This is only a preview of the January 1993 issue of Silicon Chip. You can view 65 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 Flea-Power AM Radio Transmitter":
Items relevant to "High-Intensity LED Flasher For Bicycles":
Items relevant to "A 2kW 24VDC To 240VAC Sinewave Inverter; Pt.4":
Articles in this series:
Articles in this series:
|
SERVICEMAN'S LOG
The customer is always right?
In this business, one not only has to deal with
technical malfunctions, which by themselves
are bad enough, but with the customers as
well. I sometimes wonder which are the
worst.
As I have mentioned before in these
notes, one of my regular customers is
a local motel owner who has about 30
Samsung TV sets spread amongst his
guest rooms. Most of these sets are
50cm CB15F units and were purchased a little over three years ago. It is
one of these sets that is the main subject of this story.
At the time of purchase, the sets
carried a 3-year warranty and the
owner used to call me in whenever
one gave trouble. There was no contract between us; each job was treated
on its merits and the charge depended
on the warranty situation. If the set
was still under warranty, I simply
made the necessary claim, Samsung
re-imbursed me, and there was no
charge to the customer
If, on the other hand, it was a set
FRACARRO
DELIVERS...
.. .best television antenna
performance tor less cost.
Peter C. Lacey Services Pty. Ltd.
P.O. Box 678 (74Fulton Rd.) Mount Eliza 3930
Tel:03 787 2077 Fax:(03) 787 3460 ACN006893438
44
SILICON CHIP
that was out of warranty, I charged the
customer accordingly.
I have presented this brief background because it does have a bearing
on the sequence of events, although
not in a technical sense. Basically, the
story involves a complaint by the
motel owner to Samsung, with yours
truly as the meat in the sandwich.
In greater detail, not long after these
sets were originally installed, several
of them developed the same fault.
This fault involved several components in a front corner of the main
board: inductor L401, capacitor C414
and resistor RH0l - the latter described as a safety resistor. Any or all
of these components could have dry
joints to the copper pattern and, as I
later found out, this could cause arcing and damage to the board.
And to digress here for a moment,
the resistor type number (RH0l)
caused a good deal of confusion. While
it was so designated on the PC board,
it did not appear at all on some early
circuits and nor did its companion
component, diode DHl. And when it
eventually did appear, it was as RHl.
It might be worth making a note of
that.
Anyway, after the first two or three
encounters, I realised that this was
something to watch out for on a routine basis. From then on, as other sets
came in for attention over the next
two years, I made a point ofresoldering
all the connections in this area, regardless of their appearance. In fact,
all but one, as I recall, appeared to be
quite OK but I wanted to be sure.
Eventually, the time came when
there was only about six months of
warranty period left. I had a full list of
serial numbers for these sets and, on
checking my own records, I realised
that six sets had never been serviced,
so this potential problem remained.
More to the point, if Murphy had anything to do with it, there would be a
faulty one in this group.
At the first opportunity, I drew the
owner's attention to this, emphasising that these sets should be checked
now, while still under warranty. Furthermore, as this was an off-season
for the motel, it would be a good time
to take them out of service.
He said, "Yeah, sure mate", but that
was as far as it went. I subsequently
prodded him a couple of times but
again there was no result. I then went
off on a few weeks holiday and by the
time I returned, the warranty had virtually run out. I did make one final
plea but nothing happened and so
that was the end of it.
Or so I thought until a couple of
months later when the owner ap proached me to have the work done under warranty!
Naturally, I had to knock him back.
The warranty period had now expired
and I was in no position to break the
rules. Had I done so, I could have
been left holding the financial baby.
On the face of it, he seemed to accept
the situation and went off to think
about it.
Call to Samsung
What, in fact, he did do was ring
Samsung and complain - not about
me directly, but about the strictness
imposed on the warranty period. To
give Samsung their due, they didn't
muck about. If this customer was dissatisfied then, regardless as to who
might be at fault, they weren't going
to haggle over a few weeks beyond the
warranty period. I was instructed to
go right through the system and make
whatever repairs I considered necessary, all under warranty.
At the same time, they made it clear
aggravating what was already a delicate situation. In this situation, some
diplomacy was called for. So, without
showing any reaction to the implied
aspersion, I simply asked what the
problem was.
This brought a more conciliatory
response; apparently the set was drifting off channel after prolonged use
and was also delivering a snowy picture. Well, one thing was certain; I
wanted to see these faults in-situ. The
snowy picture, at least, could be an
antenna distribution problem, so that
point had to be cleared first.
s
'I,'< 1.0.WS.
~'S, ..,.etJ'f S, .
