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This is only a preview of the December 1996 issue of Silicon Chip. You can view 28 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Build A Sound Level Meter":
Items relevant to "Build An 8-Channel Stereo Mixer; Pt.2":
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SERVICEMAN'S LOG
There’s a long, long trail a’winding
Well, in the words of the popular song, that’s
what it seemed like; a long, long trail through
circuits and PC boards in search of the most
elusive combination of intermittent faults that has
been my misfortune to encounter for many years.
It is a story about an Akai video
cassette recorder, model VS-35 and
the problem was an intermittent sound
fault. Not just any intermittent sound
fault, mind you. This was an intermittent sound fault the like of which
I have never seen before – and I hope
I never see again.
The machine came to me from a
colleague, who felt that he could no
longer cope with the problem. And
so it landed on my bench along with
a “best of British luck” attitude.
While there was some history of the
problem, I suspected that there
had been more than one finger in the
pie before it reached me.
Anyway, I was saddled with it. The
gist of the problem was faulty sound
on playback although, as far as I could
determine, the sound always recorded
normally. To test this theory, my first
step was as to make a test recording
on the machine immediately after
observing the sound problem. This
tape subsequently played perfectly
on another machine, which seemed
to clarify this point.
But putting such doubts aside, the
extent and nature of the faulty sound
was the real problem. Sometimes, at
switch on, the sound would be perfect for a few minutes, then become
intermittent. And when it went intermittent, the whole machine became
mechanically sensitive; the lightest
tap anywhere could turn the sound
on or off, as the case may be.
Initially, I couldn’t believe what I
was observing. It seemed impossible
that there would not be some variation
in sensitivity which, in turn, would
give a clue as to the general location
of the fault. But no; touching anything
– boards, leads, the deck itself – produced an equal response.
Making a start
The setup consists of a large mother
board, a small chroma board, a power
supply board and a preamp
lifier/
audio board. Initially, to the extent
that there was any particular sensitivity anywhere, I sensed that the
area around the preamplifier/audio
board might be marginally more sensitive. I even reached the stage where
I could achieve a response by flexing
or tapping the board, without it being
supported in any way but simply connected by the various leads.
I was convinced that this was where
the trouble lay. And I wasn’t particularly impressed by the soldering on
the underside. There had been some
work done on it but there was still a
large number of original joints which
were, at least, suspect.
I set to and checked every suspicious joint and remade anything which
looked even remotely suspect. I spent
a lot of time on it and by the time I
had finished was prepared to bet my
reputation on it.
Initially, it appeared that the fault
44 Silicon Chip
had been cured. But not for long. After
a few minutes it was back just as it was
before. Well, not quite; the equipment
as a whole seemed just as sensitive as
before but it now appeared that the
area of sensitivity had moved to the
motherboard. To the extent, that is,
that I could be sure of anything.
But if it was on the motherboard
the implication was that I was chasing
two faults; one which I might have
found on the preamplifier board and
one still to be found. So was it a case
of two faults producing essentially
identical symptoms? It seemed like an
impossible long shot but I was ready
to believe anything.
Assuming that there had been a fault
in the preamplifier board, and that it
had been fixed, the next logical step
was to investigate the motherboard.
Parts of the underside of this had
also obviously been worked on, while
other untouched areas needed closer
scrutiny.
The upshot of this was a major
overhaul of this board. Any suspect
connection, whether it had already
been reworked or not, was tackled. It
was a long process but when I finished
I felt reasonably confident that I had
done a thorough job.
And so it seemed. When I replaced
the board and set everything working
the machine came good. There was
no sign of the sound fault and the
machine was seemingly immune from
its previous mechanical sensitivity. I
let it run for about half an hour or so,
giving it an occasional prod or bash,
and all seemed well.
Or at least it was until I put
everything back into place and prepared to fit the cover. Then it was back
into fault condition, exactly as it was
before. I won’t bore the reader with all
the emotions and rude words which
resulted from that discovery. Suffice it
to say that it was back to taws.
