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SERVICEMAN'S LOG
How long is a couple of months?
How long is a couple of months? This may not
seem to be very important, until an irate
customer disputes the time and details of your
last service job. A properly kept record system
could then prove vital but more on that later.
It’s been rather quiet as far as work
goes during the last month. Either not
much has failed or I need to change
my brand of breath freshener.
However, as well as the normal
boring repairs there have been a few
interesting cases. First, there was a
Casio SF-7900 Digital Diary which
had been literally drowned in shaving
cream (not too sure whether I can fix
that one yet). This was followed by
an NEC remote control that had been
68 Silicon Chip
savaged by a dog (not much of that
left), in turn followed by a dropped
Hitachi 34cm TV set sporting a smash
ed tube (its only remaining function
being landfill).
Difficult repairs take time, impossible ones take a little longer. One
set was delivered in a van, with two
very fit young men visibly straining
under the weight of a Philips Matchline 83cm TV set. Fortunately, I had
a trolley available – and where they
put this set down was where it was
going to be when they picked it up!
This monster was a model 33CE
7538/42R, employing a 3A chassis
(circa 1986-1988), which had been
bought at auction. Ironically, I already
had seen this very set at a competitor’s
secondhand store and I knew the tube
was virtually defunct, which was
why the proprietor had put it back to
auction. A new tube costs over $2000.
Anyway, the new owner also had
this information but because he had
bought it so cheaply, he wanted a
few other supposedly simple faults
fixed. These included a blue line at
the top and east-west distortion in the
corners. He was prepared to live with
the washed out picture.
To make things more difficult, there
was no remote control and no instruction booklet but, on the bright
side, I did have the service
manual on this 3A chassis.
At switch-on there was
nothing except the blue line
at the top. After 15 minutes
or so, a very dingy picture
appeared, with four vertical
kinks in the left and righthand
sides about 10cm from the corners.
The picture tube carried no label
and was already running flat out,
with the heater filaments glowing like
torches. Someone had shorted the two
series inductors feeding them in an
attempt to extract extra performance.
The line at the top of the picture
and the delay in coming on was
reminiscent of the Philips 2B chassis
described last month. And, in fact, the
circuits are very similar.
I removed the chassis (not easy)
and, with the help of the service
manual, set about replacing as many
electros as I could around the eastwest circuitry and vertical output
stage (especially C2555). I also fitted
an 82kΩ resistor from pin 26 to pin
6 on IC7355 (TDA4580), in the multi-standard decoder module. As mentioned last month, this is a standard
modification to cope with tube ageing.
When I switched on, the picture
came on much faster than before but
the east-west geometry was still a
mess. It was time to delve deeper into
the likely causes.
The geometry adjustments of this
set are performed using a remote control but not necessarily the one that
comes with the set. The remote control must have a Print or Sleep Timer
function on it which, when pushed
simultaneously with the mono button
on the front panel, will bring up a
display of four bars. You then select
two numbers corresponding to the
geometry control you wish to adjust
(e.g. 08 for east-west pincushion) and
then adjust the value with the remote
volume control.
Of course, I didn’t have one. But
while I waited to see if I could beg,
borrow or steal a remote control (models RC5991, RC5275, RC5375, RC53,
RC5, RC5610 are the only ones that
can do the job), I tried my hardest to
fix the other problems.
I found that by varying the screen
preset pot (R3472) on the neck of the
tube, as well as the greyscale, I could
vary the colour and shape of the line
at the top of the picture. I also found
that freezing the components around
the east-west output stage, along with
an increase in the beam current, also
varied the corner distortions.
I spent a long and fruitless time
replacing everything in the east-west
circuitry and found that the only
way to achieve any worthwhile improvement was to fit a 10kΩ resistor
across C2602. However, this still left
a pincushion effect which would
have to wait until a remote control
became available to see if it could be
corrected.
In the meantime, I came to the
conclusion that some attempt to
rejuvenate the picture tube might be
worthwhile. And so it was that I dug
out my old home-made monochrome
picture tube booster and analyser.
This basically consists of a 15W lamp
in series with the cathode and grid of
the CRT across 240V AC.
