This is only a preview of the April 1998 issue of Silicon Chip. You can view 32 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. Articles in this series:
Articles in this series:
Items relevant to "An Automatic Garage Door Opener; Pt.1":
Articles in this series:
Articles in this series:
Items relevant to "40V 8A Adjustable Power Supply; Pt.1":
Articles in this series:
Items relevant to "A Chook Raffle Program For Your PC":
Purchase a printed copy of this issue for $10.00. |
SERVICEMAN'S LOG
Lightning can cause strange faults
Further to last month’s story on lightning
damage, another one comes to mind, along
with a story about of a couple of VCRs and
a service manual that went walkabout.
First, another lightning story. Mr
Knight’s wife was in the living room
looking out the window at the never-ending sheets of rain when there
was an unbelievably deafening crack
as lightning struck about 100 metres
down the street.
Everything electrical went off and
then the lights slowly came back on.
The initial shock of being so close to
so much power cannot be understated
and one is always amazed when people are actually struck by lightning
and survive! I suppose the immediate
physical effect is the deafness from
such a loud noise plus the intensity
of the flash, which may cause temporary blindness. However, these effects
wear off and you begin to weigh up the
cost of the damage inevitably inflicted
by nature’s fury.
In Mr and Mrs Knight’s case, the
telephone was dead, as were some of
28 Silicon Chip
the house lights and appliances. The
outside light proximity sensor switch
had also gone. And all the neighbours
suffered multiple failures.
The TV set, a Panasonic TC-29V50A
(MX-2A chassis), had been on at the
time but, as soon as the lightning hit,
reverted to standby mode. As I later
learned, the remote control restored
the picture perfectly but there was
no sound.
Because the phone had been
knocked out, Mr Knight had to drive to
my shop to arrange for me to fix the set.
I showed up that afternoon, intrigued
as to why only the sound had failed.
I suspected that, in the confusion
after the strike, someone may have
pressed the wrong buttons on the set
or the remote control. However, after
spending 10 minutes checking all the
controls, only faint clicking noises
could be head in the speakers.
I didn’t have a circuit for the set and,
as it was another large model, I was
reluctant to move it to the workshop
– as was Mr Knight. I hoped that a
temporary fix could be organised until
I could better prepare myself.
First, I tried feeding in signals from
their VCR but as expected, there was
still no sound. However, the TV set
on-screen displays showed that the
set’s stereo decoder was working and
could distinguish between mono signals from the VCR and stereo off-air
transmissions.
Next, I decided to try feeding the
AV (audio/video) outputs from the
VCR directly to the AV inputs of the
TV set. I fetched some RCA leads from
the truck, connected the two machines
together and selected the AV mode. As
before, the picture was fine but there
was still no sound. This could only
mean that the problem lay somewhere
in the audio amplifier stage.
I took the back off and, by tracing
the speaker connections, established
that IC2303 (AN7169) was the stereo
output amplifier. Rubbing my fingers
over the solder produced hissing
noises from each channel. Although
not a definitive test, it did suggest,
even without the benefit of a circuit
diagram, that the fault lay between
the input AV sockets and this chip –
possibly in the volume control and
mute circuits.
Fortunately, the family had a portable radio/cassette player with line
inputs for recording. By connecting
it to the audio output sockets on the
TV set and pressing the cassette record
buttons, I was able to hear sound from
the TV set at last.
I decided to leave things set up in
this manner while I ordered a circuit
diagram. The only inconvenience the
family had with this arrangement was
that they had to physically adjust the
volume control on the cassette player to the level they preferred, as the
remote control had no effect. And of
course, the cassette player had to be
switched on and left permanently in
the record mode.
I received the circuit about one
week later, only to find that there were
no less than seven ICs involved with
the sound circuits (not to mention
the muting and control processors).