~e,tJ~"'o,JS
1
e"~•~t',,!1
,-fllitr~!Q:
Ill. f,A N 1111I,
" ~~,a.,1111if'S,
,~-···
t-f('(SAALL
wr>,Riv. & vott:>
Back to the workshop
8E. f'JU~S1\CK'I
I\OVl-~l'AA~WT~
t ~P,.W\l ~T"/~
Oil ~ ~,. ~
Well, he was right; it was snowy. A
second set plugged into the same outlet cleared the antenna system, so it
was back to the workshop with the
set. I set it up on the bench, checked
the adjustment of each channel, and
let it run. This confirmed the snowy
picture but I wanted to check out the
drift problem first. If it was drifting, a
new tuner would be quickest solution, and this might also sw eep away
the snow, killing two birds with the
one stone.
I let it run for a couple of hours but
there was no noticeable drift during
that time. However, I wasn't going to
be caught that easily; I'd been there
before and a few comments by the
owner had put me on alert. I switched
to another channel, which I knew was
correctly tuned, but the AFC wouldn't
pull it in. I then switched back to the
first channel - same thing, it wouldn't
lock in.
I don't know why it happens but it
has caught me out before. The set can
run OK on a particular channel for
hours but, as soon it is switched to
another channel, everything goes haywire. The only practical cure is to
change the tuner.
So I fitted a n ew tuner, set the channels up again and let it run. And that
cured the drift problem; the tuner was
now as steady as a rock.
But it had done nothing for the
snowy picture. Based on exp erience,
I went first to VR151 , a 10kQ pot on
pin 12 of IC101. This pot is the AGC
delay adjustment and is a common
offender. But not this time; my checks
and adjustments achieved nothing.
Next I checked the UHF reception
against the VHF reception and established that it was just as noisy on
either. So, with the tuner cleared, it
-
~
~11'1't
,':,o~Ai
'"~-f
'l'y f::
11-IE. WAR~N1'< HA~ EX? \RED & t WAS
IN NO ~ Sl1",0N "'rt> ~RE.A..\<. °™e: ~\JL-E. S ...
that this was a on e-off situation, so
there could be no misunderstanding.
It was not to establish a precedent
and that was fair enough ..
So I went to work. Apart from the
aforementioned routine problem, most
of the sets needed little or no attention . A few did, however, require minor adjustments and I did what was
necessary to bring the whole system
up to first class condition.
It thus came as rather a shock when,
a few weeks later, the motel owner
was back on the phon e, complaining
that, "The set in room so-and-so is
still playing up". It was the word
"still", spoken with some emphasis
and with a bit of an edge to it, that
upset me somewhat. It implied that I
knew about a fault in the set and
hadn't fixed it, whereas I could not
recall that my attention had been
drawn to any particular set or fault.
I assumed that the most likely explanation was that it was an intermittent fault and I had no intention of
J ANUARY
1993
45
TUOOI
ER1U 2.-nc
+ EC104
.-----+-----,t--(JF
's.~"
ED111
ll:2Ui
1N4141
1001(
I
R211
'-'"
SQ10t-S0103
KSA 539
25A 1115
T1t1
PSF 11AT01
,11:101
...
c,..
f ·"
C1i1
1,11
C1H
1.01
1V
""
'"
iSv
1
l;zss:.
'-+---+--.=,c.c.=_ _ _ _ ♦ 12'11-----------R\Di
t - - - -- ---+--------+---+---'"""-'"-"'L----~TP-•7_,_,_ _ _ __ _ ____,<at> 11 :~1s10
~
Fig.1: the tuner & IF circuitry of the Samsung CB15F. The IF output from the tuner (TU001) is fed to IF amplifier
stage Q161 and this, in turn, drives SAW filter 2101 & the IF processing chip (IC101) at bottom right.
had to be something further down the
line.
Signal tracing
The signal from the tuner goes to
the 1st IF amplifier (Q161, 2SC388),
then to SAW filter Z101, and subsequently to pins 9 & 10 of IC101.
So what to test next? From experience, my best suspect was the SAW
filter, because I've had several faulty
ones in the past. Fortunately, it is easy
to change, involving only five soldered connections, and I had a spare
on hand. A few minutes later I had
the answer - it wasn't the SAW filter;
it was still snowing.
OK, so what about Q161? One of
the weak points of these Samsung
circuits is that they carry no voltage
values. So I spent some time checking
associated components around this
stage, then measuring the voltages on
the transistor, and doing some mental
gyrations in an - effort to decide
whether they were reasonable.
In the event, I found no obviously
faulty components and finally decided
46
SILICON CHIP
that the voltages were reasonable. So,
as a last resort I pulled the transistor
out and fitted a replacement. Alas, no
joy from that either; the picture remained snowy.