The heater filaments are powered
from a separate variable multitap
transformer and when current starts
to flow, it is limited by the lamp. I
connected the tube pins directly to
this and switched on, not expecting
very much action. To my surprise, the
globe flashed quite vigorously on each
gun in turn, telling me that AC current
was indeed flowing healthily between
the grid and the cathode, even with
the filament at the normal 6.3V.
Next, I removed the shorts across
coils L5466 and L5465 and reconnected the CRT base. At switch-on, the
picture came up quickly and strongly
with a high-definition picture. I reset
the greyscale tracking and the picture
was excellent. The blue line and the
vertical kinks in the horizontal scan
on the left and right had all gone but
the east-west fault remained. I needed
the correct remote control.
Three weeks later, I finally managed to obtain one and dialled in the
digits. Not all the controls displayed
the value of the adjustment but apart
from that, the east-west pincushion
command (08) did function properly,
without any modifications to the set.
I did find it necessary to set up the
horizontal EHT Compensation (11)
as well as controls 07, 09 and 10, and
then save the settings with the remote
control’s PP and Standby buttons on
program 1 (PR1); the default condition
when the set is switched on.
Summing up the repair, it was really just a matter of rejuvenating the
tube and adjusting the controls. Of
course, I cannot guarantee how long
the rejuvenated tube will last, as the
procedure knocks off the oxide on the
poisoned cathodes. Nor can one tell
how much material is left behind, nor
how long before it, too, becomes poisoned. Anyway, the new owner has,
for the time being at least, acquired a
cheap large-screen TV set.
The Akai video
My next story concerns an Akai
play-only video machine, model
VP170. This set was only just out of
warranty and the problem was that a
cassette had jammed inside it and it
was switching off. When I shook the
unit, I could hear loose components
rattling around inside – not a good
sign.
I took the cover off and a small
piece of white plastic fell out but the
cassette couldn’t be moved. There was
nothing for it but to dismantle and
remove the entire deck. Once it was
out, the cassette ejector mechanism
was disengaged from the loading gears
and removed.
The fault causing this mess was
the righthand loading arm on the
ejector; a fault which is well known
to Akai service agents but not to lesser
mortals such as I. A replacement was
ordered, now modified with a fourcoil spring instead of three-coil type
(BL438155C).
After a lot of fiddling around, I managed to remove a sliding gear rack and
the mode select switch, to free the cam
gear. The ejector was then reassembled. This is not for the fainthearted
or “mechanically challenged” the first
January 1999 69
time one is confronted with this operation. However, after reassembling
everything I was relieved to find it all
worked properly.
Badged sets
With so many TV sets now manufactured in Asia, it is sometimes only
the badge which really distinguishes
different brands. I had two such sets
this month with vicious faults. The
first was an Akai CT1406A, while the
other was badged as a JVC C-14K1AU
but was exactly the same set electrically.
The CT1406A sounded straightforward enough, with the words “went
dead” written on the job sheet. Having
done many of these by now, I found
the usual ZD402 12V zener short and
resistor R425 (5.6Ω) burnt open on the
secondary of the horizontal output
transformer (pin 3 of T402). These
failures were caused by two electros
– C911 and C909 in the power supply
– drying out and allowing the two DC
supply rails to go high.
It was at this stage that I found I
had no vertical deflection. I replaced
IC401 (LA7830) but that wasn’t it. I
then re
placed tantalum capacitors
C417 and C416, along with C409 for
70 Silicon Chip
good measure – still no difference.
Some quick voltage checks showed
that I had 29V on pin 6 of IC401 and
24V on pin 3 but the CRO indicated
no vertical drive from pin 31 of IC301
to pin 4 of IC401. I replaced IC301 –
an AN5601K jungle IC – and finally
restored the vertical timebase. But
now all I had was a distorted blue
raster and none of the front controls
were working. A visual inspection
revealed that IC802, a TMS73047
microprocessor, had been corroded
by some liquid that had dried there,
so I replaced that as well.
