This is only a preview of the November 1996 issue of Silicon Chip. You can view 23 of the 96 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 An 8-Channel Stereo Mixer; Pt.1":
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SERVICEMAN'S LOG
Of ships & shoes & sealing wax
Of ships & shoes & ..? Well, something like that.
More exactly, this months column is a collection
of items which have accumulated over the past
few months, typically letters from readers.
Unfortunately, good intentions
don’t always work out in practice. For
one reason or another, there always
seems to be a space problem and so,
finally, they simply had to be dealt
with in one hit. So here goes – and
my apologies to all those concerned
for the inevitable delays.
My first item is from one of my
regular contributors of the past, J. L.
of Tasmania. We haven’t heard much
from J. L. in recent months and I had
gained the impression that he has
scaled his activities down somewhat.
38 Silicon Chip
Anyway, here is his latest story.
I had a job last week that was totally
wasted on an old bloke like me.
The manager of the local gymnasium had asked me to have a look at
their cassette deck, the one used by the
aerobics instructors to play the music
for their students to puff by.
I was told that the deck would only
play half a tape and would then slow
to a stop, just when the exercises
were getting up to full speed! What I
wasn’t told was that I would have to
do the job on the job, during a class,
because their spare deck was not up
to continuous use either! They had to
swap from one machine to the other
to get through a session.
I don’t know if you have ever
tried to work in a room
with 50 young women, each scantily
clad in lycra tights
and bouncing about
all over the place to
the thumping beat of
loud rock. As I said
at the top, the job
was totally wasted
on me.
The deck turned
out to be a Tascam Ministudio, a
4-track 4-channel
recorder designed
primarily as a semipro deck for rock
bands and the like.
It is quite a rugged
unit, which was
why it had been
chosen for play-only use in the gymnasium. Unfortunate
ly, it had “play-onlyed” just a little
too often and was now due for some
serious maintenance.
Flying blind
There was no way I could do a
proper test of the deck in situ. It was
accessible enough, on top of a small
wooden cabinet, but there just wasn’t
enough space around it for tools and
test gear. Quite obviously I was going
to have to fly blind with this one.
Fortunately, I have had quite a lot
of experience with audio tape decks,
going back to long before VCRs became
common. I also have a useful collection of specialised tools, including a
torque meter which proved to be the
saviour in the present job.
First up, I loaded a 1kHz test tape
into the machine and set it playing.
With a pair of headphones plugged
into the unit’s “phones” socket, I was
just able to hear the test tone and con
firm that it was running at about the
correct speed.
As I stood watching the frenetic
activity all around me, I became aware
that the tape was definitely slowing.
This was after only about five minutes,
so I could just picture all the exercising
slowly grinding to a stop.
My next move was to replace the
test tape with the torque meter. A tape
deck in good condition should give a
take-up torque reading of around 4050 gram/centimetres. This one was
hard pressed to reach 5g/cm, which
explained why the tape was so loosely
spooled in the cassette that I could see
daylight through the layers.
I can’t remember when I first learned
the usual cause of this problem, so I
can’t tell our younger readers just how
I first found it. I do know that I have
since found hundreds of stretched
or hardened main drive belts in both
audio and video cassette decks. And
so it was in this case.
I opened the machine and exposed
the bottom of the mechanism. I used
a finger to rotate the main flywheel,
Well, that’s J. L.’s story and it is
good to hear from him again. But I
can’t help wondering whether he is
perhaps coming on a little too strong in
his protestions – that he is promising
to not enjoy himself just a little too
emphatically!
The Marantz VCR
which should have caused the motor
pulley to spin. It didn’t move! When I
powered the mechanism, the wedging
action of the motor pulley managed to
drive the belt but the flywheel barely
moved.
That was enough – I removed the old
belt which had become quite hard and
highly polished. I selected a replacement belt about 1cm shorter than the
original and fitted it into place.
When the machine was reassembled, the torque figure had gone up to
55g/cm – a whisker high but one that
will provide a margin for the future.
By this time the noise and activity
around me were beginning to wear
me down. So rather than hang around
to test the deck, I set it running and
made my departure. When I checked
next day, I was told that it was working
perfectly, so my diagnosis must have
been correct. I promised to go back
later to complete the cleanup and
general maintenance of the unit but
only if they could assure me an hour
free from the distractions of young
women and loud rock!
When I related this story to one of
my younger colleagues he couldn’t
believe that I was more interested in
repairing a tape deck than in watching
the girls do their aerobics exercises.
