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SERVICEMAN'S LOG
A clear case of sabotage
As I have shown very often in these notes,
servicing involves much more than merely
finding and fixing faults. All too unwittingly,
one can easily become involved in a
domestic battle of wills; a situation which
can call for the diplomacy of an ambassador.
One tends to relax more on Saturday mornings, knowing that the weekend starts when I close the shop in a
couple of hours and that I can leave
my worries behind. This morning was
particularly glorious; it was a beautiful day, I was relaxing with a cup of
coffee which tasted really good, and
I had only a couple of routine jobs to
finish off.
So I didn’t really notice Mr Roberts
carry in his huge TV set – a Mitsubishi
C6343 – all by himself. I must have
been half asleep because you really
can’t miss Mr Roberts; he is a giant
of a man with muscles everywhere.
His presence brought me back to
earth with a bump. I booked the set
in, noting that the complaint was that
it was dead and that it was needed
urgently. I went back to my coffee but
somehow it didn’t taste quite the same.
The lovely morning had been spoilt.
There was nothing for it but to attack this huge TV set. I removed the
back and checked the mains fuse – it
was OK. I plugged it in and checked
if the HT rail was OK – it wasn’t.
Being fairly familiar with this model, I knew that the grey power switch
gives a lot of trouble and decided to
check it. This wasn’t really necessary
as it had already been replaced but I
checked it anyway. It was OK.
The multimeter indicated that there
was no AC input and I traced the fault
back to the AC power lead, which was
open circuit. I noticed someone had
wound some insulating tape along
the first 7cm of the lead, next to the
moulded mains plug. I cut off about
10cm and checked continuity – it was
now OK. I fitted a new mains plug,
plugged it to the power socket and
switched on. The set burst into life
and both picture and sound checked
out OK.
A case of sabotage
Intrigued as to why the mains lead
had failed, I removed the insulating
tape. To my amazement, I realised
immediately that the set had been
sabotaged; the blue lead had been
neatly cut and bent back. But why?
Had someone set up a bodgie
fault, designed to trap some
unsuspecting serviceman –
muggins in this case – into
making an embarrassing diagnosis? Perish the thought.
There was nothing for it
but to confront Mr Roberts
and find out what was going
on. When the phone was answered, I informed the listener
who I was and asked could I
speak to Mr Roberts. A juvenile voice replied “I am Mr
Roberts’ son, Roger, and I know all
about the TV set. Have you fixed the
lead OK?”
“How do you know about that?”
I asked.
40 Silicon Chip
“Well, it’s like this. My dad watches
TV all night and none of us can get
any sleep, and we don’t dare tell him
to turn it down or off. My HSC exams
are due and I need all my sleep, so I
cut the wire last night before he came
in, intending to reconnect it first thing
this morning. When I got up, the set
had already gone.”
“Well, you realise you will be up
for the minimum service charge and
the plug”.
“Yes, yes, that will be all right but
please, please, don’t tell Dad. Just
write on the invoice that you repaired
the power supply and don’t mention
the cut lead.”
Apparently, Mr Roberts senior was
not a new age bloke and definitely
would not understand the situation.
I took pity on Roger and was suitably
vague with the report’s repair details.
Mr Roberts paid up without comment
and left with the set under his arm –
figuratively speaking, that is!
And I hope Roger’s HSC exam went
off well.
Fig.1: Akai VS-303EA. The
review guide (A) mounts on
the review lever arm which
is pivoted at (C). The pinch
roller mounts on the play arm
and this pivots on the top hole
at below right.
Akai video recorder
My next story is about an Akai VS303EA video recorder. This is getting
a bit long in the tooth now but, in
its day, has proved to be a good and
reliable performer. The owner carefully reiterated what “the boss” (his
wife) had told him. He cheerfully
confessed he knew nothing about it
but the message was that there was a
cassette stuck inside it and it wouldn’t
eject or play.