By now, you would have thought
that all this hard work would have
delivered some decent results but my
misery continued. I couldn’t believe
how much was wrong with this 3-year
old TV set which still looked brand
new from the outside. Should I call it
quits and write off all the work done
so far? I decided to continue.
At this juncture, I still had the
distorted blue raster and limited intermittent control of everything. The
distortion was hard to describe but the
top quarter of the picture was black
with retrace lines and the remaining
three quarters was all blue as though
there was severe “hum” in the picture.
I went back to the jungle IC (IC301)
with the CRO and checked the RGB
colour outputs on pins 21, 24 and 25.
The CRO showed large square pulses to all guns. I checked diodes D307,
D308 and D309 and disconnected the
CRT socket (CN302) in case something was dragging it down. However,
after a lot of mucking about, it turned
out that diode D306 was leaky. I found
this only by comparing the voltages
and waveforms with the second set of
this story, as I had to literally fix them
side by side. D405 measured OK and
the “hum” was a distorted vertical
pulse going into pin 14 of IC301.
By now I had a good picture and
sound but still only had intermittent
control of the set, especially when
switching on. Eventually, I noticed
that flexing the board near the
micropro
cessor (IC802) varied the
symptoms and it didn’t take long to
find a hairline fracture in one of the
copper tracks. Resoldering this fixed
the last problem.
Now for the second set – the JVC
C-14K1AU. As already noted, I was
actually working on both sets together, alternating between the two. And
the JVC also suffered from no vertical
deflection.
I followed the same search pattern
sequence as on the Akai, and was
beginning to suspect the jungle IC
(IC301) again. In addition to the three
capacitors I had already changed, I
also had a go at C321 and C326 but to
no avail. I was just about to remove the
IC and was comparing the impedance
of each pin with respect to chassis
when I noticed a sizeable difference
on pin 33. On the working set (the
Akai) it measured about 100kΩ but on
the non-working JVC set it was 1MΩ.
The soldering on this pin didn’t
look too crash hot so I reworked it,
after which it read the same as the
Akai. I fired it up and found to my
relief that it was working perfectly.
Fortunately, there were no other faults
and I sat back sipping my coffee and
contemplating why I ever became a
TV technician.
A surly customer
And now about that matter of service records, mentioned earlier. My
next customer was a surly Mr Borland
(not his real name) who brought in
his Sony VTX-100M (this is a multisystem TV stereo tuner, part of an expensive modular Profeel TV System,
COLOUR CCD CAMERAS (42X42mm)
2 lux. colour with
one of these lenses
3.6mm-92 deg./4.3mm
-78 deg.5.5mm-60 deg.
Special introductory
Price of just $189
+ $8 for audio module
B/W cameras also available.
PARROT VOICE RECOGNITION DIARY
Access up to 350 names
and their Ph./Fax No’s
just by the sound of your
III
voice. Easy to use ,small
pocket sized unit. Also up
to 13 min of vocal memo
notes, appointment book,
clock, Ph assistant etc.
Quality product made by
IBM, retails for $300, we
Parrot
have limited quantity at:
$130
IR SWITCH KIT: Ref: EA March 96.
Uses a commercial coded IR TX & a RX
kit. The Tx has one button & req. 9V. Size
115 x 33 x 22mm. The Tx uses an UM3750
(code/decode chip). The Rx uses a RX
module to pick up the 40KHz IR signal.
This demodulated code is detected & not
decoded. The detected level switches a
4013 (wired as a toggle flip flop) then a
relay. Can be used as a high security
remote. With UM3750 IC in the Tx PCB as
a decoder, a second IR Tx is used. UM3750 has 12 coding inputs. Use your own
PIN in the Tx & Rx. Note: you two TXs &
one Rx kit for high security remote. * IR
Switch Kit (with 1 TX):(K66S) $20 *Secure
IR Switch Kit (with 2 TXs): (K66C) $28
IR RECEIVER FRONT END MODULE
Contains an IR receiver diode,
amp tuned to 38KHz, a bandpass filter, an AGC section & detector circuit. $2 Ea or 10 for $15
WIRED IR REPEATER KIT: Ref: EA
March 96. Simple kit which uses a
commercial IR Transmitter and a Rx kit
which works with most remote controls.
