I had a 2002 Sony PFM-42B2E flat panel plasma monitor come in
with lines on the screen. This unit uses a Fujitsu PDP (plasma display panel).
Fortunately, we had another identical set which had just been fixed and was on
soak test, so we were able to swap modules and isolate the problem to the B
As described in a previous article, when working on these sets,
we use the cardboard packaging that protects new plasma screens. The raised foam
blocks glued onto this cardboard support the screen, while fragments of mirror
tiles placed between the blocks enable us to observe what is happening on the
screen during the course of the repair.
A small problem is what to do with the enormous amount of
screws and hardware that accumulate as you strip down the machine to access the
boards. Most times, you have to have an array of plastic boxes to put all this
stuff into. Unfortunately, with the pressure of work and the demand for quicker
and quicker service, the screws are invariably left resting on the back face of
the plasma panel.
Items Covered This Month
Sony PFM-42B2E plasma monitor
Sony KV-E29SN11 TV set (BG1L chassis)
W6900S/SF MkII TV set
Sanyo WF2-00 TV set (EB7-A32 chassis)
CA-20F898 TV set (MC-994A chassis)
Panasonic Dimension 4 Genius Convection Microwave oven
PT-LC50E LCD projector
In this case, I was under pressure and being distracted by a
colleague who has a strong accent and can sometimes be difficult to comprehend.
Anyway, I unscrewed the metal screening cover and instead of putting it to one
side, I left it lying on top of the power supply (Switching Regulator APS-179).
And as you can probably guess, when I put the new board in, I switched the set
on to check whether all was OK and completely forgot about this screen.
Of course, that was a big mistake. The metal screen shorted out
one of the live heatsinks to an earthed heatsink, which completely killed the
My colleague thought that this was hilarious. "Mate", he said
with this heavy accent, "you know why you are called the wonder technician?
Because you are one day technician, no? Ha, ha!"
Oh well, humour is all in the eye of the beholder. One day,
I’ll give him his "one day technician" stuff right back in his face!
OK, so I’d started with a working plasma panel, got it to give
a good picture and then, because of my carelessness, had made it to stop working
altogether! However, it really wasn’t that bad – power was getting into the
board and some voltages were present. What’s more, there were no obvious burnt
components, nor had there been any major noises or explosions when it
Encouraged by this, I removed the board and checked all likely
fuses and fusible resistors. Nothing was immediately obvious until I measured
R101, a 10Ω resistor, which was open circuit. Replacing it fixed the problem and
I was back in business.
But I was lucky – you really can’t afford to be careless or
complacent when working with this kind of gear. And I’ll remember my colleague’s
"one day tech" jibe.
TV sets hit in electrical storms are always dodgy – especially
when it comes to warranty claims later. Lightning, being the erratic animal it
is, can damage all sorts of odd circuits in a set, causing all sorts of bizarre
trouble. And it’s not just the parts that immediately fail that cause problems.
Many other parts can be put under so much strain that they barely survive but
then fail later.
Recently, I had a Sony KV-E29SN11 (BG1L chassis) which was dead
after an electrical storm. The switchmode power supply was still working, though
its output was low.
I removed the screen covers over the microcontroller and found
that the 5-pin 5V IC regulator (IC601, L78LR050-MA) had burnt up, creating a
hole through the board. I also found that a 22Ω 2W resistor had been fitted
where link JW158 is shown on the circuit. This resistor had been getting very
hot from the current it feeds to IC601.
Further up the food chain, I found that Q601 (2SA1315-Y) and
R606 (18Ω) were similarly getting hot supplying current for IC601.
Interestingly, this is only a back-up circuit for the main 11V rail that also
feeds IC601 via a diode. This back-up circuit is also part of the power-on
circuit and the 135V control circuit (which caused the 135V rail to drop).
I replaced all the parts and though I had some success, the
results still weren’t satisfactory, with too much current still being drawn –
probably by faulty internal diodes and zener diodes in the microprocessor IC set
on the 5V rail. In the end, this set was written off and the client got a new
one with the insurance money.
