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SERVICEMAN'S LOG
The day my multimeter lied to me!
I have had a real mixture of sets this month,
including one that bounced from last month
and a couple more than 10 years old. They
all produced their fair share of frustration
but they were all beaten in the end.
I do make mistakes. There, I’ve admitted it; I’m not perfect. So shoot me.
Well, Mrs Evans may very well
have felt like doing just that, with
good reason. Last month I described
how her Sony KVF29S Z 2 (G3F chassis SCC-G711-A) had no sound and
18 Silicon Chip
intermittent east-west pincushion
distortion. I had traced the fault as
being down to IC303, the 12V switchable regulator.
It wasn’t a hurried job. I did soak
test it for well over a week before
giving it back and it did go for almost
a month after that before the original
fault re-occurred.
While I’m in full flood, I should
confess that there was another symptom which I hadn’t really taken any
notice of. The picture had looked
slightly washed out, as though the
tube had low emission which would
have been surprising for a set less
than four years old.
But when the fault was fixed, the
picture had improved. So I thought
nothing of it.
After delicately smoothing down
some ruffled feathers and generally
eating humble pie, I got back into
Mrs Evans’ TV problem. I thought it
not unreasonable to apply the same
medicine as before, namely replacing
the PQ12RF21. It was quite possible
that the new one had failed.
These PQ12 switchable IC regulators are made by Sharp and come in
a series, with a choice of different
numbers for the last four digits for
which I am unable to find any data. I
could obtain PQ12RF11 and PQ12R04
from my local supplier for only $4.00
or so, but to get hold of the PQ12RF21
would cost me $30 at trade price.
Why the last four digits should
mean such a massive increase in price
was beyond me as all the packages
looked identical (TO-220 with four
legs).
As the fourth leg is only the switchable pin, I eventually decided to use
a 7812 3-pin regulator as a cheaper
test substitute and only to prove the
point. It was just as well as it made
no difference to the symptoms. Obviously I was barking up the wrong tree.
Because I was making voltage
measurements where only a few volts
seemed to be making a difference,
I thought using a digital meter was
sensible and more accurate to monitor
the voltages.
As the 12V rail was down to 11V on
pin 2 and varying, I thought I would
try downstream and see if the load
was too great. I changed the Pin Con-
SETS COVERED THIS MONTH
•
•
•
•
•
Sony KVF29S
Sharp CX-4814
Sharp VC-H865X
Masuda T1092
Bell & Howell VS-IC
trol IC2504 (UPC393C) but to no avail.
Next, I checked a bit further upstream to find that the voltage on pin
1 was also 11V and the 15V source
was down to 13V.
I hung a few additional electros
on the printed circuit side to see if
this would change anything, on the
off-chance that there were faulty ones
on the component side, but it made
no difference.
I then checked the main 135V rail
to find that it was also low. Indeed,
all the voltage rails were low, which
would account for the poor picture.
I also found that the voltages I read
were different every time I switched
the set on and off.
A bit disconcerted, I continued my
quest. I thought that if the secondary
voltages were low, the voltage reference must be faulty, so I replaced
IC602 SE135. This is a common 3-pin
regulator that controls the optocoupler IC600 PC111 to the main switchmode chopper, IC601 STR-S6708. The
voltage on pin 2 of IC601 was 73V
instead of 64.7V.
This made no difference until I
changed the optocoupler as well,
whereupon the fault suddenly cleared completely and the
sound reappeared. Great!
But a glance at the multimeter showed that the voltage
had now soared to 150V. I
quickly shut down the set.
This left just the STR-S6708
to replace, which I did.
Then just as I was switching the set back on, in the
corner of my eye I noticed a
spark and I heard a crackling
noise.
I’m not sure that you would
call this a lucky break, because in one sense it wasn’t.
I was indeed fortunate in
seeing where it occurred but
it surely caused some sort of
damage – probably expensive! I had of course immediately switched the set off.