The receiver uses a receiving module to
pick up the 40KHz IR signal. The output of
the receiver module is connected to the IR
LED driving circuit of the IR Tx. This retransmits, giving an extension in range of
up to 15 metres: (K66R) $20
CIGARETTE LIGHTER LEAD & PLUGS
Heavy duty 1.6M lead
Removable
4A fuse
$1.50
DRIVER/ RIDER COMMS SYSTEM
Ideal for rally driver/co-driver communications or bike intercom. This is a new
(surplus) professionally made unit and
requires some minor wiring
& a cheap pair of head
phones for this & many more
applications. With 2 high
quality unidirectional
electret Noise
Cancelling Microphones with wind
filters &mounting clips.
Appears to have been
designed for a comm
unications system.
(AP3)$18
NEW STEPPER MOTORS
30 oz./in. torque, 2.5 deg. 144 step, low
voltage, compact 57 x 38mm: $14
POWERFUL 80 IR ILLUMINATOR
With strong universal swivel
mount & 50X50X50mm
housing:$36 Just $30
With any camera purchase
X-RAY MACHINES, HEART MONITORS, SATELLITE TV
EQUIPMENT, OSCILLOSCPOES, OTHER TEST EQUIPMENT
These are some of the items that may still be for sale at our Web Site. See our
BARGAIN CORNER, TRADERS CORNER & FREE ADS
FREE ADS should be E-mailed with “FREE ADS” in the subject window
KITS OF THE MONTH
NEW DESIGN 110W CFL INVERTER
This kit is a redesign of our extremely popular inverter kit. The new
improved design uses a larger transformer and a SG3525 switch Mode
Chip.This very Efficient Driver kit can drive up to 11 X 10w CFL’s from
12vdc. And would be great for lighting the weekender or caravan Kit inc.
1 inverter & 1 CFL: $30 Extra CFLs $12
NOW TRY OUR PROFESSIONAL PIC MICRO PROGRAMER
Programs up to 39 different types of PIC
chips, Software works under DOS, WIN
3.xx and WIN 95, Quick Easy construction,
Connects to Pc’s parallel port. Download fully
functional evaluation software from the Internet
register for a small fee. More details on our web page
$35
WE BUY NEW & USED SURPLUS OR STOCK COMPONENTS, MODULES, PCBs,
MOTORS, GEAR BOXES, HOUSINGS, PLUGS, SWITCHES, METERS,
ASSEMBLIES. CALL OR FAX WITH DETAILS. LARGE OR SMALL QUANTITIES
Gosford Amateur Radio & Electronics Field Day
On Sun Feb 28,1999. Wyong race course 830am. This is the biggest sale day of new,
used & new surplus electronics, radio and computer equipment in Australia. Don't miss
out. If you can't attend on the day some Gosford bargains will be posted on our web site
from Fri Feb 26th until Mon Feb 29th and orders can be placed via the normal method
on those days only. Just take the F3 to the Alison Rd. turn-off in to Wyong or a 300M
walk from Wyong train station. More at.... www.terrigal.net.au/~rosser/fieldday.htm
BARGAIN PACK HIGH QUALITY 1.6 /
5.6 SERIES SIEMEMS CONNECTORS,
92 date code, See Siemens web site.
Compatible with new series. Some gold
plating. High Quality co-axial
connectors.. Just.$19 for 24. You get.....
2x...43 - Panel or Line Push on Female
5x...106 - Straight Line Male Push On
2x...172 - Line Male Push On 45 Deg
7x...169 - Panel or Line Female
3x...171 - Line Female 90 Deg
2x...173 - Straight Line Male
2x...105 - Line Male 90 Deg
1x...30 - 90 Deg Line Male
An international supplier
Lists similar connectors
for more than 10 times The price!!!
ENCODER / DECODER CHIPS
As used in remote control devices AX5326 encoder, AX532-7 decoder with 4 bit
latched outputs, AX532-8 with Valid Data
output. These chips could be used to
control up to 16 items (relays etc.) down
one wire all with brief application sheet:
$3.50 or combo. of any 10 for $25
EXPERIMENTERS LENS SET:
Set of 4 high quality Military spec.
lenses. Experiment with convergence,
divergence and magnification.