Apparently, at his gymnasium, they
have segregated classes and he would
have given his left leg to get into the
girls’ class. He tried to convince me
that he was only interested in “...all
that stretchy Lycra” but somehow I
don’t believe him!
Next up, is a letter from A. M. of
North Turramurra, NSW, concerning a
problem with a Marantz 740A VCR. It
is similar to the situation described in
the Serviceman’s Notes for December
1995. Anyway, here’s his story.
Like many readers of SILICON CHIP,
I usually read your column first when
the magazine arrives. The December
issue was, however, a double treat
because it contained the solution to
a problem that has consumed many
hours of head scratching and frustra
tion. I refer to the power supply fault
in the Marantz 740A.
The problem first showed itself as a
curious sort of “rotating echo” effect
in the sound. After some fiddling, I
found that if the Audio Play switch
which had been set on Mix was set to
Hi-Fi or Linear, the effect disappeared.
As a result, I dismissed the episode as
an intended gimmick.
Some months later, I noticed that the
picture was somewhat degraded when
the machine was first switched on but
that this cleared after a few minutes.
This slowly became worse, until it
took about 15 minutes for the picture
to clear. At this time, I also found that
with the Audio Play switch in the HiFi position there was an intolerable
frame buzz, while the picture was
degraded.
The fault was obviously heat affected and the service manual showed that
the Video Head Amp PWB-Y was the
only common signal path for hifi audio
and video. I heated and froze every
component on the board but without
any result.
Incidently, the circuit diagram for
PWB-Y and others have been helpfully
(?) marked to show the signal paths on
playback and record. I assume this was
originally in colour but the manuals as
supplied are black and white copies,
so that all the path markings succeed
in doing is to obliterate the details of
the circuit!
On the grounds that most faults are
mechanical but without any clear idea
as to where the thermal effect would
arise, I carefully cleaned the heads,
November 1996 39
Serviceman’s Log – continued
aligned the tape path, and replaced
the pinch roller – all of which effected a slight improvement but did not
correct the real fault. It was clear that
there was no problem with the record
function but, when we could no longer
stand the playback delay, we bought a
Sanyo VHR-310. It was a nice luxury to
have two recorders, even if one could
record only.
You can no doubt imagine my joy on
reading your December 1995 column
and I wasted little time in diving into
the monster again. Your comments
about access are spot on but I’ve delved
into this device so many times that the
operation didn’t take long.
I was glad of your encouraging
comments about bending the support
bracket out of the way. This is something I would normally be reluctant
to do but which almost cannot be
avoided in this design. Of course I
was rewarded with immediate and
40 Silicon Chip
complete success.
It is not at all obvious why the fault
manifests itself in this way, or why
the ±9V regulator is designed in a
way which is bound to make it more
fault prone.
Thank you once again for the
solution to a most obscure problem
and for an always entertaining and
informative column.
And thank you, A. M. for your interesting report of this exercise. I have no
doubt someone else will benefit from
your time and effort.
The GC181 colour TV set
My next letter comes from N. B. of
Epping, NSW and concerns a story in
the Serviceman’s Notes for January
1989. The set on the Serviceman’s
bench was a GC181 colour TV set and
it had suffered an imploded picture
tube and sundry other damage but
without any clear reason for the fail-
ure. So here is N. B.’s story about his
GC181 TV set.
The set was bought in 1977, together
with a small trolley that held it about
half a metre off the floor. Around five
years later, my wife and I were snoozing on a Sunday morning to the usual
restful background of our three sons
squabbling over which cartoon show
they would watch.
Suddenly there was a thump, fol
lowed by absolute silence. After
some minutes, our eldest son put his
head around the bedroom door and
announced that the TV set was dead.
Feeling that it would do them no harm
to miss the cartoons, I told him to
switch it off at the mains outlet.
When I later went to investigate, I
found that the set had been pushed
backwards off the trolley and was lying
backwards at about 45 degrees. The
middle of its base was against the back
edge of the trolley and the extension
of the back cover was resting against
the wall. The soft old plaster of the
Glebe terrace house had been dented
by the case but had not come away
from the wall.
Now, I know almost nothing about
how TV sets work but I had been
building audio gear since about 1960.
I figured that the damage was probably
physical and that I might be able to
repair it.