These were: sound IF IC2206, stereo
decoder IC2201, AV control IC3001,
surround sound IC2301, audio control
IC2302, preamp IC2306 and output
amplifier IC2303. The audio muting, simply put, was controlled by
IC1102 to Q2301 and Q2302, as well
as Q3015, Q3016 and other circuits,
such as Q1113 audio defeat and Q1111
volume.
As my provisional sortie had
already eliminated half of these, I
decided to take a signal tracer (a little battery-powered amplifier) and a
signal generator on my next trip. Mr
Knight was delighted to see me back
but exasperated to learn that I still
didn’t know where the problem was
and that I was only there to attempt
to identify the faulty part(s). After all,
this was just a simple sound failure –
at least, as far as he was concerned.
Selecting the left channel and using
the tracer, I managed to monitor sound
from pin 5 of AV control IC3001 (pin
1 was for the AV OUT) to pin 8 of surround sound IC2301. There was also
sound from pin 3 of IC2301 to pin 6 of
preamp IC2306 and from pin 7 to pin
3 of audio control IC2302. But there
was nothing from pin 9 of IC2302 to
pin 2 of output amplifier IC2303.
I unsoldered the collector of Q2301
to ensure that the muting circuit
wasn’t doing its thing but there was
still no sound. So, by a process of
elimination, the fault had to be in
IC2302, a CXA1279AS, and/or its
control circuits. A meter check established that the control voltage to pin
16 varied with the volume control,
which was correct.
I felt I had to be pretty certain as
to which part to order, as Mr Knight
was becoming rather “tetchy” about
the speed of this “simple” repair. To
be safe, I decided the best course was
to order the IC chipset in case of a
misdiagnosis. If nothing else, I would
have them in stock for what is a fairly
popular model.
Anyway, as luck would have it
and to my great relief, my diagnosis
was spot on – replacing IC2302 fixed
the problem and restored the sound
completely.
But why, in the multitude of components in this TV set (there are 25 ICs in
all), did the lightning destroy only this
IC and nothing else? Unfortunately,
this is one aspect of the job I am not
qualified in so I don’t have the means
to explain it. Perhaps no mortal can!
A tale of two VCRs
My next story is about two VCRs,
both Akai VS-F10EA models. This
model VCR is old by present day
standards but is a reliable performer
and a popular choice as a rental unit.
And this is the story of two such rental
units which landed in the workshop
together.
To minimise confusion, I have
designated them as VCR 1 and VCR
2. Mr Carton’s set was VCR 1 and the
symptoms were no video on playback
or even AV in or out. Mr Darnay’s set
was VCR 2 and the symptoms were
described as intermittent stopping
when playing back. But by the time I
tried it, it was completely dead.
These units presented a major problem; I had no circuit. Originally, I did
have a complete manual but this had
gone walkabout. I had a good idea as
April 1998 29
Serviceman’s Log – continued
to where it had gone but recovering
it called for some diplomacy. More
of that later but, for now, I was trying
to manage with a slightly different
circuit, namely for a VS-F16. Though
close, this was still significantly
different in parts and led me to doubt
conclusions I had made on the basis
of this schematic.
Mr Darnay’s set, VCR 2, was the more
urgent so I tackled it first. This model
VCR features two power supplies: (1) a
main switchmode power supply which
provides seven rails (23V, 16V, 3 x
12V and 2 x 5V); and (2) an auxiliary
miniature switchmode power supply
on the motherboard which generates a
-35V rail and a 5V rail, the latter called
a “filament” supply for the display
system. The auxiliary supply operates
from the 23V rail.
I checked all seven voltage rails at
the output plug (WP201) of the main
power supply. There were voltages
on all seven, though not exactly correct. However, I often find that Akai’s
marked voltages are not necessarily
exact, often contradicting themselves
on various parts of the circuit.
30 Silicon Chip
I checked both voltages generated
by the auxiliary supply. Both were
present but somewhat low. The
question was the degree of error and
what was critical. The 5V rail was
less than 4.5V, while the -35V was
down to -27V.