An easy intermittent
A Chinese-built 2002 AWA W6900S/SF MkII came in with an
intermittent no picture fault.
Of course it took a very long time for the fault to re-occur
and as luck would have it, it only appeared just after we had told the client to
come and collect it. Fortunately, it was an easy fix; the CRT filaments were
intermittent and the fault was traced to a hairline fracture on R920, the 1Ω
series resistor from the flyback transformer to the CRT heaters.
I wish that all intermittent faults were as easy as
Doing a Lazarus
A newish Sanyo WF2-00 (EB7-A32 chassis) came in DOA (dead on
arrival) and still under warranty. Our job was to raise it from the dead – just
Anyway, the set’s switchmode pow-er supply had spat the dummy,
blowing chopper transistor Q613 (2SC4429), its driver (Q612, 2SC3807) and the
surge resistor (R695, 1.8kΩ 7W). These parts were all replaced and the set
indeed rose from the dead.
The only awkward part of this miracle was that its OSD was in a
foreign tongue. I didn’t have the instruction book but I struggled on with the
menu before I worked out that it was stuck in Croatian! Once I knew what I was
doing, I was able to set up the tuning for the local stations.
Personally, I feel that all manufacturers should get together
for a convention on the OSD language options and colour code, or place them in
the same position in all sets. It would make it all so much easier if, for
example, English was always in red and Croatian in orange (say) and if the
language option was always the fifth item on the menu list. Still, who listens
to the technician?
The ticking LG
A dead 1999 LG CA-20F898 (MC-994A chassis) 48cm TV was brought
in to be repaired. Nowadays, these sets really aren’t worth looking at,
considering the price of new ones, but the owner insisted.
Anyway, this set was ticking (pulsating), which meant that the
switchmode power supply was in protection mode, probably due to a short
In a beachside suburb, the usual culprits are the flyback
transformer and the line output transistor which would make the cost prohibitive
in such a budget model. However, the owner was persistent – he felt sure it
could be fixed within his budget and told me to do the best I could.
Well, of course, the line output transistor (Q402, 2SD2499) had
gone short circuit so I replaced it, half-expecting it to instantly destruct
again at switch-on. Instead, I got a vertical white line down the centre.
This was unusual and is almost always due to a dry joint in the
deflection stage – which in a 48cm set is really simple to fix. However, there
were no dry joints and the yoke and the horizontal linearity coil were both
That just left yoke coupler C412, a 0.33μF 400V high-current
capacitor. Replacing it fixed the problem and the repair came in within budget –
a win-win situation for both of us.
Squashing the cockroaches
Edith Montague is in her late seventies and still drives a car
– well, barely. One hot summer day she arrived at our place unannounced and her
first words to me were that she had got horribly lost and it was all our
We are obviously in the wrong place and all our signs are
When she finally finished blowing us up for getting lost, I
learnt that there was a microwave oven in the back of her car, which Edith
repeatedly told me would not give a display. I carried the microwave into the
workshop for her and placed it on my bench.
This particular unit was a 1995 Panasonic Dimension 4 Genius
Convection Oven, model NN-C855B. I plugged it in and the display came on
straightaway. What could she have been thinking? Well, it might be intermittent
so I thought I would give it the once over and took the covers off.
As soon as I did this, you could hear the rustle as hundreds of
cockroaches ran for cover. I smartly replaced the cover and took it outside
where I sprayed a large amount of insecticide inside and beat a hasty retreat.
The effect of this poison was pretty rapid and I subsequently blasted the
corpses out with an air compressor before carrying the oven back into the
The cockroaches alone would be easily enough to cause
corrosion, in turn giving an intermittent display. However, the problem had to
be investigated further, so I removed the front control panel and the PC board.
When this was out, I removed a few more jammed corpses and egg sacs before
examining the PC board itself. This carried the general mess from a cockroach
infestation and there was also some of the infamous brown glue spread over some
of the components. And as if that wasn’t enough, there were dry joints on pins P
and E of the mains transformer (T1).