Taking the back off, I found that the
neck board had broken in half. I had on
hand some single-strand copper bell
wire, so I carefully cleaned the tracks
each side of the break and soldered a
short length of bell wire across each
broken track. The neck board is so
small and light that the soldered joint
provided enough mechanical strength.
However, this did not fix the set.
On investigating further, I found that
(as in the set you described) the main
board was cracked from front to back
about 50mm from its lefthand side. I
carefully removed the various plugs
and boards from the main board, noting where each went, and considered
the problem.
Obviously, wire links would not
provide enough mechanical strength
for this board, so I used Araldite first
to repair the break. When this had set,
I soldered bell wire links across each
of the broken cracks and then reassembled the set. Much to my surprise, it
worked perfectly.
The point is that a fairly minor fall
backwards snapped both the neck
and main board. If the set had fallen
all the way to the ground, it is quite
possible that it would have suffered
the same damage as your customer’s
set. I wonder how much difference
there is between an impact that snaps
a neck board and one that breaks the
neck off the tube.
Incidentally, about five years ago the
set again fell off the trolley. That time
it fell forwards and landed flat on its
screen. There was no damage. It still
works, although now as a second set.
Its only problem is that it really needs
new volume and colour pots – the
sliders have to be wedged in place with
bits of cardboard so that the contacts
bear on a relatively unworn area of
the track. Not bad for a TV set that is
almost 20 years old!
Servicing at sea
Finally, there is a most unusual
story from A. D. of Whangarei, New
Zealand. And this is another of those
letters which has languished too long
in the “too hard” file, while trying to
fit everything in. Here is his story.
Intermittent faults are the most
annoying things, especially when it’s
your own equipment and is currently
in use.
To set the scene of this story I
should explain that I live on a yacht
and I picked up a TV set while
cruising in Australia. It is a 12.5cm
Trakka 15 colour set made by Philips
(KA212).
I have another TV set, a 25cm model,
so the 12.5cm model was stored in a
box and didn’t get much use over the
winter after I bought it. It was a very
wet winter and very damp on the boat.
When the set was eventually pressed
into service so that I could watch TV in
bed, it wasn’t long before it developed
a couple of faults.
When first turned on, a hissing
sound could be heard from the EHT
supply but this usually stopped after
about five minutes, so I wasn’t too concerned about it. The more serious fault
would cut out the picture and sound
completely. The channel selector indicators would remain lit, but I couldn’t
change channels. Then the picture and
sound would return but the set would
always revert to channel 1.
I thought a few hours in the sun
might dry the set out, as my suspicions
were that the EHT system was being
loaded and that this set might have an
overload trip that was resetting itself.
But after sunning the set the fault
became more frequent and annoying.
And it always seemed to fail right
on the punch line of the show I was
watching.
I found that I could sometimes get
the picture back by rocking the monitor/TV switch on the back of the set
and, since I had stowed the set resting
on its back, I suspected that I had
damaged the switch.
So, finally, the cover came off. This
set has two large circuit boards with
quite a few discrete components, plus
a sub-board with the monitor/TV
switch on it. The switch was going to
be impossible to remove so I simply
linked the respective switch points,
as the set was never going to be used
as a monitor anyway.
I thought that would be it and sat
back to watch Fawlty Towers but, five
minutes later, it was in fault status
again – just as Basil was annoying
some German diners.
I removed the cover again and this
time did some probing with a meter.
The sub-board has a number of cables
to it: video/audio, in/out and power.
I did consider trying to remove the
whole sub-board but settled for leaving
the meter connected to the 9V supply
rail and waited.
It read 8.92V when the set was
normal but dropped to 4.2V when the
fault occurred. And so the tracking
down began. I had to find the start of
the 9V stabilised rail, which involved
checking other supply rails carrying
higher voltages. I finally came to a TO220 package with 12V on one pin and
around 9V on the other two.
So out came the magnifying glass
and, sure enough, I discovered a dry
joint. I can only assume that flicking
the switch may have applied extra
load to the circuit and temporarily
remade the contact. I cleaned the EHT
connections, applied silicone grease to
the EHT cap and put the cover back
on for the final time.
I am not a TV technician but an
experienced general technician.
Well, that’s A. D.’s story of his life
on the ocean wave. And I don’t know
about not being a TV technician, A.
D. I reckon you’ll make a pretty good
substitute in the meantime.
And that’s the roundup for this
month. Which isn’t a bad effort considering that we have been from Tasmania
to New Zealand and points in between,
and covered everything from aerobics
SC
to marine electronics.
November 1996 41
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