I checked it out and it behaved
exactly as he said it would. I also discovered that it could still fast forward
and rewind. Removing the covers and
checking the controls, I noticed that
the loading motor made no attempt
to turn in either direction. By disconnecting one lead and applying an
external DC supply, I could make it
eject the cassette. So the motor was
OK but there was no drive to it.
When the cassette ejected, I noticed
that the tape hadn’t retracted fully
back into it. I didn’t pay much attention to this at first, as the reel motor
wasn’t being told that I was artificially
operating the loading motor.
It was when I put the cassette back
in that I noticed that the cue/review
guide pin had not moved back into its
correct position, although the loading
guides were correct. Fig.1 will make
this easier for the reader to follow.
Fig.2: Akai VS-303EA. This mechanism
is on the opposite side of deck to the
parts shown in Fig.1. The play arm is
shown dotted and is engaged by the
pinch roller link. This in turn engages
the cam plate and is moved by cams in
the mode selector switch.
The guide, indicated by arrow “A”,
is mounted on the review lever arm
(partly ob
scured), which pivots on
nut “C”.
Well it should but it didn’t – pivot,
that is. Instead, it had seized solid.
A little lubrication and the arm soon
came free and the tape now played
and ejected OK.
Unfortunately, the story didn’t end
there. A day later, while soak testing
it, it became erratic and intermittent
in playing and ejecting. I returned it
to the workbench and removed the
covers again. After much erratic behaviour and red herrings, I eventually
concluded that there was a problem
involving the mode selector switch/
cam assembly and the pinch roller. In
broad terms, the pinch roller wasn’t
being moved far enough.
This takes us back to Fig.1. The
pinch roller is mounted on the larger
plate – sometimes called the “play
arm” – and this is shown with two
circles at its larger end, at extreme
right. The upper circle indicates a
pivot point, while the lower circle
indicates the point where the plate
is engaged by a lever.
This lever is part of the “pinch
roller link”, on the other side of the
deck, and is shown in Fig.2 (the play
arm is shown dotted at the left of the
diagram). This link in turn engages
the cam plate; a triangular plate
which pivots near its apex. The axle
on which this pivots was also suspect
but more of that in a moment.
The cam plate engages cams in the
mode selector switch. So the cam action is transferred, via the cam plate
January 1998 41
and the pinch roller link, to the play
arm carrying the pinch roller.
So, why wasn’t the pinch roller
moving through its full range. Basically, the problem was due to wear
between the cam plate and the pinch
roller link. I consulted Akai and
learned that the pinch roller link
has been modified, with extra metal
reinforcement where it moved against
the cam plate. Ordering and replacing
this part helped considerably but still
left something to be desired.
The problem now involved the
support for the axle which supported
the cam plate. This was slightly loose,
although this movement could be felt
rather than seen. As far as I could
ascertain, the axle and its associated
support was fastened to the deck
simply as a push-fit arrangement.
I couldn’t do much with this other
than treat it with Loc-Tite. It wasn’t
the most elegant solution perhaps but
should prove to be both adequate and
economical.
42 Silicon Chip
Anyway, that seemed to provide the
final answer, the machine functioning
perfectly during a prolonged soak test.
More to come
But it wasn’t really the end of the
story and, as it turned out, in more
senses than one. There is one part of
this story which I bypassed initially,
for the sake of clarity. But it needs to
be told.
Not all the erratic behaviour I en
coun
tered after the initial simple
lubricating exercise were due to
the mechanical problems already
described.
And the reason wasn’t very clear
initially, until I re
alised that this
model recorder can be fitted with two
different types of ejector assembly.
The first type is a conventional one,
as shown in the service manual, and
uses a 7-pin plug (P134/132).
The second type (not mentioned
anywhere) has a 9-pin plug to accommodate an optical slack tape sensor.
This consists of an infrared LED sender and an optocoupler receiver, which
creates a monitoring light path. And if
the tape isn’t fully withdrawn into the
cassette and clear of the infrared light
path – or if anything else temporarily
obstructs this path – it will prevent
the system from playing or ejecting.
At the time, neither this mechanism nor its function were obvious.