I also noticed that one of the oven’s lamps had failed and so
gave her a quote to fix all the above. She wasn’t prepared to just take my word
for it though and wanted a detailed explanation of all the faults. However,
after having a good whinge about the very reasonable estimate I had given her,
she finally agreed to the work.
The job was straightforward and when I had finished, I gave it
a thorough test to make sure it was working properly. The display was now
excellent, especially after a good clean on the outside of the window. I then
advised Edith that the oven was ready and she came straight over.
When she eventually arrived, I got another tongue lashing over
her getting lost a second time. Now, I can understand someone getting lost once
but twice is somewhat careless. After all, we have been at this address for over
six years and no-one else had ever got lost.
I wasn’t in a position to demonstrate the oven but I assured
her that the display had been fixed and that I had tested it. With that, she
paid and left, presumably to get lost on the way home. Anyway, I thought that
would be the end of the matter but it wasn’t to be.
A few hours later, Edith was on the phone, not only complaining
vociferously that she still had no display but demanding that I come round
immediately and fix it in her home. I was incredulous. How could this be? There
had to be another factor. "Are you quite certain there is no display?", I asked.
"Absolutely", she insisted.
Perhaps she hadn’t plugged the oven in – after all, that would
give no display. I asked her if the light came on when she opened the oven door
and was told that it did, so scratch that theory.
This had me baffled. As a compromise, I grudgingly said I would
call around but explained that I had no facilities to fix anything in her home.
If there was a genuine problem, it would have to go back to the workshop.
When I got to her place, I finally understood what she meant by
"no display". The display wasn’t blank at all but was instead showing just the
two little dots which form the colon between the hours and minutes digits. And
that to her meant no display.
I switched the unit off and then on again at the power point
and the usual "WELCOME" display message
immediately came on. However, it would then go into "9 LAMB" and 10 CHICKEN" alternately. Apart from the STOP/RESET button, nothing else worked and I was just left with the time colon. I
couldn’t even set the clock and it was fairly obvious that the membrane switch
was jammed on the 9 Lamb/10 Chicken selection.
Back at the workshop, I removed the membrane switch altogether
and it was indeed faulty. Switch Q17 was intermittently short circuit and had
probably only previously worked in my workshop because it had been a hot day,
which kept the air expanded between the two halves of the membrane switch.
Clearly, the unit would have to be replaced but by now, Edith
had lost confidence in both me and her microwave oven. She phoned to inform me
she had bought a brand new microwave and asked what part of her money was I
going to refund because she "hadn’t got anything for it"!
There was really no point arguing but I still wanted to recoup
my labour costs for the work I had already done. Anyway, I told her that if she
left her microwave behind, I would tear up her cheque.
For some reason, this didn’t register with her and she just
kept repeating her question over and over again – like a needle stuck in a
groove. Eventually, I was forced to put a stop to her tirade. I bluntly told her
to stop talking and listen very carefully. What had I said to her?
Well, she couldn’t remember. So, very slowly, I repeated myself
to her until the message sank in.
She was relieved to get her money back and I got to keep the
old oven as compensation for my time and effort. In the end, I replaced the part
and then sold it, so I did manage to make some money after all.
We had a Panasonic LCD projector PT-LC50E come in requiring a
new lamp unit (Part No: ET-LAC50 – a mere $600!)
However, after replacing the special globe, the projector
intermittently wouldn’t work, giving no functions except an error code CØ in the
display. This means an incorrectly installed air-filter.
After fiddling around with the unit for a while, we discovered
that the cause was due to the door and its sensor switch. However, measuring the
plug with an ohmmeter showed there was perfect continuity when the switch was
This was quite baffling until it dawn-ed on us that when measuring the plug,
the meter probes were pressing in the pins and making contact. The fault was
actually in the plug itself, with the connectors being dry jointed. We just kept
missing this with the way we were measuring it. Resoldering them fixed the