated
that he was in no particular hurry.
When I did finish the job, about a
week later, the thing was dead. Over
the next week or so, I tackled it several
times in between bigger jobs but without success. Sometimes I could make
it work, sometimes I couldn’t. Unfortunately, because of the on-again-offagain approach, I didn’t recognise any
pattern; I simply assumed it was one
of Murphy’s sick intermittent jokes.
When the owner subsequently
dropped in to see how I was progressing, I gave him a rundown of the
above sequence and advised him that
an intermittent fault in one of these
devices might be more costly to find
and fix that the unit was worth.
He thought about it briefly, than
decided to write it off, and asked me
to get him a new one, which I did. And
the old one finished up in the scrap
box as a possible source of spare bits
(eg, the keypad).
But now, alerted by the sequence
of events with the unit on the bench,
I suddenly realised that the supposed
“intermittent” behaviour, could easily have followed an upside down/
right side up sequence, without my
realising it.
All of which was food for thought
but not of much help with the immediate problem. But I was determined
to track it down now.
I pulled the board out and went over
it with a glass, looking for dry joints.
64 Silicon Chip
The only suspects were the two for
a 47µF electrolytic capacitor, which
looked a trifle dodgy. I un
soldered
the joints, pulled the capacitor out,
checked it (it was spot on) and soldered it back in. This made no difference to the behaviour.
I tried bashing and prodding to try
to make it stop when it was in the
working position, or to try to make it
work when it was the other way up.
There was no response either way.
What about the ceramic resonator?
Was this operating in both positions?
With some delicate fiddling I attached
the CRO in a manner which I hoped
would hold when I turned the device over. Thankfully it did and this
confirmed that the ceramic resonator
operated in both positions.
So what else was left? Not much, it
seemed. It had to be a mechanical fault
of some kind, but where? I thought
about the IC but, without ruling it out
completely, put it at the bottom of
the list. For one thing, it contains the
oscillator circuitry and I knew that this
was working.
Then I had a wild idea. It wouldn’t
be the first time that an IR LED had
given trouble, although only as a total
failure. But what if...? Well, it was a
long shot but it was easy to try; I had
spares on hand and it involved only
two soldered joints.
And believe it or not, that was it.
The new LED cured the problem and
the unit has now been back with the
customer for several weeks, with no
sign of trouble.
Naturally, as soon as I had proved
the point, I could hardly wait to fish
out the junked unit and confirm my
suspicions. And I did; it was an upside
down/right side up fault and when I
fitted a new LED it came good, just as
the other one had. So I now have a
spare unit, which will come in handy
for testing.
But what could possibly be wrong
with the LEDs? I dunno please – as
they say in the classics. Visual inspection is pointless; both units are totally
opaque to visible light and appear
black. One of the good ones I fitted is
clear but there is little to be seen that
would provide a clue.
Electrically, the faulty LEDs measure exactly as one would expect them
to; ie, like a diode. Nor is there any
indication of position sensitivity.
Which is about all I can say about it.
Not only is the fault almost unbelievable but it has turned up in two units.
Well, two that I know of. I wonder if
this story rings a bell with any readers.
It would be easy to be deceived the first
time, just as I was.
Food for thought
My next story is in a quite different
vein. In fact, it is not particularly profound technically but the symptoms,
and their possible effect on how the
job might have been tackled, provide
some food for thought.
In particular, the sequence of events
demonstrates just how easy it is for
there to be a breakdown in communications between customer and serviceman. And while it turned out to be
unimportant in this case, it emphasises
that the risk is always there. And it can
prove costly, both in terms of money
and reputation.
The device was a colour TV set,
Palsonic model 345, now about 12
years old. And the same chassis was
sold under the Princess label.
It belonged to one of my lady customers and was brought in by a friend,
who was simply acting as a courier.
When I asked if he knew the nature
of the complaint, he answered simply,
“No picture”.
Well, as we all know, that can have
a couple of interpretations. If there is
a raster on the screen but no image,
then there is a fault somewhere in the
signal chain, anywhere from the tuner
right through to the video amplifier
system. If, on the other hand, there is
no raster, it is a fair bet that the trouble
is somewhere in the horizontal deflection system.