The methodical approach
By now it appeared that the mass
soldering approach had served its
purpose. If it had done any good at all,
it wasn’t good enough. What was now
needed was a methodical approach.
A major problem here is how best to
convey all the circuit ramifications to
the reader. We are talking about several A3 sheets, covering both circuit
and PC board patterns. Obviously,
reproducing these is out of the question and the best I can do is present a
Fig.1: part of the audio preamplifier board in the Akai VS-35. Audio from
the A/C head (top, right) goes to pins 3, 4 & 5 of IC700, comes out on pin
16, and then goes to the motherboard via pin 12 of connector WF10.
word picture which I hope will help
the reader follow the story.
First, it is it necessary to visualise
the audio signal paths where a fault
is likely to be. And there are, initially, two sources of audio signal. One
comes from the audio track on the tape,
feeding the Audio/Control (A/C) head
on the deck. These signals are fed to
the preamplifier board, on pins 3, 4 &
5 of IC700, come out on pin 16, and
go to the motherboard via pin 12 of
connectors WF10 (“A.OUT”).
The other source comes from the
tuner and the IF system on the motherboard, which demodulates the sound
IF and delivers an audio signal. These
signals involve a longer path but
eventually find their way back to the
preamplifier board, where the two audio signals are combined into a single
audio path which then reappears on
the motherboard.
This path eventually goes to the
modulator but, on the way, connects to
an RCA socket, designated as A.OUT,
December 1996 45
Serviceman’s Log – continued
on the rear of the chassis. This provides a convenient audio check point
and the first thing I did was to organise
an external amplifier to monitor the
audio at this point, which is close to
the end of the combined audio line.
At the same time I arranged things so
that the output from the VCR was fed
into a TV set.
This simple test confirmed that the
fault was present at both the RCA
A.OUT socket and the TV set.
At this point I had to choose between
the two audio paths. I was convinced
that, whatever I did, it would be wrong
(Murphy would see to that) but I had
to start somewhere. So, for better or
for worse, I elected to check the line
coming from the IF system.
The signal from the tuner is pro-
cessed to IF level, designated on the
circuit as VIF, and applied to pins 4
& 5 of IC1 (M51496P) – see Fig.2. The
audio signal then comes out on pin
11 of IC1, which was where I started
tracing the signal.
For this purpose, I used a simple
audio probe which I have mentioned
in previous notes. This confirmed
that the signal was intact out of the IF
system at pin 11 of IC1, even though
the fault was evident at both the TV
monitor and the A.OUT socket. Well,
that was a good start.
From here, via a long path on the
circuit, I traced the audio signal to pin
5 of IC201 and then out again on pin
4. It then went to the base of transistor
TR213, the signal from the emitter then
going to line “SELECT.A” and thence
to pin 11 of a 12-pin connector WF10.
And the audio signal was still fault free
at this point.
Now, from pin 11 of connector
WF10 on the motherboard, the circuit
goes to a similar WF10 connector on
the preamplifier board, then into pin
13 of IC700 and out on pin 16.
And, as mentioned earlier, the audio
signal from the A/C head on the deck
also appears at pin 16. In short, the two
signals are combined at this point and
the combined signal goes to pin 12 of
WF10 and then back to pin 12 of WF10
on the motherboard. And I still had a
clean signal at this last point.
I was getting close now because
there was not much circuitry left between this point and the A.OUT RCA
socket where the fault was obvious.
But where was it?
From pin 12 the signal goes direct-
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46 Silicon Chip
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Fig.2: IF signals are applied to pins 4 & 5 of IC1 on the mother board (Akai
VS-35) and appear as demodulated audio on pin 11. From there, the signal
eventually goes to pin 11 of connector WF10 and then to the preamplifier
board where it is combined with the audio from the A/C head.
ly to a 680Ω resistor, R276, which is
mounted alongside the WF10 connector. There was a clean signal on both
sides of R276 so I traced the copper
track to the next convenient point, a
jumper link about 12cm away, near
the edge of the board.
And bingo! – there was a faulty
signal at this point. The fault was
somewhere along that 12cm of track.