Consequently, I wasted a lot of time
following red herrings, especially
considering the fault’s intermittent
nature. But there it is; one to make a
note of in your own records.
The machine bounces
Anyway, I was relieved when every
thing was finally fixed and the machine returned to the owner. I thought
no more of it until a few weeks later
when he returned with it, complaining of exactly the same fault.
After spending so much time and
repairing three faults for the same
symptom, I was shocked – not to
mention embarrassed – to find that
the machine had bounced. I checked
it again in front of him and he was
quite correct.
I apologised and assured him that I
would get back onto it straight away.
Fortunately, he is a very pleasant,
easy- going fellow and remained
unfazed.
I pushed aside the work I was doing and went straight onto the recall.
Looking straight at the top of the
ejector, I couldn’t see anything wrong
with the optical slack sensor but the
cassette was in the way. I removed
it by disconnecting and power
ing
the loading motor from an external
source, as before.
With the cassette removed, I had a
clear view of the infrared diode sender
and optocoupler receiver and could
see immediately what had happened.
A small sticky label belonging to the
tape cassette had come away and become stuck over the receiver sensor,
causing the same symptoms as before.
I was quite relieved that it wasn’t
due to any carelessness on my part
and the owner accepted that it was just
one of those one-in-a-million chances.
So it all ended happily.
stock of the kit. No matter; Bob just
ordered the IC separately from another
supplier. He had a resistor in stock
and plenty of heatsink compound left
over from previous kits.
When it arrived, he fitted the IC and
resistor just as he had done on dozens
of previous occasions. However, when
he switched the set on, absolutely
nothing happened. He had subse
quently spent hours investigating why
and had run out of inspiration, before
finally asking if I would take a look
at it for him.
As he had often done the same for
me, I was happy to return the favour
and in due course he dropped the set
around, along with a list of all the
things he had tried. The only problem
with a list is, of course, the possibility
of missing something somewhere. I
would have to recheck his list as well
as add my own checklist.
Measuring the set’s vital signs, I
established that 330V was present on
pin 3 of IC801 but there was only 40V
at pin 5 instead of the 113V shown
on the circuit. My checklist included
T801, R803, Q833, IC601, Q500, T500,
Q451, T501, Q504, D502, R511, C836
C808 and all stops along the way but I
wasn’t getting anywhere either.
As a precaution, Bob had ordered
two STR50113 ICs and had tried them
both to no effect. Fortunately, I had a
scrapped set (broken tube) and was
confident that it had worked correctly.
This meant that I could check suspect
components by substitution with the
knowledge that they should work. At
this stage, more in desperation than
anything else, I decided to swap IC801
from my set into Bob’s.
And would you believe it? – it
worked perfectly and continued to do
so even after I replaced all the original
parts. So why did Bob’s two brand
new STR50113s not work in this set?
It’s interesting to note that the part
number specified in the circuit and
the parts list is STR50113-M but the
M isn’t marked on either the original
or any of the substitute. The internal
circuit of the IC (see Fig.3) is shown
as consisting of two NPN transistors
and one resistor but this may be only
a simplistic block diagram.
In the end, the only conclusion I
could come to was that the two ICs
Bob fitted were cheap clones that
weren’t up to the job in the Panasonic circuit. Significantly, neither the
A friend in need
Bob is a colleague who works for
an opposition service organisation
and has been in the game for as long
as I have - too long perhaps, or so it
seems at times. Anyway, we help each
other out on odd difficult problems.
It’s good to be able to discuss and
think through a problem, or maybe
get a different perspective – a spot of
lateral thinking and all that.
So it was with a National TC2038
TV set with which he had been lumbered. This set uses an M14 chassis
which has been highly reliable over
the years, making it a good rental set.
Most problems are well known and
understood and this one should have
been the same.
Its symptom was simply that it was
dead and he had discovered that resistor R841 (4.7Ω, 7W) was open circuit
and that the switchmode IC (IC801,
STR50113) was short circuit. The
usual reason why this switchmode IC
fails is poor heat transference.