The spark occurred at the
ground (pin 10) of the chopper transformer, T601, and the printed circuit
to this pin is bisected exactly at the
pin so that it couples the negative
sides of C613 and C616 through pin
10 to ground.
The spark was caused by an invisible hairline fracture under the lacquer and screen-printed component
markings, right on the edge of pin 10
which is a solder rivet joint. Indeed it
certainly was an expensive crackling
noise because the damage caused was
quite extensive, requiring the replacement of all my familiar friends, IC303,
IC602, IC600 and IC601.
When I had done all this, it fired
up correctly but the digital meter was
still reading high secondary voltages.
I couldn’t bear this. What had I done
wrong?
I left the digital meter monitoring
the 135V rail and used an old analog
meter to check the rails, especially
the 15V & 12V ones. To my surprise,
these read correctly.
This wasn’t making much sense
any more so I checked all the rails
with the analog meter.
Guess what? They all read correctly, including the one that the digital
meter was showing as 20V higher!
The two meters were flagrantly
arguing with each other, so I got out
Fig.1: the relevant portion of the Sony KVF29S Z2. If you think that the reproduction is poor, it’s not! This is typical of
the circuit diagrams that service technicians have to work with. But it’s a positive work of art when you compare it to
the PC board component layout (right) which shows the same section.
April 1999 19
yet another meter to determine which
was right.
Fortunately, the analog one was
correct which also meant that the
set was now fixed properly. I think.
And hope!
So why didn’t the digital meter
read correctly? I’m not certain; possibly because its 9V battery was low.
Later on, after I had replaced the
battery, I checked the digital meter
on a known power supply and the
voltages were correct.
However, I feel that I can no longer
trust that meter. Call me old but I
prefer the analog meter. After all, it
is WYSIWYG – what you see is what
you get!
The TV set is still on test as I write
because I don’t want to do any more
grovelling than is absolutely necessary. As Clint Eastwood’s Dirty Harry
once said, “A man has got to know
his limitations”.
I certainly know mine.
The 10-year old Sharp
Normally, I don’t touch 10-year20 Silicon Chip
old sets but Ms Bell smiled at me
so sweetly I was beguiled. She had
struggled in with a Sharp CX-4814,
which really isn’t very heavy, but
she had got this far so I guess I just
had to fix it!
I certainly helped her carry it into
the workshop. The set was dead but
there were so many really bad solder
joints on the motherboard I wasn’t at
all surprised.
Anyway, at switch-on I could hear
the 15,625Hz timebase whistle, the
crackle of the static from the EHT and
could see the filaments in the CRT
light but that was about it.
The 115V, 24V and 15V rails were
all OK and running my fingers along
the pins of IC201 produced noise in
the loudspeaker.
It appeared as though we had lost
all the small signal circuits. I have
had a lot of experience with this series
of sets and I immediately suspected
I201 (IXO506CE) as I had had many
fail on me before.
The major nuisance of this IC is
that it has 30 legs and you can’t buy
IC sockets for it. Instead, you have to
use socket strips. The other bugbear
is the metal screening cage around
it, which makes access poor. I really
didn’t want to change this IC as it is
also very expensive.
So before I started I thought I would
just check the voltage rail feeding it
in case an unseen dry joint was the
culprit. Good move. There was no
12V at pin 4 and after chasing it back
all the way to Q603 I could measure
15V on the collector but nothing on
the emitter or base.
The transistor turned out to be
OK and there were no shorts on the
emitter or base circuits. The bias that
feeds Q603 is derived from the +115V
rail via R624 and R645. Both seemed
to measure correctly in circuit but
on removal R624 (68kΩ) was nearly
open circuit.
I fitted a new one and the set burst
into life. Ms Bell would surely smile
on me again. Sigh!
Not so the boss. She had noticed
me booking in the set rather slowly
and reminded me curtly of company
policy on old sets. I slyly shrugged it
off, saying things were a little quiet so
we should take on some of the dross
and besides I did have a little expertise on these, from all our rental sets.