Akai service bulletins warn that low
or dark displays may be due to two
electros drying out in the auxiliary
power supply. I decided to replace
C446 and C447 with two 100µF
electros (they are marked 47µF in
the circuit diagram but 120µF had
already been fitted by the factory). It
was a futile gesture which made no
real difference.
I next checked all the crystal clocks
with an oscilloscope, especially X701
(4.43MHz) on the video board. This
also supplies a clock signal (fsc) to the
digital servo (IV401, pin 22) on the
motherboard. Everything seemed OK.
My next step was to see if the loading motor mechanism was aligned
correctly but as expected, I could find
nothing wrong here (after all, when it
worked, all functions worked properly). By now, I was coming around to
the idea that either a microprocessor
or the servo itself was intermittent.
At this point, I decided to switch
my attention to Mr Carton’s set, VCR
1 (the one with no video input). The
plan was to tackle what now appeared
to be the simpler fault, then use this
set as a donor to fix the problem
in VCR 2. The latter could then
be returned, while VCR 1 could
wait for parts to be ordered and
installed (I hope all this makes
sense).
Because the fault was lack of
video, I decided to work with a
colour bar generator rather than risk
a faulty tuner. The only problem was
that I stupidly plugged the generator
RCA plug into the wrong socket on the
rear panel, namely the audio out (the
back was facing away from me, it was
dark and the sockets all looked the
same – well, that’s my excuse anyway
and I’m sticking to it).
So, following the colour bar signal
with the CRO, I tried tracing the colour bars to pin 1 of IC101, a TC4066
analog switching IC. When I found
that it never reached it, as the VS-F16
schematic showed, I abandoned this
approach and assumed the VS-F16
circuit differed from the VS-F10.
However, I did find a video signal on
pins 2 and 4 of IC101.
I followed this video signal all over
the motherboard to pin 5 of IC602
(AN3247K) and out again on pin 9 to
pin 13 of character control IC102. And
that’s where the trail went cold, with
no signal out from pin 12 to the video
output. It looked as though IC102 was
the culprit and so, to confirm this, I
momentarily shorted pins 12 and 13
together and the picture was restored.
Of course, it was possible that the fault
could still be external to the IC.
But I was happy to accept that it
was the IC and so I desoldered the
corresponding IC from VCR 2 and
donated it to VCR 1. Success – well,
sort of; the picture was fine but there
was a buzz in the sound and I still had
to solve the mystery of no video in.
The missing manual
At this point, it is appropriate to
reintroduce the subplot of the VSF10EA service manual which had
gone walkabout and detail the history
and order of events.
Being a small service organisation,
it is impossible to stock all circuits for
all models, especially as new ranges
appear about every five months from
every manufacturer. The only way
to survive in this environment is to
co-operate with the opposition – you
lend me your manuals and I’ll lend
you mine. Normally, this arrangement works well but occasionally,
when you deal with a large service
centre where there are many people
involved, manuals can get lost.
And so it was with my VS-F10EA
manual – I lent it to this centre about
six months ago but they didn’t return
it. When I reminded them, they didn’t
think that they had ever bor
rowed
it. Anyway, I didn’t want to alienate
them by pressing the point too strongly and simply assumed that it would
eventually turn up and be returned.
As it turned out, my luck was with
me. I called into the centre recently to
borrow some other circuits and on the
spur of the moment I asked if I could
borrow their VS-F10 service manual.
Obligingly, the technician went to the
filing cabinets and pulled it out, only
to discover that it was my copy with
my writing all over it!
The technician was most apologetic
and so I departed, much relieved at
recovering my lost manual. And none
too soon, because I was still puzzling
over the buzz in the sound and the
confusion over the RCA sockets and
the colour bar generator. Fortunately,
it didn’t take long to realise my error
and sort out the confusion.
So, with VCR 1 working properly at
last, it was now reassigned as a donor
and I could swap parts out of it and
into VCR 2. The first step, of course,
was to refit IC101 into VCR 2 (I know
it sounds silly but that was the way
it had to be). This done, I swapped
the entire front panel with the timer
microprocessor and display on it. I
thought I had correctly diagnosed this
too but it wasn’t long before it started
to fail intermittently and ultimately
failed completely.