So where did that leave me? I'd
cleared the tuner, 1st IF amplifier and
the SAW filter, all by replacement. So
the next suspect was chip IC101. I
(,4-~""\
\
\
\
\
I
\
This photo shows the replacement
component pack issued by Samsung
for the CB515F/CB515Z TV receivers.
The parts involved are C414, L401 &
RH01. A detailed instruction sheet
accompanies the component pack.
didn't fancy replacing it if there was
an easier way and so I resorted· to a
trick that I have used in the past.
As mentioned on a number of occasions, I have a self-contained tuner
unit, made up from an old turret tuner
and a small power supply assembled
from junkbox parts. It has proved
invaluable in tricky cases like this,
because it delivers a known good IF
signal.
At this point, the reader should refer to the accompanying drawing,
which the SILICON CHIP draughtsman
has prepared from my rough sketch see Fig.2. It shows the copper side of
the PC board around the tuner, the
tuner itself being mounted on the component side. What we are looking at
here are four tuner mounting lugs and
eight tuner connection pins.
For the sake of clarity, the top seven
. pins are neither identified nor shown
connected. The eighth pin, at the bottom of the drawing and hemmed in by
the main copper pattern, is the IF
OUT pin. This is connected to a small
copper track which forms part of an
inverted "T" pattern. Capacitor C161
then connects to the bottom of the
inverted "T" and feeds the base of
Q161.
With me so far? Right. I fired up the
substitute tuner and clipped its ouput
lead onto C161's pigtail where it was
soldered to the copper track. The result: a perfect picture, with not a snowflake in sight. This cleared the IF system, along with the following sections of the set.
So what was going on? The external tuner gave a perfect picture whereas the set's internal tuner, which was
known to be OK, gave a snowy picture. It just didn't make sense.
As far as I could see, the only difference between to two setups was a
10mm length of copper track between
the IF OUT pin of the set's tuner and
the lead from C161 (which was fed
directly by the substitute tuner).
There was only one way to resolve
the situation. I soldered a short length
of wire to the IF OUT pin, clipped the
substitute tuner lead to it, and tried
again. Result: it was snowing again.
So the fault was somewhere on that
10mm length of copper track.
I fished out the jeweller's loupe, set
EARTHY COPPER PATTERN
1111~----+
•
I
I
\
•
TUNER
MOUNTING
LUGS
.
.
~
<
z
:i
a:
.
~
a:
w
z
~
-TUNER
•
•
0161
C1~
7
Fig.2: this drawing gives some
idea of the layout around the
tuner. It shows the copper side of
the board, with the tuner (shown
dotted) on the component side.
Note that the connections to seven
of the tuner terminals have been
omitted for the sake of clarity.
Electronics Workbench®
up a good light, and moved in close.
Sure enough, there it was - the finest
of hairline cracks about halfway along
the strip. And a continuity check confirmed what was now obvious - the
track was open circuit.
After that, the cure was simple; a
run of solder along the strip put everything back to normal. So the set was
returned to the motel owner, with my
assurance that this latest fault had
now been fixed - under warranty.
By now, the atmosphere had thawed
somewhat and the owner actually expressed his gratitude for my efforts,
and his appreciation of Samsung's cooperation. So everything ended happily.
But how did that hairline crack occur iif1cl. why did it show up after the
seyhad been running for over three
years? Well, I don't think that it was a
recent happening. The set had not
been dropped or otherwise abused
and my bet is that the crack had always been there, from the time the
board was made or while it was being
fitted with parts.
My theory is that, in spite of the
crack, there was, initially, some bridge
across it; a tiny area perhaps, but ad-
EmonaLAB4
Four instruments in one package!
The electronics lab in a computer!
0
...
<at>)
[l!
©
t:l
A simple, intuitive and very
powerful teaching tool, Electronics Workbench lets students and
hobbyists design and test both
analog and digital electronic
circuits, without the delays and
expense of a laboratory .
FEATURES
• Quick and simple circuit entry
• Digital and Analog Modules
included, complete with all componen1s • Simulated instruments: dual
trace scope, spectrum analyser,
function generator, multimeter, digital
word generator and logic analyser
• Complete control over all
component values and parameters
• Print: circuit schematics, parts list,
instrument readings, macros • Logic
conversion - truth table to Boolean
formula to logic gates • Customisable hypertext help system
THREE VERSIONS
• Professional Version: EGA/VGA
colour display; unlimited components
• Personal Plus Version: Monochrome display; unlimited components
• Personal Version: Monochrome
display; limited to 20 components per
..
EMONA
INSTRUMENTS
NSW ......................... (02) 519 3933
VIC ............................ (03) 889 0427
OLD .......... ................ (07) 397 7427
DISTRIBUTORS
Perth ............................. (09) 244 2777
Adelaide ............ . ...... (08) 362 7548
Hobart ..