Could be used to
extend the range of
infra-red systems,
like IR communicators, PIRs & lasers. $7
12VDC - 240AC INVERTER Features
include modified square wave output,
Auto start with load sensing, Uses six
power MOS-FETS with minimal
heatsinking required. 200 - 600VA.
Dependant on trans former size. To save
money you can use an rewind your own
transformer. Basic kit includes pcb & all
on-board components +
4 X 60A MOSFETS. $35
Requires 240V to 8-0-8 V
Transformer..
Ring or
E-Mail for
More Details.
BEST VALUE $1
for our famous wiring kit with any order
COMPUTER CONTROLLED STEPPER
MOTOR DRIVER KIT
can drive larger motors,
Has optoIsolation. Inc.
Software & notes: $40 Or
$50 with two Used 23
frame 200 step 1.8 Deg. motors!!
NEW DIGITAL BAR CODE WANDS:
USA made, with 2.5m
curly cord & 5pin 240°
DIN plug. With an
Optical sensor, visible
Red LED, a photo IC detector & precision
aspheric optics. Converts bar codes to
digital pulses. Uses a Sapphire tip, pot
size 0.19mm. TTL / CMOS compatible
open collector output. Req.5Vsupply:
KEY-CHAIN LASER POINTER
Very bright 650Nm laser pointer
in a high quality machined
metal housing
$20
FOR SALE TO ADULTS ONLY
VERY BRIGHT LASER MODULE
650Nm laser module
as used in the
above pointer.
(Lm2)
FOR SALE TO ADULTS ONLY
**************NIGHT VISION***************
IMAGE CONVERTER TUBES
Hard to find deep IR tubes
As used in night
viewers. Tube plus
EHT power supply
kit plus suitable
eyepiece: $50
UNIDIRECTIONAL ELECTRET
MICROPHONE
New quality product
with clip, 3M lead,
2.5mm plug: $4
Make a stage quality
wireless microphone
by combining it with
our FMTX MK2 transmitter kit:
$16 for the kit plus the microphone
************ OPTICAL PRISMS *********** Series I, 3,4 CHANNEL UHF RECEIVER:
Ref: EA Mar 94. Control up to 4 output
These are military spec.
optically pure glass prisms
relays. Uses a pre-built and pre-aligned
set in a diecast mount
UHF (304MHz) receiver module & security
(removable).They are in
coding ICs. Output relays have 5A contact
A1 condition and stored in
PO Box 89 Oatley NSW 2223
ratings and can be configured for toggling
plastic
(no
scratches)
Ph ( 02 ) 9584 3563 Fax 9584 3561 They will show colours
operation at each press of a Tx button or
orders by e-mail: oatley<at>world.net of the spectrum on a wall
momentary operation when Tx button is
www.oatleyelectronics.com
pressed. 1 X 3ch transmitter plus 1 X4ch
when placed in sunlight $12.50
major cards with ph. & fax orders,
We also have a small Quantity of very receiver:$50 extra Tx $15 is req. to access
the fourth relay. 12V operation. (K39) $70
large prisms “RING FOR DETAILS”
Post & Pack typically $6
OATLEY ELECTRONICS
$35
$18
NICAD CHARGER & DISCHARGER:
Professional, fully assembled & tested fast
NICAD battery charger & discharger PCB.
Switch mode circuit. Has 6 ICs, 3 indicator
LED's, 3 power MOSFETS, a toroidal
inductor & many other compon-ents.
Nominal unreg. input 13.7V DC, 900mA
charge current. Appears to use volt slope
detection to end charge, also a timer
(4060) to end charge. We supply a
thermistor for temp sensing. Probably for
fast-charging 7.2V AA nicads. 3 trimpots
for adjustment + Basic info. $9 or 3 for $21
LARGE LED DISPLAYS
70mm HIGH 7 SEG.
STANDARD TYPE DISPLAY .