Just to prove the point about old
sets, a young man brought in his Bell &
Howell VS-IC TV/Video which would
have been over ten years old too.
This set is an Australian hybrid
of a Sharp X-3434D television and
a National N-180EN video recorder
put together with a bizarre add-on
extension case.
Dead flyback transformer
The set was dead but I took the job
on anyway in case I was accused of
being inconsistent.
This was a mistake of course,
because the headache of this model
is detaching the extension VCR and
case. It is held on with two plastic
screws on the rear and four concealed
screws on the side.
To make it worse, these screws are
concealed with little plastic covers
and are three inches deep inside the
cover, too dark to see. It took me ages
with the aid of a small torch to work
out that they were 4mm Allen screws
and you needed a really long shaft!
Despite all this, I guess the easy part
was taking it apart.
So was the diagnosis and replacement of the flyback transformer T601
(TRNF1412CEZZ). The really difficult
part was getting the whole thing back
together, especially as the screw retaining plastic washers had got lost.
Trying to get four Allen screws in
simultaneously without one falling
inside was extremely difficult. In
fact, trying to get it all back together
took longer than disassembling and
repairing it.
The other major drama was that the
TV set wouldn’t work without being
connected to the VCR via an interface
which was too hard to juggle whilst
disassembled. Things got even worse
when I discovered after I had got it all
back together that there was no sound.
There was nothing for it but to
go back in. Swearing and sweating,
many hours later I finally diagnosed
and replaced IC301 (IXO250CE) and
got it all back together again.
In the process of dying, the old
flyback transformer had punctured
the insulation and arced over and
killed the IC. The reason I hadn’t
picked it up before was because of
Fig.2: the trouble that one little half watt resistor got me into (and I'm not just
talking about a difficult service job)! Still, she it was worth it in the end!
the additional TV mute button on the
rear extension!
Aptly named . . .
Mr Bradley brought in his beautiful
Sharp VC-H865X hifi VCR. I say beautiful because not only did it look good
but it was immaculate, as though it
had just come out of the box. He had
really looked after it.
This was a great shame because
when he told me what the problem
was I thought that might well be the
end of it.
Its fault was that none of the controls worked and the display randomly showed different segments. This
rang alarm bells with me,
spelling out “expensive microprocessor”. Not
only that but
sometimes they
are very difficult to replace,
especially if it
is a 64-pin surface mount and
sometimes it
can be either
o n e ( Ti m e r
or Syscon) or
even both ICs.
The only way to
confirm these large
scale ICs is to replace
them – but which one first?
Anyway, I told him that I doubt-
ed that it would be worth fixing and
explained, as best I could, my dilemma with the above scenario. Initially
crestfallen, he took it badly, so I said
I would have a quick look and see
if anything else might be causing it;
otherwise I would advise him to get
a new one.
The only things I could check were
voltages, clocks, dry joints and cracks
and possibly corrosion from the old
brown glue – (aptly named gorilla
snot.) It didn’t take long to ascertain
that all was perfect in the peripheral
circuits to the ICs and it really looked
as though replacing one or both was
April 1999 21
Fig.3: the electros were easy – pity they also decided to take out the switches
in this Sharp VCR.
the only cure and that was uneconomical.
In the course of examining it, I noticed that of all the random displays
the VCR was giving, one seemed
more persistent than all the others.
The “tuning up” symbol was flashing
as though someone was pressing the
button. I thought it was worth having a
closer look on the timer control board
behind the front escutcheon. Because
of its complexity, it wasn’t easy to
remove but finally I got it out.
Under the mag lamp I checked for
cracks and found none but when I
examined the tuning board, I noticed
the metal was dull and slightly discoloured.
On the copper side the pattern was
all corroded around this area. Now I
was getting excited – here was a possible cause of all the strife. What had
caused all this corrosion? I doubted
that it could be from external sources
as this location was too far inside the
VCR and was localised.
22 Silicon Chip
Back on the component side it didn’t
take long to find the cause or causes.