I pulled the motherboard out and
examined that length of track in the
minutest detail, using my most powerful glass. I couldn’t pick it so I resorted
to cleaning away small areas of lacquer
on the track, allowing the probe to
make contact, until I narrowed the
fault to a small length near resistor
R270 and transistor TR214.
It was then that I noticed a hole in
the board through which a mounting
screw was fitted. Suppose someone
had been a mite too heavy handed in
fitting that screw; could it have cracked
the track?
Now that the search area had been
narrowed to within a couple of centimetres I took a long hard with the glass.
And, yes, there was no doubt about it;
the finest and faintest of cracks could
be discerned but only because I knew
where to look.
As usual, once a fault is found, the
whole thing becomes something of an
anticlimax; a spot of solder was all that
was needed to bridge the gap. Then,
just to be on the safe side, I fitted a
wire link anyway.
That fixed it. No amount of bashing,
prodding, or soak testing produced any
sign of the fault. I progressively put
everything back in place and there was
no sign of trouble. Finally, I phoned
the customer and told him to collect
it. He indicated that he would come
around immediately.
The unbelievable
Now you’re not going to believe
this. No sooner had I hung up than
the machine went into its act again. It
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December 1996 47
me; no, that wouldn’t be anywhere
near strong enough.
Unfortunately, no amount of wailing and gnashing of teeth was going
to solve the problem. I just had to
get stuck into the monster and start
all over again. Nor was it any help to
know that the customer was on his way
and that I would be under pressure to
either come up with a quick fix or some
kind of an excuse, although I couldn’t
imagine what it would be.
As it transpired, I had a reprieve.
The customer had been delayed and
rang back after a short time to say that
he couldn’t make until the next day.
But a reprieve was all it was; I still had
to start all over again.
By the time I had delved back to
where I started I was shocked to
find that the fault now appeared to
have shifted back to the preamplifier
board. Indeed, I quickly found that by
flexing it at one corner I could create
the fault.
After seeking desperately for some
kind of inspiration, I finally decided
to look more closely at a number of
surface mounted components on this
board. I was clutching at straws but it
was all I could think of.
To tell the truth, surface mount
components do not usually cause
problems – at least not in my experience. Of course, there are a lot of
factors that must be considered at the
manufacturing level but, provided due
care is taken, a surface mount assembly
is extremely reliable.
So had I overlooked something? As
I said, I was clutching at straws but,
after going over this part of the board
and re-soldering everything yet again,
it did appear that the problem was
fixed. Mind you, I could be pardoned
for being sceptical.
Anyway, all seemed well for a while
until I started to put everything back
together again. Then the fault reappeared only this time it was pretty
clear that it was on the motherboard.
What was more, after more prodding
and flexing, it appeared that the sensitive area was now back near where
I had found the original crack.
I fished out the audio probe and
began tracing the audio path as before.
But this time there was a difference.
Originally, the audio signal had appeared as a clean signal on pin 11 of
socket WF10 on the mother board, having originated from IC201. And, after
its journey to the preamplifier board,
it was still clean when it reappeared
on pin 12 of WF10.
But not this time – the fault was now
obvious on pin 11 of WF10. Now the
important point about this is that the
copper track running from pin 11 runs
parallel to the track from pin 12 –the
very track containing the fault which
I had originally repaired.
Another crack
It didn’t take Sherlock Holmes to
suspect that there might be another
crack in the adjacent track. And so it
was; this crack was even more difficult
to see and the audio probe gave the
only positive indication. Anyway, I
bridged it as before and tried again.
And this time the job held in spite
of all I could do it. I even finished the
job before the customer turned up.
Considering everything, I would have
preferred to give the machine a much
longer test but the customer wanted it
back as soon as he could get it.
Yes, I thought the worst every time
the phone rang for the next few days
but there was no sign of a bounce. A
phone call to the customer several
weeks later confirmed that the machine hadn’t missed a beat. Still, I’m
SC
keeping my fingers crossed!
48 Silicon Chip
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