Normally, one purchases this IC as
part of a small kit that includes a new
insulating washer and thermal grease.
However, on this occasion, our mutual suppliers were temporarily out of
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January 1998 43
Serviceman’s Log – continued
suppliers nor anyone else have been
able to come with another theory.
The end of the weekend
After a fairly lax weekend, I arrived on a Monday to find three jobs
awaiting me. One was a large stereo
Teac CT-M631S TV set which was
described as “dead and burning”.
Well, fortunately, the latter symptom
had long since stopped but the acrid
smell of burnt plastic was obvious.
As soon as I removed the back, I
expected the cause would be obvious
and straightforward – for example,
burnt horizontal output transformer
insulation. I was surprised, therefore,
to find that the AC power line input
filter coil (L901) had burnt and taken
fuse F901 with it. The adjacent 0.47µF
capacitor across the AC mains (C901)
was also badly melted.
So had the coil shorted and melted
the capacitor or vice versa?
When I measured the capacitor it
still read OK but I have had cases in
the past where capacitors like this
develop and clear their own shorts,
blowing the fuses on a random basis. However, I don’t think that this
was the case this time as the heating
process must have lasted for quite
some time.
But that wasn’t the end of it. In the
course of cleaning and resoldering the
board, I noticed that the main electrolytic (C905, 220µF 400V) appeared
to be loose. I unsoldered one leg and
the whole capacitor came away in my
hand. The other terminal had been
corroded by the electrolyte.
I replaced this along with C908 and
Fig.3: National TC-2038. The circuit
shows that IC801 (STR50113-M)
consists of just two NPN transistors
and a resistor. Note the suffix “M”
which is not marked on either the
original or any of the replacement
units.
C910 which can also cause the HT rail
to fail. On reassembling and powering up, everything was now OK; the
sound and picture were perfect. But
it left a niggling thought in my mind
as to what was the original fault and
what caused what to fail in what order.
The second set was an NEC N-3419
with a Daewoo C-43M chassis. The
fault description on the job sheet stated
that the picture height decreased after
about half an hour, so I left it to run
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while I dealt with the third set. This
set was a Teac Televideo MV-1440
combination TV/VCR and it was dead.
Removing the covers revealed a
chassis which is quite difficult to
work on, as access to the PC board
is restricted by the short connecting
leads to the video recorder beneath it.
I measured the main HT rail, which
should be at 112V, and there was
nothing. Nor was there any voltage
on the 12V secondary rail.
Next, I checked the 320V rail to
IC1501 (STK7348) and again there was
nothing. I had to remove the chassis
altogether to get the ohmmeter to
finally confirm that the IC was short
circuit and that R1501, an 8.2Ω 5W
anti-surge resistor, had gone open
circuit. I replaced the IC (along with
fresh heatsink compound) and replaced C1507 (2.2µF, 50V) and R1501
for good measure. I also found that
D1506 was short circuit and replaced
that as well.
This restored both the 112V and
12V rails but the set was still dead. I
quickly established that there was no
voltage on the collector of Q1401, the
horizontal output transistor, but it was
there on R1407, the supply resistor.
When I subsequently removed
the horizontal output trans
former
(T1401), I found that its primary was
open circuit due to a corroded lead to
one leg. After mucking around with
extra wire splints and microsurgery, I
eventually managed to repair the coil
and reassemble the set again.
And that was the end of it. The set
now worked fine and it must surely
be bad luck to find so much wrong
all at once.
Shrunken picture
In the meantime, the NEC TV set
had been doing its thing and the
picture had indeed shrunk from top
and the bottom. I measured the main
HT rail and found that it was 125V
instead of 103V.
Freezing IC Q801 (STR5412) caused
the HT rail to drop and the height to
increase. I replaced the IC, applying
fresh heatsink compound in the process, and changed C806 (100µF). I
also removed some brown goo from
around IC 1502 (the 12V regulator)
and reworked a few potential dry
joints before trying it out.
The set now worked perfectly and
it was still going strong after a 3-hour
SC
soak test.
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