(no data available)
JUST $20 FOR 7 (Dl2)
TELESCOPE Build your own,
with our high quality components: 1 X
eyepiece lens worth $5 + 1 X prism (to
invert the image) worth $12.50 + 1 X large
object lens worth $27 + construction plans
all for the price of just $35
NEW SUPER LOW PRICE + LASER
AUTOMATIC LASER LIGHT SHOW KIT:
MKIII. Automatically changes every 5 - 60
secs, & is adjustable. Each motor has 8
speeds, one motor is reversible, & one can
stop. Countless great displays from single
to multiple flowers, collapsing circles,
rotating single and multiple ellipses, stars,
etc. Easy mirror alignment with “Allen
Key”. Kit inc. PCB, all on board components, three small DC motors, mirrors,
precision adjustable mirror mounts: (K115)
+ very bright 650nM laser (LM2) module.
$59
UHF DATA TRANSMISSION
Stamp sized Xtal locked 433.9MHz
superhetrodyne receiver module $25
Small matching transmitter kit: $12 (K122)
SOLID STATE 4-6A PELTIER EFFECT
COOLER / HEATER 3.3A<at>14V(GP1)
PELTIER: $27, 6A<at>15V(GP2) Peltier:
$35, both approx. 40X40X4mm, temp.
Control via supply voltage /current, will
even work from a 1.5V battery!! With data
sheet, diagram
& circuit for a
Fridge /
Heater.
OVERSPEED MONITOR KIT
Ref EA Feb. 97.Gives a pulsed tone signal
when preset speed is exceeded. 12V
operation. A small PCB is provided for a
Hall Effect pick-up sensor. This assembly
is mounted near the drive shaft and
connected to the main PCB by three wires.
Kit inc. two PCBs & all on-board components, a small speaker, & two small
powerful 'rare earth' magnets: (K99) $22
MEGGER METER / INSULATION
TESTER For testing for insulation
breakdown or moisture ingress etc. of
cables or connectors etc. This kit will
deliver a genuine 500Vdc in to a 1M ohm
load!!! This means that unlike other cheap
kits it performs to AUSTRALIAN
STANDARD AS-3000. Kit inc. PCB, all onboard components, surplus meter movement plus instrument case for just $50
*** NEW *** NEW *** NEW ***
HIGH POWER IR FENCE / DRIVEWAY /
DOOR MINDER Uses include powerful
Passive IR detector, invisible fence / gate
& doorway monitor. Range: with 5 IR LEDs
40m (can drive 50 LEDs), can be boosted
with a torch reflector. The kit has on board
relay + active HI & active LO outputs for
relays etc Simple to construct PCB can be
cut into two for active mode Kit inc. PCB,
all on-board components, 5 IR LEDs +
salvaged new plastic case All for $18
Extra box + swivel mount $3
SC-JAN-99
Serviceman’s Log – continued
circa 1986). His manner was aggressive from the start. “Remember this?,
he said. “I brought it in a couple of
months ago? Well, it’s doing the same
thing again”.
“And what was that, Mr Borland?”
“You know, it doesn’t work. There’s
no picture – just like before”.
Summoning up as much poise and
dignity as I could, I booked the job
in without further comment. When
he had gone, I looked up its service
history. It was just as I thought. The
unit was last in some three and a half
years ago for a “no stereo” problem,
caused by IC203, C251 and C253.
Hmmm! – that’s a bit different from
being in a couple of months ago for
no picture.
When I had a chance to look at it,
the fault was no off-air reception.
Once the lid is off, access is easy and
I started by checking the four voltage
rails from the power supply, which
were all OK.
Next, I checked the voltages to the
tuner, TU101, while in the search
mode. There was 12V at this point
and the band switching was all OK,
as indeed was the tuning voltage, varying from 0-30V. The only clue was
that the RF AGC didn’t seem to vary
at all. I checked the video output from
pin 13 of IC201 while still searching
on all bands. There was no output at
any time.
This brought me to IC201, a TDA–
4429T. In view of all the symptoms,
it was the most likely culprit. This IC
ran very hot even though it was fitted
with a heatsink. However, it was also
possible that either the tuner or IF
circuits were at fault. To be certain,
I substituted a test tuner that I keep
for just such occasions. This is a
freestanding rotary VHF tuner with
its own manually adjustable AGC.