Next, I swapped IC403 (syscon),
which means desoldering and resold
ering 64 pins twice over. Once again,
it started to work and then died. I was
becoming rather dispirited but decided to swap the digital servo IC (IC401).
This had exactly the same effect as
before and so I put the machine aside
and waited for inspiration.
While I was catching up with routine work, I kept thinking about the
symptoms of this rogue set. More often
than not, it worked when cold rather
than hot or failed after it had been on
for a while. So why not try the freezer
treatment?
To cut a long story short, I expended an expensive can of freezer and
achieved nothing. Well, not quite;
there was some momentary activity
in the power supply, which made me
put my thinking cap on again. Perhaps
some of these voltage rails were more
critical than others but the question
was, which ones?
It was at this time that I had the
chance to talk again to my mate from
the opposition. He was very familiar
with Akai VCRs and told me the 23V
rail from the main power supply was
the one to watch and if it was down
to 19V to change the bridge rectifier
(D1, D2, D3 & D4). This was the lead
I needed and when I measured it, it
read only 20.5V.
Unfortunately, replacing the diodes
made no difference and so I decided to
check the main filter electro (C3). Why
not connect another electro across C3
and see if that made any difference?
To my delight and surprise, it fixed
the problem completely. I replaced
C3 (2200µF 35V) with a new one and
reassem
bled the VCR. It was now
working perfectly.
I left all the good parts in VCR2 and
put VCR1 aside to wait for the new
IC101 to be delivered.
And so it all ended happily – for the
customers. But in retrospect, I didn’t
come out of it particularly well, either
financially or technically. I had missed
the obvious; ie, the need to follow up
any suggestion of a power supply fault.
My only excuse is that I got sidetracked by the need to work (initially)
with a substitute circuit, an apparently intermittent fault and by my confusion over the voltage values. Still,
I should have known better and I do
know better. I simply didn’t follow
the rules and paid the price.
The flea-marker computer
To finish up, here is a story on a
brighter note. It comes from a reader
and was inspired in part by these notes
in the December 1997 issue, describing a service job on an AST Ascentia
Colour Notebook computer. It comes
from a VK5 amateur, S. M. of Elizabeth
Downs, South Australia, describing a
tentative approach to laptop computer
servicing. This is how he tells it.
It all started when my 14-year old
stepson, Peter (not his real name),
at high school and up to his ears in
computers, wanted to visit the local
computer flea market. My wife said
OK; she hoped she might get some
clip art for her craft hobby, while
Peter might get a CD ROM or some
more SIMMs.
The place was chock-a-block
when we arrived. There were trestles
sag
ging with games, programs, old
computers, VDUs; you name it, it was
there. After it had thinned out a little
EVERYONE KNOWS...
2694.VET.SIL.1/4.1
If you‘re concerned about viruses while surfing the Net, you can rely on Vet –the all-Australian software that offers you superior protection and full
local support. Vet protects against thousands of conventional and macro viruses and is suited to any PC platform as well as Novell NetWare & NT Server.
Unlike other software developers, we only make anti-virus software so it‘s natural that we‘re the experts at it.
Confidence in Vet‘s abilities extends from fellow surfers to governments, banks and companies in over 30 countries world wide. So if you‘re on
email or surfing the Net, Vet has all the protection you‘ll need, including products with free mailed upgrades to make sure you stay protected.
And, after all, when you go surfing who wants to worry about the quality of the water.
Evaluate Vet for Windows 95, Windows NT Workstation and Windows 3.x at www.vet.com.au
For your nearest reseller or an information pack telephone 1300 364 750 Email: info<at>vet.com.au
All the Anti-Virus you need
April 1998 31
I wandered around and saw a chap
offering a laptop. A notice on it said,
“A Mr Fixit Special”.