There were two very small electros,
C5020 and C5021 (220µF 6.3VW),
which were a complete mess, leaking
down onto the SW5001 and SW5002
tuning/tracking tactile switches.
Cleaning them up and replacing
the switches was easy; finding small
replacement electros was a little harder. Anyway, this effected a complete
repair and the unit is now back in
service.
Masuda trouble
These days I tend to shudder when
I hear the name “Masuda”. It’s not that
it sounds like a Japanese food dish,
it’s just that it spells T.R.O.U.B.L.E.
These sets were originally imported
by Brashs and are no longer supported
at all.
I feel sure that the reason why there
was one on my desk to be repaired was
a reprisal by the Boss over the Ms Bell
affair – which wasn’t an affair and was
of course quite innocent.
Anyway the written command on
the job card was “Dead – fix”; not even
a “please fix”.
This was a Masuda T1092 which
is an AC/DC 27cm remote control
TV from Taiwan, also sold under the
name of Akai, Aiko, Hanimex, Tandy
and Silver.
The most common problem with
this set is the failure of IC402, a custom-made 3-pin 11V regulator block.
However, in this instance it was a
different problem. F402, a 5A fuse,
was actually glowing and there was
no sound or picture.
The 11V rail was low and got even
lower as it reached the flyback transformer. I switched the set off and
started looking for shorts to ground
with the ohmmeter but could find
none, despite the huge current.
It took a long time disconnecting
and measuring backwards and forwards between IC402 and T404. The
line output transistor was OK and so
were all the connections to the flyback
transformer.
I was beginning to actually suspect
the flyback transformer of breaking
down under load when, more by luck
than judgement, I measured D410
FR605 out of circuit to find it was very
leaky. I replaced it with an FR607, a
1000V 6A high-speed diode which
actually fixed the problem.
However, after the set had been
on for only fifteen minutes or so, the
diode was amazingly hot to touch. I
added another diode in parallel but
even then they were still running
very hot.
I left the set to soak test but it seemed
quite stable and a week later showed
no further sign of stress. I checked all
the voltages again (11V on the anode
and 22V on the cathode), reboxed it
and sent it home.
I couldn’t help wondering why the
diodes, now rated at 1000V and 12A,
could still be so hot! No wonder the
original didn’t stand a chance. What
a weird TV!
The easy NEC
With some sets you can’t help
thinking, “this has got to be an easy
fix”. The symptoms are clear and logically, the answer has to be equally
so. So I thought I had it made when a
Thai-built NEC N4850 came in. I think
this is an NEC design as the board has
PWC3607A printed on it.
Fig.4: the relevant circuit section for the NEC N4850. The “A” in the type
number for IC601 makes all the difference.
Now how easy is this? The fault is:
goes dead after one to two hours and
if you freeze IC601 (just ever so slightly) it comes good. Probable solution:
replace IC601; no need to even look
at the circuit. Did that. I ordered the
STR50115. It came the next day and
I fitted it –piece of cake. Switched on
confidently. Goodness gracious me!
A complete new set of symptoms –
the set was still dead but the two front
LEDs were pulsating very slowly. I
must have made a mistake.
Silly me. I checked everything; it
was 100% OK.
To be doubly sure, I removed the
new IC and refitted the old one. The
set came on perfectly and then went
off an hour or two later. My conclusion
was that I had a duff new one.
These things happen, so I ordered
another. When it arrived, I slammed
it in – the set was still dead and pulsating. I compared the new ones with
the original – no difference.
It was time to get technical. I dug
up the circuit diagram of the set. No
clue immediately hit me. There are a
couple of modifications to this set, one
being to add a .001µF 2kV capacitor
across C613 and C612 is upgraded
to .0047µF but doing these made no
difference.
I finally found the reason. It stares at
you so obviously in the circuit and you
probably picked it straight away – the
answer is, of course, the IC.
It is marked on the circuit as STR
50115A. The A is not always printed
on the component but it makes all
the difference, and fitting it with new
heatsink compound (it gets very hot)
fixes the problem completely.
SC
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April 1999 23
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