So I normally try to clarify this
point, using terminology appropriate
to the customer (I avoid the word “raster” –it only produces a blank stare).
But there was no point in trying to
delve further in this case; the good
samaritan courier was in no position
to help.
Oh well, no worries; I’d know as
soon as I turned it on. Or so I thought.
But when I did turn it on, the result
did not really slot into either category,
although I had to admit that the customer’s description was not strictly
wrong.
There was a raster on the screen,
and there was evidence of video on
the screen too, but there wasn’t a
picture because the horizontal scan
was out of lock. So much for trying to
pick the faulty area from a customer’s
description.
But that was a minor hiccough;
the real question was why was it out
of lock. The horizontal hold control
(R451, 10kΩ) on these sets is not on
the rear apron of the chassis, as in
most sets, but towards the front of the
chassis. This means that the chassis
has to be pulled in order to adjust it
but it also means that it is unlikely that
anyone has fiddled with it.
So I pulled the chassis, located the
pot, and gave it tweak. The picture
locked up immediately, even though
it was only a very small tweak. It gave
me the distinct impression that it
could have been due to drift in any of
the associated resistors, particularly
considering the age of the set.
And, of course, it was a situation
where one might be tempted to make
a good show by returning the set to the
customer within a couple of hours. But
I’m too old to be caught that easily. I
let the set run for the rest of the day
and planned to do the same the next
day too.
The set was still running when I
pulled the master switch that night
and I naturally expected it to come on
when I turned the switch back on next
morning. But it didn’t, it was dead; no
sound, no picture and no raster.
My reaction was to wonder whether
this was the condition the customer
had experienced, when she nominated
the fault as “no picture’. Either way,
it seemed likely that the set had two
faults; the one I had just fixed and the
one I was now facing.
At first, I suspected a power supply
fault but a few quick meter checks
tended to rule this out – the HT rail
was normal at 112V. However, it was
clear that there was no horizontal circuit activity. This prompted a voltage
check on the collector of the horizontal
February 1995 65
Fig.2: the horizontal & vertical oscillator circuit in the Palsonic 345 is
based on IC301 (top, left). The horizontal output transistor, Q404, is at
bottom right, while the driver transistor, Q402, is immediately to its left.
The horizontal hold control, R451, is below pin 1 of the IC.
output transistor (Q404), which was
correct at 112V, and also on the collector of the horizontal driver transistor
(Q402), which was correct at 42V.
Similarly, there was correct voltage
on pin 11 of IC301, the horizontal and
vertical oscillator generator.
Those points cleared, it was necessary to delve a little deeper. I reached
for the CRO leads and checked for any
horizontal signal coming out of the IC
at pin 4. There was none.
A faulty IC? It could be, of course,
but there were other possibilities. One
was that some other circuit fault could
shut down the horizontal oscillator,
particularly involving the x-ray protection (over-voltage) circuit which
connects to pin 3.
So, before rushing in to replace the
IC, I made some more checks. One of
these involved the x-ray protection
circuit but, as far as I could tell, it had
not activated.
Another check was at pin 10,
which provides the output from the
vertical oscillator. And this was quite
revealing because it appeared that the
vertical oscillator was dead also. And
that immediately threw suspicion
back on the IC. I made a few more
voltage checks around the IC and
finally decided that the only logical
step was to replace it.
They are not particularly expensive and it is no big deal to make the
change, the only snag being that I
didn’t have one in stock. So it had to
be ordered and I put the set aside for
a couple of days until it turned up.
When it did, I lost no time in fitting
it, whereupon the set leapt into life.
But, interestingly, the horizontal system was now out of lock again and I
had to reset it to what was virtually its
original position to restore the lock.
And that was it. After a couple of
days running on the bench it went back
to the customer and it hasn’t missed
a beat since.
But I did wonder about the faulty
IC. Was it two separate faults or two
different degrees of the one fault? We
can never be sure, of course, but my tip
is that it was one fault in some part of
the internal circuitry common to both
oscillators. At first, its effect was quite
mild, being sufficient only to upset the
vertical oscillator frequency slightly.
Subsequently, it went all the way and
shut down both oscillators.
It’s all rather academic, anyway.
What was more important was that
I could easily have been caught out
by it. After all, just what did the
customer mean when she said “no
SC
picture”?
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