It was a Tandy 1100HD with an
LCD green screen, a 20Mb hard disc
drive, a 3.5in floppy disc drive and
MS DOS version 5. It included two
batteries (one of them new) and all
manuals and discs. The notice said,
“power supply will not operate the
PC, or charge the battery”. He wanted
$45.00 for it.
I pointed it out to Peter. With a
gleam in his eye he asked, “Do you
think you can fix it?” I asked the
vendor if I could have look at the
instruction book. “Yes, go ahead”, he
said. “It just won’t run, that’s why I
bought a new battery”.
The book indicated a 6V battery,
and the external power supply unit
(PSU) was 9.5V at 1.2A. This went
into the computer via a standard DC
connector, similar to most plugpack
PSUs. I said to Peter, “If it’s the PSU,
I could easily make a new one”.
By this time my wife had taken
some interest in it. And, in answer to
Peter’s unspoken question, replied,
“Yes”, and handed him the money. We
32 Silicon Chip
wandered around a bit, then headed
for home.
On the way, my wife and I called in
at a delicatessen, leaving Peter in the
car. When we came out, Peter could
not contain his excitement. “It works,
it works! I connected the second battery, turned it on, the ‘Charge Light’
blinked, the display came up with a
start-up routine, and the beeper beep
ed. Then it died”.
When we arrived home Peter wanted to fix everything straight away. I persuaded him put the battery on charge
while we had lunch. With the battery
removed and a couple of makeshift
pins in the battery plug, we applied
7.5V across the battery at 500mA from
a constant current regulated supply
and left it for about 45 minutes.
When the charged battery was installed, the computer fired up straight
away. Peter’s fingers flew over the
keyboard. Everything appeared OK.
“Right”, I said, “let’s check the
PSU”. I plugged it into the mains and
switched on. There was no smoke
and the DVM indicated 10V DC at the
plug. A 12V 3W festoon lamp lit up
when connected across the plug, with
9.2V still indicated on the meter. So
it wasn’t the PSU.
“Ah”, I said to Peter, “Are you game.
Shall we take it apart?” “Yes; what
have we to lose?”
It took us 10 minutes to undo all the
screws and the little plugs and sockets, after which we were able to remove the covers. Continuity tests with
a DVM and a DC plug with test wires
showed that the switching action of
the DC socket was OK. Further checks
showed that the DC was applied to the
PC board and that the battery plug was
connected to the board.
So what was wrong? I was inspecting the board for burnt or damaged
components when I spied four
miniature fuses marked F1, F2,
F3 & F4, each about the size of
a 0.25W resistor. A quick check
with the DVM revealed that F2
(2.5A) was OC.
I decided to use one strand of
multi-strand hookup wire which I
guessed would fuse at about 2A. This
was soldered to one end of the dodgy
fuse and a short piece of plastic insulation slipped over it (in case it blew
and splattered everywhere). The other
end was then soldered to the other
end of the fuse and a DVM used to
confirm that it was intact.
It took us a careful 15 minutes to
get it all back together again, with all
the right screws and bits in the right
places. No wonder computer techs
charge $50-$60 just to look at a repair.
With it all back together we tried it
on the battery first. It worked OK. We
took the battery out and tried it on the
PSU – OK again. We then reinstalled
the battery and the battery charge
light came on, so all functions were
OK. Peter couldn’t get inside quickly
enough to show his Mum.
I obtained two spare fuses, one for
the computer and a 4A one for the
other battery pack (a miniature one
under the heatshrink cover). They cost
$1.50 each. We subsequently checked
various Tandy stores and established
that the 1100HD was on the market in
1991 for around $2000 and that it uses
a 386 processor. It prints OK on two
Canon printers and an old Panasonic
dot printer – all for $45.00 for the unit
and $3.00 for fuses.
Thanks S. M. for an interesting
story. I wonder how many other old
(and not-so-old) machines have been
consigned to the scrap heap for relaSC
tively minor faults.
|