.. ........ (002) 34 2233
The LAB4 has four full
function test instruments:
■
DC POWER SUPPLY
Triple output; 5V, 15V, 0-50V
■
with neatly laid out and labelled
front panel controls. Also, the
LAB4 is compact and only takes
up a small space , compared to
four bench-top instruments.
DIGITAL MULTIMETER
Full function, auto-ranging
■ FUNCTION GENERATOR
0.02Hz-2M Hz, with SWEEP
■
FREQUENCY COUNTER
1Hz-100MHz input range
INDEPENDENT INSTRUMENTS
Each instrument in the LAB4 is
completely independent and can be
switched ON or OFF as required.
The LAB4 has only one 240V mains
input, saving on multiple power
points.
Each instrument is clearly defined
EMONA
INSTRUMENTS
NSW ......................... (02) 519 3933
VIC ................ ............ (03) 889 0427
OLD .......................... (07) 397 7427
DISTRIBUTORS
Perth ............................ (09) 244 2777
Adelaide ..................... (08) 362 7548
Hobart .......... ............. ... (002) 34 2233
]ANUARY
1993
47
TETIA TV TIP
Samsung Model VB 711 VCR
Symptom: machine will not record
sound and a newly recorded pictu re shows flashes of the original
ch roma . Any previous ly recorded
tapes are unaffected.
Cure: bias oscillator not working.
This oscil lator not only provides
the bias fo r record ing the sound
but also powers the fu ll erase head
to remove the original picture. The
trouble is caused by an intermittent contact at the AC head and is
cured by replaceme nt of the fau lty
parts and a modification of the AC
head connections .
TETIA TV Tip is supplied by the
Tasmanian branch of the Electronic Technician's Institute of Australia. Contact Jim Lawler, 16
Adina St, Geilston Bay 7015.
-~
.•r.--
~-c:::w~~...l.
--'=r"'-~ .~
"-#ANO 'tl-\\S M\GH, P.,\...$0 ~W€EP
A'WP.."1 1ltE: SNOW, KU..l.1"1G1WO 'B\~t>S W\1'\i ~ ONE. CS1'0~e:....
equate for a signal path. But copper,
by its very nature, will eventually succumb to corrosion. So th e tiny contact
area was eventually destroye d.
Coincidentally, as I started to write
these notes, Samsung issued a recall
notice con cerning the dry join ts
around C414 , 1401 and RHOl. Ap parently I wasn't the only one to rep ort
this problem. As a result, I and oth er
accredited service organisations have
been issued w ith a detailed instruction sheet and kits containing replacements for all three compo nents .
Samsung is not taking any chances.
Computer repairs
Now for a ch ange of scene. In the
December 1992 issue, I detailed how I
was conned into repairing a computer - or, more accurately, a computer
monitor. As it turned out, the fault
was relatively simple but , even so, I
expressed the hope that it would be
my last computer repair.
48
SILICON CHIP
It was to be a forlorn h ope because
it wasn't long before another customer,
on e of my regulars , fronted up with
another computer monitor. This time
there was no image at all and the
customer, rightly, thought that the
monitor was at fault.
This w as a different brand; an Acer,
mo del MM211 , marketed by Dick
Smith Electronics. This was a helpful
aspect; it's always reassuring to know
that there is someone to turn to for
spare parts or technical advice , if
n eeded.
Undoing four screws released the
p lastic cover and revealed a very
neatly laid out board. It also revealed
th e immediate cause of the failure; a
blown fuse. And, significantly, there
w as a second fuse nearby which was
still intact. So where did they both fit
in?
By pulling off some plugs fitted to
the board and removing some screws ,
I w as able to juggle the board clear
and turn it over. This enabled me to
make a qui ck assessment of the power
supply circuit and even to make a
rough draw ing of it.
In many respects, the circuit w as
very similar to that use d in the previously mentioned MAGIC model. It
consisted of a transformer, w ith a fuse
in the primary circuit, that delivered
about ZOVAC to a bridge rectifier. This
was then followed by another fus e, a
filter capacitor, and a 12V voltage regulator - just about as ordinary as one
could get.
It was the fuses that pinpointed the
culprit. The blown fuse was in the
transformer primary circuit, while the
one following the bridge rectifier was
intact. That indicated that either the
rectifier or the transformer was at fault.
It took only a few minutes of checking with the ohmmeter to find a dead
short across one of the 1N5401 bridge
rectifier diodes; and only a few minutes more to clear the small capacitor
across it. So that was it; whereas the
MAGIC model had an open circuit
diode , this unit had a diode that was
shorted.
As before, a new diode and fuse
restored everything to normal. The
biggest job had been getting things
apart and finding my way around the
innards. There's nothing to this computer business; so far, it's been-a snack.
But don't tell anybody.
SC
|