There was still no picture and I was
now virtually certain that it had to
be IC201. When I checked the price
of this device, I was horrified to be
quoted a trade figure of $86. The
real problem now was informing Mr
Borland.
I phoned him and presented the
facts. His response was typical: “But
you replaced that part before and I
haven’t used it since then – I don’t
think much of your guarantee”.
72 Silicon Chip
I couldn’t make much sense out of
his claim that the unit had not been
used since I last serviced it and said
so. I also calmly pointed out that it
had in fact been 31/2 years since I
last serviced it – not two months as
he claimed – and that I had replaced
entirely different components for
an entirely different problem. And
I couldn’t resist pointing out that in
12 years, this was only his second
problem with the unit.
It all washed right over him. He
abused me for a few minutes but I
was past caring what he thought. I
told him that that was my price for
the work that had to be done and that
I guarantee only the parts I fit and the
work I do, and nothing else. He could
either take it or leave it.
Finally, and ungraciously, he decided to accept the estimate and I placed
the order for the IC.
Three weeks later it arrived, I installed it and tuned in the stations;
my diagnosis was correct, much to
my relief. However, I was dismayed
to find that the pictures were very
poor and distorted, with severe
“pulling”. Adjusting the tuner RF
AGC pot, RV201, varied the quality
of the picture, which suggested an
AGC problem.
There were three electros near the
IC that could have dried out from
the heat. I thought it worth a shot
and replaced C216, C215 and C214
in that order. It wasn’t until I got to
the last one that I fixed the problem.
I installed 105°C capacitors instead of
the original 85°C types and mounted
C214 underneath the board on the
copper side, away from the heat
source.
Mr Borland was still firing broadsides when he unappreciatively
picked the set up a few days later,
despite the extra work I had done at
no further charge. I guess there’s no
pleasing some people.
Sony monitors
In striking contrast, my last story
concerns a small firm of accountants
who have a fleet of Sony PD-1704S
computer monitors. These are rather
nice 17-inch SVGA multiscan monitors but are now getting long in the
tooth – they were made in 1992. So
far I have had four of them in, mostly
suffering from east-west distortion
problems.
The difference between this firm
and my last client was entirely in their
attitude. These accountants were in
no way pushy, understood that the
equipment was complex, and understood that the faults were intermittent
and required time to soak test. They
also appreciated the costs involved.
The problems were nearly all confined to a plug-in module DA(DC-1)
on the righthand side of PBG-626-S
(looking at the rear). This board
has nearly all the 23 preset controls
mounted on it and it didn’t take a
mental giant to determine the cause
of the faults. There are no less than
20 subminiature surface mount
ed
electrolytic (CHIP) capacitors mounted on this board, some of which are
now leaking electrolyte and causing
corrosion.
There is a very simple test to determine whether a capacitor is faulty or
not and that is to heat its terminals. If
faulty, it will produce a pungent fishy
odour from the vaporising electrolyte.
However, it is an expensive exercise
to replace all these capacitors at one
time, even though it probably makes
more sense in the long run. (The
capacitors are expensive and it is
extremely time consuming and fiddly
to replace them).
So which are the critical ones? The
most common fault I had experienced
was a trapezoidal picture. By heating
and freezing the board, I eventually
concluded that C349, C331 and especially C334 were the worst culprits
and so I replaced them with ordinary
105°C 35V electros. When the old
capacitor is removed, it is necessary
clean off the electrolyte and repair any
corrosion to the PC tracks.
When freezing these capacitors,
slightly misleading results can sometimes occur due to water vapour
condensing on the electrolyte that has
oozed onto the PC board. One fault I
encountered caused the set to come
on, intermittently go off horizontal
frequency and then shut down. This
turned out to be C110 on pin 5 of
IC101, plus C311 and C309.
Anyway, the fault in this unit responded to the replaced capacitors
and to a general clean up of the leaked
electrolyte. And the accountants were
extremely happy and better still, paid
SC
on the spot.
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